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TECHNICAL NOTES
PREPARED FOR THE
UNITED STATES ARMY
U . s . SCHOOL of MILITARY AERONAUTICS
UNIVERSITY of ILLINOIS
BY
THE TECHNICAL STAFF
1918
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PBEFAOE
In a coarse of intensive training such as is given at the Schools of Military
Aeronautics, it is difficult for the cadets to absorb information as rapidly as it
is imparted by the instructor in the lecture room or laboratory. Hence it is
desirable that cadets be furnished with a syllabus containing the essentials of
the several courses arranged in the order of presentation in class, so that during
their study periods cadets may have the means at hand for reviewing the work
of the day, for obtaining a better understanding of the principles explained, and
for arranging their notes on the subject matter in a more orderly manner than
would otherwise be possible. It is with these objects in view that this volume of
"Technical Notes" is prepared.
Each chapter is preceded by a short outline which gives the main divisions
and sub-divisions of a subject. Since instruction is given in accord with the
outlines as herein presented it is unnecessary for the cadet to make an outline.
His note^keeping is aided and reduced to a minimum, thiU9 making it possible
for him to follow the class- work closely. Such personal notes as he does take, how-
ever, should be classified under the headings tused in this volume.
Each subject is covered as fully as is consistent with the aims of the School.
At the same time the sub-divisions of each subject are presented as briefly as is
consistent with clearness. The aim throughout is to present the information more
in the form of a syllabus than in that of a treatise.
In a few instances, illustrations from other books are used. Where this is
done, credit is given to the author. Acknowledgment of our indebtedness is
hereby made to these gentlemen whom we trust will pardon the unauthorized
use of their work in the present emergency.
AcADEMi€ Board, S. M. A., U. of I.
Urbana, Illinois, June, 1918.
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CONTENTS
Chapter I — ^Airplanss
1. Tbbobt of Flight 104
2. NOICSNCLATUUE 112
3. EZJEICSNTS OP AlBPLANB DSSION 113
4. A88B1CBLT 183
5. Aligkicbnt 134
6. Pbopkllebs 137
7. BiPAiE 141
Chapter II — Cooperation with the Artillery
1. The Artillery and Its Uses 208
2. The Obganization op the Abtillebt 205
3. Methods op Fiee (Banging) 206
4. Methods op Pike (Continued) 209
6. psocedube in cooperating with the artillery 211
Chapter III — ^Engimeb
1. Fundamental Principles 804
2. Carburetion 308
8. Ignition ... . .' 814
4. Cassipigation op Airplane Engines 321
5. Types op Rotary Motors 826
6. Essential Bequirements and Allied Factors 832
7. Engine Troubles 841
8. Notes on Timing 346
Chapter IV — Gunnery
1. Nomenclature op the Lewis Qun 403
2. Stripping and Assembling — ^Lewis Oun 411
8. Mechanism op the Lewis OtN 412
4. Drill and Immediate Action— Lewis Gun 414
5. Stoppages and Jams— Lewis Oun 415
6. Care op the Lewis Qun T '. 416
7. Standard Tests op Stripping — ^Lewis Oun 420
8. Range Practice— Lewis Oun 422
9. Nomenclature op the Marlin Airgrapt Oun 423
10. Stripping and Assembling — ^Marlin Aircrapt Oun 427
11. Mechanism op the Marlin Aircrapt Oun 430
12. Drill— Marlin Aircrapt Oun 433
13. Stoppages and Jams — Marlin Ascrapt Oun 434
14. Care op the Marlin Aircrapt Oun 486
15. Aerial Sights 487
Chapter V — ^Instruments
1. Instruments por Determining Direction 502
2. Instruments por Determining Altitude 504
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3. INSTBUHENTS FOB DETERMINING BPEED THROUGH THE AlR 505
4. Instruments for Indicating Stabilitt 506
5. Instruments pob Detebmining Gbound Speed 506
6. SpECUL iNfiTTBUMENTS USED IN TESTING 507
7. Miscellaneous Instbuments and Theib Specifications 507
Chapter VI — ^Map Reading
1. Classification 602
2. Scales 602
3. Peace Maps and Wab Maps 604
4. Sketches 604
5. Obibntation 605
6. Map Symbols and Abbbeviations 605
7. Oontoubs 605
8. Landscape Sketches 610
9. Photoqbaphic Maps 610
10. The "Squabed" Map 611
11. Map Beading Pbopeb '. 611
12. Pbactical Applications of Map Beading 612
Chapter VII — ^Meteorology
1. The Atmosphebe 702
2. Heat 704
3. Wind 704
4. Clouds 708
5. Ant CuBBENTS 713
Chapter VIII — Signalling
1. AntPLANE Badio Equipment 802
2. Use of Badio in Abtilleby Obsebvation 803
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Chapter I.
AIRPLANES
OUTLINE
1. Theory of Flight
A. Airplane Wings
1. Lift
2. EfBeien<7
B. Airplane Performance
C. Airplane Equilibrium
D. Types of Machines
1. Introduction
2. Land and Water Machines
3. Classification as to Number of Planes
4. Classification as to Position of Propeller
5. Military Maiehines
6. Recapitulation
E. History of Aviation
2. Nomenclature
A. Wings
1. External
2. Internal
B. Fuselage or Body (Nacelle on Pushers)
1. External
2. Internal
C. Undercarriage, Chassis, or Landing Gear
D. Tail, or Empennage
E. Control System
F. (General Terms
3. Elements of Airplane Design
A. Introduction
B. Decision as to the (General Form of the Machine
1. Three Gfeneral Types
2. Choosing among these Three Types
C. Gkneral Conclusions as to the Materials which will be required
1. Necessity for Rigidity and Strength
2. Necessity for Lightness
3. Airplane Material must be Reliable
4. Airplane Material must possess a High Strength-Weight Ratio
5. Occasional Necessity for Special Properties
101
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102 AIRPLANES
D. Data Relative to the Recognized Airplane Materials
1. Woods
2. Steel
a. MildStedB
b. Alloy Steela
3. Metals other than Steel
4. Wires and Cables
a. Aircraft Wire
b. Aviator Strand
c. Aviator Cord
5. Covering Material
a. Fabric
b. Coating
6. Miscellaneoas Materials
E. Fitness of Certain Materials for Certain Classes of Parts
1. Woods
2. Steels
a. Mild Steels
b. Alloy Steels
3. Metals other than Steel
4. Wires and Cables
a. Aviator Wire
b. Aviator Strand
c. Aviatpr Cord
F. Necessity for Streamline Form of Parts
G. Design of small Parts Capable of (General Application
1. Wire and Cable Tension Members
a. End Connections
b. Tumbackles
c. Shackles
2. Hinge and Clevis Pins, Bolts, Nuts, Cotter Pins
3. Qeneral Design of Metal Fittings
a. Stamped Fittings
b. Forged Fittings
c. Stampings and Forgings used together
4. Ferruling of Struts
5. Protective Coatings
a. Wood Parts
b. Steel Parts
H. Actual Design of the Varioiug Elements of the Machine
1. The Wings
2. The Wing Truss
3. The Body
a. The Fuselage of the Tractor Biplane
b. The Fuselage of the Tractor Monoplane
c. The Pusher Biplane
4. The Landing Oear
5. The Tail
4. Assembly
A. Undercarriage
B. Center Section
C. Tail
D. Wing
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AIBPLANES 103
5. Alignment
A. FojBelage
B. Undercarriage
C. Center Section
D. Tail
E. Wing
F. Ailenma
6. Propellen
A. Manufacture and Structure
B. Inspection
1. Balance
a. Static Balance
b. Dynamic Unbalance
2. Trackage
3. Pitch Angle and Pitch
4. Camber
5. Joints
6. Condition of Surface
7. Mounting
7. Bepair
A. Wings
B. Longerons
C. Soldering
D. Brazing
E. Welding
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CHAPTER L .
AIRPLANES
1. THEOBT OF FLXGHT
A. AIRPLANE WINGS
1. Lift An airplane is supported by the air just as surely as an auto-
mobile is supported by the pavement. The supporting force is called lift.
(See Fig. 101) The lift is obtained by drawing the wing through the air
thiU9 allowing the small particles in the air to strike the wing and bounce off.
In bouncing off they exert upward forces the sum of which is the lift. The
resistance which the wing offers to being drawn through the air is called
drift.
The lift may be varied by changing the angle of incidence. Increasing
the angle of incidence will increase the amount of air striking the wing and
will increase the lift until an angle of about IS"" is reached. At this angle the
lift decreases as the angle of incidence is increased. This is called the
stalling angle.
The lift will vary as the density of the air which strikes the wing varies.
Density of air means the weight of the air. The heavier the air the more
air particles it contains, and therefore the greater the upward force given to
the wing. The denser the air the greater the lift will be.
The velocity of the air affects the lift because it affects the number of air
particles which strike the wing in a given period of time. The greater the
velocity the greater the number of reactions in a given time and the greater
the lift. The lift varies with the square of the velocity.
The greater the area of the wing the greater the lift because the total
upward force on the wing depends on the total area of the wing which
the air may strike.
There is an interdependence of angle of incidence and velocity. As an
airplane maintains its usual horizontal flight through the air there must at
all times be a total lift equal to the weight of the machine. The density of
the air will be constant because of the horizontal flight and the wing area
will be constant. But if the speed is changed the lift is changed and in order
to bring the lift back to a value equal to the weight, the angle of incidence
must be changed. As the speed is increased the angle of incidence is de-
creased and as the speed is decreased the angle of incidence is increased.
The limit of increasing the angle of incidence is the above mentioned 15"*
where increase of lift is not obtained.
2. Efflciency A good wing is a wing which will give the greatest
lift with the least drift This kind of a wing would be the most efficient so
efficiency is defined as lift/drift. This maximum lift/drift occurs at small
angles (l%*-8*) because the wing is not tilted enough to allow very much
of the total force of the air to go into drift.
The particular shape of the wing would naturally have some effect on
the efficiency of the wing, the same as the shape of a body will affect the re-
sistance of that body. In general, wings are thick toward the front and
taper down a curve to a thin trailing edge.
104
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AIRPLANES
105
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106 AIRPLANES
Above a wing is a zone of reduced pressure which produces two-thirds
of the lift. (See Fig. 101) Below the wing is a zone of increased pressure
responsible for only one third of the lift. When one wing is placed directly
above the other its zone of increased pressure interferes with the zone of
reduced pressure of the other. (See Fig. 101) In order to partially obviate
this the wings are staggered.
The aspect ratio is defined as the span/chord. A stream of air naturally
follows the path of least resistance ; therefore the air will not only escape over
the trailing edge but also over the ends. This means loss of lift. But, a
smaller percentage of the total area is affected on a high aspect ratio wing.
Therefore, the higher the aspect ratio the higher the efficiency.
B. AIRPLANE PERFORMANCE
Resistance is impediment to motion. Resistance depends upon the shape
and speed of a body. The body which offers the least resistance will be so shaped
that few eddies will be catused by the rush of air. The air is easy to split asunder
with little eddying but it does not unite until some distance behind the body.
This is why a streamline section has a blunt nose and long sharp tail. The resis-
tance of a body varies as the square of the speed. The thrust of the propeller over-
comes the totid resistance of the machine. Since it takes power to drive the pro-
peller the greater the total resistance and speed the greater the power required.
The maximum speed of an airplane then is that velocity at which the power
available is equal to the power required to drive the propeller. If at any time
there is more power available than required to fly the machine at any particular
speed then that excess power may be utilized to climb. The climbing ability of
an airplane therefore depends mostly upon excess power. An airplane climbing
is the same as an automobile climbing a hill.
At all times there must be a force pulling the machine forward equal to or
greater than the total resistance. If the motor stops this force is lost It can
only be regained by borrowing some of the force of gravity. This is accomplished
by gliding downward. The angle at which the airplane approaches the ground is
the gliding angle. An airplane in a glide is the same as a sled coasting down hill.
(See Fig. 101)
C. AIRPLANE EQUILIBRIUM
The fundamental principle of airplane equilibrium is the correct position
of the four forces, thrust, center of resistance, gravity, and center of lift. The
tendency of the machine to dive due to thrust and center of resistance must be
counteracted by the tendency to rear due to gravity and center of lift. (See Fig.
101)
Longitudinal stability is aided by the decalage. Due to the fact that the
angle of incidence of the wings is greater than that of the horizontal stabilizer
the airplane tends to come back to an even keel when disturbed by some external
force. The elevator controls longitudinal motion. (See Fig. 102)
Lateral stability is aided by the dihedral angle. When the airplane rolls
over, the low wing lifts more than the high one because the low one is nearly
horizontal. (See Fig. 103) The ailerons control lateral motion.
Directional stability is aided by the keel surface of the airplane. The keel
surface is the silhouette view of the side of an airplane. This is sometimes called
"weather cock" stability because a weather cock must have more keel surface
behind its axis than in front ; otherwise it would not head into the wind. An air-
plane then must have more keel surface behind the center of gravity than in front.
The rudder controls directional motion.
Banking is one of the many combinations of the three motions. It combines
lateral motion with directional motion. If a turn in the air is made without
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AIRPLANES
107
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108
AIBPLANES
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AIRPLANES 109
banking the skidding effect is called side slip. A spiral nose dive is a combination
of all of the motions.
Airplanes sometimes do not correctly regain their even keeL They some-
times oscillate back and forth longitadhially due to the inertia of liie tail.
Damping is the stamping out of these oscillations. This is accomplished by
making the fuselage as short and light as possible thereby making the inertia of
the f u»Blage less.
D. TYPES OP MACHINES
1. Introdnction The classification of airplanes into very definite and
fixed groups is a rather difficult matter, since there is such a variety in con-
struction and design, and since one machine may possess features that will
place it under two or three different headings. So that any classification
attempted will necessarily be general, and based on some fundamental prin-
ciple of use or design, tho the overlapping of classes may lead to some dight
confusion. With some form of dasedfication however, the identification of
the various airplanes becomes an easier matter, since it is always easier to
see some definite thing in the construction of a plane if one knows what to
look for.
2. Land and Water Hachinei The broadest and simplest classifica-
tion of machines divides them into the two groups of land and water ma-
chines, that is, machines so designed that they may alight on the land, or so
designed that they may alight on the water. Considering the former dass, it
will be seen that the main requisite is some form of landing gear that will
permit o£ rolling or sliding motion on fairly smooth ground. So that the
use of an undercarriage embodying wheds or skids is sufficient to class a
plane as a land machine. In the water machines, all that is required is some
form of undercarriage that will float on the water and have buoyancy enough
to support the weight of the machine, yet will also offer a minimum resistance
to motion on the surface. There are two principal types of water machines.
The seaplane, or flying machine adapted to lighting on the water, is
exactly similar to a land machine except that the wheels are replaced by
floats or pontoons of proper design. Most land machines are so constructed
that the wheeled gear may be detached and replaced by a float construction.
The use of one or two pontoons is a matter of designers' preference. Several
typical seaplanes in American practice are the Curtiss N-9, the Thomas sea-
plane, the Burgess seaplane, and the Aeromarine hydroaeroplane. This
latter term is as general as the term seaplane, and the type of machine is often
referred to merely as a ''hydro". The second type of water machine is
really a boat so designed that it will rise out of the water and fly in the air.
The hull is similar to that on a hydroplane, and all points of design are mod-
ified to give the machine good stability and control when operating on water.
Typical examples of the flying boat are the Curtiss modd "P,'* tibe Curtiss
"America" and "Canada" types, and the Curtiss H-12.
3. Classiflcation as to Number of Planes A second very general
dassification divides airplanes according to the number of supporting sur-
faces. The simplest is naturally the monoplane, or single surfaced machine.
This type has certain advantages in speed and handiness of control, but is
gradually being displaced by the small biplane which has greater advantages
in the matter of structural strength. Typical monoplanes are the earlier
Bleriot and Nieuport machines, the Morane, and the Pokker. The second
class is the machine carrying two main surfaces, or the biplane, which is
the most familiar type, being known in the first Wright machines and coming
down thru the Parman and Curtiss tjrpes to its present high stage of develop-
ment in the Caudron, Caproni, Gotha, Sopwith, S. P. A. D., Nieuport, etc.
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110 AIRPLANES
This type has advantages in constructional strength, surface capacity
without excessive span, and adaptability to various needs that assures
its being the prominent type of plane for all time. The third division is the
multiplaned machine, of which the triplane is the only practical example.
This type has only recently found its place in modem construction, for the
inefficiency of the surfaces, the great height of the structure, and various
difficulties in construction have led many to consider it impractical. Tet in
the latest Caproni machines this form has been found very good, giving great
weight-carrying capacity and reliability without any very difficult design.
Some smaller speed planes, such as the Curtiss triplane scout and the Sop-
with triplane are proving that the triplane type need not be confined to the
large machines only, tho it is in the latter class that it will find its greatest
application.
4. Olassiflcation as to Position of Propeller As the third general
classification, airplanes may be divided into groups based on propeller posi-
tion. This sub-divides naturally into three main classes — ^the tractor type,
with the propeller ahead of the main planes ; the pusher type, with the pro-
peller behind the main planes ; and a combination type, in which it is sought
to retain the advantages of both tractor and pusher by some modification in
design. In the first class, the propeller or propellers are so situated that
they pull the machine thru the air much as a locomotive draws a train of cars,
with the same inherent idea of stability. This construction has advantages
in the more efficient streamlining of the body — the fuselage — ^and in many
ways gives the most satisfactory results. For the modem fighting and obser-
vation planes however, there is the disadvantage that the observer and pilot
sit well back in the body, under the planes, and so have their vision cut off
in certain directions, as well as having the propeller obstructing the range
of gun-fire. Yet this type continues the favorite for many uses, and the
finest types of machines — Nieuport, Curtiss, Vickers, Albatross, Fokker,
etc. — ^are all d^ this dass. It is with the idea of giving a better arc of fire
and a wider range of vision that the pusher type of machine is being devel-
oped. Structurally this type is inferior to the tractor, for the body — ^nacelle
— ^is not so efficiently streamlined, and the outriggers or booms supporting
the tail present a point of weakness and greater head resistance. Yet where
speed is not a prime requisite, and where good arc of vision is imperative,
this type is superior. Therefore the observation and bombing machines of
the present time use this design, and the Farman, Voisin, F. E., and De
Havilland are most valuable. If a machine could be designed that would
have the structural advantages of. the tractor and the observational advan-
tages of the pusher, it would be most desirable. To this end certain machines
such as the Caudron,Gk)tha,Handley-Page and Caproni have a motor and pro-
pdler arrangement that is desirable in many respects. The machines men-
tioned are the multi-motored planes, and in the Caudron each of the two
motors is carried out on the planes in separate streamlined shells, while the
main nacelle, carrying the fuel tanks, men and supplies much resembles the
tractor fusdage, except that the obi^rver sits at the front end, as in the
pusher nacelle. In the Ootha twin-motor machine is found a very similar
construction, and also in the twin Handley-Page. In the Caproni three-
motored machine however, the two motors driving tractor propellers are
carried in regular tractor fuselages about one-third of the distance from each
end, while the third motor drives a pusher propeller, and is carried in a
main body or nacelle at the center of the planes. In this nacelle are also the
supplies, tanks and men, so that all the observational advantages of the
pusher are obtained, while the fuselage construction for the other two motors
serves to support the tail and so give the structural tractor advantages. It
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AIEPLANES 111
is this overlapping of the pusher and tractor types that tends to lead to con-
fusion in the matter of classification.
5. Military llaohines For the last general classification of machines
they are grouped according to their use for military operations. As the
main sub-divisions they are classed as Reconnaissance, Fighting, Bombing
and Training machines. The first use of airplanes for war purposes was as
reconnaissance or observation means, and the French attest their value dur-
ing the battle of the Mame. These first planes were of course crude com-
pared to those now in use for this purpose, and the development has been
specialized and rapid, so that now the Farman, Yoisin, Caudron and F. E.
bear small resemblance to the first small fliers. The later observation planes
carry special cameras, trained observers, and protective guns. Th^ are
designed for good climbing capacity, for speeds up to 100 m.p.h., for dear
arc of vision and for good carrying capacity. But tho the primary use for air-
planes in the war has been for the purpose of observation, yet there speedily
developed the realization that the airplane must have some means of defense,
or it could be driven down by an armed enemy plane, and so lose its obser-
vational value even if it were not destroyed. Thus the planes began to arm
with rifles and machine guns, and the natural outcome was the development
of a purely fighting type of machine, canying two or more quick-flring guns
and with a speed and manoeuvering ability tiiat made it most deadly to the
slower, heavier observation machines. This flghting type represents the
acme of airplane design and construction, with its high-powered,, light weight
engine, its minimum factor of safety, its high rate of dimb and tremendous
speed, and its ease of control and ability to perform all manner of
*' stunts". The S. P. A. D., Sopwith, Nieuport, Morane, Fokker, Albatross
Scout, and Halberstadter are all of this type, carrying usually only the pilot,
and as deadly as waspsi This flghting type is of course a special develop-
ment, and really of no great utility except as it serves to protect the obser-
vation and bombing planes. These latter — ^the bombers — are gradually be-
ginning to assume greater importance as an arm of offense, for they pene-
trate the land defenses of the enemy and attack where least expected. They
are heavy, moderately fast machines capable of carr3ring loads up to two or
two and a half tons of bombs. They carry generally several protective guns,
for their operations involve flights of too great duration for the flghting
machines to accompany them. Even so they would be vulnerable to the at-
tacks of the enemy flghters but for the fact that these bombing squadrons
have developed a flying formation that leaves no single machine open to in-
dividual attack, and the combined flre of the squadron is usually too much
for the attacking flghters, just as a fleet of cruisers can protect itself against
a superior nxunber of destroyers. These large bombers, such as the Gotha,
Caudron, and Handley-Page or Caproni can make flights of four hundred
to six hundred miles, carr3ning tons of explosives to drop on ammunition
depots, aerodromes, railway centers and industrial centers of the enemy, and
being usually of the multi-motored type can manage to survive pretty severe
anti-aircraft fire and return to their base. The bombing type will in peace
times become the carrier of costly merchandise, or the passenger airship.
A type of machine that is gradually becoming specialised is the military
training plane. For the work of training thousands of embryo aviators a
machine is needed that has standard control, a reliable motor, a good degree
of inherent stability, and the structural strength to stand the severity of
amateur landings. Such a machine is the Curtiss JN-4, and the Standard
J-1 in America, or the B.E.-2C in England. These machines do not need
climbing ability or speed, they do not demand refinement of construction or
perfection in design, yet they are valuable for the one purpose of teaching
the fundamentals of airplane control.
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6. Recapitulation It will be noted that many of the machines fit into
several of the general classifications, and that the last classification and the
first classification are general enough to include any machine that may be
designed to travel in the air. Yet certain necessities of use as a rule involve
certain necessities of construction or design, and the habit of analysing the
use for which a machine is intended will in time lend greater facility in
identifying the various makes of machines — a faculty that is much needed
in the present war times, when the pilot of a machine must be able in a few
seconds to positively state whether a machine is friend or foe, and that often
under adverse conditions. Ability in observation implies intelligent appli-
cation of knowledge.
E. HISTORY OP AVIATION
Four important men, dates, and achievements in the history of aviation
are: —
Lilienthal— 1893— Camber.
Chanute— 1896— Biplane Truss.
Wright Bros.— 1903— Stability.
Nieuport— 1909 — Fuselage.
Lilienthal was a German who made extensive experiments with gliders.
He discovered that a wing with camber was more efBcient than a flat wing.
Lilienthal was killed in a glide, having lost control of his apparatus while some
distance from the ground.
Chanute was a Chicago civil engineer who applied the principles of bridge
construction to the airplane. Instead of only one surface Chanute used two
surfaces rigidly braced together. He is called "The Father of the Biplane*'.
The Wright Bros, were bicycle repairmen in Dayton, Ohio. When they
heard of the death of Lilienthal they commenced thinking over the problem.
They did not like Lilienthal's method of balancing the glider, namely, by shifting
the pilot's weight. After experimenting for some time they found that they
could control the longitudinal balance by means of surfaces supported some dis-
tance in front of the Uf ting planes. These they called elevators. They observed
that the birds maintained their lateral balance by warping their wings so they
tried that method on their gliders. It proved to be successful. They found that
they had to use a rudder which operated with the wing warping control to
counteract the increased wing resistance of the warped wing. With gliders of
this type they experimented for three years at Kitty Hawk, N. C. After becom-
ing proficient in the manipulation of the apparatus they installed a motor and on
Dec. 19, 1903, ma^e the first flight with a heavier than air machine propelled by
a motor. Their success was due to their solution of the problem of stability.
Nieuport recognized that in order to obtain speed the resistance had to be
reduced. He then brought out a machine with no open structure of the airplane
exposed to the resisting action of the air. This type of structure gave to the
airplane a distinct body. The fuselage is associated with Nieuport.
2. NOBDENOLATXTBE
WINGS
End edge
Aileron control horn
«a • '
Fabric
Aileron control wire
External
Dope
Aileron balance wire
Bight and left
Mast
Interplane strut
Wing panel
Mast wire
Flying wire
Leading edge
Wing skid
Landing wire
Trailing edge
Aileron
Diagonal wire
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Follow-thru wire
Drift wire
Wing fitting
Strut socket
Sidewalk
Wing hinge
Wing hinge pin
2. Internal
Wing hinge
Wing beam (spar)
Compression rib
Compression strut
Filler rib
Filler web
Cap strip
Drift wire
Anti-drift wire
Internal fitting
Tumbuckle
Terminal
Stringer
Nose veneer
Leading edge
Trailing edge
End edge
Box rib
B. FUSELAGE OB
BODY
1. External
Cowl
Cockpit
Windshield
Covering
Turtleback
Tail skid
2. Internal
Longeron
Strut
Ferrule
Fire screen
Fitting (clip)
Stay wire
D.
E.
Engine bearer
Floor board
Nose plate
Foot bar
Control frame
Seat rail
Tail post
UNDBRCABBLA.GE,
CHASSIS, OB
LANDING GEAB
Wheel
Tire
Axle
Strut
Bridge
Cross strut
Fairing
Stay wire
Saddle
Shock absorber cord
TAIL, OB EMPEN-
NA6E
Horizontal stabilizer
Vertical stabilizer
(fin)
Elevator (flipper)
Budder
Brace
Control horn
Control wire
Brace wire
Shackle
TaU skid
CONTEOL SYSTEM
Control wheel
Control bridge
Joy stick
Budder bar
Stick
Dep.
F. GENEBALTEBMS
Aerofoil
Angle of incidence
Bank
Cabane
Camber
Center of gravity
Center of pressure
Chord
Crash
Dihedral
Dive
Drift
Droop
Gap
Glide
Lift
Motion
Lateral
Longitudinal
Directional
Nacelle
Overhang
Propeller
Pusher
Side-slip
Skid
Span
Spin
Spiral
Stall
Stagger
Stability
Streamline
Stress
Stunt
Sweep-back
Taxy
Thrust
Torque
Tractor
3. ELEMENTS OF AIRPLANE DESIGN
A. INTBODUCTION
The problem of design is that of incorporating into a practical machine the
theoreticid elements which are shown to be necessary by a study of the Principles
of Flight. These elements are :
1. The Wings 4. The Horizontal Stabilizer
2. The Propeller 5. The Vertical Stabilizer
3. The Engine 6. The Control Surfaces and Bigging
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m^cro/r sz/^l/i/v^
/=^USH£:/f 3/FL/lN£
Pio. 104
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B. DECISION AS TO THE GENERAL FORM OP THE MACHINE
1. Three general types We find that about ninety-five per cent of
the machines wMch are built today may be classified into three general types.
These are:
a. The Tractor Monoplane
b. The Tractor Biplane
c. The Pusher Biplane
These three types are illustrated by Pig. 104. We shall consider that
the greater part of the problem of decision as to the general form of the
machine has already been solved for us in the selection of these three tyi>es.
2. Ohoosing among these three types Whether in any particular case
we should select a monoplane or a biplane, a tractor or a pusher, would
depend largely on the purposes for which the machine was to be used.
C. GENERAL CONCLUSIONS AS TO THE MATERIALS WHICH WILL
BE REQUIRED
1. Necessity for rigidity and strength Our wings receive the upward
force or lift of the air distributed more or less uniformly over their surfaces,
whereas the weights or loads which this lift supports are concentrated at
points within the body. This means that the wing structure must possess
the strength and stiffness necessary to transmit the lift to the points where
the wings are attached to the body. Similar reasoning would show the
necessity for strength and stiffness in the body itself, in the stabilizers and in
the structure connecting them with the wings, in the control surfaces and in
the rigging for their operation. In short, our structure must be possessed
of a certain amount of rigidity and strength throughout.
2. Necessity for lightness A study of the Principles of Plight shows
that a set of wings of a certain size, moving through the air at a certain
velocity, will produce a certain amount of lift, and no more. We must take
the total lift which we expect to obtain from our wings, subtract from it the
weights of the useful loads which we expect to carry, and select our materials
in such a way that the weight of the structure itself will be kept within the
renuiinder.
3. Airplane material must be reliable One way of securing lightness
is by cutting down the sizes of our parts until there is left in them only a
moderate surplus of strength over that which will actually be demanded.
This makes it necessary to select materials of high character. We note
reliability as the first requisite of airplane material.
4. Airplane material must possess a high strength-weight ratio It is
obvious that having set down the requirement of reliability, our next demand
would be that our materials must be those giving us the greatest amount of
strength in proportion to the weights employed. The second requisite of
airplane materiid is a JiigJi strength-weight ratio,
5. Occasional necessity for special properties Por certain parts we
may require such special properties as compactness, non-corrosiveness, duc-
tility, etc., and we may find it necessary to sacrifice strength or lightness in
order to secure these properties.
D. DATA RELATIVE TO THE RECOGNIZED AIRPLANE MATERIALS
We fipd that ali^ost the entire field of present practice is covered by a com-
paratively small number of materials which, of course, satisfy the requirements
stated under Section C.
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AIRPLANES
For oonvenience, we may divide materials into the following general claases:
1. Woods 4. Metals other than steel
2. Steel 5. Wires and Cables
3. Covering Material 6. Miscellaneous Materials
1. Woods Table I below gives the names and a few of the physical
properties of the woods which we may consider for airplane construction.
Tabub I — ^Physical Pbofertiks ow Woods
Wood
Spniee
White Pine
Port Oxford Cedar
Aflh
Boek Elm
Oak
Hickory
Weight
lb. per ca. ft
27
27
81
40
44
46
60
Mod. of Bnpt.
lb. per sq. in.
Gomp. Strength
lb. per sq. in.
7^00
7,400
10,300
12,700
12,500
12,000
16,800
4,800
4,600
6,800
6,000
6,800
6,000
7,800
Strength-weij[ht
Batio
189
167
171
160
188
128
140
2. Steel For this material it will be sufficient to mention two broad
classifications —
a.
b.
Mild Steels
Alloy Steels.
a. MUd steels The composition of the mild steels used for air-
plane parts will run about the same as that of ordinary structural steel.
The tensile strength is from 60,000 to 70,000 lb. per sq. in., and the
weight about 490 lb. per cu. ft. The strength-weight ratio figures out
about 135.
b. Alloy steels By an alloy steel we meau one containing one or
more ''special" elements such as nickel, chromium, and vanadium.
These elements act to make the material harder, stronger, and tougher
than the "mfld** variety. The tensile strength of most alloy steels will
run about 100,000 lb. per sq. in. while the density is practically the same
as that of mild steel. The strength- weight ratio is accordingly higher —
about 204.
3. Metals other than steel The use of metals other than steel is prac-
tically limited to those which appear in Table II following:
Table n — ^Physical Properties oi< Metals other than Steel
Metal
Weight
lb. per en. ft.
Tensile Strength
lb. per sq. in.
strength-weight
Batio
Aluminum Alloy
Brass
160
523
552
3fe.S
112
76
Bronzo
128
4. Wires and cables These are made from steel, but are worthy of
consideration as a separate materials group. This group is divided intc
three principal classifications (see Fig. 105) as follows:
a. Aircraft wire A solid steel wire closely resembling piano wire.
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Table ni— Phtsioal Pbopkbtdes or AmoBArF Wibb
117
Gauge No.
Dedmal
Teiunle
Weight lb.
(Brown and Sharpe)
Diameter
Strength |
per 100 feet
14
.064'
830
1.10
12
.08r
1,300
1.74
10
.lQ2f
2,000
2.77
8
.128"
3,000
4.40
b. Aircraft strand The result of twisting a number of wires
tightly together.
Tablb IV— Physical Pbopebtiss oi< 19 Wm AntcsAFF Stbakd
Diameter in
inches
Tensile StrengOi
Weight per lb.
per 100 feet
H.
2,100
8.50
9b
3,200
SM
9U
4,600
7.70
%t
6,100
10.00
K
8,000
18M
9is
12,500
20.65
c. Aircraft cord The result of twisting several strands together,
known as *'7xl9", ''7x7", or ''6x7 Cotton Center'', depending on the
construction.
Tablx Y— Physical Pbopebtiss of Aibcbajt Cobd
Diameter in
inches
7x19
7x7
6x7 Cotton Center
Tensile
Strength
Weight lb.
per 100 ft
Tensile
Strength
Weight lb.
per 100 ft
TensUe
Strength
Weight lb.
per 100 ft
9i«
2,000
2,800
4,200
5,600
7,000
2.88
4.44
6.47
9.50
12.00
1,350
2,600
3,200
4,600
5,800
2.45
4.67
5.80
8.30
10.50
1,150
2,200
2,750
4,000
5,000
2.20
4.20
5.30
7.43
9.50
5. Ooyerizig material
a. Fabric The standard is unbleached linen weighing about four
ounces per square yard and having a strength of from 60 to 70 pounds
per inch of width. There is high probability that linen will he replaced
soon by some cotton substitute, due to scarcity of the former material.
6. Coating The material used is commonly called dope. Dope is
a cellulose solution of some sort. It adds strength and makes the linen
taut, air tight, and water proof. The usual application is about four
coats of dope followed by two coats of spar varnish.
6. Miscellaneoiu materials Under this heading it will be sufficient
simply to mention such items as rubber, used in various shock absorbing
devices, leather, used for straps, etc., upholstering materials used about the
seats and cockpits, and sheet copper, used for bindings or ferrules for the
ends of struts. We shall mention aliso the use of various coating materials
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AIRPLANES
7x19
7x7
19 WntE Strand
6x7 Cotton Center
Fio.106
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119
other than dope ; chief among these are yamishes for the wooden parts, cop-
per plating, nickel plating, and various paints and enamels for the metal
parts.
B. FITNESS OF CERTAIN MATERIALS FOR CERTAIN CLASSES OF
PARTS
1. Woods Woods are fitted to take compression, or stress tending to
shorten or telescope a part along its own axis. The compressive strength
of a material in lb. per sq. in. is the amount of load required to crush a block
of the material, for each square inch of cross section which we have resisting
the load. The compressive strengths of our various woods are shown in the
tables. The compressive strengths of the metals are not given but they are,
roughly speaking, the same bb tiie tensile strengths. We see that weight for
weight our woods give us greater compressive strength than any other of our
materials except alloy steels. A part which is subjected to compressive
stress is usually called a strut. Now a peculiar thing about a strut is that
if it is very long in proportion to its sectional dimensions, it will buckle side-
ways and either break under a rdativdy low load or else allow its ends to
move far enough toward each other to make it worthless in our structure.
Wood struts can be made large enough at the center to prevent the side
buckling without unreasonable addition of weight, but this would not be
true of steel struts, unless they were made tubular in shape, which is a
possibility.
Woods are also fitted to take bending stresses. A part which is sub-
jected to bending has compressive stresses in its concave half and tensile
stresses in its convex lialf . These stresses are greatest at the outer faces
of the part and less toward the center. We would expect all of this from the
effect of the load on the lengths of the fibers, as illustrated by Fig. 106.
Fio. 106
Fia. 107
On account of the particular distribution of stresses in a part sub-
jected to bending, the center can be cut away, decreasing the weight in
greater proportion than the strength. The result of such cutting away
is a section of which Fig. 107 is typical.
Wood parts are easily shaped to such sections as these. In making
bending tests of wooden parts in materials testing laboratories there is re-
corded a sort of theoretical compressive or tensile strength, which is called
the Modulus of Rupture, and is expressed in lb. per sq. in. The
modulus of rupture is significant to us in that it is a measure of the ability
of a material to withstand bending loads.
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120 AIRPLANES
The question of which woods are suitable for struts and parts subjected
to bending would naturally be determined from an examination of the table.
We notice that the first three woods, the soft woods, are stronger, weight for
weight, than the last four, the hard woods. The data given favor the
cedar, but there are other reasons why spruce is preferred above all others.
Spruce can be obtained in straight-grained pieces of great length and is
not only strong but very stiff. It may be said that spruce is perhaps the
most important single airplane material. The heavy demand for spruce
makes it possible, however, that Port Oxford Cedar, White Pine, and even
Douglas Fir may soon come into fairly general use as substituteis.
Sometimes it seems advisable to use hard wood parts in order to secure
great strength or toughness without making the parts unduly bulky. This
is a case of sacrificing strength-weight ratio in order to secure compactness.
Our table shows that ash would be the logical selection here. Practice bears
this out ; ash is by far the most important hard wood, although it is possible
that it will be replaced to some extent by Rock Elm, on account of scarcity.
Bickory is occasionally used where very great strength or toughness is
desired.
2. Steels
a. MUd steels Steels have the property of being uniformly strong
against compressive stress and tensile stress. The latter is that type
of stress which tends to elongate a part or pull it in two along its own
axis, the tensile strength of a material in lb. per sq. in. being the
amount of load required to pull a bar of the material in two, for each
square inch of cross section which we have resisting the pull. Because
of the uniformity mentioned, steels are particularly fitted for the
manufacture of fittings for rigidly connecting various parts. Such
fittings are often subjected to different t3rpes of stresses in different
regions or at different times. To secure lightness, steel in the sheet
form is often employed, different parts being stamped out, bent to shape,
and then brazed or welded together. While alloy steels possess the
* higher strength-weight ratios, they are so hard that they cannot be
bent in the way described ; so we resort to the use of mild sheet steel
which is sufficiently ductile.
6. Alloy steels Alloy steels are mostly used in the bar form, from
which they can be reduced to such parts as bolts, hinge pins, and turn-
buckle ends by machining operations which are not prevented by the
hardness of the material. They find some use also in the shape of
forged fittings; as a rule, however, stamped fittings of mild sheet steel
will give us the necessary strength at the expense of less weight.
Parts made from liloy steels are frequently heat treated after
machining, to give them still greater strength and toughness.
3. Metals other than steel Aluminum is used wherever some light
weight part that will keep its shape but will not have to withstand any
great amount of stress is wanted.
Brass and bronze are used principally for turnbuckle barrels because
of the excellent wear of the threads which is obtained. It will be remem-
bered that the ends which screw into the barrels are alloy steel. Occasion-
N ally brass or bronze is used for the manufacture of such parts as bolts and
hinge pins for seaplanes and fiying boats. Whenever this is done it is a
case of sacrificing strength-weight ratio to secure non-corrosiveness.
4. Wires and cables The great superiority of this material is, of
course, for all parts which are subjected to pure tension.
a. Aviator wire For its size, the solid wire is the strongest ma-
terial of this class. It is used therefore, except where we require more
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flexibility than it possesses. We can not nse it in large sizes because
it becomes impossible to form the necessary end loops.
&. Aviator strand Where tension members must be handled fre-
quently, or coiled up, as with the wires of the wing trasses, we use the
strand, which is next in strength to the solid wire, and more flexible.
c. Aviator cord Where we require even more flexibility than
strand can give us, we resort to the use of cord. This material is the
least strong and the most flexible of all. It is particularly useful for
control wires.
F. NECESSITY FOB STREAMLINE FORM OF PARTS
We are now almost ready to begin the actual decision as to methods of
building up the diifferent elements of our machine and selection of materials
for the various parts decided upon. Before doing this it will be well for us to
consider one item which has not been mentioned as yet. Every external part
of our machine is going to offer Kead-resistance, that is, it will require a cer-
tain amount of force to move each part through the air at the velocity of our
machine. Now since every bit of this force must be counter-balanced by pro-
I>eller thrust in addition to that necessary to overcome the drift on the wings,
and since every bit of additional propdler thrust means additional engine power,
it behooves us to keep down the head-resistances of our external parts as far as
possible.
It is known that the resistance which a body meets in passing through the
air is largely due to the eddy currents of air which are set up around the
body. The left hand body of Fig. 108 is surrounded by such currents.
Fio. 108
The body on the right is of such shape, however, as to be practically free from
eddy currents. A body of this shape is known i^ a streamline body, and area
for area its resistance in passing through the air is the practical minimum. We
must make the attempt to design the external parts of our machine, both large
and small, in such a way that their sections approach streamline forms to as
great an extent as is practical.
Q. DESIGN OF SMALL PARTS CAPABLE OF GENERAL APPLICA-
TION
In taking up the actual design of our machine we shall find it convenient
first to settle upon the forms of certain items which we shall be able to apply
generally over our entire machine. These items may be grouped under several
different heads, as follows:
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AIRPLANES
1. Wire and caUe tension members These will of course be made up
in units of ddSnite lengths. Each unit, including whatever fastening de-
vices are permanently assembled with it, is called a stay.
a. End connections In making up stays from wire, strand, and
cord, different end connections are used for each. The methods of con-
struction of some of these are clearly shown by Fig. 109.
Fia. 109
The ferrules used in forming end connections of Aviator Wire stays
are themselves made of Aviator Wire. The detail form is shown clearly
by Fig. 110.
The thimbles used in forming end connections of Aviator Strand
and Aviator Cord stays are stamped from galvanized steel. A typical
thimble is shown in Fig. 111.
6. TumbucJdes One end of a stay always includes a turnbuckle,
which is capable of adjusting the length by means of right and left
threads. It is important in connecting a turnbuckle that both ends
be started into the barrel at the same time ; otherwise the full adjust-
ment cannot be realized. It is also important that the ends be screwed .
into the barrel far enough that all threads are completely covered when
the full load is applied. When a turnbuckle is finally adjusted after
setting up the machine, it is wired for the purpose of preventing its
loosening up. The wire so used is called a safety-wire. Fig. 112
shows a turnbuckle with the safety-wire properly applied. The barrel
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AIRPLANES
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ROEBUNG STANDARD STEEL
AIRCRAFT FERRULES.
Seetionof
Gauge Bar
bavrngBemi-
ctrcttlar edges.
Fio. 110
10 WiKO
■^^i
Fig. hi
of the tambuckle is made of brass or bronze, while the ends are made
of alloy steel. The standard material for the safety-wire is copper,
c. Shackles At the opposite end of a stay from the tumbuc^e,
the eye is usually formed about a small steel forging called the shackle.
Fig. 113 shows examples of the use of shackles.
2« Hinge and devia pins, bolts, nuts, cotter pins For fasteners a
great many hinge pins, clevis pins, hex head bolts, and eyebolts of practically
standard designs are of course used, these parts being practically always
made from alloy steel, heat treated.
Fio. 112
Nuts are usually slotted for cotter pins, or castellated, as we say. The
material for nuts is occasionally a mild bar steel, being case-hardened after
the machining operations are over although best practice is to make these
of 3^% nickel steel. The cotter pins are usually the standard commercial
article, made from spring steel.
3. (General design of metal fittings While fittings for different uses
have, of course, rather different forms, certain similar features in the de-
signs are noticeable throughout. Some of these similar features will be
observed from a study of the fittings shown in Fig. 113.
a. Stamped fittings These are usually made from one or more
stampings of mild sheet steel, bent to the proper shapes, and brazed
or welded together. ''Open-work'' or lightening holes are character-
istic of these parts.
b. Forged fittings These are not so common as they are usually
heavier. Drop f orgings of either mild or alloy steel may be used. The
latter are stronger, weight for weight, but more expensive.
c. Stampings and forgings used together Occasionally it is
found convenient to make up a fitting by welding or brazing one or
more forgings to one or more stampings.
4. Fermling of struts It is characteristic of airplane construction
that wherever we have a strut fitting into a clip or socket of any sort, the
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AIRPLANES
Fig. 113
OojMIe
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end of the strut is usually banded with sheet copper. The band is called a
ferrule. The purpose of the ferrule is, first, to prevent the end of the strut
from splintering, and, second, to distribute the bearing pressure of what^
ever bolts are used over a somewhat greater area of the wood.
5. Protective coatings
a. Wood parts Wood parts are always coated with shellac or
clear varnish only. Paint would conceal imperfections in the wood,
and allow them to pass unnoticed.
6. Steel parts Steel parts are practically always given some
sort of a ** rust-proof" coating. It is the practice of one prominent
American manufacturer to copper-plate every steel part. For the sake
of appearance, nickel-plating or enamel of some sort is then applied
over the copper.
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126 AIRPLANES
H. ACTUAL DESIGN OP THE VARIOUS ELEMENTS OF THE
MACHINE
We are now through with all preliminary considerations and may proceed
at once to the work of direct design. This wUl include the selection of the con-
struction to be employed for each element of the machine, as well as that of
the materials to be used for tiie various parts. "We shall, of course, take up one
element at a time.
1. The wings The usual method of construction is clearly shown by
Fig. 114. The exact relationships of the different parts are shown more
in detail by Fig. 115. The covering transmits the lift of the air to the
ribs, which in turn transmit it to the spars. From the spars it will be
transferred to the wing struts and the truss wires at their points of at-
tachment, and it will finally be delivered by the wing trussing system to
the body. It will be seen that both the spars and the ribs are subjected
to bending loads; they are therefore usually made of wood in approxi-
mations to the section shown by Fig. 107. Fig. 115 shows a typical rib
design and the methods of rib attaclunent.
The materials commonly used for the various parts are listed below:
Spars — Spruce, sometimes ash or steel tubing.
Ribs:
Compression
"Webs: Spruce. Cap strips: Spruce
Filler
"Webs: White pine. Cap strips: Spruce
Leading Edge — Spruce
Trailing Edge — Steel tubing or steel cord
End Edge— Ash
Drift and Anti-Drift Wires — ^Aviator Wire
Stringers — Spruce or white pine
Nose Veneer — Two- or three-ply wood veneer
Small Wooden Braces (Miscdlaneous) — Spruce, sometimes yellow
pine
Metai Fittings — ^Mild sheet steel.
As indicated above, steel has been used for spars, although the practice
is not by any means general. The principal reason for the non-adoption of
steel for these parts is that the weight of steisl is greater than that of equiv-
alent wood parts.
The use of steel has not been limited to spars, for there have been in-
stances where all-metal wings have proved quite successful, and as the sup-
ply of available wood becomes less, we may expect to see more and more
steel used.
In Fig. 114 it will be noticed that the tail portion of the wing is
discontinued at the outer end. This is for the purpose of inserting a hinged
flap called the aileron. The aileron usually has a thick wooden leading
edge which serves as its only spar, and ribs, trailing edge, and small parts
of the same materials as employed in the wing proper. After the aileron is
covered it is equipped with vertical steel struts or levers called horns for
attaching the operating cables. These are mounted and connected in the
same way as shown for the tail surfaces in Fig. 118. The horns are usually
made up by brazing together several mild sheet steel stampings. Aviator
Wire is commonly employed for tying the horns to the trailing edges.
2. The wing truss The general method of trussing a monoplane may
. be seen in Fig. 104, and the general truss construction of biplanes is too
familiar to need further discussion as regards form.
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AIBPLANES 129
As to the materialB of a wing truss, these are practically determined
by general considerations already discussed. The wires must be flexible
enough to handle easily and to make coiling possible. Aviator Wire
would not permit this; therefore we use Aviator Strand. The various
struts are, of course, compression members, and are consequently made of
spruce. Spruce is easily worked also, and lends itself to streamline shaping.
The use of tubular steel struts can not be said to have progressed further
than the experimental stage.
A typical method of fastening the strut to the wing spar is shown by
Fig. 116. In this particular case the strut socket is made up by brazing
or welding a stamping and a forging together, while the bearing plate is
a single forging. Often the bearing plate is made up from several stamp-
ings, and includes a socket which takes the end of the strut directly. The
bearing plate is practically always fitted with lugs or jaws for taking the
tumbuckles of the stays of the truss.
3. The body We shall discuss rather in detail the fuselage of the
tractor biplane since this will give us a good insight into the general prin-
ciples involved in the design of all bodies. It will sufBce simply to mention
certain peculiarities of the monoplane and the pusher bodies.
a. The fuselage of tTte tractor biplane A good idea of the usual
construction may be obtained from Fig. 117. Oxir fuselage is
essentially a long girder which is supported by the wings at their points
of attachment and which is loaded at the points where the engine, the
occupants, and the tail are located. The heavy stresses in our fuselage
are naturally in the front half; the rear half does nothing but sup-
port the tail and transmit air forces from it to the main supporting
surfaces. Reduced to its elements our fuselage consists of the long
members at the comers or the longerons, the vertical and horizontal
cross pieces or struts, the sockets for attaching the struts to the longerons,
and the bracing wires. The longerons are subjected to high stresses
but for convenience in construction it is highly desirable to keep their
sectional dimensions down ; so we resort to the use of hard wood, usually
ash. This is a case of sacrificing strength-weight ratio to secure com-
pactness. Sometimes we splice the longeron about mid-way of its
length, using spruce for the rear half. The struts are all held under
compression by the system of wiring, and are therefore made of spruce.
The strut sockets are ordinarily formed from mild sheet steel. For
the brace wiring in the rear half of the fuselage, we usually employ
Aviator Wire. In the front half where air stresses are high, however,
the wire if employed would have to be of such size that its stiffness
would practically prevent the formation of the end loops necessary for
fastening it. This forces us to select Aviator Strand or Aviator Cord
for this region.
Of course, there are certain modifications of most of these basic
parts. The vertical strut at the extreme rear becomes the tail post,
which is often subjected to severe shocks, and is usually made of ash.
The vertical struts at the points where the wings are attached and where
the rear ends of the engine beds rest are necessarily heavy, but they
are still struts and usually they are made of spruce. The two hori-
zontal struts at the points of attachment of the lower wings are sub-
jected 4o extreme tension because of the lift acting on the wings, and
for this reason are made of hard wood or often of steel tubing. Where
steel tubing is used the wing hinges are commonly forgings which are
welded to the tubes. The tension is then transferred from one wing
over to the other without being imposed to any degree whatever upon
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AIRPLANES 131
the faselage structure. At the very nose of the body both horizontal
and vertical struts are discarded, their place being taken by the nose
plate, which is ordinarily a single stamping of mild sheet steel.
Mounted in the fuselage we have certain accessories. Among these
are the seats with their mountings, the floor boards, the control oper-
ating devices, the instrument boards, the fuel tanks, the sheet aluminum
fire-screen betwieen the fuel tanks and the engine, and the engine beda.
The latter must be very strong and in order to keep down their size they
are frequently made of ash. They are parts subjected to bending
and are therefore often hollowed out to the I-beam section of Pig. 107.
The Curtiss Company makes the beam rectangular in section, but lami-
nates it, using layers of ash at the top and bottom, and layers of spruce
in the center.
The rear half of the fuselage is usually enclosed in a pen^anent
linen covering, except at the top, where we ordinarily find the fuselage
cover, a light detachable wooden structure for streamlining out the
arched upper portion of the body. The fuselage cover is in turn
"roofed over" with linen. For housing over the cockpits and all round
the engine, we use light sheet metal parts called the cowls; these are
practically always stamped from sheet aluminum.
There are two variations from the conventional design which are
worthy of mention. The fuselage is sometimes made a continuous cir-
cular shell, say of five-ply wood-and-canvas veneer, this shell being
braced at intervals of its length by steel "bulkheads". The other
variation is the use of steel tubing for practically the entire structure.
This has not become common because it usually means increased weight,
decreased flexibility, and high manufacturing cost.
6. The fuselage of the tractor monoplane The construction of this
body is practically identical with that of the biplane, and a general
consideration of the details involved will show that the location of the
wing hinges would be the only item causinjg material changes in the
design.
c. The pusher biplane Here the fuselage degenerates into the
nacelle, as is shown by Pig. 104. The construction of the nacelle is
practically the same as that of the fuselage, save that lighter parts
and sometimes weaker materials are employed.
The structural functions of the tractor fuselage are in the pusher
assumed by the system of outriggers connecting the wings with the
tail. The outriggers are usually tied together by means of horizontal
and vertical struts, and braced by means of diagonal wiring. The out-
riggers and struts are made sometimes of wood, sometimes of steel, and
sometimes of bamboo. The wiring is usually of Aviator Strand.
4. The landing gear This part of the machine is of course necessary
for support when standing on the ground and for partially absorbing the
shock of landing. It must resist not only the upward shock of landing but
also the horizontal force tending to sweep the machine backward when run-
ning on the ground. The simplest design for withstanding such forces
seems to be a combination of a continuous axle carrying two wheels, a pair
of V-struts running from the lower side of the body down to the axle, and an
elastic connection between the axle and the system of struts. To make the
system of struts rigid it is necessary to add cross struts or braces connecting
the apexes of the two " V*s", and diagonal bracing wires. The general scheme
is illustrated in Pig. 117. The struts of course are attached to the fuselage
at their upper ends by steel sockets, and the two members going to make up
one "V" are tied together at their lower ends by means of a complicated
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132 AIBPLANBS
steel piece which we may call the bridge. The axle most be connected to
the strut system in such a way that it can move upward freely, but not back-
ward. Tldfl is usually accomplished by running the axle through a long
vertical slot in the bridge, and then tying it down against the bottom of
this slot with several turns of a heavy rubber cord. At the point where
the cord passes over the axle it is usually confined within a '^floating'' metal
saddle of some sort. The axle is sometimes restrained from moving back-
ward by means of a system of steel levers or "radius-rods** instead of
by a slot.
The materials commonly employed for the various parts are as follows:
Strut Sockets — ^Mild sheet steel.
Struts — ^Spruce, ash, or steel tubing with wood "fairing".
Bridge — ^Mild sheet steel.
Cross Struts — Spruce, or steel tubing with wood "fairing".
Brace Wiring — ^Aviator Strand.
Axle — ^Mild or alloy steel tubing.
Shock Absorber Cord — ^Made up of many fine strands of rubber
bundled together and enclosed within a fabric cover.
Wheels — Much like automobile wire wheels except that the bear-
ings are plain, no balls or rollers.
Hubs — Formed from steel. Bronze bushings.
Spokes and Nipples — Steel.
Rims — Steel.
Tires*— Usually of "stubby" proportions, e.g., 26''x4''.
Casings — ^Plain clincher cord casings. Very light treads.
Tubes — Same material as automobile tubes.
Types of landing gears other than that above described are frequently
used, particularly with the heavier machines, where we may find pneumatic
or glycerine shock absorbers, and often a complicated multiple-wheel struc-
ture.
5. The tail . The standard arrangement is that illustrated by Fig.
118. The horizontel stebilizer is mounted directly on top of the rear end of
the fuselage, and the vertical stabilizer on top of that. The horizontal
stabilizer is braced to the bottom of the fuselage with stays of Aviator Strand
or with braces which are frequently of steel tubing with wood "fairing".
The peak of the vertical stebilizer is usually braced by steys of Aviator
Strand. '
The construction of the tail units themselves follows very closely the
general idea of the wings and the ailerons. One rathet distinctive feature,
however, is the absence of any interior wiring, diagonal wooden webs being
commonly employed to give whatever bracing of this nature is required.
Another feature is the frequent employment of steel tubing for both lead-
ing and trailing edges.
The ten skid is obviously for the purpose of keeping the rest of the
teil ott of the ground. It is usually made of ash or hickory, protected on
the lower side by a steel shoe. The skid is sometimes mounted as shown in
Fig. 117. In this case the metal fittings would be formed from mild sheet
steel. More recently, however, the tail skid has been pivoted te the fuselage
forward of the tail post, to reduce breakage caused by heavy landings. The
forward end of the teil skid is usually connected with some sort of a shock
absorbing device located within the fuselage. The elasticity is practically
always obtained from rubber cord identical with that used in the land-
ing gear.
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AIRPLANES
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A. UNDERCARMAQB
1. Bolt wheels to axle.
2. Attach taU skid.
3. Lift fuselage onto horses or raise it by hoist high enough to let
landing gear run under it.
4. Bolt landing gear struts to fuselage, and attach cross-bracing wires.
5. Align*
6. Safety tumbucUes and nntf.
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134 AIRPLANES
B. CENTER SECTION
1. Insert struts in sockets on center section.
2. Pick up assembled center section and struts and bolt struts into
sockets on the longerons.
3. Fasten wires.
4. Align.
5. Safety.
C. TAIL
1. Attach horns to control surfaces.
2. Attach horizontal stabilizer and its braces or cables.
3. Attach vertical stabilizer and its braces or cables.
4. Align both stabilizers.
5. Safety tumbuckles and nuts.
6. Attach rudder and its control cables.
7. Attach elevators and their control cables.
8. Cotter nuts and hinge pins.
9. Align rudder and elevators.
D. "WING
1. Unpack upper and lower panels.
2. Attach horns and brace wires to ailerons.
3. Set upper and lower panels on leading edges about a strut's length
apart.
4. Attach intermediate pair of struts and their diagonal wires.
(Makes wing self-supporting.)
5. Attach other struts and diagonal wires.
6. Attach flying and landing wires.
7. Attach wing skid, masts, and ailerons.
8. Hang assembled wing to body.
9. Align.
10. Safety.
5. ALiaNMENT*
A. FUSELAGE
1. Set bare fuselage on horses placed well forward.
2. Level up top longerons lengthwise and crosswise in fourth bay by
adjusting horses and side bracing wires.
3. Repeat for each of the other bays back to the tail thus getting top
longerons in same plane all along. ^
4. Stretch string from center of nose plate over top of fuselage and
adjust cross bracing wires between top longerons until centers of top hori-
zontal struts line up.
5. Adjust transverse and bottom bracing wires at each set of struts
until a plumb-line dropped from top longeron just touches the lower lon-
geron, being careful to keep top longerons level crosswise.
6. Set engine beds equidistant from each other and from sides of
fuselage at both ends.
7. Adjust wires in front two or three bays until fuselage is square
at each station and engine beds are level lengthwise and crosswise.
8. Check every bay and the stem post.
*See figures 119 and 120.
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AIRPLANES
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B. UNDERCARRIAGE
1. Make front croBsbracing wires on undercarriage same length.
2. Make lengths from rear wing hinge to axle the same on both sides
of fuselage.
3. Safety tumbuckles.
C. CENTER SECTION
1. Tighten and equalize follow-thru wires.
2. Set fuselage in flying position.
3. Set stagger by dropping plumb-line from leading edge or hinge
face and measuring back to lower front hinge, on both sides of fuselage.
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136
AIRPLANES
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AIRPLANES 187
4u Set center section square over fuselage by equalizing diagonal brac-
ing wires. Check by measuring distances from plumb-line to center section
and to top and bottom longerons ; they should be equal.
5. Safety tumbuckles.
D. TAIL
1. Level fuselage crosswise.
2. Level horizontal stabilizer by adjusting brace wires, if any.
3. Plumb vertical stabilizer or line up rudder hinges on it and tail post.
4. Set control bridge in neutral and line up elevators with hori-
zontal stabilizer.
5. Set rudder bar in neutral and line up rudder with fin.
6. Safety tumbuckles.
E. WINO
1. €kt leading edges straight.
2. Oet both wings same height.
3. Put in dihedral, if any, by stretching string between masts (or
between tacks placed in leading edges of top panels near the outer struts)
and adjusting landing wires until string is at required distance above the
wing beam (or tack in leading edge of center section).
4. Align trailing edge by vetting, i. e., by sighting under leading
edge and moving eyes up or down until trailing edge is just visible next
the fuselage. Then the whole trailing edge should just be visible. Check
by setting one comer of straight-edge under rear spar, leveling up straight-
edge and measuring up from it to under side of leading edge or front spar.
This measurement should be the same under the intermediate and outer
struts as it is next the fuselage.
5. Droop left wing, if motor is clockwise, to counteract the propeller
torque. Use straight-edge and level as in 4, and make measurement under
outer struts % inch more than at the fuselage.
6. Check eta^gger in front of each set of struts and next to fuselage.
7. Qet up on a ladder and by vetting check top panels for straight-
ness of leading edge and alignment of trailing edge.
8. Be-check all measurements.
9. Safety tumbuckles and nuts.
10. Make following over-all measurements:
a. Measure distance from propeller hub to points on bearing plate
at outer front strut on both wings. These lengths should be equal
to within % inch.
b. Measure distance from stem post to similar points on rear of
wings. These should be equal to within l^ inch.
P. AILERON
1. Adjust balance wire so that each aileron droops % to ^ inch below
trailing edge of wing.
2. Tighten control cables so that they do not bind on the drum but
are not loose enough to allow wheel to turn without moving ailerons.
6. PBOPELLEBS
A. MANUFACTURE AND STRUCTURE
Propellers are made with two, three or four blades. The widest part of
the blade is usually at about six-tenths of its radius. The maximum width is
about one-tenth to one-fifteenth the diameter of the propeller. The thickness
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138
AIRPLANES
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PROPELLERS
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AIRPLANES 139
of the blade near the hub is very great, but diminishes rapidly to about half
blade length and then* gradually to the end. This increased size near the hub
is for strength. The cross' section of a propeller assumes somewhat the same
shape as a wing section. In fact the propeller is nothing more than a wing
travelling in a helical path, every point of which has different velocities due to
the varying distance of the sections from the axis. Since the velocity increases
along the blade, the lift or in this case the thrust would increase too. To make
this thrust equal along the blade the angle of incidence or the Pitch Angle must
become smaller along the blade.
The propeller is built up from five to ten laminations according to size.
The laminations are laid out and sawed to outline. They are then surfaced to
the required thickness and slightly roughened by tooth planing. After warm-
ing they are assembled together with the best hide glue and finnly clamped.
The entire process is carried on in a room at 100' F. After 18 to 24 hours the
clamps are removed and the center hole bored. The propeller is now left for
ten days to dry. After drying the propeller is placed on a horizontal table
and carved to shape. A few days are given it to dry then it is given a finish
and balanced.
B. INSPECTION
To the pilot the inspection of propellers is the most important phase of the
whole subject, because the success of the fiight depends a great deial upon the
dependability of the propeller. The propeller is usually the last thing gone
over on a machine and the tendency is to slight it.
1. Balance The correct balance of the propeller is the most im-
portant consideration if good running is to be obtained. A propeller which
is out of balance a small amount will destroy the crank-shaft in a few
hours. Vibration is the result of unbalance and vibration is the worst
enemy of materials. Vibration causes materials to crystalli!ze and break off.
It therefore behooves the pilot to see that the propeller is correctly balanced.
a. Static balance is all that the name implies: it is balance when
stationary, not rotating. That is, the propeller should be balanced
when it is placed on a stand such as shown in Fig. 121. It should stay
in any position desired. This is the test for static balance. This means
that the centrifugal force of the blades due to rotating the propeller
will be equal and no vibration should result.
b. Dynamic unbalance refers to a propeller which even though
it has been balanced statically still continues to vibrate. This is due
to the centers of gravities of the blades not being in the same plane.
If the pedals of a bicycle are rotated fast without the chain the frame
is seen to shake back and forth. This is because the pedals are rotating
in different planes. The centrifugal forces even though they are the
same, act in different planes and do not counteract each other. This
can be tested only by running the propeller at high speeds and observ-
ing whether the propeller vibrates or ** flutters."
2. Trackage Each point on the trailing edge of a blade should track
with the corresponding point on the other blade. This can be checked on
the parallel bar testing stand used for static balance. Bring the point in
question around to graze some fixed point, then turn the propeller com-
pletely through a half turn and see if the corresponding point on this
blade grazes the same point. This should check within 1/16 inch and will in
a measure show something about the dynamic balance.
3. Pitch angle and pitch The pitch angles of the blades at various
points should be checked up. This is done as indicated in Fig. 122. The
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140 AIRPLANES
propeller is mounted horizontally on a level face plate, a stub axle set
securely on the face plate going through the axle hole in the hub of the
propeller. Then at fixed distances, say about every foot from the center,
the pitch angle which the blade makes with the level face plate is measured
by means of a protractor.
Imagine a propeller rotating in a paper cylinder (Fig. 125). The tip
of the blade would describe the helical path shown on the cylinder. A point
on the blade at A would have its helical path on a smaller cylinder be-
cause its diameter is less. If the cylinders are cut and spread out the mark-
ings would appear as shown in Fig. 124. ABCD is the large cylinder spread
out and AEFD is the small cylinder spread out. The path is now shown
as a straight line. The pitch is the same for section A as it is for the tip.
If it was not the thrust would be different for the two sections and the blade
would tend to bend. The line DF is the circumference of section A and
the line DC is the circumference of the tip. It now should be evident that
the pitch angles sls shown by this diagram should check those measured by
the method of Fig. 122.
. 4. Camber The camber or curvature of the blades should be equal
and should decrease evenly towards the tips of the blades, and the greatest
depth of the curve should, at any point, be approximately at the same per-
centage of the chord from the leading edge as at other points.
It is difficult to test the top camber without a set of templates, but a
fairly accurate idea of the concave camber can be secured by slowly passing
a straight edge along the blade as shown in Fig. 123. The camber can now
be easily seen, and as the straight edge is passed along the blade, the ob-
server should look for any irregularities of the curvature, which should
gradually and evenly decrease towards the tip of the blade.
5. Joints The usual method for testing the glued joints is by re-
volving the propeller at a greater speed than it will be called upon to make
during flight, and then carefully examining the joints to see if they have
opened. This test might be made when the propeller is mounted on the
engine, but it is not a very convenient test to make especially in the field.
Under either circumstance, test or no test, all the joints should be examined
very carefully, trying by hand to see if they are quite sound. Inspect a
propeller, the joints of which appear to hold any thickness of glue. Some-
times the joints in the boss may open a little, but this is not dangerous unless
they extend to the blades, as the bolts will hold the laminations together.
6. Condition of surface The surface should be very smooth, es-
pecially towards the tips of the blades. Some propeller tips have a speed
of over 30,000 feet a minute, and any roughness will produce a bad drift or
resistance and lower the eflSciency. The best propellers have copper tips
covering the ends of the blades. These tips protect the propeller from the
splitting action of weeds and grass and offer some security against damage
from moisture. However to insure against any moisture collecting on the
inside of the tip, holes are drilled in the ends to allow the centrifugal force
to throw out what moisture might collect. All metal tipping should be in-
spected to see that the metal lies dose to the wood, that nails or rivets have
not split the blade, and that solder is placed smoothly and properly. Pro-
peller hubs should be pressed into propellers on an arborpress, and great
care should be taken to see that the wood has not been injured in the process.
The hub should be inspected for flaws.
7. Mounting Great care should be taken to see that the propeller is
mounted quite straight on its shaft. Test in the same way as for trackage.
If it is not straight, it is possibly due to some of the propeller bolts being
too slack or to others having been pulled up too tightly.
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AIBFLANES 141
7. BEPAIB
A. WINGS
Very few repairs are made to the internal parts of a wing panel which
do not necessitate the complete removal of the fabric ; also it is often necessary
to remove the fabric for inspection, because damage to a panel cannot be located
with covering in place. The covering when once removed cannot be used again.
The fabric used for wing panel covering is usually the best grade of Irish
linen; some cotton is used but is not as satisfactory as linen. Grade A linen
should weigh not more than 4.5 ounces per square yard, have a thread count of
90 to 105 threads per inch, be capable of withstanding a tensile stress of 75
pounds per inch of width, and be free from all imperfections in thread and
weave.
The fabric is attached to the wing panels by sewing. Stitches from 4 to
6 inches apart are taken around the rib, thru the top and bottom covering.
The connecting thread is always carried on the convex surface. After sewing,
a 2-inch strip is doped down on both top and bottom for reinforcement and
protection of the thread and to reduce stdn friction.
Dope is a cellulose acetate or nitrate lacquer with which the fabric is coated.
The chief function of the dope is to tighten up the fabric, and give a taut, smooth,
weatherproof surface, resistant to the weather and preferably also to oil and
gasoline. It also adds to the tensile strength of the fabric and prevents the
tension changing with the hygroscopic conditions of the atmosphere. The
solvents for dope which are commonly used are acetone and tetrachlorethane.
Dope is highly inflammable and poisonous. Care should be taken to keep it away
from fire, and the vapors, which are heavier than air, should be allowed to escape
thru openings in the floor. Moisture is very detrimental to dope; it should
therefore always be used in a dry place and applied when the fabric is dry.
Spar varnish protects the dope, makes the covering more weatherproof, and
gives the covering a smooth finish, thereby reducing skin friction.
The most frequent repair work done on a wing panel is the patching of the
fabric. Most of the ruptures will be tears or long narrow slits because of the
weave of the fabric. In case of the long narrow sUts a few stitches may be taken
before the patch is applied. The hole or slit should first be trimmed out to
remove the loose ends of the threads. The dope and varnish must then be re*
moved from where the patch is to stick on. This can be done in two ways ; first,
by applying dope remover ; and second, by applying new dope. The new dope
softens the old dope because it is an unsaturated solution while the dope re-
mover is nothing more than the pure solvent. The old dope when soft may
be scraped oflE.
Patches should be rectangular that the edges may be frayed out so the
dope can hold the patch better. The lap varies from % inch on small patches
to 2^ inches on large ones. A fabric patch is attached with dope instead of
glue because dope gives sufficient adhesion, gives a patch homogeneous with
the repaired surface, is waterproof, and by shrinking makes a very smooth
repair.
Dope is brushed on the fabric around the hole and on the underside of the
patch. The patch is then placed on the hole, care being taken to stick down
the edges evenly. One coat of dope is placed over the patch and allowed to
stand. As soon as it is dry another coat may be applied, and so on. In all,
about four coats are sufficient. One or two coats of weatherproof varnish should
be applied to make the job thoroughly weatherproof, and to reduce skin friction.
The most frequent wood replacement or wood repair is that of the ribs.
Each rib web is made in three sections; nose, intermediate, and tail. These
sections are held together and in place by cap strips, which can be removed
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142 AIBPLANES
without disturbing the beams. Stringers run lengthwise of the panel to hold
the intermediate web in place when stressed.
To remove the intermediate section of a rib, the drift and anti-drift wires
must first be removed. The stringers are then cut diagonally with the expecta-
tion of splicing in new sections. In replacing the intermediate section of a rib,
the web is first placed in position, and the cap strips tacked and glued in place.
A new section of stringer is spliced in by gluing the diagonal cuts together,
and wrapping with copper wire, which is soldered over to form a sleeve. The
drift and anti-drift wires are replaced and the alignment of the panel checked.
The veneering if injured is replaced only at the section damaged. Nose sections
of the rib web should be distributed along the repair for reinforcement. The
method of repairing the trailing edge depends upon the material of which it
is made. Broken wires are always replaced. Fittings must be replaced unless
facilities are at hand to test the repaired members before they are placed in
service.
The wing beam, if damaged, is never repaired. It is too important a mem-
ber to trust after having been repaired so a new one is always put in place.
Care should be taken to see that all wooden parts have at least two coats of
weatherproof varnish, and that all metallic parts are well enameled before the
fabric is replaced.
B. LONGERONS
The longerons are the most important members of the fuselage, and for
this reason longeron splicing is a highly specialized type of work. Every break
in a longeron necessitates at least one splice. Ash is the wood spliced in, even
when that part of the longeron repaired is made of spruce. A diagonal cut, of
not less than eleven inches on the horizontal, is made on each side of the break.
The use of a miter box is advisable in order that the longeron and piece to
be spliced in may be most conveniently cut to the same angle. The surfaces
of the splice must be as smooth as possible and perfectly flat so they will come in
contact with each other at all points.
Glue which has been approved by Signal Corps inspection should be used
if possible. If this is not obtainable, use the best grade of hide glue. The
glue is prepared for use by first putting in cold water until thoroughly soaked
and softened. Between 28 and 36% by weight of glue is the amount gen-
erally used. When the glue is soft it should be melted over a water bath, the
temperature not being gJlowed to go above 150 ** F. The glue pot should be
kept covered as much as possible in order to prevent the formation of a skin
or scum over the surface of the glue. Glue should always be applied at a tem-
perature between 140* and 150' F. Always use fresh glue, i.e., glue which
has been prepared on the same day it is used.
Be sure that the wood is dry and both pieces are at the same temperature.
The joint is first sized, i.e., a coating of glue is spread over the surfaces
and allowed to stand a few minutes before the sticking coat is applied. This
will prevent the glue used to stick the joint from soaking into the wood. Care
should be exercised in clamping to see that the surfaces match. The pressure
applied should be at least 100 lbs. per square inch. After thoroughly drying
(about 24 hrs. at temperature of 85 ** F.) the clamps may be removed and a
light cut taken over the joint with a wood plane to ascertain if a thin layer of
glue has been obtained. A good joint shows no glue and can scarcely be
detected except by the variation in the grain of the wood. The joint is now
wrapped with whip cord. In order to make the cord secure, so that if any one
turn should break the entire wrapping will not come ofif, a knot or half-hitch
is taken in each turn. The cord is covered with a coat of glue. Sometimes
an additional wrapping of linen tape and another coating of glue are applied.
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AIRPLANES 143
Always varnish the finished splice when dry. A well made glued splice is as
strong as the wood.
Bolts are sometimes used in addition to the glue, and when used should
divide the joint in equal sections. They should be put in place and tightened
up at the same time the clamps are put on.
All repairs made to the fuselage, with the exception of the longerons, are
by replacement.
C. SOLDERING
The alloys used for joining other metals with the aid of heat are called
solders. The variety and number are considerable, but are easily divided into
two general classes, namely, "hard" and "soft'* solders.
Hard solders are alloys of zinc, copper, silver, etc., which melt at fairly
high temperatures.
Soft solders are alloys of tin, lead, bismutji, etc., which melt at compara-
tively low temperatures.
Soft solder is used extensively in the construction and repair of airplanes
because of its extreme handiness of application and because the heat of working
is not high enough to appreciably weaken the metals joined.
The theory of soft soldering is that the solder adheres to, but does not unite
with, the surface of the metal unless the latter has a melting point lower than
that of the solder. In fact soldering usually consists of uniting two or more
pieces of similar or dissimilar metals by means of another metal of lower melt-
ing point. That constitutes soldering; all the rest of the operation is detail,
which may be varied to suit conditions.
The following are the commercial soft solders :
Lead Tin Melting point
Hard 1 2 340''F
Medium 1 1 370T
Soft 8 1 400*F
Medium or ''Half and Half" is the most generally used solder and is the
solder used in airplane construction and repair.
Before solder will stick ^the surface must be free from dirt, grease, oxide, or
any foreign substance which will prevent the adherence of the solder. The
surface may be cleaned and brightened by scraping or rubbing with sand paper
or emery cloth. To remove the oxide a flux is applied to the surface of the metal
just before soldering.
One of the most important considerations in work of this sort is the employ-
ment of the proper flux. However satisfactory a flux may be in soldering various
parts its use is prohibited if it is of a corrosive nature as it may cause corrosion of
the parts soldered after the soldering has been done. Bosin is the best non-
corrosive flux for use on airplanes. It may be prepared by dissolving in alcohol.
Other fluxes are salammoniac, zinc chloride, boracic acid, etc., all of which are
more or less corrosive. The main prerequisite of a good airplane flux is that it
be non-corrosive and yet perform its functions satisfactorily.
Solder is used in an airplane in the construction of tanks and radiator, con-
nections on fuel and water lines, electrical connections, and the end connections
on all stay wires.
Under no circumstances should any great reliance be placed upon it for
mechanical strength, although it may add to the strength of a joint by its adhe-
sive qualities and by filling up voids and thereby stiffening the joint Its func-
tion in the construction of tanks, radiators, and connections on fuel and water
lines is to fill up the voids and make the joints tight. The usual method of ap-
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144
AIBPLANES
plying the solder in this construction is by the use of the so-called ''soldering-
iron'' (see Fig. 126 B) which is really a copper bit placed on the end of an iron
handle.
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AIRPLANES
145
Before a soldering-iron can be used, the tapered part of the bit must be
coated with solder by the process known as "tinning". This is done by heating
the bit and applying flux and solder to the surfaces. A bit can never remain in
good condition if it is overheated. Once a bit is made red hot the solder will be
burned off and its usefulness is gone until it has been re-tinned ; overheating will
also cause the copper to become rough or pitted. Heating in a soft coal fire is
detrimental.
When using, the heat of the soldering-iron melts the solder and heats the
surface so that the solder will adhere, the bit being so manipulated that the solder
is placed in the desired places. The soldering-iron may be heated electrically by
a heating element placed inside the bit, or it may be of the more common kind
which is heated by a blow torch or fire pot.
The blow torch, Fig. 126 A, is used to produce a hot flame for heating the
soldering copper, melting solder, etc. Gasoline, the fuel used, is maintained
under pressure by means of an attached air pump. This type of torch bums
with a blue flame of intense heat.
Electrical connections are soldered to hold the parts more firmly in place
and to reduce the contact resistance.
Solder is used extensively in making end connections in stay and control
wires. These wires are never repaired but always replaced. The replacing of
a wire necessitates the making of two end connections, the type of end connection
used depending upon the kind of wire.
Aviator wire is a solid wire used for stays, and is always replaced if it is
worn, kinked, or shows signs of corrosion. When replacing, first ascertain the
correct length, making allowance for the wire used in the end connections.
PULIS A
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146 AIRPLANES
The ''ferrule" end connection used in aviator wire is shown in Pig. 2,
Plate A. This consists of a snugly fitting oval wire ferrule made up from the
same sized wire as that upon which it is used. Eight to ten convolutions consti-
tute the standard length. The ferrule is slipped on the wire first, the eye is then
formed, care being taken to make it as nearly round as possible, and with the
radius of the curve at A and B the same as the radius at C. The ferrule is
then slipped back to the shoulders over the free end of the wire which is in
turn bent back over the ferrule. Flux is applied and the connection is dipped
into a pot of melted solder. The solder should never be hotter than necessary
as heat is very detrimental to the wire. This connection is 85% efficient.
Pig. 1 and Pigs. 3 to 15, Plate A, show various experimental types of
ferrule end connections.
Aviator strand wire usually consists of either 7 or 19 wires stranded together
and is used for stays. It is replaced whenever it has broken wires or shows signs
of wear or corrosion. The '*end wrap" end connection, Pig. 127, is used. This
p/^77
^mes Wire Diom:
'D--^ ^ — C' Coarse Copper
'3 - rhimb/e '^f'^ ^^P
-^'/7ne Copper y/ire IVnsfp
Pig. 127
is made up as follows : The strand is first bent to approximate shape and the
loop wrapped with small copper wire A; the thimble B is then inserted and
the strand drawn around it firmly. The length of the lap covered by the
wrapping C is 15 times the diameter of the strand. Two openings D are left
in this wrapping for the solder to run in. Plux is. applied and the connection
dipped in a pot of melted solder. This connection is 100% efficient.
Aviator cord wire is made by twisting 7 strands together forming:* a cord or
rope, the strands being either 7 or 19 wire. The end connection' made in this
wire at the factory is usually the *^end splice". Pig. 1, Plate B shows a thim-
ble spliced in 7 by 19 aviator cord. Pig. 2 shows the splice after, the serving is
applied. Pig. 3 shows the result of a test to destruction; five strands have been
broken at the last tuck in the splice. The splice usually fails at this point, and is
only about 80% efficient. In repair a modified end wrap end connection is used;
" the only difference being that the wrapping A is omitted, otherwise it is made
in the same manner as on strand wire.
D. BRAZING
Brazing is the uniting of two pieces of metal by a thin film of soft brass. It
is practically the same as soldering except the brass or spelter takes the place of
the solder. The common fiuxes are borax and boracic acid. The parts to be
brazed are brought into contact, dusted with borax or coated with some other flux,
and the spelter is melted into the joint. The spelter will automatically run to
. the hottest part of the joint and no space need be provided for it.
E. WELDING
Welding is the uniting of metals by heat without using either flux or com-
pression. The heat is ordinarily obtained from oxygen and acetylene gas and is
usually about 6000'P. The pieces of metal are held together and the heat
applied until the fusion takes place. If the pieces are large they should be heated
all over to a red heat to avoid warping.
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AIRPLANES
147
FiatbB
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CHAPTER II.
COOPERATION WITH THE ARTILLERY
1.
OUTLINE
The Artillery and Its Uses
A.
The Characteristics of the Types of Artillery
1.
Gun
2.
Howitzer
3.
Mortar
B.
Principal Types of Projectiles
1.
High Explosive
2.
Slmtpnel
8.
Gas
C.
The Duties of the Artillery
1.
Barrage
2.
Counter Battery
3.
Bombardment
(Under each division)
a. Object
b. Projectiles Used
c. Artillery Used
d. Aviator's Duties
e. Occurrence
4.
Special Duties
a. Night Fire
b. Fleeting Opportunity Targets
5. The Artillery Preparation for an Infantry Attack
a. During the Weeks and Months Previous to the Attack
b. When the Zone of Attack has been Selected
c. During the attack
d. When the Objective has been Attained
2. The Organization of the Artillery
A. The Allotment of Pieces and Objectives
1. Divisional
2. Army Corps
B. Principles of Organization
C. Organization of the Artillery Units
3. Methods of Fire (Banging)
A. Terms Used in Ranging
B. The Accuracy of a Piece
C. Method of Reporting Shots
4. Methods of Fire (Continued)
A. Fire for Adjustment
B. Continuous Fire for Effect
^ C. Pre- Arranged Shoot
D. Impromptu Shoot
201
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202 COOPERATION WITH THE ARTILLERY
5. Procedure in Cooperating with the Artillery
A. General Suggestions as to Work of the Observer
B. Difficulties of Observation
C. Methods of Signalling
1. Airplane to Qround
2. Ground to Airplane
D. Organization and Routine for a Pre- Arranged Shoot
E. Report Forms and Reports
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CHAPTER II.
COOPERATION WITH THE ARTILLERY
1. THE ABTILLEBY AND ITS USES
A. THE CHARACTERISTICS OP THE TYPES OP ARTILLERY
Type
Compar
Appearance
ative
Mobility
Muzzle
Vel.
(m. p. 8.)
Max.
Bange, Km.
Max.
Angle
Path of
ProjectUe
Targets
1. Gun
Longest,
slender
Highest
500 to
800
27
20*
(except
Anti-
Aireraft)
Flat
Vertical,
aircraft,
barb wire,
men in
open,
long range
work
2. Howitzer
Shorter,
breech
heavier
Lower
(except
field
how.)
100 to
300
10
45«
Curved
Horizontal,
reverse
slope work
3. Mortar
Shortest,
very
heavy
breech
Lowest
Max.
200
8
90*
Very
Curved
Horizontal
B. PRINCIPAL TYPES OP PROJECTILES
Name
Case
Fuse
Contents
Action On
Explosion
Targets
1. High
Explosive
Thick
Percussion
Disruptive
explosive
as lyddite,
T. N. T.,
amatol, etc.
Penetrates,
disrupts
Material
2. Shrapnel*
Thinner
Time and
percussion
Propellant
explosive,
bullets,
matrix
Nose blown
off, bullets
spray out
Aircraft,
barb wire,
men in open
3. Gas
Very
thin
Percussion
Disruptive
explosive
and liquid
Shell bursts
and liquid
vaporizes
Men under
cover
•The effect of shrapnel depends on:
1. Number of bullets
2. Energy of bullets on impact
3. Spread of bullets
4. Curve of trajectory
5. Position of the burst
For best results shrapnel must be fired from guns in order to obtain:
1. Sweeping effect due to flat trajectory,
2. High energy due to high remaining velocity
203
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204
COOPERATION WITH THE ARTILLERY
Other Types of Projectiles are:
a. Tank shells (armor piercing H. E.)
b. Liquid Are
c. H. E. shrapnel
d. Shrapnel gas
C. THE DUTIES OF THE ARTILLERY
Name
(a)
Object
(b)
Projectiles
Used
(c)
Artillery
Used
(d)
Aviator's
Duties
(e)
Occurrence
1. Barrage
1.
2.
3.
4.
Isolation
Neutraliza-
tion
Sweeping
Defense
1. Gas, H. E.
2. H. E. &
Shrapnel
3. Shrapnel
4. Gas and
Shrapnel
Guns 75 's
and
at times
How. and
guns 100
Location
of
batteries,
troops,
amm. etc.
During an
attack
2. Oounter
Battery
1.
Destruction
1. H. E.
Gun 100 up
How. 155 '*
Mor. 220 "
Location.
Control of
fire and
ammunition
All the
time
2.
Neutraliza-
tion
2. Shrapnel
and gas
Guns 100 to
155
3. Bombard-
ment
i:
Destruction
of material
1. H. E.
Gun 155 up
How. 155 "
Mor. 220 '*
Location
and fire
control
At all
times
2.
Destruction
of barb wire
2. Shrapnel
Guns 75 to
155
4. Special duties
a. Night fire This type of work is usually carried out with
75 's or some of the medium guns with shrapnel for any one of the
following purposes:
1. To prevent reconstruction of works bombarded during
the day
2. To prevent rest at rest camps and billets
3. To prevent bringing up supplies, etc.
6. Fleeting opportunity targets Targets (use shrapnel and
sometimes H. E.) which last only for a short time; e.g., battery on the
move, trench R. R. train with troops or supplies, troops in the open on
the move, etc.
5. The artillery preparation for an infantry attack
a. During the weeks and months previous to the attack
1. Constant counter battery work
2. Bombardment to disorganize the defensive system —
bridges, roads, command posts, observation posts, trench sys-
tems, etc.
3. Fire to prevent organization and supply — ^night fire, etc.
4. Destruction of all opportunity targets such as moving ob-
jects of all kinds, etc.
b. When the zone of attack h<is been selected
1. Destruction of the trenches and entire defensive system,
that is to be the objective
2. Destruction of all hostile artillery which would interfere
with the attack
3. Destruction of wire in the path of the infantry
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c.
d.
COOPEEATION WITH THE ARTILLERY
During the attack
1. Barrage fire
2. Counter battery work (neutralization)
3. Work on fleeting opportunity targets
206
When the objective has been attained
1. Prevention of reinforcements and counter attack
2. Prevention of hostile artillery interference with the con-
solidation of the position
2. THE ORGANIZATION OF THE ARTILLEBT
A. THE ALLOTMENT OP PIECES AND OBJECTIVES
There are two main branches of the artillery — ^the Divisional and the Army
Corps Artillery. The first includes the light or field artillery; the second is
composed of the medium and heavy artillery.
Branch
Pieces
Objectives
a. Field guns
Guns 75 mm
a. Barrage in aU attacks
b. Destruction of barb wire
c. Cantonments within range
d. Night fire to prevent reconstruction
b. Howitzers
How. 155 mm
a. Bombardment within range
(inc. mortars)
Mot. 220 mm
b. Reverse slope work
Mot. 270 mm
c Fleeting opportunity targets
2. Army Corps
/
a. Counter
Gun 100 mm
a. Destruction of batteries
battery
105 mm
b. Neutralization of batteries
120 mm
c. Night fire on cantonments and lines of
155 mm
supply
b. Heavy
Gun 140 mm
a. Heavy bombardment — ^batteries, forts, con-
artiUery
240 mm
crete workS; deep dug-outs
805 mm
b. Long distance work — on cantonmentS| H. Q.,
supply and ammunition dumps.
340 mm
How. 370 mm
400 mm
520 mm
Mor. 270 mm
280 mm
293 mm
B. PI^INCIPLES OP ORGANIZATION
As the pieces of the Divisional Artillery have a short range they are assigned
the immediate front, including our first line trenches and the enemy's first and
sometimes his second line trenches ; that is, they will cover very intensively the
enemy's territory to a depth of about four kilometers.
The task of the Army Corps Artillery is to cover the region back of
that covered by the Divisional Artillery. Further they will work upon objec-
tives which require heavier projectiles than the Divisional Artillery is able to
use even tho these objectives may be situated within the territory normally
covered by the Divisional Artillery. Forts, batteries with heavy cover, and
almost any concrete works will come under this type of work.
There are three principles that will determine the organization of the
artillery.
1. The Divisional Artillery must be able to cover the immediate
front intensively with a large number of pieces placed well forward.
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206 COOPERATION WITH THE ARTILLERY
2. The region covered by the Divisional Artillery must be covered
twice — once by the Field Gun group and once by the Howitzers. This
is due to the difference in the objectives for which the field guns and
the howitzers and mortars are best suited.
3. The Army Corps Artillery will cover the region farther back,
that is the enemy's rear. This region will be covered less intensively,
as the region assigned to each piece will be much larger. This however
does not mean that the Army Corps Artillery will be located behind
the Divisional Artillery. The heavy howitzers, due to their short range
and the fact that their objectives are situated well behind the enemy
lines, must be put well to the front.
C. ORGANIZATION OF THE ARTILLERY UNITS
AA, AB, AC, DA, DB, DC, are battalions of Divisional Artillery Field
Guns, allotted to cover the enemy zones A'A', ATB', A'C, D'A', D'B', D'C,
respectively. (See Fig. 201)
BA, BB, CA, CB, are battalions of Divisional Artillery Howitzers, allotted
to the zones A'A' to D'C inclusive.
GA, GB, GC, are battalions of Army Corps Artillery allotted to cover the
zones G'A', G'B', G'C, respectively.
Each battery is commanded by a Captain. Each battalion, or group, as
AA, AB, etc., is commanded by a Major.
Headquarters of Commanding Officers : Lieutenant Colonels at A, B, C, D, ;
Colonels at E, F, G.
E', F', G', are aero squadrons and E", F", G", are balloon companies under
the Commanding Aeronautical Officer, K, assigned to work with the commands
of Colonels E, JP, G, respectively.
A special aero squadron, J', assigned to general H. Q.
A General at J commands the whole unit.
8. METHODS OF FIRE (RANaXNG)
A. TERMS USED IN RANGING
1. The range of shot is the straight line distance from the piece
to the point where the shot strikes.
2. The range of a piece is the longest straight line distance that
a given piece can place between it and a shot from that piece. This is
usually referred to as the maximum range,
8. The correction to the range is the number of meters that a shot
lands short or over the range correct line.
4. The correction to the deflection is the number of meters that
a shot lands to the right or left of the Battery-Target (B-T) line.
6. The time of fliight is the time in seconds that it takes a shot
to travel from the piece to the point where it strikes.
6. In salvo fire all the pieces of the battery are aimed at the same
point, and the pieces then fire at the target one at a time at intervals of
five seconds. The appearance of a salvo about the target will be four
bursts separated by intervals of five seconds. In ranging, the shots of a
salvo must hit fairly close to one another. In case one piece is wild it will
be corrected before further work is done with the salvo.
7. The extreme right deflection of a salvo is the correction to the
deflection of the extreme right shot of a salvo.
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COOPERATION WITH THE ABTILLEBY 207
G'A'
G'B*
G'C
A'A'
A'B'
A'C
D'A'
D'B'
D'C
%^iyi~
' ^T^ ^T^ ^T^
AC n^aHIib Vdc
Fia. 201. — Organization op the Abtillebt UnitS'*'^®*^ '^^
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208
COOPERATION WITH THE ARTILLERY
8. The center of impact for any group of shots is the average center
of all the shots. It is the point winch has the average correction to the
deflection and the average correction to the range. In reporting on a salvo
the aviator gives the extreme right deflection and the range of the center of
impact See Fig. 202.
9. A bracketing salvo is one in which half of the shots land over
the range correct line and half short
d
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202. — ^Diagram Showing Location of Shots by Means of Coordinateb
WITH Reference to the Battery-Target Line
For the salvos, x=:extreme right defleetion
7=:raiige of the center of impaet
Salvo; left 075, over 155.
Bracketing salvo. (Brackets the target, two over, two short)
Salvo; right 155, over 101.
Salvo; right 202, range correct
Salvo in which one piece is shooting "wild". Such a piece must be corrected before
ranging by ealvo is continued.
Single shot; deflection correct, short 155.
Single shot; left 125, short 101.
B. THE ACCURACY OF A PIECE
The accuracy of a piece is measured by the size of its 100% zone. The
100% zone is the region where all the shots fired from a rigid piece (data kept
constant) would fall. In general the 100% zone is an ellipse with the longer
axis pointing in the direction of the piece. The zone for a gun is an elongated
ellipse whereas that of a howitzer is more nearly circular. This shows that the
gun is much more accurate in deflection (line) than in range. On the other
hand the howitzer is as inaccurate in line as in range.
Ranging a piece means moving the center of the 100% zone over onto the
target. When this is accomplished, fire to destroy the target f qUowb.
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COOPERATION WITH THE ARTILLERY 209
The reasons for the existence of the 100% zone are :
1. The unsteadiness of the piece. Although the recoil mechanism of
the piece may be very good, it is not perfect. Some movement of the piece
occurs which causes the next shot fired with the same data to fall in a slightly
different place.
2. Wear on the piece. This will include the wear on the piece during
the firing which may be appreciable in the case of the larger pieces, also
the previous wear on the piece. Both contribute to the inaccuracy of
shooting.
3. Irregularities in projectiles and ammunition. This will include
variations in the weight, polish, explosive power, etc.
4. Irregularities in atmosphere. This includes density, winds,
eddies, humidity, pressure, etc.
6. Irregularities in fire. If the piece is not fired at regular intervals
the projectile with its explosive will be heated more or less depending on
the length of time it remains in the piece. An explosive has more force
when heated. Unless the firing is regular, serious irregularities in the
range will result.
C. METHOD OP REPORTING SHOTS
In reporting on shots the following rules must be observed by the aviator.
1. General rules
a. All messages must be preceded by the battery call given twice.
b. The correction to the deflection will be given first, then the
correction to the range.
c. All distances will be reported in meters.
d. All reports on distances will consist of three figures, e.g.,
075, 025, 125.
e. No set of figures representing a distance may end in a zero;
send 055 for 50 m., 101 for 100 m., 202 for 200 m., etc.
2. Rules for the correction to the deflection
a. The deflection will be reported as either left, right, or correct
b. The amount of the correction to the deflection will be esti-
mated in multiples of 25 meters.
8. Rules for the correction to the range
a. The range will be reported as short, over, or correct
b. The amount of the correction to the range will be estimated
in multiples of 50 meters.
The code for the designation of the target and necessary information to
be sent to the battery is confidential and will be given to the cadet in class.
4. HETHODS OF FIRE (OONTINnED)
A. THE FIRE FOR ADJUSTMENT
The object of the fire for adjustment is to range the piece or battery. Thus
the battery will fire and the aviator will report where the shots landed and
then the battery will correct and fire again. This will continue until the bat-
tery is ranged. The defiection must be much more accurately corrected than
the range.
Fire for adjustment is divided into: 1, preliminary adjustment; 2, final
adjustment.
1. PreUminary adjustment The object of the preliminary adjust-
ment is to correct the line or deflection. There is the additional effort
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210 COOPEEATION WITH THE ABTHJiERY
made to correct the range. The work is carried out by means of salvos.
The aviator will report on each salvo, giving the extreme right deflection
and the range of the center of impact. This work will stop when a bracket-
ing salvo is obtained. The aviator will report the bracketing salvo as hav-
ing hit the target.
2. Final adjustment The preliminary adjustment has corrected the
deflection completely but as a rule the range needs further adjustment. This
is carried out by the battery firing groups of three salvos (twelve shots)
and the aviator making his report after each group of twelve shots and
calling for the next group. In this work the aviator will report only the
number of shots over and the number short, e.g., 8 over, 4 short, as what
he is trying to do is to correct the range. This continues until the battery
is satisfied and signals to the aviator to take up the next stage — continu-
ous fire for effect. In this work, as in all the work, the aviator must report
only what he sees. In case some of the shots fail, only those which appear
will be reported upon ; e.g., 7 over, 4 short. The shots may be lost due to
their falling in places where observation is impossible or to the mis-fire of
the piece.
B. CONTINUOUS FIRE FOR EFFECT
The object of the continuous fire for effect is either to destroy the target
or to neutralize it. In this part of the fire the aviator ceases to tell the bat-
tery when to fire and the battery fires as rapidly as possible on the target. The
aviator makes no reports on this fire except in two cases.
1. In case the fire drifts ofT tiie target due to movement of pieces,
change in temperature of pieces, etc. Then the aviator will report this
fact to the battery by giving either
a. the range and, if necessary, the deflection of the center of
impact, or
b. the number of shots over and number short for a certain num-
ber of shots which are fired successively. This will be at least three
salvos.
2. In case the target is hit and destroyed he will at once signal to
the battery "result has been attained '\ The battery will then signal further
instructions. The aviator will always inform the battery when he starts
for home.
C. THE PRE-ARRANGED SHOOT
In the pre-arranged shoot the aviator is given all the necessary informa-
tion about the battery and target before he takes the air. As to the battery,
this information will be its range, time of flight, rapidity with which it can
deliver flre, its location on the map, the identification panel it will use, and finally
the program that it wishes to follow.
The information of the target will consist of its number or squared map
location, photographs of the target taken from the air and ruled off to aid in
the spotting of shots, and the amount of work which is to be done on the target.
On the other hand the battery knows which aviator is to direct the fire
for it, the wave length he will use, and the time at which he will appear over-
head. When he leaves for the battery this latter information is telephoned.
This kind of a shoot is the most successful and is made in the majority
of cases.
D. THE IMPROMPTU SHOOT
The impromptu shoot will occur when the aviator is out on some work,
such as artillery patrol or work on some particular target and finds a target
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COOPERATION WITH THE ARTILLERY 211
which needs immediate attention. This might be any one of the targets which
are classed as fleeting opportunity targets, or some particularly active enemy
battery.
In this case it is necessary for the aviator to call the particular battery
which can cover the target in question. For this purpose he carries with him a
zone map on which are marked the arcs of fire of the various batteries. After
determining the battery which can reach the target he gets into communication
with it. Once communication is established the impromptu shoot is conducted
in practically the same manner as the pre-arranged shoot.
5. PROOEDURE IN OOOPERATINa WITH THE ARTILLERY
A. GENERAL SUGGESTIONS AS TO WORK OP THE OBSERVER
As the aviator is to serve as the ''eyes of the artillery'' it is necessary that
he be familiar with the artillery which he is to direct. Part of this knowledge
can be obtained from books but much must be obtained from actual contact.
"Observers should utilize every opportunity of visiting the batteries allotted to
them. On non-flying days squadron and balloon company commanders should
arrange to send observers to the batteries with which they work, in order to
watch the procedure during a shoot so that they may get some idea of the diffi-
culties and delay with which the artillery have to compete."
The observer must necessarily be mentally keen and alert; he must be
persevering and have a clear-cut understanding of the problems connected
with his work. He has a mission to perform and is, therefore, acting under
orders. His duties are essentially two-fold: 1, location and surveillance of
targets; 2, observation of fire effects. He must have a thorough knowledge
of tiie artillery map. He must become expert in judgment of distance and di-
rection. He must gain the confidence of his battery by accurate and painstak-
ing work.
B. DIFFICULTIES OF OBSERVATION
1. Phyiioal
a. Speed of airplane
b. Atmospheric conditions, (clouds, mist, haze, etc.)
c. Conditions on the earth's surface — difficult country, snow, shell
action, burning buildings, dust
2. Imposed
a. Enemy planes
b. Anti-aircraft guns
c. Camoufiage — dummy guns, etc.
d. Trajectories of shells from numerous batteries
8. Observation will be aided by
a. Photography — ^its careful interpretation (paths, snow, horses.
ete.)
b.
Moving objects, gun fiashes
c.
Ruses, false attacks*, sudden returns to suspected regions
d.
Balloons
e.
Ground observation posts
f.
Sound ranging
g-
Cross bearings
*Dtiring the great battles around Verdun, the French launched a false attack thereby
eanaing the Germana to open fire with all their batteries. Observers were thus enabled to
locate the positions of the batteries. During the following days these batteries were in-
cessantly pounded and a large per cent destroyed so that the actual attack later was successful.
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212 COOPERATION WITH THE ABTILLERY
C. METHODS OF SIGNALLING
1. Aixplane to ground
a. Wireless is used almost exclusively. If the wireless is out of
order, plane will return to landing ground and another be sent up.
b. OtTier methods that may be used at times are : dropped written
messages, searchlight or lamp-flash signals, fusee (Very's lights)
signals.
2. Ground to airplane
a. Ground panels of white cloth, 4x4 m. and 4x^ m. If snow
covers the ground, black cloth will be used.
b. Panneaus, using Morse code.
c. Wireless receiving sets have as yet been used only on a few
planes with transmitting sets at receiving stations near important H. Q.
D. ORGANIZATION AND ROUTINE FOR A PRE-ARRANGED SHOOT
1. Cooperating planes Careful organization and strict discipline are
necessary to reduce tibe chances of confusion to a minimum.
a. Identity of planes Planes may be identified by the "call"
adopted by each receiving station, the use of varying wave-lengths,
and the loudness of emission.
h. Zone of operation Due to wireless limitations, i.e., the inter-
ference of messages one with another, the number of planes operating
on the front for observation is restricted to one plane for each 1000
meters. In some instances chronographs (watches with colored dials),
have been used to enable neighboring planes to send messages at
alternate specified intervals, but this interferes with the continuity
of observation. It is important that planes keep out of neighboring
zones when signalling, and that they do not come closer than two kilo-
meters to their own receiving stations. Messages sent when directly
above the antennae interfere seriously with other messages. Technical
matters concerning wireless are prescribed in each Army Corps by
the Chief of the Radio Service concerned.
c. MetJiod of designating location of target Target locations will
be given by coordinates on the artillery or squared map, as 88-97, or
possibly in three figures, as 695-486. In order to prevent the enemy
from knowing the meaning of a message, letters may be substituted for
certain of the figures. In the event of a pre-arranged shoot, the target
may be designated by a number, as N9. In case of a hostile battery,
the right gun (as viewed from our line) is chosen as the center of
the target.
d. Position of plane when observing The plane should be ap-
proximately over the target when observations are made. Observa-
tions may be made at angles up to 45*.
e. Position of plane when sending Plane should be headed di-
rectly towards receiving station when sending wireless messages. Don*t
send messages when turning or when headed in opposite direction for
wireless reasons. (See Fig. 203)
2. Battery and receiving station
a. Identification Each receiving station is provided with identi-
fication panels, which enable the observer to distinguish his own re-
ceiving station. The squared map location of these panels is known
to the observer.
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COOPERATION WITH THE AETILLERY
213
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214
COOPERATION WITH THE ARTILLERY
b. Receipt of messages All information by wireless is received
at a special receiving station. Information as to targets and correc-
tions is then transmitted to Battery Officer usaally by telephone, then
by megaphone or otherwise to men at the battery.
c. Signals to plane Ground panels should be placed where the
observer expects to find them, either behind the second gun in a bat-
tery of four guns or nearer the receiving station.
d. Laying of pieces The observer must remember that some time
is required to correct data and to change from one target to another
and diould make necessary allowance. . After ranging has begun, how-
ever, the observer should expect prompt firing aftOT he gives subse-
quent signals. The Battery Commander will ddiver regular, accurate,
and rapid fire. Time is important, as observation by plane is limited
to three hours at a time and liable to interruptions from atmospheric
conditions, engine trouble, gasoline supply, aerial combat, etc The
piece should fire in ten seconds from the time the signal is received.
The observer knows the time of fiight of the projectile and using a
stop watch will be prepared to observe the shell burst. If battery
does not respond within 30 seconds after '*fire'' signal, they must wait
until the observer again signals to fire.
3. Bontine Observer leaves the aerodrome and when at a height
of about 500 feet, he lets out the aerial and puts in the safety plug, and
then calls up the wireless operator at the aerodrome to test his apparatus.
If the wireless is working properly, he files toward his battery, calling re-
peatedly as he approaches. After communication is established with his
battery, he then flies out over the target and proceeds with the work.
Fig. 203 illustrates the position of the plane at various stages of the shoot.
When shoot has been completed, he sends the proper signal before leav-
ing. Before returning to landing ground, he takes out safely plug and winds
in aerial. After landing, he makes proper records and talks over the shoot
with the Battery Commander.
E. REPORT FORMS AND REPORTS
Reports must be accurate. DaUy records will be made. Especially in the
case of hostile batteries detailed reports must be kept, including the history of
the battery day by day, when engaged, when active, together with photographs
taken. Destruction of enemy batteries should be continually in progress ex-
cept during and immediately following an infantry attack. (Usually from 250
to 300 rounds from 8.2 or 9-inch howitzers or 500 rounds from 6-inch howitzers
are required to destroy a battery). The following are sample forms of reports:
A card index record of hostile batteries is maintained. The face of the
card contains a record of the coordinates and a photograph of the region. The
reverse side of the card has the following form :
Card Index Record of Hostile Battery
Date
No.
of
pieces
Targets
Adjustments
Results
86611
active
Calibre
Shots fired
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COOPERATION WITH THE ABTILLERY
A form of report of 8ummar7 of work done includes the following:
SUMMARY OF WORK DONE
Enemy Battesoes Discovered
215
No.
Coordinates
No. of pieces
In action
or not
How
Their objective
(if possible)
Were they under
bombardment
1 1
Fire Adjustments
No.
Battery whose fire'
was adjusted
Objective
Difficulties
No. of successful adjustments..
Causes of failures
Photographs taken (region)
Airplanes met: Allied.
Sundries
Squadron No. ...
Pilot
Observer
The day of.
Hr. of start „. _.. Hr. of return....
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CHAPTER m.
ENGINES
OUTLINE
1. Fundamental Principles
A. Four-Stroke Cycle
1. Suction Stroke
2. Compression Stroke
3. Expansion Stroke
4. Exhaust Stroke
B. Definitions
1. Work
2. Power
3. A horsepower
4. Charging of the Cylinder
C. Two-Stroke Cycle
2. Carburetion
A. Requirements for Maximum Economy
1. Combustible proportions
2. Complete vaporization
B. Single Jet Carburetors
1. An Auxiliary Air Valve
2. A Movable Needle Valve
C. Multiple Jet Carburetors
D. The Zenith Principle
1. Construction of the Zenith
2. Care of the Zenith Carburetor
E. The Effects of Altitude
F. Duplex Carburetors
3. Ignition
A. Elementary Principles
B. Sources of Electrical Energy
C. Ignition Circuits
D. Magnetic Flux
E. The Armature
F. The Primary Circuit
G. The Condenser
H. Short-Circuiting Switch
I. The Contact Breaker
J. The Secondary Circuit
K. The Distributor
L. Spark-Plugs
M. The Safety Spark-Gap
N. High-Tension Magnetos
O. Connections
301
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302 ENGINES
4. Classification of Airplane Engines
A. Pour-Stroke vs. Two-Stroke
B. Air-Cooled vs. Water-Cooled
C. Classification According to Cylinder Arrangement
1. Vertical Type
2. V-type Eight and Twelve
3. Horizontal, Opposed-cylinder Type
4. Y and Broad-arrow Types
5. Radial, Stationary Cylinder Type
6. Botary Engines
5. Types of Botary Motors
A. Clerget
B. Le Bhone
C. Gnome
D. **Monosoupape" Gnome
1. Ignition
2. Working Stroke
3. Exhaust Stroke
4. Induction Stroke
5. Compression Stroke
6. Essential Bequirements and Allied Factors
A. Factors that aflfect Power and Weight
1. Causes of Decreased Weight of Charge
a. Besistance to Flow of Gases
b. Heating by Hot Clearance Gases
c. Improperly set Valves
d. Improper Valve Clearance
e. Worn Cams
f . Weak Valve Springs
g. Too High a Speed
2. Improper Treatment of Fuel Charges
a. Leaks
b. Incomplete Vaporization
c. Improper Ignition
d. Insufficient Compression Pressure
e. Too Early Belease of the Burned Gases
f . Poor Form of Combustion Chamber
3. The Weight Factor
B. Fuel and Oil Economy
C. Beliability
D. Desirable Qualities
1. Low Air Besistance
2. Controllability or Flexibility
3. Freedom from Vibration
4. Accessibility
5. Silenee
6. Cleanliness
E. Demands Dependent Upon Weight Efficiency and Fuel Efficiency
1. Ability to Ascend Bapidly
2. A Combination of Fast and Slow Flying Speeds
3. Saf eness of Handling in Winds
4. Ability to Eemain in the Air for Long Periods
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ENGINES 303
7. Engine Troubles
A. Essential Conditions
1. The Mixture
2. SufScient Compression
3. Means for Igniting the Explosive Mixture
4. Proper Lubrication and Ccioling
B. Oarburetion Troubles
1. Mixture too Lean
2. Mixture too Rich
c.
Compression
1. Leakage at the Valves
2. Leakage at Spark-plug Gaskets
3. Leakage at the Pistons
D.
Ignition Troubles
E.
Lubrication Troubles
P.
Cooling Troubles
Notes
on Timing
A.
Designation of Valve Events
B.
Valve Clearance
C.
Direction of Rotation
D.
Grinding Valves
E.
Principles of Valve Timing
F.
Setting the Engine
G.
Principles of Ignition Timing
H.
Timing the Distributor
I.
Firing Order
J.
Setting the Engine
K.
Wiring Up
L.
Double Ignition
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CHAPTER m.
ENGINES
1. FUNDAMENTAL PBINOIPLES
The function of an airplane engine is to rotate a propeller against air resis-
tance, and in performing this operation the expansive force of a combustible mix-
ture of gasoline vapor and air is caused to act against the face of a piston within
a cylinder so as to produce motion, this * * reciprocating ' ' motion being transmitted
through a connecting-rod and transformed into '* rotary *' motion at the crank-
shaft which carries the propeller. The action within the cylinder is not unlike
the explosion of gun-powder in a cannon, in which the pressure created by the
sudden evolution of heat drives the ball out with great force. In other respects,
the action of an engine is quite similar to the operation performed by a man
turning a grindstone ; the stone corresponds to the fly-wheel or propeller, the
handle corresponds to the crank on the crankshaft, the man's arm acts as con-
necting-rod, and the force exerted by him represents the expanding gas. Thus
the heat generated by the combustion of fuel (gasoline) is converted into mechan-
ical energy, or power.
A. FOUR-STROKE CYCLE
A Cyde, in gasoline engine operation, constitutes a complete series of events.
Pour-stroke operation requires four strokes of the piston to complete a cycle, and
in that case there is an explosion or working impulse only once in two revolutions
of the crankshaft. The operation of a four-cycle engine may be summarized as
follows, with reference to Pig. 301 :
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304
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ENGINES 305
1. Suction stroke (a-b) The piston moves away from the cylinder
head, the intake valve I is opened, and a charge is drawn into the cylinder
from the carburetor. (Note : The illustration shows the cylinder horizontal
so that the movement of the piston may be more easily followed on the
pressure diagram.) The suction pressure is below atmospheric, and it is
the "pressure head", or difference of pressure, that causes the fresh charge
to flow in.
2. Compression stroke (b-c) With both valves closed, the piston
moves back and compresses the charge into the clearance space, and Ignition
takes place near the end of the stroke.
3. Expansion stroke (d-e) After the charge is ''fired", the pressure
rises rapidly and forces the piston out on the power stroke.
4. Exhaust stroke (e-a) The exhaust valve, £, opens near the end
of the expansion stroke, the pressure decreases, and on the return stroke the
piston drives out jnost of the remaining burned gas. Owing to resistance
to flow through the valve, the pressure within the cylinder during the ex-
haust stroke is somewhat above atmospheric.
Since the pressure in the crankcase of a four-stroke cycle engine is always
approximately atmospheric,- the net pressure tending to move the piston is the
difference between the absolute pressure in the cylinder and atmospheric pressure
underneath the piston. Let Pw be the average pressure above atmospheric during
the expansion stroke, ?« the average pressure above atmospheric during the ex-
haust stroke, P. the average pressure below atmospheric during the suction stroke,
and Pc the average pressure above atmospheric during the compression stroke.
It is obvious that Pw is a ** forward" pressure, and that the "back" pressure of
the other three strokes tending to slow down the engine is overcome only at the
expense of a part of Pw, or energy stored up in the rotating parts of the engine
during the expansion stroke. What is left after the losses have been deducted is
called the Mean Effective Pressure, Algebraically, M.E.P.— Pw — Pe — ^P. — ^Pc-
B. DEFINITIONS
1. Work, by definition, is the product of force times distance, and the
common unit of work is the foot-pound. The distance through which the
piston reciprocates in any engine is a definite measurable quantity, called
the Stroke. But the force exerted upon the piston by an expanding gas
doing work is variable, and can only be determined by means of a Pressure
Diagram taken while the work is being done. A pressure diagram shows the
actual pressure within the engine cylinder at any point of the cycle. The net
area of the diagram is proportional to the work done.
2. Power is defined as the ''rate of doing work"; i.e., work divided
by time. Indicated Power is the power developed by action of the gaseous
charge within the engine cylinder ; it is the net power delivered to the face of
the piston. Indicated power depends upon the weight of charge burned, and
the efficiency with which the heat of combustion is converted into mechanical
energy (thermal efficiency).
The Delivered Power at the propeller is always 10 to 20 per cent less
than the indicated power of the engine, because of f rictional losses in engine
bearings^ and power used to drive the camshaft, magneto, water and oil
pumps, etc. The percentage of the indicated power which represents the
amount actually delivered to the propeller is called the Mechanical Efficiency.
In other words, mechanical efficiency is the ratio of delivered power divided
by indicated power.
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306 ENaiNES
3. A Horsepower is the equivalent of 33,000 foot-pounds of work per
minute, or 1,980,000 ft. lb. per hour. A heat unit, or B.T.U., which is the
amount of heat necessary to raise the temperature of 1 pound of water 1
degree Fahrenheit, is equivalent to 778 ft. lb. of mechanical energy. One
horsepower-hour is therefore equivalent to 2545 B.T.U.
If the M.E.P. is known, it is a simple matter to calculate the indicated
horsepower of an engine from a well-known formula, which is derived as
follows:
Let: P=M.E.P. (lb. per sq. in.) .
a^area of piston face, which is Trr* or ^ird* (sq. in.)
L^length of piston stroke (feet)
N=number of explosions per minute.
The effective force acting against the piston — Pa (pounds) ; the dis-
tance through which the force acts = L (feet). The work done by one
charge is, then, P a L (foot-pounds) ; and since the cylinder is charged N
times per minute, the work done by the gas per minute is P a L N foot-
pounds, whence
PLaN
I.H.P..
33,000
The "lower loop'' of the pressure diagram is a measure of the power
lost in overcoming the resistance of the valve openings and gas passages ; in
other words, it represents the **pump work" required to force gas into and
out of the cylinder in the very short time available in gasoline engine opera-
tion.
4. Charging of the cylinder The space between the piston and the
cylinder head when the engine is on top dead-center is called the Clearance
Volume. This volume is determined by the amount of compression that
can be used, and is usually about 25 or 30 percent of the stroke volume, or
Piston Displacement, Only the piston displacement is available for filling
with fresh charge, because the clearance volume cannot be ''scavenged".
The dead gas at exhaust pressure remaining in the clearance space at the
beginning of the suction stroke must expand to below atmospheric pressure
before actual suction of the fresh charge can begin. This delay reduces the
volume available for charging, because of the increased volume occupied by
inert gas expanded down to the suction pressure. Furthermore, the fresh
gas in the cylinder at the end of the suction stroke is in a rarefied condition
due to the low pressure, and is consequently lighter than an equal volume of
gas would be at atmospheric pressure. Finally, the high temperature at-
tained by the gas within the cylinder dilring suction causes it to expand and
exclude some that would otherwise follow in, thus affecting the actual weight
of charge inhaled. The efficiency with which the cylinder is charged with
fresh gas by the displacement of the piston during normal operation is called
Volumetric Efficiency. This is equal to the weight of charge actually drawn
in divided by the weight that would fill the volume of piston displacement
at atmospheric pressure and atmospheric temperature. Anything that rare-
fies or reduces the pressure of the gas entering the cylinder, or which in-
creases its temperature, lowers the volumetric efficiency, and in turn the
M,E.P. and the horsepower of the engine.
C. TWO-STROKE CYCLE
The two-stroke engine, while possessing certain advantages that make it the
ideal type of motor for certain classes of work, has not yet attained the state of
perfection and reliability so requisite in airplane service and therefore is not
used to any large extent. The principal causes of its deficiency lie in the method
of operation.
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ENGINES
807
Bef erring to Fig. 302, the cycle starts with the cylinder containing a fresh
charge. Compression, ignition and combustion, and expansion occur as in the
other cycle. But near the end of the expansion stroke, the piston uncoyers ex-
haust port E in the side of the cylinder, allowing the burned gas to escape and
the pressure within the cylinder to drop. Immediately afterwards, a fresh charge
under sufiScient pressure is admitted through an intake i)ort I in such a way as
i
Fig. 302. — Two-Stboke Ctcle Engine wtth Crankoase Compression
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308 ENGINES
to further scavenge or clean out the dead gas (a baffle plate on the lace of the
piston deflects the incoming gas upward). After the piston covers the ports on
the return stroke, compression begins, and the cycle is repeated.
Engines operated on the two-stroke cycle principle commonly utilize ''crank-
case compression", wherein the piston on its regular compression stroke sucks
the mixture from the carburetor into the crankcase through an automatic intake
valve, and during the expansion stroke this charge is compressed in the crankcase
to sufficient pressure so that when the intake port is uncovered by the piston
(shortly after the exhaust port is opened) the fresh gas at higher pressure rushes
into the cylinder and sweeps out much of the remaining burned gas. The baffle
on the face of the piston greatly assists this scavenging action, and prevents the
fresh charge from passing directly across to the exhaust port.
It is obvious that exhaust and intake must take place while the i)orts are
uncovered, that is, during a small part of the piston stroke. To bring about
transfer of the mixture and displacement of the burned gases requires precom-
pression of the full charge to 5 to 10 pounds pressure, and this work of pr^om-
pression is lost as far as useful work is concerned. A pressure diagram takeli on
the crankcase would represent the work done on the gas in preparing it for "&[%•
trance to the working cylinder, and the M.E.P. of this diagram must be subtracted
from that of the power diagram in order to get the true M.E.P. of the cycle. In^
order to avoid waste of fuel, the main bearings of the crankshaft must be kept
in dose adjustment to prevent leakage from the crankcase.
Scavenging is never as perfect in a two-cycle engine as in a four, because in
the latter the piston shoves out the products of combustion, while in the two-cycle
engine the whole cylinder volume is full of hot gas at the time the fresh gas rushes
in. Some of this inert gas will be driven out by displacement, but much of it
will mix with the new charge, diluting and heating it to such an extent as to
seriously reduce the power capacity of the engine. Although it has a power
stroke every revolution, a two-cycle engine can develop only 1.4 to 1.6 times
as much power as a four-stroke engine of equal size and running at the same
speed ; and at the same time the fuel and oil consumption of the former is 30 to
60 per cent greater. And while a two-stroke engine is limited to less than
1500 revolutions per minute, a four-stroke engine may develop its maximum
power at a speed as high as 2500 R.P.M., so that if comparison is not restricted
to the same speed of rotation the high-speed four-stroke engine is fully as light
per horsepower, and its efficiency and fuel economy are much better.
Another serious disadvantage inherent to two-cycle operation is the fact
that the engine will not throttle down well, because the amount of unscavenged
burned gas increases as the fresh charge is decreased, and the effect of this
increasing dilution is veiy detrimental to efficiency and reliability. The charge
becomes weaker and weaker, and finally non-explosive.
The main advantages of a two-cycle engine are simplicity and cheapness.
There are very few working parts, and no valves or valve-operating mechanism
to give trouble. The engine will run equally well in either direction by simply
providing for sufficient movement of the spark-advance lever both sides of the
normal retarded (top center) position. There is a working stroke from each
cylinder every revolution, which tends to give more uniform torque or turning
effort than would result from less frequent application of power impulses; but
since four-cycle airplane engines can revolve at much higher speed than is pos-
sible with two-stroke operation, this does not necessarily count as of any con-
sequence.
2. OARBUBETION
A. REQUIREMENTS FOR MAXIMUM ECONOMY
Gasoline will bum efficiently in an engine cylinder only when the following
conditions are fulfilled:
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1. The proporttons of gasoltaie and air must be eorreet for every
variable condition met with in engine operation, such as variations in com-
pression, due to throttling or to atmospheric changes. Ordinarily there
should be between 10 and 16 parts of air to one part of gasoline vapor, by
weight. If there is a larger proportion of the gasoline, the mixture is said
to be too ''rich" in quality; if too little of the gasoline vapor is present,
the quality is designated as ''lean". In the case of a rich mixture, there is
not enough air present to support complete combustion of the gasoline, and
unconsumed fuel will be decomposed into soot by the action of the intense
heat, which will either deposit in the cylinder and foul up the engine or
pass out through the exhaust in the form of black smoke. Too lean a
mixture does not necessarily mean incomplete combustion, but invariably
results in slow burning of the fuel and loss of power. If the burning is
slow enough, there may be flame still present in the cylinder when the
intake valve opens, and a "back-fire" follows.
2. The gasoline must be completely vaporized, or gasified, in order
to make possible a perfectly homogeneous mixture, uniform distribution to
all cylinders, and rapid and complete combustion of the fuel. Gasoline
vaporizes quite readily at ordinary summer temperatures, but will not va-
porize in sufiicient quantities to supply an airplane engine unless it is first
broken up into a fine spray and the vaporization accelerated by the artificial
application of heat Whenever vaporization takes place, there is an absorp-
tion of heat during the process that tends to cool off the surrounding medium.
The intake manifold must be kept warm enough to prevent condensation on
the way to the cylinders. Liquid particles of gasoline, if present in the
swiftly moving column of gaseous mixture, will be thrown out by their own
inertia wherever there is a bend or branch in the pipe, so that the end cyl-
inders are apt to get a proportionately larger amount of gasoline while the
intermediate cylinders are starved, with the result that the power of the
engine is lowered, fuel is wasted, and the end cylinders foul up rapidly and
overheat. When little drops of gasoline are present in the cylinder at the
time of combustion, only the surface of any drop is in contact with air, and
the inner part of the drop is entirely excluded from chemical union with
oxygen. The intense heat, however, liberates the hydrogen and "carbon-
izes" the unconsumed residue, which deposits in the cylinder or on the valves
in the form of carbon. Whenever vaporization is poor, as in cold weather
or when starting a cold engine, more than the normal amount of gasoline
will be required to make the mixture combustible, because it is partly in the
liquid form and much goes through unconsumed.
; B. SINGLE JET CARBURETORS
, The process of mixing gasoline vapor and air is called "carburetion", and
■ the instrument used on gasoline engines to supply proper mixtures of gasoline
! vapor and air is called a "carburetor". The basic principle upon which most
carburetors are founded consists of a single jet or nozzle located in a fixed air
* passage and supplied with gasoline from a vessel in which the level is always at
! or a trifle below the opening at the jet. The air passage is connected to the
! engine so that suction, when the intake valve is open, induces a flow of air and
j a spray of gasoline simultaneously, the relative proportions depending upon the
suction and the size of the spray nozzle. The amount of mixture fed to the
engine is regulated by a butterfly valve, or throttle, located above the jet.
The deficiency of a simple arrangement of that sort is that an adjustment
for one speed would not be correct for any other speed, because with varia*
tions in suction the gasoline-flow changes more than the air-flow changes. This
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gives increased richness with increase in speed; or, vice versa, if properly ad-
justed for high speed the mixture becomes altogether too lean at low speed.
There are two principal schemes in use to automatically maintain proper
proportions in a single- jet carburetor :
1. An auxiliary air valve The ''throttle" by which tie speed of an
engine is controlled is always located between the spray nozzle and the
intake manifold. When the amount of throttle opening is increased, the
suction at the mixing chamber becomes greater. By admitting part of the
air supply above the jet, through an air valve provided for that purpose,
less air and less gasoline will be drawn from below, although the total amount
of air going to the engine remains approximately the same. By properly
proportioning the opening of the auxiliary air valve to the speed of the
engine, the tendency for increased richness previously mentioned can be
overcome in a more or less satisfactory manner. The auxiliary air valve
may be controlled by a light spring, properly adjusted as to tension, or it
may be linked with the throttle-operating mechanism so that its action is
regular and positive under all conditions. The latter is by far the better,
because a spring-controlled valve will not proportion the mixture satis-
factorily over a very wide range, it has a tendency to ** flutter *' during
operation, and in time dirt or grease will collect and interfere with proper
action of the valve.
2. A movable needle valve Jets or spray nozzles are often provided
with adjustable needle valves for varying the size of opening to give the
required amount of gasoline. With a needle valve under automatic or me-
chanical control, there is no need for an auxiliary air valve, the needle being
arranged to slightly decrease the size of the jet with increased throttle
opening. In some carburetors, however, the mixture is controlled by a
combination of auxiliary air and a movable needle valve (Schebler).
C. MULTIPLE JET CARBURETORS
In general, multiple jet carburetors may be likened to a series of small,
single-jet carburetors, arranged to be brought into action consecutively to increase
the speed of the engine. Fig. 303 illustrates the principle. The jets are in sepa-
PiG. 303. — ^Illustrating Multiple-Jet Principle
rate air passages over which the throttle fits. In the illustration the throttle is
shown as a plain slide, for simplicity, but it might be a cylindrical throttle if
slotted to open the air passages in succession. For slow spc^ operation, the ve-
locity of the air past the spray nozzle is much higher thanf it would be in a single-
jet carburetor of equal capacity, and the nozzle itself is small, which tend to pro-
duce better vaporization and mixing at low speeds. Furthermore, the possibility
of adjusting the mixture accurately for a number of throttle positions makes cer-
tain more nearly correct mixtures over the entire range.
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311
T^
Pig. 304. — Simple Single Jet Arbakge-
MENT WITH Nozzle Connected Di-
rectly TO Float Chamber
—-■^ — 1
1
"U
[J
.
— J—
= :
— ,
r
^^
n
Fig. 305. — ^Arrangement of Com-
pensating System in Zenith
Carburetor
P— float;
Gj — afloat valve;
I — eompeiiBator ;
Q — main jet;
H — compensating jet;
X— Choke;
0-N — sidling adjustment;
T— throttle valve.
— Sectional View op Zenith Carburetor
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D. . THE ZENITH PRINCIPLE
The Zenith compound-nozzle carburetor, which appears to be a standard
on airplanes today, utilizes a principle entirely different from those previously
described. The simple jet (Fig. 304) with its characteristic deficiency is used as
a basis, and around it is built a ** compensating" jet which makes up for the
deficiency of the first by its tendency to produce exactly the opposite results as
regards variation in mixture proportions with variable speed or suction. The
compensating jet, which is illustrated in Fig. 305, is not connected directly to the
constant-level source of supply (float-chamber), but is connected to an open well,
J, into which the gasoline feeds by gravity through a small hole, or ** compen-
sator", I. Since the well is open and under atmospheric pressure at all times,
increased suction at the jet has no effect on the rate of flow through the compen-
sator into the well, hence the flow is constant and depends only on the size of the
opening I. The primary jet, calibrated to give almost enough gasoline at high
speed, will give much too little at low speed, while the constant flow from the
compensating jet is quite sufficient to make up the deficiency at low speed and
is proportionately very little at high speed. Thus a substantially constant pro-
portion is maintained by the two together.
1. Construction of the Zenith Referring to the sectional drawing of
a Zenith carburetor. Fig. 306, the arrangement of the various parts may be
seen. The float-chamber is a common type, located at one side of the mixing
chamber. A removable fine-mesh screen is inserted at the union which con-
nects the float-chamber with the pipe from the main gasoline tank. The
compensator and the primary jet are screwed into place through plugged
holes at the bottom of the carburetor. The plugs also serve as little pockets
or wells into which dirt or water may collect, and should be cleaned occa-
sionally. The compensating jet is screwed in from above, and probably
never need be removed, since the compensator determines the flow and the
size of the jet has practically no effect if large enough to handle the limited
supply from the well.
The ** choke-tube", which gives the Venturi or stream-line shape to the
mixing chamber, increases the suction and the velocity of the air at the jets,
thereby giving better spraying and vaporization, especially at low speeds
where it is most difficult to effect proper carburetion. The passage must
not be restricted too much, however, because of the throttling effect which
affects the volumetric efficiency of the engine at full speed and reduces the
maximum power.
The ** idling- tube" through which gasoline is sucked directly into the
air passage above the butterfly throttle- valve when the throttle is nearly
closed, greatly assists in running the engine slowly when the suction, and the
velocity of the air past the jets, are not sufficient to spray the gasoline. There
is a needle- valve adjustment on the priming-tube by which a slight inleakage
of air is permitted in order to reduce the suction enough to give the correct
amount of gasoline. The priming-tube goes out of action entirely as soon
as the throttle opens enough to induce spray from the jets.
The Zenith carburetor is usually equipped with a **strangler- valve" at
the air intake, which may be used to partially restrict the flow of air and
increase the suction on the jets for starting. In some other carburetors, a
needle valve is arranged to increase the size of jet opening temporarily so
that more gasoline will flow to enrichen the mixture. The latter scheme has
the disadvantage that spraying is not as good with the opening enlarged,
and the mechanism for moving the needle valve is not as simple.
There are two possible ways of supplying heat for vaporization. One
is to preheat the air by passing it through a jacket or stove around the ex-
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haust pipe of the engine. This method is often used in connection with
Zenith carburetors, and the carburetor itself is then provided with a sleeve*
valve or shutter whereby cold air may be admitted to keep down the temper-
ature and prevent excessive heating. More heat than is necessary to produce
complete vaporization would result in reduced volumetric efficiency and
loss of power. The other way to furnish heat for vaporization is by cir-
culating hot cooling-water or hot oil through a jacket around the carburetor
and the intake manifold, thus at the same time cooling off the water or the
oil, which is advantageous.
2. Oare of the Zenith carburetor The Zenith carburetor is of the non-
adjustable type, there being no external adjustments, except that on the
idling-tube and a screw to limit the amount of throttle closure. The only
way to change the ''setting" of the carburetor is by removing the main jet,
the compensator, or the choke-tube, and inserting different sizes. Jets and
compensators are numbered according to their inside diameter in hundredths
of a millimeter. Chokes are numbered in millimeters of smallest diameter.
Great care should be used to prevent dirt or lint from getting into the
jets. If a carburetor suddenly develops indications of a lean mixture, it is
more than likely that one of the holes bias become clogged. In removing
the obstruction, care should be taken not to enlarge the opening. Water
in the gasoline sometimes gives trouble from skipping and irregular opera^
tion by collecting below the jets, but it may be drained off by removing the
plug underneath after first shutting off the gasoline.
Flooding of the carburetor may be due to imperfect seating of the
float-valve, or to a leaky float. The former may be caused by careless
handling, or by collection of foreign matter between the valve and its seat.
A leaky float can not always be repaired satisfactorily. Before making any
attempt to effect a repair on a metal float, it will usually be necessary to
make a second perforation in order to thoroughly dry out the inside. In
sealing up the holes, solder must be used very sparingly, so that it will not
weight down the float appreciably. Cork floats may be dried and shellaced.
Flooding of the carburetor might frequently happen during flight by
tilting. Some carburetors are built to obviate this to a large extent by
placing the float concentric with the mixing chamber (Schebler). There are
also apt to be indications of flooding immediately after an engine is stopped,
because of unvaporized gasoline running down the sides of the intake mani-
fold momentarily. This condition is not as likely to obtain when the engine
and pipes are thoroughly warmed up.
E. THE EFFECTS OF ALTITUDE
High altitude is characterized by low barometric pressure and low tempera-
ture. The atmospheric pressure at the earth's surface, due to the weight of air
several miles high, is in the neighborhood of 15 pounds per square inch, but for
every 1,000 feet elevation there is a decrease of approximately 0.5 lb. per sq. in.
The rarefied condition of the air makes it impossible for the engine to inhale so
much, and the weight of the mixture and the compression are consequently less.
The power of the motor decreases at about the same rate as the air pressure.
The mixture tends to vary in quality, and to prevent excessive richness at high
altitudes there is usually provided an auxiliary air port between the carburetor
and the intake manifold. Sometimes provision is made whereby the compres-
sion may be kept high for altitude work by simply removing large washers or
spacers from beneath the cylinder flanges before flight, which decreases the
clearance volume.
The temperature decreases at the rate of about 1 degree for every 300 feet
rise. At an altitude of 20,000 feet the temperature is usually below zero, which
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makes vaporization of the gasoline difScolt There is danger of freezing the
water in the cooling system, and if any water is present in llie gasoline it will
freeze in the pipes or the jets. There is always danger of moisture condensing
out of the air in the gasoline tank when the temi>erature is reduced. Further-
more, extremely low temperature introduces excessive strains in metal parts of
the machine, and may even produce ' ' crystallization ' ' and fracture. The lowered
** boiling-point" must also be considered.
P. DUPLEX CARBURETORS
Engines of very high power require so much gasoline that the jet in a single
carburetor would have to be too big for successful spraying. For that reason, it
is common practice to use double carburetors on all but the very small engines.
Two mixing chambers are cast side-by-side, and the jets in each are supplied from
a single float-chamber. It is well to use a separate carburetor, or a separate
mixing chamber, for every three or four cylinders, and care must be taken to
synchronize the throttles to work in unison.
3. lONinON
A. ELEMENTARY PRINCIPLES
Electrical current flows through a wire much the same as water flows in a
pipe. The amount flowing depends upon the pressure and the resistance to flow.
In measuring electrical quantities, the amount of current is expressed in Amperes,
the pressure is measured in Volts, and the resistance is expressed in Ohms. The
''jump spark" system of electrical ignition is always employed on airplane
motors. The electrical resistance of the gap at a spark-plug is so great that an
extremely high voltage is necessary to force the current across. The flame or
spark which accompanies the flow of current across the gap sets flre to the com-
pressed mixture in the cylinder, providing the spark is properly ''timed".
B. SOURCES OF ELECTRICAL ENERGY
The source of current may be a "battery", in which electricity is stored and
ready for use at any time. Common dry-cells, such as are used with "buzzers",
give off current by virtue of chemical action within the cells, but the capacity of
a dry-cell is so little that it is not suitable for prolonged use. So-called "storage
batteries", in which the action is electrolytic, have much greater capacity, and
can be re-charged when exhausted. Electrical energy can be pumped into a
storage battery from a dynamo, and used when wanted, just the same as water
is pumped into a reservoir on top of a building for use in case of flre. The switch
in an electrical circuit corresponds to the valve in a water system. Storage
batteries have not been used much in airplanes because of the constant danger
of spilling and losing the liquid "electrolyte" (sulphuric acid), but with tiie
increasing demand for self-starters on airplane engines it is probable that elec- .
triced systems will be perfected for this work. To start an engine by electrical
means, it is necessary to make use of a storage battery; but once an engine is
running, the current for ignition and lighting can be supplied by a dynamo.
An electrical generator, or dynamo, acts like a pump, and builds up a certain
pressure or voltage which is more or less dependent upon the speed of rotation.
If the djmamo is properly connected to a storage battery, it will pump electricity
into the battery. A dynamo can also be used for ignition purposes. When
current flows continuously in one direction, it is said to be Direct Current.
Current that flows momentarily in one direction, then in the other, surging
back and forth like a pendulum, is called Alternating Current. A "magneto" is
a special type of alternating current dynamo used for ignition purposes. There
are "low tension" and "high tension" magnetos, both identically the same in
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principle but differing in constructive details. High-tension magnetos are almost
invariably used, but for the sake of simplicity the low-tension type will be ex-
plained, and then the essential differences pointed out.
C. IGNITION CIRCUITS
The jump-spark system requires a high-tension current at the spark-plug.
This gives rise to ''primary'* and "secondary" circuits in the ignition system.
The primary circuit carries a low-voltage current, which is transformed or
**8tepped-up" to a high voltage in the secondary by means of an induction coil
or transformer. This coil consists of a few turns of heavy wire, forming a part
of the primary circuit, together with several thousand turns of very fine wire,
wrapped around but insulated from the primary, and connected with the sec-
ondary circuit. A soft-iron core completes the transformer.
In addition to the primary transformer coil the primary circuit consists of
an armature winding, an interrupter or contact breaker, a condenser, and a short-
circuiting switch for cutting out the ignition. The secondary circuit includes,
besides the transformer coil, the safety spark-gap, and a distributor for con-
necting in the different spark-plugs in proper sequence. (See Figs. 307, 308
and 309.)
D. MAGNETIC FLUX
When electrical current is caused to flow through a coil of wire around a
soft-iron core, a magnetic field is established and the core is given magnetic prop-
erties. One end or ''pole*' of the magnet would point towards the North if free
to turn, while the other pole would be attracted simultaneously towards the
South. The ' ' flux ' ', or lines of magnetic force, can easily be traced by sprinkling
iron filings over a piece of paper in the neighborhood of the electro-magnet.
These lines of force practically disappear when the circuit is broken so that the
current stops flowing, i.e., the magnetic flux collapses. If the core consists of
hardened steel, however, a permanent magnet is formed. In an ordinary dynamo,
the magnetic field is maintained by field coils wound on iron lugs, but in a
magneto permanent magnets are employed.
E. THE ARMATURE
If a closed coil of wire is moved through a magnetic field, so that there is a
change in the flux interlinking the coil, a current is induced in the wire. Then
if the coil is moved across the field so that the direction of flux through the coil
reverses, the flow of current is reversed. The armature of a low-tension magneto
consists of a coil of wire rotating between tJie poles of a horse-shoe magnet and
cutting lines of magnetic flux in such a way as to produce an alternating current
in the primary circuit. The voltage depends upon the intensity of flux, the
number of coils cutting the flux, and the speed of cutting. Both the current and
the voltage increase to a maximum, and then decrease to zero and build up to a
maximum in the opposite direction, a reversal occuring twice per revolution.
The current wave lags behind the voltage because of "inductance". Magnetos
are ordinarily arranged to "spark" in the secondary when the current generated
in the primary is maximum and the breaker in "advanced" position. A re-
tarded spark, then, is necessarily weaker in case only the breaker-box is moved,
but in some magnetos special provision is made to have the spark of equal
intensity for all positions (Dixie).
F. THE PRIMARY CIRCUIT
The current generated in the armature windings of a low-tension magneto is
carried outside to the primary coil of the transformer. This current establishes
a magnetic field around the secondary coil. When the breaker in the primary
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cireoit is opened, the current stops flowing and the flux about the coil collapses.
In coUapsing, the lines of force move with extreme rapidity, and in so doing
they ''cut'' the large number of turns of wire in the secondary coil, which induces
an extremely high voltage impulse. The high voltage is sufficient to break down
the air gap and cause a current to flow across the gap at the spark-plug.
G. THE CONDENSER
The condenser plays an important part when the primary circuit is inter-
rupted. It consists of several thin metallic plates (tin foil) carefully insulated
from one another by mica or waxed paper. Alternate plates connect to one side
of the interrupter, the other plates connect to the other side, thereby forming a
''parallel" connection. When a "break" occurs, the current tends to keep on
flowing because of its inertia, which causes a hot spark between the breaker points
if the condenser is not functioning properly. The condenser acts like a shock
absorber, absorbing the current abruptly, thereby preventing destructive spark-
ing at the breaker points and at the same time increasing the intensity of the
secondary spark by causing more rapid collapse of the flux.
H. SHORT-CIBCUITING SWITCH
Magneto ignition is usually discontinued by short-circuiting the breaker, so
that the opening of the points does not interrupt the flow of current in the
primary. Thus no high-tension voltage is generated in the secondary because
the flux about the coil does not coUapse.
I. THE CONTACT BREAKER
The device that interrupts the primary circuit at regular intervals consists
of a movable contact point operated by cams, together with a stationary point,
all mounted in a housing called the Breaker-box. The breaker-box may be sta-
tionary around one end of the armature shaft, and the cams integral with the
shaft. The sparW is advanced by simply moving the breaker-box opposite to the
direction of rotation of the cams. One end of the armature coil is grounded
while the other end connects to one of the breaker points. The other side of the
contact-breaker is connected to the primary coil of the transformer, and the
circuit then completed through the ground. In some magnetos the cams remain
stationary, and the breaker is fixed rigidly and rotates with the armature. The
action is identical to that of fixed brei^ker and rotating cam.
J. THE SECONDARY CIRCUIT
Current is generated whether a coil of wire crosses a magnetic field or a
magnetic field moves past a stationary coil. The action of a transformer corre-
sponds to the latter principle, the flux collapsing across the secondary coils with
extreme rapidity and inducing an enormous voltage in the secondary circuit.
This voltage is, of course, only momentary, but it is quite sufficient to break down
the resistance of the spark-plug gap and produce the desired spark.
K. THE DISTRIBUTOR
The function of the distributor is to make connection with each spark-plug
of the engine in proper sequence at the time a spark should occur. It consists
of a hard-rubber disk, usually, with as many insulated contact^points as there
are cylinders. The contacts are equally spaced in the path of a rotating brush,
which turns at a predetermined speed and "distributes" the high-tension cur-
rent just as the breaker points separate.
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L. SPARK-PLUGS
The essential elements of a 8i)ark-plug are simply two stationary conductors
or electrodes, insulated from each other and separated by a short air-gap about
the thickness of a dime. One electrode usually forms the threaded outer shell
of the plug, and grounds directly to the cylinder, while the central electrode is
insulated by porcelain or mica, both of which have heat-resisting properties.
The high-tension cables from the distributor connect to the insulated electrodes
of the spark-plug.
^r, THE SAFETY SPARK-GAP
If a spark-plug should be disconnected when the magneto is in operation
the pressure or voltage in the secondary circuit would tend to become excessive.
The safety spark-gap in an ignition system is analogous to the safety valve on
a steam boiler. It is simply a l^ to V^ inch gap, with one side grounded and the
other terminal connected to the secondary circuit between the secondary coil and
the distributor. It acts as a safe outlet for the induced charge before the voltage
can become great enough to break through the insulation of the coils, or damage
the condenser by excessive ''inductive kick" in the primary.
N. HIGH-TENSION MAGNETOS
A low-tension magneto delivers current at a very low voltage, which requires
the use of a spark transformer as a connecting link between the magneto and the
spark-plug. In a high-tension magneto, the transformer is integral with the
armature, so that high voltage is generated directly. The spark from a high-
tension magneto is probably of longer duration than one produced in a high-
tension system by a low-tension magneto, because of the voltage induced in the
secondary by virtue of the continual flux change in the armature core produced
by rotation after the contacts separate.
The armature may either revolve or be stationary. In the latter type, the
rotor consists of iron lugs so arranged that every 90 or 180 degrees (depending
upon whether it is a *' four-spark'* or a '* two-spark*' magneto), the flux flowing
through the armature core from the N-pole of the magnet to the S-pole is suddenly
reversed. Magnetos provided with stationary armatures are said to be of the
''inductor*' type. When the rotor comprises iron segments in the form of a
sleeve around the armature, the machine is called a Sleeve Inductor Type Mag-
neto. One in which the iron rotor operates in conjunction with an external
armature core is called Polar Inductor Type Magneto. The Dixie is represen-
tative of the polar inductor type.
Magnetos in which the armature rotates, as in the Berling and the Bosch,
are called Rotating Armature Type Magnetos. These require a "collector ring"
and brush to take off the secondary current. The brush connects to the rotating
arm of the distributor.
0. CONNECTIONS
In all types of magnetos, the primary current flows through the breaker-
box, and since this current is of low voltage it is obvious that the contacts of
the interrupter must be kept smooth and bright. Poor contact when the points
come together, due to rust, grease or corrosion, will cause a large decrease in pri-
mary current. This, in turn, affects the voltage induced in the secondary and
poor ignition results. All permanent connections in the primary circuit should
be soldered, and the breaker points should be smoothed up and adjusted when
necessary.
Poor contacts in the secondary circuit are not of much consequence except
for heating effect, because the high voltage will jump across any slight obstruc-
tion. This very fact, however, is frequently a cause of ignition trouble when
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ENGINES 321
foreign matter accumulates either on the spark-plugs or on the distributor and
forms a path of low resistance where leakage of current readily occurs. Dis-
tributors with little carbon brushes are very apt to foul up in this way, the
carbon dust being spread along the path of the rotating arm where it acts as
an electrical conductor from one segment to the next. This causes trouble
because of the fact that a spark passes more readily in a cylinder that is not
under compression.
The spark-gap type of distributor and brush, in which a metal brush is
used and so arranged that there is a small air gap between it and the dis-
tributor segment, obviates the trouble from "tracking" experienced with the
ordinary type of distributor.
4. OLASSIFIOATION OF AIRPLANE ENGINES
A. POUR-STROKE VS. TWO-STROKE
The comparative advantages and disadvantages of four-stroke and two-
stroke cycle engines for airplane service were discussed under Part I. There are
two-cycle motors in- use, the Roberts being a notable example, but in the present
stage of deyelopment the two-cycle engine is easily outclassed by the more
efficient and flexible four-cycle type.
B. AIR.COOLED VS. WATER-COOLED
Air-cooled engines are necessarily of low power. Air cooling, at its best,
is far less efficient than water cooling, because of the comparatively low specific
heat and the low specific weight of air. With inefficient cooling, cylinders must
be smaller in order to prevent overheating. Furthermore, compression in air-
cooled engines must be lower in order to avoid pre-ignition. These factors limit
the power of a cylinder. Only certain cylinder arrangements will permit of
air cooling, because all of the cylinders must be swept by the current of air
with equal intensity. The radial cylinder arrangement, in which the cylinders
are distributed around a circular crankcase like spokes around the hub of a
wheel, is best suited to air cooling, but only a limited number of cylinders can
be so arranged. Air-cooled engines are not built in units much larger than 100
horse-power, and are used only for short flights in very light scout and pur-
suit planes.
C. CLASSIFICATION ACCORDING TO CYLINDER ARRANGEMENT
1. Vertical type Motors of the vertical type have four or six cylin-
ders in line, and are not ordinarily built in sizes much larger than 150 horse-
power. More than six cylinders in line is not a practical arrangement,
because the additional cylinders would require not only an increase in
the length of the crankcase, crankshaft, camshaft, etc., but also greater
thickness and weight of the whole length to insure rigidity. Thus the
increase in power would not be proportional to the increase in weight, and
the resulting weight-power ratio (weight per horsepower) would be too
great. The length would also be objectionable in an airplane. For school
purposes, and elsewhere where engines of small power are suitable, the
vertical type of motor is ideal, because it is less expensive than the more
complicated types, it is more economical of fuel and oil, and it is easier
to overhaul and adjust. It offers less *'head resistance" than any of the
other types, so at high speeds of flight less of the power of the engine is
wasted in pushing itself through the air. This is a considerable item, when
it is realized that the power absorbed in this direction increases with the
cube of the velocity. Engines are usually enclosed in streamline housings
to minimize air resistance, and a narrow housing naturally offers less re-
sistance to high velocity than one of wider construction. (See Fig. 310.)
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322
ENGINES
Fio. 310. — Austro-Daimler Aeroplane Engine
This engine hM giVen wonderful service in the present wftr. It is very similar to the MeroedM-Dftim-
ler. At BB cftn be seen the two wftter jacketed, annular float feed single spray, noule type carbnreton,
identical in construction and adjustment, and each supplying three cylinders. The interchangeable, oone
seated vaWes below are each about one-half the cylinder bore in throat diameter and are inclined thirty
degrees to the vertical. The exhaust valve seating and stem guides are formed in the combustion head cast-
ing and are also water cooled. The inlet valve on the right is in a separate casing fixed in one posHion
by the removable hollow flange nut 0. If this cage is removed, the exhaust valve on the left may be with-
drawn. A single laminated spring D clamped at the center on the fulcrum post supporting the rocker armt
keeps the valves seated. A single push rod positively operates both valves through the use of two cams in
conjunction with a bell crank lever. At H the tubular push rod M is pin-connected to the bell-crank HLK,
which is pivoted at L. The arms, however, are not coplaner as LH is located behind LK. 0am E aetnatea
roller K, and as it revolves clockwise its plus part strikes K, M descends, and the inlet valve is opened. A
similar cam behind E acts on H, the plus part raises M, and thus opens the exhaust valve. Relative
proportions may be seen as the diameter of the cylinder is 4.78 inches for the 00 horsepowar engine
illustrated. — ^From The WUeontin Bnginewr.
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ENGINES 828
2. V-type eight and twelve By far the larger ntimber of airplane
motors in use today are of the V-type. Cylinders are arranged in two rows,
each of four or six cylinders in line, and the two rows placed in V forma-
tion. The angle between the V will usually be 60 degrees for a 12-cylinder
en^ne, and 90 degrees for an 8-cylinder, in order to have the explosions
occur at regular intervals. Take an 8-cylinder engine, for example, and
remember that there must be eight explosions in two revolutions of the
crankshaft (720 degrees of crank travel). If equally spaced, the explosions
will occur 90 degrees apart. The connecting-rods of any two opposite
cylinders are necessarily attached to the same crankpin, and ignition in
either of these cylinders must take place as the crankpin approaches the
center-line of the cylinder (the amount of ignition advance ahead of center).
Therefore, if ignition in the two cylinders is to occur 90 degrees apart, or
any multiple of 90, the center-lines of the two cylinders must make an angle
of 90 degrees. By similar reasoning it may be shown that the cylinders
of a 12-cylinder engine should be set at 60 degrees. The Liberty avia-
tion engine, however, as well as some foreign makes, has cylinders set
closer in order to make tbe engine narrower to decrease head resistance. The
period between explosions in such an engine running at 1,700 B.P.M. is
so short that one cannot detect the slight irregularity due to the unconven-
tional angle of the cylinders; and it is claimed that the difference can
hardly be detected at speeds as low as 500 B.P.M. Another disadvantage
of the unconventional arrangement with magneto ignition is that a separate
magneto is required for each row of cylinders.
Engines of this class are not unlike the Y-type automobile engines in
general appearance, but are of far more expensive materials of construc-
tion and refined workmanship. The multi-cylinder types produce more
frequent working impulses, hence a more uniform flow of power into the
crankshaft and less vibration. Until recently, the highest powered motors
developed for airplane propulsion were of the 12-cylinder high-speed type.
400 horsepower is not an unusual size, and it is claimed that one engine
of this date actually developed over 600 H.P. under test. (See Fig. 311.)
3. Horizontal, opposed-cylinder type These motors, of which the
Ashmusen is one of the few survivors, differ from the V-type only by having
the cylinders set at an angle of 180 degrees (horizontally). The main ad-
vantage lies in the fact that the inertia forces on one side are counter-
balanced by equal and opposite inertia forces on the other side, thus making
a well-balanced and smooth-running engine. The principal difSculty ap-
pears to be the impossibility of uniformly cooling and lubricating the
cylinders.
4. T and broad-arrow types A notable endeavor to increase power
capacity and decrease weight per horsepower simultaneously is apparent
in the recently developed motors built by the Sunbeam Company in Eng-
land. They are merely outgrowths of the V-type, made necessary by the
constantly increasing demand for more powerful machines. In the Y-type,
two rows of four or six cylinders are in standard V formation, and a third
row is suspended vertically below, making either a 12- or an 18-cylinder
engine. The broad-arrow type differs only in that the third row is placed
vertically upright between the two inclined rows, and that oil is carried in
the crank chamber.
The lower cylinders of the Y project a short distance into the crank-
case, and oil is drained away as fast as it tends to accumulate. The shaft
is mounted on ball-bearings, and the arrangement of connecting-rods is sim-
ilar to the Gnome construction. One rod has a bearing on the crankpin, and
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324
ENGINES
Renault
245.H.R
Aviation
Engine
Section through RenOMt*
twelve-cylinder oirplano
engine which develops
245 hp. It ia used with a
geared down propeller.
Like the Merqeaea avia^
lion engine ihia conatrue-
tion usea at eel cyliiuiera
with ahte>t ateel water
iacketa welded on It ,
an oveihead camshaft
with two valvea per cyl*
inder inclined in the head
Fig. 311. — Sectional View of Modern V-type Airplane Engine Used in France
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ENGINES 325
the other two rods are pinned to this ** master" rod. Details of newly
developed motors are guarded with more or less secrecy in times of war,
and very little information about these two types is available. It is per-
fectly obvious, however, that engines of this type may be built to develop
over 600 H.P., and a few more months of development may bring out
engines with four or five rows of cylinders, each additional row making
possible higher power and a lower weight-power ratio.
6. Radial, stationary cylinder type In radial engines, the cylinders
are grouped in a single plane around a circular crankcase, like spokes in a
wheel, either completely around as in the ''star'' arrangement (Anzani),
or the cylinders may all be above the horizontal plane in "fan" form
(Masson). In either case, the connecting-rods work jointly upon a single
crank throw, one rod serving as a master rod to which the other connecting-
rods are attached by individual wrist-pins. By virtue of the shortness
of the crankcase, crankshaft and camshaft, and the elimination of a num-
ber of intermediate bearings and their supporting webs which would be
necessary with the usual tandem construction, the radial form is inherently
lighter than any other. It is also adapted to air cooling, which is also con-
ducive to lightness. But in actual service air-cooled radial motors with
stationary cylinders will overheat and become distorted to such an extent
as to render them useless except for very short flights. The blast of air
impinging directly on the front side of a cylinder cools the front side more
efficiently than it does the back side, and the temperature difference warps
the thin-walled cylinders. There is also the disadvantage that a radial
engine must overcome considerable ''head resistance" at high speeds of
flight, which absorbs a large percentage of the power output of the engine.
Because of the shortness of the motor, the mass cannot be distributed
fore and aft to any extent and must hang out over the end, which tends to
make the airplane '*nose heavy." There is also difficulty in lubricating the
cylinders uniformly in the star arrangement, which perhaps accounts for
the fan form previously mentioned.
6. Rotary engines Radial engines in which the cylinders revolve are
better suited to air cooling than the stationary radial type, and have
practically displaced that type of motor. The circulation of air set up by
the motion of the cylinders results in more uniform cooling, and the fly-
wheel effect of the revolving cylinders and the absence of reciprocating
motion lead to a smoothness of operation not ordinarily possessed by any
other type of gasoline engine. Centrifugal force keeps down the speed
of rotation to about 1,200 R.P.M., however, and the power is further limited
by low compression necessitated by air cooling, and by the limited number
of cylinders that can be arranged around the crankcase. (See Pig. 312.)
Fio. 312. — General Scheme of Rotary Engines
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326 ENGINES
Radial engines of the four-cycle tjrpe must have an odd number of
cylinders in order to have firing balance. The cylinders fire alternately
so as to distribute the explosions over two revolutions. It can be easily
shown that an even number of cylinders, symmetrically arranged, would
not fire with regular sequence, and would therefore lack smoothness of
running while at the same time the sound would be far from pleasing.
The most decisive disadvantage of a rotary engine is that it is impractical
for flights of more than 2 or 3 hours duration, primarily because of over-
heating, and also because of the excessive fuel and oil consumption. An
economical stationary engine will not use more than 0.55 pound of gaso-
line per horsepower-hour (about % pint), while a rotary engine usually
consumes nearly twice that amount. Rotary engines, as a rule, use castor
oil for lubricating purposes; and due to the fact that ample lubrication
must be provided at all times, and because, on account of centrifugal force,
the oil is necessarily wasted instead of being circulated over and over again,
the amount of oil used in a rotary engine exceeds that of a stationary water-
cooled engine by 5 to 8 times. The water-cooled engine uses about 0.03
pounds of oil per horsepower-hour, while the rotary engine requires about
0.18 pounds per horsepower-hour.
To illustrate by an example, suppose it is required to determine which
of the two above-mentioned types would be more suitable for a flight of
three hours in a 100 H.P. machine, if the rotary engine weighs 240 pounds
and the stationary engine weighs (with cooling water) 425 pounds. From
the economy data given in the preceding paragraph, the wat,er-cooled engine
would require 165 pounds of gasoline and 9 pounds of oil, making a total
weight of 599 pounds, fully equipped for the flight. The rotary engine,
on tiie other hand, would require about 330 pounds of gasoline and 54 pounds
of oil, which added to the weight of the engine itself makes the aggregi^te
weight 624 pounds. Another similar example is presented graphically in
Fig. 313, which shows that a heavy economical engine is ''lighter in the
long run'' than a rotary.
The poor fuel economy of rotary engines may be partially attributed
to low compression, which results in a very low M.B.P. Then approxi-
mately 10 per cent of the power developed by the gas is used in turning
the cylinders over against air resistance (not head-resistance), which de-
tracts from fuel economy since fuel consumption is based on rated or de-
livered power. . In the single-valve Gnome engine, which will be described
in detail later, the early release of the expanding gas, made necessary by
the unique method of operation, lowers the M.E.P. and power develoi)ed
from a given weight of fuel.
Other disadvantages of the rotary type of motor might include such
things as difficulty in making adjustments, frequent fouling of spark-plugs
by lubricating oil and carbon, need of overhauling after 15 to 25 hours of
service, large head-resistance, difficulty in shooting past a revolving enginC)
gyroscopic effect, impossibility of using a muflSer, a tendency for exhaust
gas and oil to sweep back towards the pUot, smoke obstructs his view, etc.
6. TYPES OF ROTABT MOTORS
A. CLBRGBT
Modem rotary engines invariably operate on the four-stroke cycle prin-
ciple and are usually constructed with either 7 or 9 cylinders. The Clerget
is representative of the type in which the intake and the exhaust valves are
located in the cylinder head and both mechanically operated. The valve
mechanisms are driven independently by two eccentrics, which is a distinctive
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ENGINES
327
cv
/
A
f
/
/
x-
/
/
<3
r
y
/
/
/
J
/
^/
/
^
^
k
4
7
^
J
7
^
r|>
x^
J
/
.
4
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y^
/
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A
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5
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/
^
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/
{
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Fig. 313. — Shoyhng Effect of High Fuel Consumption op Rotabt Engine
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328 ENGINES
feature of this motor. The 7-cylinder model, with bore of 120 mjn. and stroke of
160 mm., is rated at 90 H.P. and weighs 280 pounds. The 9-cylinder model,
having the same size cylinders, weighs 350 pounds and is rated at 120 H.P.
The engine operates upon a single crank, and the hollow shaft serves as an in-
duction tube. Ignition is by two high-tension magnetos. The pistons are of
aluminum alloy. Clerget motors are used largely in Sopwith planes.
B. LE RHONE
This engine is being manufactured in sizes varying fl*om 60 to 150 H.P.
as follows:
Bated H.P. at 1200 B.P.M. 60 SO UO ISO
Number of cylinders 7 9 9 9
Bore, mm. 105 105 112 124
Stroke, mm. 140 140 170 180
Weight, pounds 199 240 808 860
The steel cylinders have cast-iron liners, and are screwed into a steel crank-
case. There are two valves per cylinder, seating in the cylinder head. Induc-
tion is via crankshaft to crankcase and via external copper pipe from crank-
case to cylinder head. Forced lubrication is employed, It is stated that these
engines consume 0.72 pints of fuel and 0.1 pint of oil per B.H.P.-hour. Le Rhone
engines are employed in Nieuport scouts.
C. GNOME
The main difference between the original Gnome and the Clerget or the
Rhone lies in the scheme whereby the gaseous mixture is taken into the cylinders.
The intake valve for each cylinder is located in the head of the piston, and is
of the automatic type. The exhaust valve is in the cylinder head, and mechan-
ically operated by pushrod and rocker-arm mechanism. The engine operates as
follows :
The exhaust valve closes 13 degrees past top center, and as the piston is
drawn away from the cylinder head a slight vacuum in the combustion chamber
opens the intake, valve and sucks in fresh mixture from the crankcase. At the
end of the suction stroke, the intake valve automatically closes and compression
begins. Ignition takes place 26 degrees before top center, this advance being
necessary to secure maximum pressure within the cylinder early on the power
stroke. The exhaust valve opens when the piston has gone only three-quarters
of its stroke, or about 65 degrees before bottom center.
The head of the piston is bored out to receive the two pieces that make up
the piston-pin bosses and the valve seat respectively. A leaf spring, with a
hole through the center for the valve stem, is attached at the ends to small
weighted levers calculated to give balance against centrifugal force. This is
naturally a delicate mechanism, and one that readily gives trouble. The valve
is removable through the cylinder head by the use of special tools, after first
dismounting the exhaust valve and its seat.
D. ''MONOSOUPAPE" GNOME
The Gnome engine described in the preceding paragraphs has practically
gone out of existence. The Rhone, manufactured by the Gnome Company since
the beginning of the war, is a very successful type; but at the present time
there seems to be a great demand for the latest type Gnome, which has but one
valve and is of extremely light construction. The 9-cylinder model, as manu-
factured in this country and abroad, is rated at 100 H.P. at 1200 R.P.M., and
weighs but 2.7 pounds per horsepower. The finished cylinder, machined from
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ENGINES 829
a solid bar of forged steel, is about 1/16-iiich thick and weighs only 5^^ pounds;
the finished distribution-case to which the cylinders are attached weighs but
15^ pounds; the ''mother" rod, that odd looking integral piece to wUch are
secured the other eight connecting-rods, weighs 5^ pounds. The entire engine
calls for extremely choice alloy steels and fine workmanship.
The construction of the 7-cylinder SO-horsepower engine is practically the
same as that of the 9-cylinder type. The bore and stroke are the same, viz.,
110 mm. and 150 mm., respectively. The cylinders, valves, etc., of both types
are interchangeable.
The crankcase of the engine is made in two halves, fastened together by
bolts, the two halves clasping the cylinders in a groove at their base. The cylin-
ders are prevented from turning in the crankcase by keys. Each cylinder is
drilled near the bottom with a number of circular ports connecting the interior
of the crankcase with the cylinder. The ports are uncovered by the piston
near the bottom of the stroke, permitting a passage from the crankcase to
the cylinder. The exhaust valve and seating is fixed at the extreme outer end
of each cylinder by means of a castellated ring nut The weight of this valve
is so adjusted that its centrifugal action, when the engine is rotating, is equal
to that of the tappet-rod and lever ; and thus the use of counterbalance weights,
such as are fitted to the rocking levers of the ordinary Gnome engine, is avoided.
The exhaust valves are operated by long push-rods from the cam-box situated
in front of the crankcase, the cams operating these rods through tappet-rollers.
The pistons are of castiron, and each has a portion of the trailing edge
cut away to prevent interference. A thin brass ''obturator" ring, of L-section,
is fitted onto the head of the piston, and is kept pressed out against the cylinder
wall by an auxiliary steel expansion ring. The gas pressure above the piston
forces the thin brass ring against the cylinder walls and prevents leakage, even
with the cylinder slightly ^storted. The obturator-ring gap goes on ti^e lead-
ing edge of the piston because the gases have a greater tendency to escape along
the trailing edge. The clearance between cylinders and pistons is 5 or 6 thou-
sandths of an inch.
The connecting-rods are of nickel steel, H-section. One of the rods, called
the "master rod" (sometimes called "mother rod"), is mounted on ball-bear-
ings on the crankpin, the outer ball-races being fitted into L-section rings integral
with the rod. These rings are bored to receive the hollow steel wristpins upon
which the other connecting-rods oscillate. Obviously the auxiliary rods are some-
what shorter than the master rod, but the difference does not appear to have
any noticeable infiuence on the running of the motor.
The crankshaft is of builtup construction to enable mounting the master
rod onto the crankpin. The 7- and 9-cylinder models use a single-throw shaft,
but in the 14-cylinder model, which has two sets of cylinders, there are two
throws at 180 degrees. The shaft is stationary, being held thus by a key
and two bearer-plates at the rear end extension. The revolving part of the
engine is mounted on large ball-bearings, and a ball thrustbearing transmits
the push or pull of the propeller, through the stationary shaft and its supports,
to the plane.
The smaller part of the crankshaft can be dismantled from the front of
the engine after the front cover or cam-box has been removed. This part of
the fidiaf t acts as a bearing for the cam-sleeve and the cover holds the cam-follower
guides. The propeller hub is bolted to a nose-piece attached to the cam-box.
The main part of the crankshaft has a long extension to the rear whereby the
engine is supported in bearers attached to the airplane. The shaft is hollow
for lightness and to admit air to the crankcase. It also carries the gasoline
and oil pipes, the former terminating in several minute holes just inside the
crankcase for the purpose of spraying the gasoline to break it up as much as
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330 ENGINES
possible. This spray is pointed downwards in the direction of flow of mixture
towards the opening ports. The amount of air admitted to the crankease is
not sufficient to render the resulting mixture combustible. This is an important
improvement, because a great deal of trouble from backfire was experienced
in former rotary engines that took a combustible mixture into the crankease.
The cam-block has a bearing on the stationary crankshaft, and is driven by
satellite gearing. A fixed timing gear keyed to the shaft meshes with a planet
or satellite gear of the same size pivoted to the revolving cam housing. A smaller
integral planetary gear meshes with the cam-sleeve gear, the latter being twice
the size of the former, which gives the half -speed motion to the cams. If one
attempts to trace out the direction of rotation from a sketch, it almost appears
as if the cams must go backwards; but as a matter of fact the cylinders and
the satellite gears are revolving and the apparent "backward" motion of the
cams is only relative, and is really the retarding action of the two-to-one gearing.
The thrust-box on the opposite side of the crankease from the valve-operating
mechanism carries a distributor, and a gear for driving the magneto and the oU
pump. The magneto and the pump are mounted on the stationary bearer-plate
by which the engine is supported. The distribirtor has an insulated brass segment
for each cylinder, connected to the spark-plug by a bare wire. The high-tension
current is led to the distributor through a stationary contact brush.
Jbn^
>hhfi
Vo/ve cpehing
on sifcfJon 'SfJvAe
Pig. 314. — Timing Diagram of ''Monosoupape'* Gnome
The engine works on a four-stroke cycle principle, as follows: Consider a
cylinder in which the piston is on the compression stroke and follow the engine
through two revolutions. (See timing diagram, Fig. 314)
1. Ignition, which is "fixed", is set to take place about 18 degrees
before top dead-center, i.e., before the cylinder reaches the upright position.
2. Working stroke From top center, the piston recedes from the
cylinder head on the expansion or power stroke. When the cylinder has
turned through 85 degrees from top center (piston in mid-stroke), the valve
in the head starts to open and the expanding gas is gradually released. The
working stroke continues, however, until approximately 20 degrees before
bottom center, the presumption being that the pressure within the cylinder
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ENOINES 831
has become just equal to crankcase pressure (approximately atmospheric)
at the time the ports are uncovered. The early opening of the valve is a
contributory cause of the low M.E.P. obtained with titis engine, but is
necessary in order to avoid a pressure difference at the end of the stroke
which would be sufficient to eject burned gases into the crankcase through
the ports.
3. Exhaust stroke During the entire up-stroke of the piston, burned
gas not previously discharged is pushed out of the cylinder/ This scavenging
action is assisted to a slight extent by centrifugal force acting on the gas.
4. Induction stroke The valve does not close at the end of the ex-
haust stroke, but remains open for 120 degrees of the down-stroke (three-
quarters of the piston travel) to admit air. After the valve is closed, the
descending piston reduces the pressure within the cylinder, and this partial
vacuum causes transfer of the rich mixture from the crankcase when the
piston uncovers the ports. The rich gas mixes with the air previously taken
into the cylinder, and is thereby rendered combustible.
5. Oompression stroke The ports are covered 20 degrees after bot-
tom center, and compression then begins.
Ignition is by means of a high-tension Dixie magneto, mounted on the
forward bearer plate and driven by gearing from the revolving crankcase.
The spark is delivered from the magneto to a stationary contact-point bearing
against the main distributor. An uncovered brass wire connects each dis-
tributor segment with the spark-plug of the corresponding cylinder. The
distributor turns with the crankcase, and therefore makes contact for each
cylinder every revolution, but the magneto is timed to spark for each cylinder
only once in two revolutions when the spark is needed.
Lubrication is by means of a double plunger pump, and the oilway sys-
tem is similar to that of the 80-H.P. Gnome engine. There are two leads
from the pump, which supplies oil at about 5 pounds pressure, and each pipe
has a branch connection to a pulsator glass in front of the pilot. The oil
is conducted by one lead through the stationary hollow crankshaft to the
cam-sleeve bearing and the timing gears at the front of the engine. Holes
through the cams permit the oil to pass out to the cam-rollers, which pick
it up and throw it into the hollow tappet. Centrifugal force carries the oil
to the outer ends of the push rods, lubricating the tappet guides and all
joints of the push-rods and rocker-arms, after which it is wasted. A branch
connection at the rear end feeds a small amount of oil to the thrust bearing
and the rear main bearing, after which the oil works out along the crankcase
to the cylinder walls.
The second lead also enters the hollow crankshaft, but has its outlet at
the crankpin. This oil is conducted through holes in the master rod to the
various wrist-pin bearings of the other connecting-rods. Prom the wrist-pin
bearing the oil passes through holes and out along the rod to the piston-pin
bearing. Overflow from this bearing is collected in circular troughs at the
sides of the bearing, and led to the piston ring and cylinder wall through
holes drilled in the webs of the piston. Centrifugal force continues to carry
the oil outward, and some carbonizes in the combustion chamber while the
remainder passes out with the exhaust, impinging on and lubricating the
valve stem. Castor oil is used, and once used it is wasted.
The speed is controlled through a limited range by simply manipulating
the gasoline shut^off valve.
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882 ENGINES
The characteristics of the ''Monosoupape" engine, as compared with the
ordinary Qnome engine, are :
a. Absence of inlet valves.
b. Solid piston heads.
c. No carburetor.
d. Non-explosive mixture in the crankcase.
The inlet valve on the older Gnome, besides adding weight to the rotat-
ing mass, was a constant source of trouble. Being inaccessible, accurate
adjustments could not be made readily, and adjustments were often aJBfected
by heat and oil. Introduction of the gasoline spray directly into the crank-
case does away with the carburetor and makes possible operation of the
engine in any position. By limiting the amount of air admitted to the
crankcase, the mixture is kept too rich to ignite until it gets into the cylinder
and is mixed with the proper amount of air, and therefore "back-firing" is
entirely avoided.
It should be noted in connection with rotary engines that the *' recipro-
cating motion" of the pistons is purely relative to tie cylinders, since both
cylinders and pistons revolve about fixed centers. The path of the cylinders
is eccentric to the path of the pistons by an amount equal to the ''throw"
of the crankshaft, or one-half of the apparent stroke of the piston. (See
Fig. 312, page 325)
6. ESSENTIAL BEQXrmEIOarrS AND ALLIED FA0T0B8
The three principal requirements of an airplane engine may be briefly
summed up as follows :
1. Low weight-power ratio, which means lightness in proportion to power
delivered.
2. Economy of fuel and oil, so as to obtain the maximum possible radius
of action with a given quantity.
3. Absolute reliability, since no repairs can be made during flight
In the demand for low weight per horsepower, the requirement of low fuel
and oil consumption per horsepower-hour is included, since for practical purposes
it is necessary to carry enough fuel for a running time of several hours. For
long continuous flights, an inherently light engine may easily prove inferior to
one of heavier construction, providing the latter has a better fuel efficiency.
This matter has already been discussed in connection with rotary engines.
Hand in hand with reliability goes the demand for durability. An airplane
engine runs almost continuously under full load, and deficient operation, however
slight, can not be tolerated. A decrease in speed of only a few revolutions below
normal is often quite sufficient to bring about a forced landing, which, should it
occur at an inopportune moment, might be attended with disastrous consequences.
A. FACTORS THAT AFFECT POWER AND WEIGHT
Referring to the formula for indicated power, it becomes evident that the
power transmitted to the piston by the worUng medium will depend upon the
M.E.P., the piston displacement, and the speed of the engine. The power that
is actually delivered to the propeller will also depend upon the mechanical effi-
ciency of the engine.
Obviously tiie larger the volume of piston displacement per cylinder, the
greater will be the power capacity of the engine. The diameter of a cgrlinder is
Umited in high-speed engines by cooling -difficulties and inertia of the recipro-
cating parts, while the stroke is fixed by the allowable piston speed.
Rotative spe^s of different airplane engines vary from 1,200 to 2,500 revo-
lutions per minute. Propellers work most efficiently at 1,200 to 1,400 R.P.M.,
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and engines that have propellers attached directly to an extension of the crank-
shaft mnst necessarily operate at speeds best suited for propeller efficiency.
Higher speed engines, when properly designed and constructed, deydop more
power for the same displacement and engine weight; but the propellers used with
such engines must be ''geared down", and are larger to absorb the increased
power. The maximum value of engine speed is limited largely by the inertia
stresses of the reciprocating parts of the engine, and obviously these stresses have
to be considered in conjunction with those resulting from the gas pressure. The
whipping action of a connecting-rod tends to bend the rod every time its direction
of motion is reversed, and this introduces large bending-moment stresses. Small
unbalanced centrifui^ forces in a rotating crankshaft are quite sufficient to bend
the shaft at high rotative speeds and cause vibration, friction, and excessive wear
in the shaft bearings. With high speeds, cooling and lubrication become more
difficult because of tiie greater amount of heat liberated to the cylinder walls in a
given time by the burning gas, and the increased velocity between rubbing sur-
faces. The rapidity with whidi valves must be opened and closed also limits the
practical speed. Needless to say, the wear and tear of an engine running contin-
uously at even ordinary propelter speed is not favorable to smoothness nor relia-
bility of running, and it is taxing tiie ingenuity of designers and manufacturers
to produce ^igines that will stand up in service for sufficiently long periods to be
practical in modem warfare. The life of an airplane engine is iriiort, and fre-
quent overhauling is necessary ; but in spite of adverse limitations, the tendency
seems to be towards higher engine speeds and geared-down propellers.
^H9/(fmi/m//or9wpoii¥Wfi.
Fig. 315.— Usual Form of Power Cubvb
Fig. 315 shows a typical horsepower-speed curve of an airplane engine.
Note that as the speed increases the power increases less and less rapidly, and
that finally a speed is reached at which the power is maximum. This curvature
•is explained by the fact that at high speeds the M.E.P. is reduced by lowered
volumetric efficiency and insufficient time for proper completion of combustion.
The practical operating speed is always somewhat less than the maximum shown
by the curve, because near the peak it requires a large increase in speed to get
but little increase in power, and the additional wear and tear more than offsets
the gain.
There is no question about the desirability of increasing the M.E.P. factor.
High compression and more complete expansion tend to increase the efficiency
of combustion ; but there are certain limiting conditions beyond which it is im-
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334 ENGINES
practical to go. Compression of a gas is accompanied by a rise in temperature,
and the limit of compression is reached when the heat of compression causes
spontaneous pre-ignition of the combustible mixture. The usual compression
for mixtures of gasoline vapor and air is about 90 pounds per square inch,
although aircraft engines are being used with compression pressures as high as
130 pounds per square inch. Small cylinders and efficient cooling make high
compressions possible.
The time necessary for complete combustion in an engine cylinder depends
upon the character of the hydrocarbon fuel, the proportion of fuel to air, the
amount of compression, the initial temperature, the homogeneity of the mixture,
the cooling effect of the surfaces in contact, and various other things. This
relative slowness of burning after ignition requires what is commonly known as
''spark advance", which means making ignition occur some time previous to the
end of the compression stroke, the amount of advance depending largely upon the
speed of the engine.
The theoretical temperature of combustion is never even closely approached
in an internal combustion engine. The maximum temperature permissible de-
pends upon the ability of the lubricating oil to withstand heat and function prop-
erly, Pre-ignition is also a limiting factor. To keep the temperature from becom-
ing excessive, it is always necessary to artificially cool the cylinders, either by
forced air circulation or by water. Water is much more efficient for the purpose,
because of its higher specific heat and greater density. The heat removed by the
cooling medium represents a direct loss in heat efficiency, but as just explained,
cooling is a necessary evil without which the engine could not run. Another source
of heat loss equally as considerate is the hot exhaust gas, which leaves the cylinder
before complete expansion and at a very high temperature. At least 75 or 80
per cent of original heat-energy of the fuel is wasted in making it practical to
transform the other 20 or 25 per cent into mechanical work. Thermal Efficiency
is the ratio of the heat transformed into work divided by the original heat content
of the fuel used. True thermal efficiency would be that based on the indicated
pov?er, but frequently it is more convenient to use the rated power, which is
approximately the delivered power in any case. A pound of gasoline has a
thermal value of about 20,000 B.T.U.
There are a number of other items that directly affect the M.E.P. within the
cylinder, and these logically divide themselves into two classes:
1. Those that decrease the weight of the cylinder charge.
2. Improper treatment of fuel charges.
1. Causes of decreased weight of charge It will be at once apparent
that all of the causes of reduced volumetric efficiency simultaneously reduce
the M.E.P. by diminishing the weight of charge. Some of the causes of
lowered volumetric efficiency are as follows:
a. Resistance to flow of gases into or out of the cylinder, due to
small or crooked intake passages and small valve openings, re-
quires a greater difference of pressure to overcome the inertia of the
gases and induce flow at high velocity, which during suction rarefies the
gas within the cylinder and results in a smaller weight of charge. This
would be indicated on the pressure diagram by a low suction line. With
a high exhaust line, the dead gas in the clearance space at the end of the
stroke must expand before suction can begin, which shortens the period
of suction. Mufflers also increase back-pressure.
6. Heating hy hot clearance gases and cylinder walls expands the
incoming mixture and decreases its specific weight.
c. Improperly set valves affect volumetric efficiency. Too early
closure of exhaust valve, for example, shuts in burned gas that would
otherwise escape and make room for fresh mixture. If this valve closes
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335
too late, on the other hand, the suction immediately preceding would
draw some inert gas back into the cylinder. The exhaust valve should
close at the time the pressure within the cylinder reaches atmospheric,
somewhat after the beginning of the suction stroke, in order to allow
maximum scavenging ^ect. The intake valve should open almost im-
mediately after the exhaust closes, a brief interval intervening in some
cases to allow for slight inaccuracies in setting and adjustments. It is
equally important that for best volumetric efficiency the intake valve
should dose when the pressure within the cylinder reaches atmospheric
on the compression stroke. Until that time, gas will continue to flow
in because of the difference of pressure ; but if the valve remained open
any longer there would be expulsion of combustible gas and reduced
compression. The point of exhaust-valve opening has no effect on
volumetric efficiency, and is considered under Class 2. (See diagram,
Fig. 316.)
b
ignltionfnjn Mv)
i ntQKe Cio3ed
s
o
a
|2
CxhQuat Cioacd
IntQKe Oo&n
ExhQu^r Open .
£
o
I
Fio. 316. — Typical Valve-Timing Diagram
d. Improper valve clearance, if too much, decreases the valve lift
and shortens the period of valve opening. In addition to increasing
the resistance to flow, the valve timing is affected, which in turn affects
the volumetric efficiency as explained in the preceding paragraph. Too
little valve clearance results in early valve opening and late closing.
The clearance should be on the order of a few thousandths of an inch,
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336 ENGINES
and account most always be taken of the fact that expansion of the en-
gine in heating up affects the clearance adjustment. The purpose of
clearance is to permit free expansion and contraction between the
cylinder and valve-actuating mechanism.
e. Worm cams result in improper valve action, giving very much
the same effect as too much valve clearance. The only remedy is a new
camshaft
/. Weak valve springs will not dose the valves quickly, and at high
speeds this is a source of considerable loss in power. StLff springs, on
the other hand, increase the resistance to opening and reduce the me-
chanical efficiency of the engine. Four-valve construction makes lighter
valves and lighter springs possible, besides offering larger openings.
The spring on an automatic suction valve, as used with crankcase com-
pression in a two-cycle engine, offers resistance to flow and reduces
volumetric efficiency.
g. Too high a speed, requiring large differences of pressure to in-
hale or exhaust the gases in the allotted time, results in low volumetric
efficiency. The amount of valve lift cannot ordinarily be as great at
high speed because of the inertia of the valve.
Changes in atmospheric conditions, as, for example, decreased
barometric pressure at high altitudes, affect the weight of charge taken
into an engine cylinder and therefore affect the M.E.P. But the vol-
umetric efficiency is not necessarily affected by atmospheric changes,
for obviously it would not be consistent to brand an engine deficient
when its iwwer is reduced by some purely external agency.
2. Improper treatment of fuel charges Under this class may be men-
tioned the following causes of reduced M.E.P. :
a. Leaks, causing loss of compression and waste of fuel, may be
due to a variety of causes. A little carbon on the valve seat may pre-
vent the valve from closing tightly, or the valve may become per-
manently warped from excessive heat. Worn cylinders, scored cylin-
ders, and broken or worn piston-rings, are often contributory causes
of leakage.
b. Incomplete vaporization or improper mixture of gasoline and
air due to faulty carburetor adjustment or inleakage of air at a mani-
fold joint, results in inefficient combustion.
c. Improper ignition To obtain high M.E.P. with high engine
speeds, it becomes necessary to use the highest grade of spark-plugs, to
have a good spark at each, and to ignite the compressed charge at more
than one point Two-point ignition is common practice in airplane
engines. It is also essential that the proper ''spark advance" be given.
Too early ignition is evidenced by "knocking'* and loss of power; late
ignition is followed by retarded combustion, loss of power, and over-
heating. The point of maximum pressure should occur soon after
the piston starts out on. the working stroke. Retarded combustion al-
ways tends to overheat an engine because of the additional surface ex-
posed to flame when the piston is further down on its stroke.
d. Insufficient compression pressure, as previously explained, af-
fects the efficiency of combustion and the M. E. P.
e. Too early release of the burned gases, thus shortening the period
of full expansion and throwing away energy that might have been
utilized.
/. Poor form of combTistion chamber, or clearance space, as has
been proved by numerous experiments, causes slow burning, greater
heat loss during combustion, and consequently low M.E.P.
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ENGINES 337
The factors heretofore mentioned affect the power applied to the face
of the piston by the expanding ^. Mechanical losses within the engine
merely subtract from the indicated power, and in that way affect only the
power delivered to the propeller. Friction between the piston and the cylin-
der wall, and in the crankshaft bearings, together with the power required
to drive the camshaft and auxiliary parts of the engine, make up most of
the mechanical losses. Some power is lost in turning the crankshaft at
high velocity within the crankcase against air resistance, and in the case
of geared-down propellers there is some loss in the gearing. A short con-
necting-rod increases the side-thrust against the cylinder wall, and causes
greater friction and more rapid wear. Tight bearings, or bearings not
properly lubricated, also decrease the mechanical efiSciency. A shaft that
is not accurately balanced, or one that is not perfectly straight, throws
itself out against the bearings at high speeds and causes excessive vibra*
tion, and consequently increased friction and wear. Heavy reciprocating
parts, the inertia of which must be overcome and reversed twice every revo-
lution, cause vibration and stress in the connecting parts at high speeds
of rotation, and heavy pressure on the bearings. Stiff valve springs, which
are necessary on high speed engines for dosing the valves quickly, increase
the power required to drive the camshaft. The use of two or more mag-
netos and oil pumps adds materially to the mechanical operation losses.
Lubrication of an airplane engine must at all times be positive and uni-
form. Only high pressure, forced circulation of large quantities of oil, wiU
permit continuous operation of an engine at full speed for several hours at
a time. Oil is supplied from a pump at a pressure of 50 or 60 pounds per
square inch, perhaps, and the bearing surfaces are thus actually held apart
by the thin film of oil maintained between them. The oil is often circu-
lated from one bearing to the next, and for that reason bearings must have
sufficient and uniform clearance, and the oil must be of proper viscosity. A
tight bearing will exclude the oil, or throttle the pressure in such a way
as to prevent the proper amount from flowing on to other parts of the engine.
Too much looseness will have the same effect, because the oil will simply
take the path of least resistance and leak out around the sides of the bear-
ing. A thick oil, because of its high viscosity, will not flow freely, while
an oil that is too thin will not maintain a film between the bearing surfaces
under high bearing pressures even though it flows in copious quantities. An
oil that appears heavy and viscuous when cold usually thins down very
materially when heated up to the operating temperature, and it is always
important that the oil retain enough ''body" when hot so that the oil
film will not be destroyed.
3. The weight factor It is perfectly obvious that an airplane engine
must be as light as possible, in order to have greater load-carrying
capacity, or higher speed and better climbing ability. To move swiftly
with reference to the earth in the face of an opposing wind requires a very
high speed relative to the sustaining medium and consequently a powerful
motor. The horsepower available for climbing is the power output minus
the power consumed in horizontal flight, and the latter varies directly as
the weight of the machine. A light motor is therefore essential, which is
the reason why the gasoline engine monopolizes the aviation fleld.
Lightness is attained largely by designing for maximum M.E.P. at the
maximum feasible speed, and by the intelligent use of materials of selected
kinds and cross-sections. Alloy steels, properly heat-treated, are nearly five
times as strong as ordinary cast steel, and are used for crankshafts, con-
necting-rods, and other highly stressed parts. Aluminum alloy and sheet
metal are used for parts that need not possess much strength, such as water-
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338 ENGINES
jackets, crankcases, pumps, and manifolds. Cylinders are often of cast
aluminum alloy, but need to be lined with an iron or steel shell to give the
necessary strength and wearing qualities. Pistons for airplane motors
are almost invariably made of aluminum composition, known variously as
Magnalite, Lynite, etc. Copper, forming about 8 per cent, is usually the
alloying material, although zinc is used to a slight extent and in some cases
nickel is a constituent. Aluminum is advantageous mainly because of its
lightness and superior heat-conductivity, although the fact that it will not
rust is important, and it makes a satisfactory bearing for piston-pins with-
out being bushed. The main difficulty that has limited its usefulness is in
properly providing for differences of expansion, since the coefficient of ex-
pansion is so high. It expands twice as much as iron or steel for a given
tempjerature change, which practically precludes its use for large block
castings. Fortunately, in using steel liners with cast aluminum outer walls,
the latter are in contact with the cooling water while the steel sleeve is in
direct contact with the hot gases, which practically equalizes the expansion.
When aluminum is used for cylinder heads, it becomes necessary to pro-
vide iron valve-seats, because duminum is too soft and malleable to stand
up under the constant hammering of a valve. There are three ways of
providing iron seats, namely: (1) casting an iron ring integral with the
aluminum cylinder head; (2) threading the iron ring and screwing it into
place; or (3) using a separate valve-cage, which has the valve seat and the
stem guide integral. Passages for oil through aluminum castings need to be
lined with steel tubes, to prevent high-pressure oil from breaking through
porous places. Threads in aluminum are apt to cut out or shear unless a
permanent steel insert is provided. Added to these complications is the cost
of the material itself, which is very expensive. This, however, is a mat-
ter of secondary importance in military airplane construction. ^
Next in importance to the kind of material is the form or shape of
the section. Connecting-rods are usually of I- or H-section, and sometimes
tubular. Crankshafts, camshafts, wrist-pins, and other round pieces are
made hollow. Such shapes get the most strength out of a given weight
of metal by distributing the metal away from the center of gravity where
it can do the most good. Pistons and crankcases are webbed for stiffness
and strength instead of using thick, heavy walls. In the case of V-type
engines, forked connecting-rods are sometimes used to minimize the length
and weight of the engine, although they are more costly and difficult to
manufacture. Bearing caps are now tied directly to the cylinders by
through bolts, thus making the stress direct and eliminating most of the
strain from the walls of the crankcase.
The arrangement of cylinders also affects the weight-power ratio. With
cylinders arranged **in line*', a piece of crankcase, crankshaft, camshaft,
etc., must accompany each cylinder, while with the same cylinders arranged
radially a short crankcase, a short crankshaft, and a short camshaft serve
all together. Arrangement of cylinders radially, then, is conducive to
lightness.
Air cooling, while not as efficient as water cooling, has the advantage
of lightness, and lends itself fairly well to the cooling of radially-disposed
cylinders, especially if the cylinders revolve. But air cooling is not gen-
erally employed in stationary cylinder engines, since uniformity in cool-
ing can be obtained only by exposing all cylinders equally to a strong blast
of air. The radiator, pump, and water necessary for a water-cooling sys-
tem adds but little to the weight of the engine, and this little additional
weight is fully compensated for by increased power, efficiency, and re-
liability.
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ENGINES 339
B. FUEL AND OIL ECONOMY
Fuel economy is dependent first of all upon liigh compression. By forcing
the particles of gasoline vapor and air into more intimate contact, and at the
same time raising the temperature of the mixture closer to the ignition tem-
perature by the heat of compression, the charge bums more rapidly and exerts
a greater force against the piston after ignition takes place. The proper pro-
portioning of the mixture and complete vaporization of the gasoline also im-
prove fhe fuel economy. It is equally important that the ignition be of suf-
ficient intensity and properly timed, and that the charge taken into the cylinder
be in every way properly treated so that the maximum M.E.P. results from
the combustion.
In the matter of oil economy, it is essential that the oil have a high ''flash
point" to prevent deterioration in so far as possible, in order that the oil
may be re-circulated continuously. Cooling of the oil is desirable, and in some
engines this is accomplished by passing the oil around the intake manifold, where
it assists in vaporizing the gasoline.
C. RELIABILITY
In this connection it should be noted that only the best of materials and
the most careful workmanship are tolerated, and that before an engine is ac-
cepted as a standard for military service it must fulfill very exacting require-
ments, and it must stand up under far more severe tests than would ever be
imposed upon it in actual service. Experience has shown that the life of a
certain type of engine is limited to approximately so many hours, and accurate
service records are kept for every engine in the field. It is false economy to
drive an engine to the limit, rather than give it the inevitable overhauling before
it is badly crippled.
Dual ignition increases the reliability of the ignition apparatus, which is
one of the most vital parts of the engine. It also adds to the power and to
the fuel economy in high speed motors.
Various indicating instruments and devices, with which most machines
are now equipped, add to the safety of fiying. Reliability is further insured
by taking definite precautions in looking over tiie machine before a fiight, strain-
ing all gasoline and oil put into the tanks to minimize the danger of dirt work-
ing into the carburetor or into the engine bearings, going over the wiring and
the piping regularly to make sure that both are secure and not wearing through
at any point, cleaning the spark-plugs and distributor frequently, and other
details that will be apparent from the section on ''Engine Troubles".
D. DESIRABLE QUALITIES
In addition to the essential requirements mentioned above, certain addi-
tional features are advantageous and desirable for increased safety and better
performance. These will be enumerated and discussed separately.
1. Low air resistance The importance of low air resistance becomes
more marked with increase in speed of fiight, as the power absorbed in
this direction varies as the cube of the velocity. A motor that presents a
large projected surface to the direction of motion is undesirable for high-
speed service. Contrary to earlier practice, it is now customary to en-
close the whole power plant in a stream-line housing, or fuselage, to mini-
mize air resistance.
2. Controllability or flexibility Although there is not the same need
for this as with engines employed on automobiles, it is none the less a de-
sirable quality since at low speeds of rotation the propulsive or tractive
effort of the propeller is insufiicient to move the machine along the ground,
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840 ENGINES
and hence the pilot will be able to start np without assistance should cir-
cumstances necessitate his so doing. Farther, as the engine is not required
to develop its full power in horizontal flight and when alighting, the ability
to vary the speed daring descent is certainly preferable to the crude method
of switching the ignition off and on.
8. Freedom from vibration The necessity for elimination of vibra*
tion as far as possible will be obvious when the slender nature of the sup-
ports upon which the engine is carried is realized, especially as vibration
of a dangerous character may be set up in the various parts of the machine.
It is also worthy of note that an engine permitted to vibrate in that way
can not deliver as much power as if more rigidly supported.
4. Accessibility is, if possible, of even more importance in aero en-
gines than in those used in automobiles, because the nature of the service be-
ing so severe and exacting requires very frequent inspection and overhauling
of the power plant
5. Silence is desirable in any machine used for pleasure or sporting
purposes, but when on military reconnaissance duties it becomes of increas-
ing importance to be able to manoeuvre without giving audible warning of
approach, especially at night.
6. Cleanliness is in the nature of a refinement, but it is none the less
necessary since a dirty appearance is generally caused by leaks, and leaks
not only cause waste but render the risk from fire very much greater. Oil
soaked wood or fabric deteriorates rapidly, weakening the structure and
rendering its use dangerous.
E. DEMANDS DEPENDENT UPON WEIGHT EFFICIENCY AND FUEL
EFFICIENCY
There is probably no form of prime mover in existence that is more highly
stressed or that has a more strenuous life than the aeroplane motor, and there
is undoubtedly no engine that has greater claims on reliability. The demands
are for extremely light economical engines, such that probably as much as 90
per cent of the factors which determine the most successful machine are gov-
erned directly or indirectly by the weight efficiency and fuel eflSciency of the
engine. By the former is meant, of course, the number of pounds of weight
for every horsepower developed.
Among the essential features of all successful aeroplanes are the following:
1. Ability to ascend rapidly is of paramount importance in warfare,
where a machine's capabilities of evading destruction depend to a large
extent on how quickly it can get out of range of projectiles. It must
also be efficient in climbing in order to rise successfully from a small field
surrounded by taU trees, as might be necessitated by a forced landing dur-
ing a cross-country fiight. The rate of climb varies directly as the power
developed and indirectly as the weight to be lifted, so climbing ability de-
pends to a large extent on the weight efficiency of the engine.
2. A combination of fast and slow flying speeds is highly desirable,
but this is only possible by the adoption of an extremely light and powerful
engine. If the machine is designed for very high speed, a slow speed is
only possible by the aggregate weight being very light.
3. Saf eness of handling in winds, with or without engine in operation,
is more characteristic of lightly loaded machines. Aeroplanes have been
built that will carry as much as 15 to 20 pounds per square foot of support-
ing surface, but the average loading on the planes today is genendly in
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ENGINES 841
the neighborhood of 4 or 5 pounds per square foot A heavily loaded
machine depends to a great extent on high speed of flight in order to main-
tain it in the air, and ediould the speed faU the control becomes sluggish
and wiU not answer quickly, and only by nosing down promptly to increase
the speed can the pilot prevent floundering. The life of the pilot of a
heavily-loaded machine is more dependent upon the good behavior of the en-
gine than is the life of the pilot of a lightly-loaded machine, and the latter
could probably go on fl3dng in search of a good alighting ground with two
or three i^linders not flring at all.
4. Ability to remain in the air for long periods depends chiefly on the
oil and gasoline consumption of the engine, and without efficiency in this
respect tiie extremely light power-plant is practically useless, as flights of
only a few minutes duration are not likely to be of much use in serious
warfare.
7. ENQINE TBOUBLBS
In answer to the question that is foremost in the minds of most embryo
pilots who are confronted with the seemingly imi>ossible task of getting thor-
oughly familiar with all the details of engine operation in a relatively short
time, there are four possible reasons why a military aviator needs to be expert
in detecting engine troubles:
First, no repairs can be made during flight. This makes it imperative that
all adjustments be attended to beforehand, and it is a safe rule never to pin too
much faith upon your mechanic. Confluence is a most efFective nerve tonic,
and only by absolute belief in the reliability of the power that responds to his
touch can a pilot render the most efficient service.
Second, a pilot must be capable of diagnosing any troubles that might de-
velop on cross-country flights, in order to make rapid and practical repairs if
possible. < ; '
Third, if impossible to make repairs with the material at hand, he must
send an intelligent message for parts.
Fourth, whenever troubles appear during flight, he should upon return ex-
plain to his mechanicians the nature of the trouble, and perhaps the remedy.
A. ESSENTIAL CONDITIONS
The following conditions must be fulfilled if the engine is to deliver its
maximum power:
1. The mixture must be of the correct strength, and its components
properly mixed.
2. There must be sufficient compression to bring the particles of gas-
oline vapor and air into intimate contact, and to raise the temperature
of the mixture to make it readily explosive.
3. Means for igniting the explosive mixture at the right time, i.e., just
before the end of the compression stroke, must be provided.
4. Proper lubrication and cooling.
B. CARBURETION TROUBLES
1. Mixture too lean is characterized by lack of power, especially at
low speeds, when the "pick-up" will be uncertain and there will be an oc-
casional popping-back through the carburetor. Backfiring of this sort is
dangerous, and should be avoided. Continued operation on a very lean mix-
ture will result in overheating of the exhaust valve, because of the slow-
ness with which the mixture bums.
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342 ENGINES
Indications of a lean mixture may be apparent when an engine is cold,
or in winter weather, on account of incomplete vaporization. The gasoline
pipe might become clogged, or the valve might be only partially opened,
thus preventing free flow to the carburetor. Similarly, insufficient pressure
in the gasoline tank of a pressure-feed system, or a clogged vent in a gravity
system, may be the cause' of too little gasoline in the float^shamber. Jets
partially closed by dirt or lint, or water in the gasoline, may cause an engine
to suddenly develop signs of a lean mixture during flight Tilting, if so
that the mouth of the jet is raised much above the level of gasoline in the
float-chamber, tends to reduce the proportion of gasoline in the mixture.
Inleakage of air at the manifold joints, or past a badly-worn intake valve-
stem guide, will affect the suction at the jets, and decrease the amount of
spray.
Water-vapor occasionally condenses out of the air inside of the gaso-
line tank, due to sudden or extreme cold outside, and running down the
sides of the tank it collects at the bottom and feeds into the carburetor.
(See "The Effects of Altitude", end of Part 2.) In cold weather, water
may freeze in the line and stop the flow of gasoline.
2. mixture too rich is characterized by black smoke at the exhaust,
loss of power, overheating, and rapid fouling of the cylinders and spark-
plugs. A rich mixture may be produced by choking the air intake, as with
the strangler valve. The same effect would be produced if some foreign
object, like waste, was sucked against the air opening during operation.
Jets of too large size will give excessively rich mixtures. Flooding has the
same effect, and may be caused by improper seating of the float-valve, by
a leaky float, or by tilting of the jet downward during flight. In the case
of incomplete vaporization, where it is necessary to increase the flow of
gasoline to make up for the unvaporized portion, the inertia of the heavy
liquid particles throws them outwards wherever there is a change in the
direction of flow, and this action ''loads up*' the end cylinders.
C. COMPRESSION
The necessity for high compression has already been explained. Loss of
compression affects the power and the fuel economy adversely, hence it is im-
portant to avoid leaks. The compression may be tested roughly by turning the
engine over by hand, with the ignition off. It should be absolutely uniform in
all cylinders to give smooth running and least vibration. Loss of compression
may be due to :
1. Leakage at the valves Particles of carbon lodge between the ex-
haust valve and its seat, and eventually the whole edge of the valve becomes
covered with carbon. Continued leakage overheats the valve and warps
it, and leakage is usually so rapid in case of a warped valve that the ex-
plosion is very weak. Lack of valve clearance will prevent a valve from
closing tight. The valve stem frequently gets dirty or sticky, and the
excessive friction retards the action of the valve spring to such an extent
as to cause late valve closure at high speeds, which of course would not
be apparent at all when turning the engine over by hand. Broken or over-
heated springs have a similar effect, and it is always best to carry spare
springs on a long flight.
2. Leakage at spark-plug gaskets, which is not an infrequent occur-
rence, is most aggravating at slow speeds.
3. Leakage at the pistons Too thin oil, or lack of oil, is a common
cause of piston leakage and loss of compression. The thrust on the cylin-
der wall, due to the shortness of the connecting-rod (angularity), wears
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BNOINES 848
the cylinder out of round in time, and there is no remedy except to re-grind
llie cylinder or replace it with a new one. Piston-rings expand as th^
wear, and the joint opens up. A plain joint tinder sach conditions allows
leakage, and for that reason specially designed rings are sometimes used
in wMch the joint overlaps so tiiat leakage cannot occur at that point. In
fitting a piston ring, a small amount of clearance is allowed at the joint when
the engine is cold, so that expansion due to heat will not force the ends to-
gether and break the ring. Broken rings not only leak, but sometimes
score the cylinders. Scor^ cylinders are more likdy to result from dirt
or insufficient lubrication, however. It is not uncommon to find a cracked
piston the cause of lost compression and leakage.
Too high compression results in pre-ignition. Compression may easily be
increased by carbon accumulating in the combustion chamber, and when coupled
with the fact that cooling is not as efficient under such conditions, it is easily un-
derstood why carbon causes knocking. Unusually hot weather will also cause
pre-ignition, as well as overheating.
D. IGNITION TROUBLES
The most common of ignition troubles is a defective spark-plug. Only the
very best plugs will stand up for any length of time at the high temperature
and the rapid functioning to which they are subjected in an aeronautical motor.
Porcelains crack and allow oil and carbon to work through and short-circuit the
plug. The points bum away and cause irregular action at low speeds. Carbon
and oil deposit all over the insulation if not cleaned frequently, or a piece of
carbon or a drop of oil sometimes lodges between the points, which completes
the circuit and no spark takes place. The presence of a drop of oil might be
the effect instead of the cause, however, for after a cylinder has bec^ out of
action for a time it will be found that tiie combustion chamber is wet with oil
that has worked up past the piston, and the spark-plug is quite apt to collect
oil under such conditions.
Upon feeling just below the water-jackets, an inactive cylinder will be
decidedly cooler than those that are firing regularly. In case the exhausts are
separate and accessible, a cylinder that is not firing may be located by exposing
the hand or a stick directly in front and noting the relative weakness of the
'^puffs''. A third test, which is always applicable, is to throttle down the engine
and ''short" each plug in succession by means of a screw-driver until one is
found that shorting makes no apparent difference in the speed. Short-circuiting
a good plug wiU immediately slow down the engine. After locating the miss-
ing cylinder, the plug should be taken out and examined. In case the plug is
not at fault, examine the wiring for loose connections and worn insulation, and
examine the distributor segment.
When a whole group of cylinders fail to get a spark, the trouble will most
likely be either in the switch connections or in the magneto. A ''ground" due
to worn insulation in the wire leading from the magneto to the switch, or stray
strands of wire short-circuiting the switch itself, will prevent ignition. Oil,
dirt, or moisture on the distributor plate will cause irregular firing; wipe fre-
quently with a clean cloth, and if corroded clean with a fine metal polish.
Pitted or burned breiEiker points make poor contact, and should be care-
fully smoothed up by means of a thin file. The gap must be accurately adjusted
by means of a gauge. The condenser is provided to prevent excessive sparking
between the breaker points, which would bum the points and possibly fuse
them together. Sparking is an indication of condenser trouble, which some-
times results from punctured insulation. The function of the safety spark-
gi^ is to provide an outlet for the current before the voltage gets high enough
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344 BNOINBS
to break through the insalation of the secondary coil and back to the condenser,
so that condenser trouble is rare. The safety spark-gap also furnishes a means
of determining whether magneto trouble is internal or not A short-circuited
condenser would shunt the breaker points, and prevent interruption of the
primary circuit
If an engine fails to stop when the switch is thrown to the ''off" position,
it is probable that the switch circuit is broken by a loose connection, either at
the magneto or at the switch. The purpose of the switch is to ''ground" the
magneto to stop the engine, hence it is always advisable to make sure that the
switch is functioning properly before moving the propeller for any other pur-
pose than to start the engine.
The importance of correct spark-advance has already been mentioned.
In case it is necessary to determine the amount of advance, first remove the
spark-plugs to relieve compression and give access to the top of the piston. Turn
the engine over until the breaker points are closing, and insert a thin cigarette
paper between them. Continue to turn the engine in its direction of rotation,
and at the same time pull gently on the cigarette paper until it can be pulled
out At that position of the engine the breaker points are just about to open
to produce a spark in the particular cylinder for which the ^stributor segment
is making contact. One can then determine the piston position by means of a
steel scale through the spark-plug hole, or if the lower half of the crankcase is
off the crank angle may be measured with a protractor.
E. LUBRICATION TROUBLES
In starting a cold engine, always run it slowly for several minutes and
then open up the throttle gradually. This gives the oil time to get warmed
up and circidating freely to all parts, and also avoids distortion and cracking
from too rapid heating of the cylinders.
In airplane engines, where the pressure system is invariably used, it is
very essential that the oil be of the correct grade and the bearings properly
adjusted. If the oil is too heavy, it wiU not flow freely to all parts of the
engine. Oil is more viscuous when cold than when warm, and the oil gauge
on an airplane will invariably show decreasing pressure until the oil becomes
thoroughly heated up. The pressure relief valve, which forms an important
part of all pressure lubricating systems, is used to prevent excessive pressure on
the system when the oil is cold or too thick. It will not give the same results
with different grades of oil, unless readjusted. If for any reason it should get
stuck open, the oil gauge would show little or no pressure as soon as the oil
heated up. The same thing would happen if the pressure became high enough
to burst an oil pipe. It is advisable to stop an engine immediately at the first
indications of lack of oil pressure. This, however, should not be confused with
the normal decrease in pressure previously mentioned. Loose bearings will cause
low worldng pressure, and tight bearings will increase the pressure. It is im-
portant that the crankshaft bearings and the crankpin bearings be properly
adjusted, because it is these adjustments that usually determine the amount
of oil that reaches the cylinders. Too little oil will increase the friction and
wear on the cylinder walls and cause overheating, while too much oil fouls
the combustion chamber and the spark-plugs and causes bluish- white smoke at
the exhaust. Bearings should have at least two or three thousandths of an
inch clearance — ^no less.
Oil pumps may be either plunger type or gear type. In either case, a
worn pump will not deliver as much oil as when new. Wear of this sort may
be indicated by low gauge pressure. One must be guided largely by experi-
ence and good judgment as to how low the pressure may be allowed to go with-
out endangering the engine.
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ENGINES 345
A strainer, or screen, is nsoaUy provided over or within the oil-somp to
eolleet dirt or grit and prevent it from getting into the pump or the bear-
ings. This may need cleaning occasionally, in order that it may not retard
the flow of oil back to the reservoir. If ordinary precautions are used, there
is little danger of any of the oil pii>es getting clogged. Ck)pper tubes are easily
jammed, however, and if knocked together the oil supply will be affected.
F. COOLING TROUBLES
Overheating may be caused by lack of water. When filling the radiator,
be sure that the system is entirely filled. To make sure of this, run the engine
for a few moments, and then see if the radiator is still full. The pump some-
times prevents the water from entering the cylinders, or air pockets may be
present Leakage should not be tolerated, however slight, because it is an easy
matter to prevent it. During long flights, enough water may evaporate to de-
crease the efficiency of cooling materi^y. A tiiermometer is usually provided
to warn the pilot when an engine is getting hot, but such an instrument in the
radiator cap is not always reliable when the water level is low. The best place
for a temperature indicator is in the outlet just above No. 1 cylinder.
Lack of circulation may also cause overheating. A centrifugal pump is the
type universally used on water systems, and if it becomes broken or shears off
its key, circulation becomes retarded. That might easily happen during freez-
ing weather, when starting the engine. Occasionally the radiator or connections
become clogged by scale, dirty water, pieces of rubber from the hose conneo-
tions, etc.
Lack of cooling surface may be apparent in extremely hot weather, when
the cooling will not be sufficient to prevent pre-ignition. Oil on the inside sur-
faces of the radiator acts as a partial heat insulator, thus cutting down the
effectiveness of cooling. Scale formation on the tubes has a similar effect. It
is good policy to flush out the radiator and the cylinder jackets occasionally, and
never use rotten or frayed hose connections.
8. NOTES ON TUmNO
Proper timing of the valves and ignition of a gasoline engine is very im-
portant When one realizes that for a four-stroke cycle engine running at 1400
R.P.M. there is only 0.085 second available for a complete cycle, it is apparent
that the valves must operate rapidly and all events occur at the proper time.
In this small fraction of a second, the mechanism must draw a deftuite amount
of liquid fuel from the source, vaporize it, mix it with the correct amount of
air, distribute the mixture uniformly to the cylinders, entrap and compress it
to a fairly definite pressure, ignite and bum it, expand the highly heated and
high pressure gases, and expel the products of combustion. Failure to have any
of these operations performed at the proper time results in a loss of power and
a waste of fuel. An improperly timed engine, therefore, weighing as much as
when properly timed, develops less power and consequently has a higher weight-
power ratio.
A. DESIGNATION OP VALVE EVENTS
Valve events are measured from the nearest dead center, either in terms
of piston travel or in degrees of crankshaft rotation. The point of valve opening
is when the tappet first touches the valve stem and the valve begins to leiave
its seat. Valve closure is when the valve just touches the valve seat in closing.
The interval during which the valve is off of its seat is called the period of valve
opening, and is measured in degrees of crank travel.
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346 ENGINES
Valve lift is the distance parallel to the stem between the valve and its
seat when the valve is folly opened. This varies from % to ^ inch. It is
governed by three things — ^the eccentricity of the cam or difference between the
radius of the point of the cam measured from the center line of the camshaft
and that of the ronnd or neutral part of the cam, the ildative lengths of the
two arms of the rocker arm, and the valve clearance. In high speed engines
there is not time enough available to utilize as much lift as would be desirable,
which adds to the difficulty of high speed engine design.
B. VALVE CLEABANCE
Valve clearance is the distance (about five to thirty thousandths of an inch)
between the end of the valve stem and the tappet on the end of the rocker arm,
measured and set with the valve closed tight and the cam follower on the neutral
part of the cam. It is necessary to have the valve clearance properly adjusted
before timing valves. To set the clearances for the valves of any one cylinder,
turn the engine over in the proper dir^tion of rotation until the piston is
near the end of the compression stroke, at which time both valves are closed
and the cam followers are down. This position may be obtained with sufficient
accuracy by turning the engine until the intake valve closes, and then turning
a quarter to half of a revolution farther. The adjustment can then be made
on each valve by loosening the lock nut that holds the tappet screw, and turning
the screw until a thickness gauge of the proper size can just be inserted. Re-
peat for all cylinders.
C. DIRECTION OP ROTATION
If the direction of rotation of an engine is not known, one can determine
it very easily by observing the occurrence of the valve events in any one cylinder.
Bear in mind that exhaust valve closure and intake valve opening occur almost
simultaneously just after top center on the suction stroke. If the engine is
turned in the wrong direction, the intake valve will dose and the exhaust valve
will open at about the same time, whereas these two events normally occur at
widely different points in the cycle.
D. GRINDING VALVES
If the valves are in need of grinding, this should be done before setting
the valve clearance. Different engines require somewhat different prdiminary
procedure to gain access to the valves. If the valves are in valve cages, or if
the cylinder heads are detachable, the cylinders need not be removed. In any
case, it is necessary to free the valve springs, which may be done by depressing
the spring and tating out the key that holds the retainer to the valve stem. In-
take valves seldom need grinding ; but the exhaust valves, exposed to hot gases
and carbon, soon become pitted or warped in service and require frequent at-
tention to keep them from leaking badly.
Valve grinding paste consists of powdered emery or other abrasive material
mixed with grease to hold it together. A coarse grade may be used first, but
the grinding should never be considered complete until a fine grade of paste
has been applied. To grind a valve, apply the paste uniformly around the
bevelled edge of the valve, and with a screw driver or valve grin<fing tool twist
the valve back and forth against the valve seat several times, and at the same
time press the valve firmly but not too hard against the seat. It is good prac-
tice to lift the valve occasionally and turn it to a new position, in order to avoid
<5utting deep grooves. Several applications of the paste may be necessary if the
valve is badly pitted. If the valve is found to be warped, it should be dis-
<!arded for a new valve, not only to save time but also to avoid grinding away
an excessive amount of the valve seat
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BNOINBS 847
The yalye should be inspected frequently during the grinding process, so
that it will not be ground more than necessary. First wipe off the paste and
clean the valve with gasoline. The seating edge of the valve will then appear
shiny. If there are any dark spots present, the valve should be ground until
these are removed. But always apply the paste uniformly all of the way around,
and dean off all traces of the abrasive substance before assembly.
E. PRINCIPLES OP VALVE TIMING
The general scheme to be followed in timing the valves of an engine con-
sists in placing the piston or the crank throw for any one cylinder in the por-
tion recommended by the maker for some particular valve event to take place,
and then turning the camshaft in the proper direction by hand, independently,
until the exact position is reached where that event does take place. The cam-
shaft should be connected to the crankshaft in that position, by meshing the
driving gears. When one event is timed, the others must be correct if the cam-
shaft is properly designed and constructed.
The event by which an engine is usually timed is the closing of the exhaust
valve, for the reason that it is most important to have this event accurately fixed.
A slight error in exhaust closure would have more effect on the power of the
engine than the same error on any of the other valve events. The exhaust
valve should close when the pressure within the cylinder just reaches atmospheric
pressure shortly after the suction stroke begins, in order to get rid of as much
burned gas as possible. Burned gas takes up space in the cylinder, and dilutes
the fresh charge. If the exhaust valve should close too late, the suc^on would
draw back burned gas (or air), with about the same effect.
P. SETTING THE ENGINE
Some V-lype engines are difficult to set by the crankshaft method, while on
some engines there is no hole in the top of the cylinder suitable for accurate
measurement of the piston position. The Curtiss OX engine, for example, should
be timed by setting the piston 1/16 inch past top center, measuring through
the spark-plug hole. Pirst turn the engine in the proper direction of rotation
until the piston of some particular cylinder (say No. 1) is exactly on top center,
as indicated by a wire or measuring device held against the face of the piston
and parallel to the center line of the cylinder. Then continue to turn the engine
in the same direction until the piston has travelled the proper distance from this
position, as specified on the manufacturers' test card. Extremely accurate mea-
surements must be made, because an error of a hundredth of an inch causes an
appreciable error in the setting.
A vertical engine, like the Hall-Scott, may be timed by the use of a pro-
tractor applied to the milled surface of the upper half of the crankcase. This
requires removal of the lower half, of course, but there is no alternative on the
Hall-Scott engine because the spark plugs are in the side of the combustion
space and there is no hole through the top of the cylinder parallel to the center
line. The crank throw by which the engine is to be set (say No. 1 cylinder)
should be placed 10 degrees past top center, as measured by the protractor.
Then, with the camshaft gear removed, turn the camshaft in its proper direc-
tion of rotation until the exhaust valve just touches its seat, then replace tiie
gear. On Model A5a a screw adjustment makes it possible to shift the gear
relative to the shaft, so that the gears may be meshed properly should they fail
to do so in the positions set. On Model A5 this may be accomplished by shift-
ing the upper half of the vertical drive shaft relative to the lower.
Many engines have timing marks on the propeller hub, placed there by
the manufacturer. One of these marks, lined up with a mark on the crank-
case, indicates top dead center for No. 1 cylinder. With this position prevailing.
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348 ENGINES
and the piston in No. 1 cylinder just starting on the suction stroke, a mark
on the camshaft gear should be in line with one on the bottom of the cam-
shaft gear housing. On Model A5 Hall-Scott the marks on the flanges of the
upper and lower halves of the vertical driving shaft should be in line and to
the front
The timing should always be checked by turning the engine over until
the exhaust valve just seats, and then measuring the piston position or the crank
angle to see if it is correct.
a. PRINCIPLES OP IGNITION TIMING
There are two distinct processes involved when timing a magneto ignition
system. First, the internal mechamsm of the magneto must be arranged so that
tjie breaker points separate when the distributor brush is in contact with a
segment, both in advanced and retarded pofidtion^. Second, the spark generated
by the magneto must be timed to occur at the right point in the engine cycle,
near the end of the compression stroke (No. 1 cylinder is most convenient).
It is also necessary to have the breaker points properly adjusted, and to have
the wires from the distributor to the spark plugs connected in accordance with
the firing order of the cylinders,
H. TIMING THE DISTRIBUTOR
Adjust the breaker points so that they open the proper amount specified
by the manufacturer. A thickness gauge should be used for this purpose. Turn
the breaker box to the advanced position (opposite to the direction of rotation
of the magneto) , then turn the armature shaft or rotor in its proper direction of
rotation until the breaker points just start to separate and note whether the dis-
tributor brush makes contact with a segment. Retard the spark, turn the shaft
until the points are just opening again, and note if the distributor brush still
makes contact with the same segment. If contact is not made in both the ad-
vanced and retarded positions, the distributor gear must be re-meshed so that
the proper condition will obtain.
I. FIRING ORDER
Before wiring the magneto to the engine, it is necessary to know the firing
order of the cylinders. Determine this by turning the engine in its direction
of rotation and noting the order in which a particijdar valve event occurs.
J. SETTING THE ENGINE
The engine should be set in much the same way as for timing valves,
except that it should be in the proper position for ignition near the end of the
compression stroke (say in No. 1 cylinder). The crank angle, or the piston
position, for either advanced or retarded spark, will be specified by the manu-
facturer. The breaker box should be placed in the position specified, and the
magneto shaft turned in the normal direction until the distributor brush is
on the segment corresponding to the cylinder that is ready for ignition, and
the breaker i>oints just beginning to separate. A thin cigarette paper between
the points will indicate when the points are separating. Having thus set the
engine in the proper firing position, and the magneto ready to deliver a spark
to the proper cylinder, the gears should be meshed and the timing checked.
K. WIRING UP
When the wires are in a casing or manifold, it is necessary to trace them
out before connecting up. Do this by means of a dry cell and electric bell or
lamp, and tag each wire at both ends after testing. From the known direction
of rotation of the distributor arm, and the firing order of the engine, each suc-
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ENGINES 849
cesgiye segment of the distributor should be connected to a spark plag in the
order in which the varions cylinders fire, beginning with the segment and the
cylinder used in timing the ignition.
L. DOUBLE IGNITION
When there are two independent ignition systems on an engine, it is abso-
lutely necessary for efficiency that they be synchronized. That is, the breaker
points in the two magnetos should separate simultaneously. The slightest varia-
tion is sufficient to make one spark occur too late to be of any use except
as a reliability factor. Extreme care should be used in setting and checking,
and in connecting up the advance-and-retard mechanism.
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CHAPTER IV.
GUNNERY
OUTLINB
1. Nomenclature of the Lewis Gun
A. Body Group
B. Barrel Group
C. Piston Group
D. Magazine
E. AooesBories
2. Stripping and Assembling— Lewis Gun
A. Stripping
B. Assembling
C. Rules to Observe when Stripping or Assembling
3. Mechanism of the Lewis Gun
A. Backward Movement
B. Forward Movement
4. Drill and Immediate Action — Lewis Gun
5. Stoppages and Jams — Lewis Gun
A. Defects Prevented by Proper Cleaning and Oiling
B. Defects Prevented by Inspecting and Testing Magazines
C. Defective Ammunition Eliminated by Testing and Inspecting
D. Defects Prevented by Inspection of Parts
E. Principal Causes of the Lewis Gun Stopping
1. Empty Magazine
2. ''Stoppages'*
8. "Jams''
F. Definitions
G. Sequence of "Immediate Action"
6. Care of the Lewis Gun
A. Cleaning and Oiling
B. Points to be Observed before Flight
C. Points to be Observed during Flight
D. Points to be Observed after Flight
7. Standard Tests of Stripping — ^Lewis Gun
A. Increase Return Spring Tension
B. Decrease Return Spring Tension
C. Change Cartridge Guide
D. Change Bolt
E. Change Ejector
F. Change Pinion Casing Complete
401
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402 GUNNERY
8. Range Practice — Lewis Gun
A. Practices
1. Practice No. I Drill
2. Practice No. II Grouping
3. Practice No. Ill Application
B. Range Discipline
1. Duties of Cadets
2. Procedure
9. Nomenclature of the Marlin Aircraft Gun
10. Stripping and Assembling — ^Marlin Aircraft Gun
A. Stripping
B. Assembling
C. Stripping and Assembling Group Parts of the Marlin Aircraft Gun
D. Points to be Carefully Observed when Stripping or Assembling the
Marlin Aircraft Gun
11. Mechanism of the Marlin Aircraft Gun
A. Backward Movement
B. Forward Movement
12. Drill — ^Marlin Aircraft Gun
A. Loading
B. Firing
C. Unloading
13. Stoppages and Jams — ^Marlin Aircraft Gun
A. Defects Prevented by Proper Cleaning and Oiling
B. Defects Prevented by Proper Inspection of Parts
C. Defects Prevented by Proper Assembly of Gun
D. Defects Prevented by Proper Adjustment of Gun
E. Defective Ammunition Eliminated by Testing and Inspecting
F. Defects Prevented by Proper Loading of Belts
G. Stoppages and Jams.
1. Position I
2. Position 11
3. Position III
H. Uncontrolled Automatic Fire
I. Reports of Stoppages and Jams.
14. Care of the Marlin Aircraft Gun
A. Cleaning
B. Points to be Observed Before Flight
C. Points to be Observed During Flight
D. Points to be Observed After Flight
15. Aerial Sights
A. Purpose
B. Types
C. Ring Sights
D. Principles Governing the Use of Ring Sights on a Fixed Gun
E. Use of Ring Foresight with Bead Backsight on a Fixed Gun
F. The Norman Wind- Vane Foresight
G. Use of Norman Wind- Vane Foresight with Ring Backsight on a
Movable Gun
H. Compensation for Depression and Elevation of Movable Gun
I. Practice with the Model Aiming Airplane
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CHAPTER IV.
GUNNERY
1. NOBOSNOLATUBE OF THE LEWIS OUN
The following is a list of the principal parts of the Lewis Automatic Machine
gon, 1916 model. Cadets should be able to identify each part named and give
its function.
A. BODY GROUP
1. Spade grip
nate 6, 92
2.
8.
5.
a. Spade grip cap
Body cover or Feed cover Plate 4,
a. Cartridge guide (1915 model) or Cartridge guide spring 4,
b. Right stop pawl or Stop pawl 4,
c. Left stop pawl or Rebound pawl 4,
d. Stop pawls spring or Magazine pawls spring 4,
e. Tangent sight or Backsight 4,
Feed arm or Feed operating arm Plate 4,
a. Feed arm latch or Feed operating arm latch 4,
b. Feed arm pawl or Feed pawl 4,
c. Feed arm pawl spring or Feed pawl spring 4,
d. Feed arm pawl spring stud or Feed pawl spring stud 4.
e. Tail of the Feed arm with stud 4,
Pistol grip or Ckiard
a. Plunger or Sear spring box
b. Trigger spring or Sear spring
c. Trigger
d. Sear
e. Spade grip catch
f . Spade grip catch spring
Ejector cover
6. Ejector
7. Body locking pin or Receiver lock pin
8. Pinion casing or Gkar casing
a. Pinion pawl or Qear stop
b. Pinion pawl spring or Oear stop spring
c. Tension screw or Collet pin
d. Pinion or Gear
e. Return spring casing or Mainspring casing
f. Return spring or Mainspring
g. Hub or Mainspring collet
Plate 8.
3i
3,
3,
8,
8,
Plate 4,
4,
Plate 3,
3,
3
3i
3,
3,
3,
3,
3,
3,
3
Body or Receiver Plate
a. Pinion casing hinge pin or Gear casing hinge pin
b. Right and left safety catch or Right and left safety
403
18
40
28
29
27
11
34
66
35
36
Q
34
42
41
30
91
7
44
21
68
61
46
49
56
52
53
55
57
26
65
62
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404 OUNNEBY
c. Ejection opening
d. Magazine post 8, 63
1. Magazine post center key or Center key
e. Ejector seating
f. Boltway
1. Longitudinal goideways
2. Locking recesses
g. Piston way
B. BARREL GROUP
1. Qbm regulator key
2. Qbm regulator or Gtas regulator cup
8. Olamp ring
a. Clamp ring screw
b. Fore sight or Front sight
c. Clamp ring positioning stud
4. Radiator casing
a. Front radiator casing
b. Rear radiator casing
1. Gas regulator key positioning stud or Regu-
lator key stud
c. Rear locking piece
5. Qbm cylinder
6. Qbm chamber
7. Barrel mouth piece
8. Barrel
a. Gas port
b. Barrel register
9. Barrel band or Qbm chamber band
10. Radiator
C. PISTON GROUP
1. Ooddng handle or Charging handle
2. Piston rod or Piston
a. Rack
b. Bent or Cocking notch
c. Striker post
d. Striker
e. Striker fixing pin
f . Cocking handle slot or Charging handle slot
8. Bolt
a. Extractors
b. Camway groove or Cam slot
c. Resistance lugs or Locking lugs
4. Feed arm actuating stud or Feed operating stud
a. Boss
b. Guide lugs
Plate 2,
Plate 2,
81
2.
84
2,
2,
2,
2,
85
88
86
87
Plate 2,
2,
2,
90
74
2,
71
2,77
2,
82
2,
89
2,
76
2,
83
2, 78
Plate 2,
2,
38
2,
2,
79
72
2,
2,
50
47
Plate 4, 37
4,45
Plate 4,
31
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GUNNERY
405
D. MAGAZINE
1. Magadne pan
a. Outer circumference
2. Separating p«gi or Interior Mp&rators
8. Magarine catch
4. Magaitne center Uock or Magarine center
6. Magarine catch spring
Plate 5
5,43
6, 70
5, 62
5, 58
I
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GUNNERY
PLATES DESCMETIVE OP LEWIS GUN PARTS
/*
I
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GUNNEBT
407
C Ji —
0)
i f i ■~nr^i ^i^^Mi^p^^i
T.
^^* ■ ' ■ vfr 1 1 ^11
2Z^^
Plate 2. — Gun parts: Barrel group, and Piston Rod
T. ^"
Plate 3. — Gun parts : Body, Return spring, and Trig-
ger Mechanism
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408
GUNNERY
Plate 4. — ^Pekd Mechanism, Bolt, Extractors, and Ejector
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GUNNERY
409
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GUNNERY
ACCESSORIES
Plate 6.— Rifle Buttstock, Loading Handle, and
Spade Grip
Plate 7. — ^Deflectob Bag complete
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Plate 8. — ^Mountino Yoke and Standard
E. ACCESSORIES
L Loading handle Plate 6, 187
2. Spanner wrench 2, 186'
3. Spring balance
4. Oilcan
6. Barrel cleaning rod with wire gauze
6. Oas cylinder cleaning rod with mop and wire bmih
7. Mounting yoke and itandard Plate 8,
8. Deflector bag Plate 7,
2. 8TRIPPIN0 AND ASSEMBUNO— LEWIS Omr
With the exception of the gas chamber and the barrel mouthpiece for
which a spanner wrench has to be used, and the clamp ring screw which is re-
moved with the gas regulator key, the Lewis gun can be stripped by means
of the nose of a bullet.
A. STRIPPING
1. Remove spade grip
2. Remove body cover
a. Remove cartridge guide
b. Remove stop pawls spring
c. Remove stop pawls
8. Push feed arm latch forward and remove feed arm
a. Remove feed pawl and feed pawl spring
4. Remove pistol grip
6. Remove pinion casing
a. Release tension of return spring
b. Remove tension screw
c. Remove pinion from pinion casing
d. Force return spring casing out of pinion
e. Remove hub
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412 GUNNERY
6. Draw back coddng handle to fuUeit aartent^ remove it, and take
out piston rod and bolt
a. Remove bolt from striker post
b. Remove one extractor
c. Unscrew feed arm actuating stud
7. Remove ejector cover
8. Remove ejector
9. Remove body locking pin and unscrew body
10. Remove gas regulator key
11. Remove gas regulator
12. Unscrew clamp ring screw and remove front radiator casing
13. Remove clamp ring
14. Remove rear radiator casing
15. Unscrew gas cylinder
16. Unscrew gas chamber
17. Unscrew barrel mouthpiece
B. ASSEMBLING
The gun is assembled by reversing the operations given above for stripping.
C. RULES TO OBSERVE WHEN STRIPPING OR ASSEMBLING,
1. Be sure that magasine is removed and that the gun is free of all
fired or unfired cartridges before beginning to strip.
2. Oare should be taken
a. That the feed arm is towards the right before removing and
before replacing body cover.
b. That the bolt is at the rear of its travel before attempting to
remove or replace the ejector.
c. That the feed arm ig at the extreme left before attempting to
insert the piston rod and bolt into the body.
d. That the feed arm actuating stud is screwed into the bolt as
far as it will go, and that the striker post is in the rear of the camway
groove before inserting piston rod and bolt into body.
e. That the cocUng handle is securely in place in its guide slot.
8. MECHANISM OF THE LEWIS GUN
A. BACKWARD MOVEMENT
1. Action of the Gkises
On the cartridge being primed, the powder is turned into gas
which forces the bullet up the barrel. Four inches from the muzSde,
part of the gases enter through the gas port into the gas chamber and,
passing through the large hole in the gas regulator, enter the gas
cylinder. Here they strike against the cupped head of the piston rod
and drive it to the rear.
In the case of the ground gun, the remainder of the gases leave
the barrel mouthpiece and spreading out evenly, strike against the
inner surface of the front radiator casing, driving the air out and
causing a suction, which draws in cool air from the rear along the
radiator fins. By this means the barrel is kept cool.
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2. Action of the Piston Bod and Return Spring
' During the first V^" of the backward travel of the piston rod,
the striker post travels along the straight portion of the camway
groove, and the teeth of the rack being engaged with the teeth of the
pinion, the winding up of the return spring commences. This V/^'
travel is the safety device to insure that the gas pressure on the face
of the bolt has dispersed before the bolt is unlocked.
8. Action of the Bolt and Extraction
The right side of the striker post now bears against the right side
of the curved portion of the camway groove, causing the bolt to rotate
%th of a turn to the left. This action unlocks the bolt by withdraw-
ing the resistance lugs from the locking recesses. The striker post
now bears against rear end of the bolt and from this point the piston
rod and bolt come back together. As the bolt comes back, the extractors
bring with them the empty case from the chamber.
4. Action of the Ejector
The left guide lug of the feed arm actuating stud now strikes the
tail of the ejector which, working on its pivot, causes the head to enter
the recess in the bolt, and travel across the face of the bolt. It strikes
against the base of the empty cartridge and knocks it out through the
ejection opening on the right side of the body.
6. Action of the Bear
The piston rod and bolt continue their backward movement and
if the trigger is rdeased, the nose of the sear will engage with the
bent of the piston rod as the latter commences to go forward. The
return spring is now fully wound up.
6. Action of the Feed Arm and Pawls
As the bolt comes back, the boss of the feed arm actuating stud
working in the curved channel in the tail of the feed arm, causes
the fe^ arm to move from right to left. The feed pawl, being en-
gaged behind a corrugation of the magazine, rotates the magazine
dockwise one corrugation. The feed pawl spring stud bearing away
from the right stop pawl, allows it to come into action. This prevents
the magazine from rotating more than one corrugation. The left stop
pawl is depressed by a corrugation of the magazine and engages be-
hind it. This prevents the magazine from rotating counter-clockwise.
7. Action of the Magazine
As the magazine rotates, the cartridge which is hdd in the maga-
zine by the outer circumference and the separating pegs, is forced
down into the feed way by means of the cartridge guide assisted by
the slope of the centre block. It is now in position in the slot on the
top of the body ready to be pushed forward by the bolt in the forward
movement.
\ B. FOBWABD MOVEMENT
Note: During the forward movement, the magazine does not rotate.
1. Belease of the Sear
When the trigger is pressed, the nose of the sear will become dis-
engaged from the bent of the piston rod. The return spring will carry
the piston rod forward, and the striker post bearing on the left side
of the curved portion of the camway groove, and tbe bolt being un-
able to rotate owing to the resistance lugs being engaged in the longi-
tudinal guideways, the bolt is carried forward also.
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414 GDNNBBY
2. Fordng of the Cartridge into the Ohamber
As the piston rod and bolt move forward, the top extractor strikes
the lower edge of the rim of the cartridge and forces it into the cham-
ber, where tiie extractors spring over and grip the rim.
8. Action of the Ejector
As the bolt moves forward, it strikes the head of the ejector and
forces the tail into the boltway, ready for the backward movement.
4. Action of the Bolt
The resistance lugs are now opposite the locking recesses. The
left side of the striker post still bearing against the left side of the
curved portion of the camway groove, causes the bolt to rotate %th
of a turn to the right and the resistance lugs enter the locking re-
cesses so locking the bolt. When the gun is fired, the resistance lugs'
being engaged in the locking recesses, tiJ^e the shock of discharge.
6. Priming the Cartridge
As the striker post travels along the straight portion of the cam-
way groove, the striker passes through the hole in the face of the bolt
and so primes cartridge.
6. Action of the Feed Arm and Pawls
During the forward movement of the bolt, the boss on the feed
arm actuating stud working in the curved channel in the tail of the
feed arm, moves the feed arm from left to right. The feed pawl rides
over a corrugation of the magazine and engages behind it ready for the
backward movement. The feed pawl spring stud bears against the
right stop pawl and forces it out of action. The left stop pawl remains
stationary and prevents the magazine from rotating counter-clockwise.
4. DRILL AND DOIEDIATE ACTION— LEWIS OUN
The object of "Drill" is to instruct cadets in the art of handling the Lewis
Gun in the air and to teach them the combined use of eye, brain, and fingers,
which is so essential to good aerial gunnery.
IMPORTANT
a. Keep away from muzzle
b. See that gun is unloaded
c. See that no live rounds are among the dummies.
Cadet to be seated on tripod, with six magazines placed fiat on the fioor
on his left. The movements of Drill are as follows:
1. Hold the magazine firmly with both hands, with fingers under the
center block and thumbs on top. Tilt front part downwards to give stream
line effect. Keep thumbs away from magazine catch. Keeping the maga-
zine in this position lift it up and place it on the magazine post.
2. With hands still in the same position, attempt to lift magazine off
post by giving a direct pull upwards, in order to see if magazine catch is
holding.
3. Orasp spade grip with right hand ; with left hand pull back cocking
handle to fullest extent.
4. Grasp pistol grip firmly with left hand.
5. Lay sight on to target (in barracks room a chalk mark on the wall
is sufficient for a target) ; press the trigger.
6. Apply ''Immediate Action'' with eye on the target by rotating
magazine sharply clockwise with left hand.
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QUNNEBY 415
7. Grasp magazine as in 1, with thumb of right hand release maga-
' zine catch, lift off magazine and place it on floor to the right still keeping
the stream line effect as in 1.
8. Bepeat these same operations with all the magazines.
9. After removing the last magazine, cadet executes (a) "Lock Gun'',
(b) "Unlock Gun", and (c) "Unload".
a. Lock gun To lock gun, pull cocking handle to rear, raise
safety, press trigger.
b. Unlock gnn To unlock gun, pull cocking handle back, depress
safety.
c. Unload To unload pull cocking handle to rear twice holding
the pistol grip with the right hand and keeping the trigger pressed aU
the time. During these operations the gun must be kept sighted on
the target.
6. BTOPPAaSS AND JAMS— LEWIS OUN
The Lewis Gun stops firing when in action because of defective mechanism
or defective ammunition. It must be borne in mind that three-quarters of the
difficulties which may occur are due to careless cleaning and inattention to small
details. If magazines and ammunition are thoroughly inspected, if the return
spring tension is properly adjusted, and if the gun is carefully cleaned, well
oiled, and correctly assembled, it will seldom stop when in action.
A. DEFECTS PREVENTED BY PBOPEB CLEANING AND OILING
1. Clog^[ed gas port
2. Excessive friction in gas cylinder
3. Excessive friction in working parts
4. Hard extractions due to grit or rust in chamber
5. Defective extractors due to brass dust under extractor hooks
B. DEFECTS PEEVENTED BY INSPECTING AND TESTING MAGA-
ZINES
1. Damaged magazine
2. Dented magazine
3. Empty efpace in magazine
C. DEFECTIVE AMMUNITION ELIMINATED BY TESTING AND IN-
SPECTING
1. Bulged rounds
2. Thick rims, deep set caps^ split cartridge cases
3. Hard extractions (partially eliminated)
D. DEFECTS PEEVENTED BY INSPECTION OF PARTS
1. Defective f^d mechanism
2. Damaged striker
Note: By firing 20 rounds into the ground the feed mechanism and striker
will be known to be in good condition initially. They are not likely to become
defective while firing a few hundred rounds.
E. PRINCIPAL CAUSES OF THE LEWIS GUN STOPPING
1. Empty magazine
2. "Stoppages''
a. Insufficient charge
b. Weak cartridge guide
c. Worn striker post or camway groove
Note: Striker post becomes worn, camway groove becomes burred,
resulting in excessive friction.
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416 GUNNERY
a. Broken cartridge guide
b. Broken return spring
c. Broken extractor (s)
d. Broken ejector
e. Full or loose deflector bag
P. DEFINITIONS
1. A *' stoppage" can be remedied by applying "Immediate Action"
2. A '*jam" cannot be remedied by applying ''Immediate Action"
fj
G. SEQUENCE OP "IMMEDIATE ACTION"
1. If the Lewis gun stops, try to rotate the magazine clockwise with
the left hand, keeping tiie eye on the target.
2. If the magazine rotates it is empty, and should be replaced with a
filled one.
3. If the magazine does not rotate, pull back on cocking handle, aim,
press trigger.
4. If the gun fires, the trouble was a "stoppage".
5. If the gun does not fire, apply "Immediate Action" again.
6. If the gun still does not fire the trouble is due to a jam.
7. Remove the magazine and proceed to determine the cause of
the jam.
a. Examine the ejection opening. An empty case in the bolt-
way indicates a broken ejector or a full or loose deflector bag. An
empty case in the chamber indicates defective extractors. If the ejec-
tion opening is clear and a live round is under the cartridge guide,
either the cartridge guide is broken or the top extractor has broken,
resulting in an "under run".
b. If the cocking handle comes back with no resistance when ap-
plying the first "Immediate Action" the defect is a broken return
spring.
Note: If the gun fires after applying "Immediate Action" twice,
the trouble was a stoppage.
6. CARE OF THE LEWIS OXTN
The reliability of the Lewis Qun depends upon the care and the attention
accorded it. It is of the greatest importance that the cadet knows how to
clean and oil the Lewis Qun properly, how to inspect the gun for defects, and
how to adjust the gun for firing.
A. CLEANING AND OILING ^
1. Time of cleaning
a. After every firing
b. Before every firing
c. At end of each week
2. Kind of cleaning
a. After every firing
1. If more than 500 rounds have been fired, thorough cleaning
2. If less than 500 rounds have been fired, partial cleaning
b. Before every firing, partial cleaning
c. At end of each week, thorough cleaning and inspection of parts
by officer in charge of guns
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%JNNE»T 417
8. Deflnitloiii
a. Partial cleaning Strip body group and gas regulator. Clean
working parts, barrel, and gas cylinder. Do not Use boiling water.
b. Thorough cleaning Strip body group and barrel group, last
part to be stripped being gas chamber. Clean working parts, barrel,
gas Qsrlinder, and other parts of barrel group. Use boiling water as
specified later.
4. Kind of oil spedfled: Light Havoline Oil
6. Procedure in deaning
a. Strip giuL part by part, last part to be removed being gas
regulator.
b. Clean each part as it is stripped, first with an oily rag, then
with a dry rag.
c. Lay deaned parts on dean end of table where they cannot be-
come dirty.
d. Having deaned working parts, dean barrd.
1. Push deaning rod through barrd from muzzle end.
2. Put oil on flanndlette. Work oil wdl into flannellette.
Thread fianndlette through eye of deaning rod. Wrap flannellette
around end of cleaning rod.
3. Pull rod through barrel from chamber end with one stroke.
4. Bepeat (1), (2), (3) with same piece of flannellette re-
versed.
5. Bepeat (1), (2), (3) with dry flanndlette.
a. If over 500 rounds have been fired, 7 pints of boiling
water should now be poured through barrel, and then barrd
should be deaned as in (1), (2), (3), (4), (5). .
b. If less than 500 rounds have been fired, oily fiannd-
lette and dry fianndlette should be used alternately as in
(1), (2), (3), (4), (5) above until barrel is dean.
6. An oily flannellette should then be drawn through bar-
rel to leave a thin film of oil in bore.
Note: If any nickeling appears on the lands of the barrel,
barrd should be sent to an armorer.
e. Clean gas cylinder.
1. Use wire brush with oil on it.
2. Use dry soiled mop.
3. Use dry dean mop.
4. If over 500 rounds have been flred, 7 pints boiling water
should also be poured through gas cylinder and body.
6. Assembling and oiling Oil parts as they are assembled in follow-
ing order
a. Qas regulator and gas regulator key (no oil). Large hole of
gas regulator should be placed to rear.
b. Body.
c. Body locking pin (no oil).
d. Ejector and ejector seating (slightly).
e. Locking recesses (well).
f. Head of piston rod (wdl, on lateral surface of head, not on
cap).
g. Bade (sUghtly).
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418 GUMNBBY
h. Striker post (well).
Insert piston rod half way into body. Assemble bolt, oiling
i. Worm of feed arm actuating stud (slightly),
j. Camway groove (well),
t Surface of bolt (slightly).
Put bolt on striker post, push in on bolt, insert cocking handle cor-
rectly, push cocking handle forward.
1. Pinion casing complete, having slightly oiled edges of coiled
return spring in return spring casing before inserting latter into pinion,
m. Oil parts on top of pistol grip (slightly), assemble pistol grip
to body.
n. Feed pawl where it pivots on stud (slightly).
0. Curved channel in tail of feed arm (slightly),
p. Around magazine post and underneath feed arm (well). As-
semble feed arm to body.
q. Around stop pawls studs (slightly),
r. Stop pawls (alightly).
s. Bibs of body cover (slightly),
t. Projections on sides of body cover,
u. Assemble body cover to body,
v.* Assemble spade grip.
Note: Always remove both extractors when cleaning the bolt in or-
der to clean the face of the bolt and prevent the accumulation of brass dust
under the hooks on the extractors. It must also be borne in mind that the
Lewis gun depends upon a sharp backward movement to unlock the bolt.
If a slow movement results, energy of the gases will be overcome in ro-
tating the bolt, hence a stoppage occurs. For this reason, all working parts
must work with as little friction as possible.
B. POINTS TO BE OBSEBVED BEFOBE PLIGHT
1. The gun must be stripped as far as the gas regulator and cleaned
and oiled. Bemove the oil from the barrel bore and see that there is no
obstruction in it, that it is dry, and that the chamber is dry.
2. See that the large hole of the gas regulator is to the rear, which
is indicated by placing the ''L^' on the gas regulator to the rear.
3. See that the cocking handle is in correctly.
4. See that the return spring is at the correct working tendon,
about 11 lbs.
Note: If the tension is too low the rack will strike the spade grip too
hard on the backward stroke which would probably result in brei&age of
working parts. If the tension is too high, the piston rod will not be moved
back far enough to feed the next cartridge.
a. To measure return spring tension, engage hook of spring bal-
ance with cocking handle, pull back ring of spring balance and read
the tension just as the cocking handle starts to rear.
5. See that the cartridge guide is in the proper position, i.e., acting
as a spring.
6. See that the sights are fixed firmly and are upright.
7. Place an empty magazine on the gun and work the cocking handle
to see that the feed mechanism is operating correctly.
8. Fire 20 rounds into the ground.
9. See that the deflector bag is on correctly. See that it is hammered
up tight and the bottom of the bag securely fastened.
10. See that the mounting yoke latch is on the right hand side. See
that the pin is pushed in from the rear and the handle turned down.
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GUNNEBY 419
0- POINTS TO BE OBSERVED DURING PLIGHT
1. Immediately npon leaving the ground place a filled magazine on
magazine post and cock the gun.
2. It is a good plan to work the moving portiona backward and for-
ward about 15 to 20 times every ten minutes in order to free the recoiling
portions from congealed oil in case they are getting clogged. The maga*
zine must first be removed.
3. During a temporary cessation of fire, a partially emptied maga*
zine should be replaced with a filled one.
D. POINTS TO BE OBSERVED AFTER PLIGHT
1. Before landing on airdrome, remove magazine. If live round is
in the feed way, lock gun, and on landing fire off round into the ground
having previously warned mechanics that you are going to do so.
2. When landed, have gun taken to gun shop and cleaned as laid down
above. (See par. A) '
3. Magazines must be emptied, examined and tested before being
refilled.
a. Testing magazines
1. Place each on a loading handle and spin to ensure free
rotation.
2. Place on magazine post and rotate slowly to test for
diameter.
3. Examine separating pegs to insure that none are broken
or loose.
4. Examine aluminum center to insure that aluminum lip
is not bent.
5. Test magazine catch to insure its working freely.
b. To fill a magazine
1. Place the magazine bottom upwards on a fiat surface.
2. Insert the loading handle and rotate the center block,
placing the cartridges horizontally in succession between the sep-
arating pegs in such a way that the lip of the bullet groove en-
gages them and leads them to place.
3. Care should be taken not to leave an empty space; an
empty space will cause the gun to cease firing when in action.
Note: When no loading handle is available, the nose of a
bullet may be used as a substitute ; it is a help to place a cartridge
vertically in one of the holes of the center block of the magazine.
4. Ammunition must be tested.
• a. The testing of ammunition is most important as the passing
of one defective round through carelessness may cause very serious
results when in the air.
b. Each round must be carefully examined for deep set caps, split
or damaged cases.
c. The best test to employ is to drop each round into the breech
of a new Lewis gun barrel in order to see that the cartridge enters
freely, and by removing the striker from the striker post each round
can be passed through the gun and tested for thick rims.
Note: Great care must be taken to insure that the striker Tuis
been removed.
5. Return spring must be left at correct working tension before gun is
put away, for if it is left at too great a tension it is liable to break during
cold weather.
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420 OUNNEBY
6. Ouns must not be left lying about with the magazine on, since
magazine is specially liable to damage.
7. Spare parts must be given proper attention.
a. Spares should be checked over at least once a week, and in-
spected by the officer in charge of guns.
b. After checking they should be wrapped in greased paper and
returned to the receptacles provided.
c. On no account should they be left lying loose in the large boxes.
d. Every gunner should assure himself that the spare parts pro-
vided fit his gun properly ; e. g., feed arm actuating studs may not
be interchangeable.
e. In view of the fact that some little time is always needed to
get a machine away from the ground, it will, as a rule, not be neces-
sary to keep guns moimted on machines, or standing in arm racks.
They should be kept in their boxes until wanted for use.
8. Weekly inspection of guns is necessary.
a. Guns and magazines, like spares, should be inspected by the
officer in charge of guns at least once a week.
b. Every gun should be stripped and the parts be laid out for
inspection. Consequently the gas cylinder must be removed; the only
other occasion upon which this should be done is when the gun hais
fired a total of over 500 rounds.
7. STANDARD TESTS OF STRIPPINCI^LEWIS OXTN
The following tests are designed to make the student skillful in adjusting
the return spring tension and in replacing broken parts of the Lewis Gun. Con-
tinued practice of the pre-arranged actions will soon enable the student to
perform these important operations quickly and automatically.
In these tests only wooden bullets or dummy cartridges of a distinctive
shape should be used. Necessary spares and a dummy cartridge should be
on table beside moimted gun equipped with deflector bag. Cadet must begin
each test anew if he makes an error.
It is assumed that initially the cocking handle is forward and that the gun
is free of cartridges in all of these tests.
A. INCREASE RETURN SPRING TENSION
1. Remove magazine
2. Remove spade grip
3. Holding pinion casing in place, release pistol grip
4. Pull cocking handle back
5. Drop pinion casing, disengaging pinion from rack
6. Push cocking handle forward
7. Mesh pinion with rack
8. Push pistol grip forward
9. Measure return spring tension
10. Replace spade grip
11. Replace magazine
12. Pull back charging handle (C. H.)
13. Aim
14. Press trigger
B. DECREASE RETURN SPRING TENSION
1. Remove magazine
2. Remove spade grip
3. Holding pinion casing in place, release pistol grip
4. Drop pinion casing, disengaging pinion from rack
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OUNNBBY 421
5. Pall cocking handle back
6. Mesh pinion with rack
7. Push pistol grip forward (Cocking handle will snap forward)
8. Measure return spring tension
9. Replace spade grip
10. Replace magazine
11. PuU back C. H.
12. Aim
13. Press trigger
C. CHANGE CARTRIDGE GUIDE
1. Remove magazine
2. Remove cartridge guide
3. Insert new cartridge guide
4. Put on filled magazine
5. PuU back C.H.
6. Aim
7. Press trigger
D. CHANGE BOLT
1. Remove magazine
2. Remove spade grip
3. Holding pinion casing in place, release pistol grip
4. Drop pinion casing
5. Pull back C. H.
6. 'Remove C. H.
7. Withdraw piston rod far enough so that bolt can be removed
8. Put new bolt on striker post
9. Push forward on bolt
10. Insert C. H.
11. Push C. H. forward
12. Mesh pinion with rack
13. Push pistol grip forward
14. Replace spade grip --
15. Put on fiOUied magazine
16. Pull back C. H.
17. Aim
18. Press trigger
B. CHANGE EJECTOR
1. Remove magazine
2. Remove spade grip
3. Remove body cover
4.. Pull back C.H.
5. Put feed arm to right
6. Remove ejector cover
7. Remove ejector
8. Slip in new ejector
9. Replace ejector cover
10. Put feed arm to left
11. Ease C. H. forward
i2. Replace body cover
13. Replace spade grip
14. Put on filled magazine
15. Pull back C. H.
16. Aim
17. Press trigger
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422 OUNNEBT
P. CHANGE PINION CASING COMPLETE
1. Remove magazine
2. Remove deflector bag
3. Remove spade grip
4. Holding pinion casing in place, release pistol grip
5. Remove pinion casing complete
6. Replace new pinion casing complete, having return spring at cor-
rect tension
7. Mesh pinion with . rack
8. Push pistol grip forward
9. Replace spade grip
10. Replace deflector bag
11. Put on soiled magazine
12. PuUbackC.H.
13. Aim
14. Press trigger
8. RANGE PRACnOE— LEWIS OUN
A.* PRACTICES
The purpose of range practice is to teach the cadet ''Drill'', to hold the
gun properly, to sight accuratdy, and to group his shots correctly.
There are three practices; Drill, Grouping, and Application.
1. Practice No. I, driU
Range, 25 yards
Six magazines, each with one round
Six-inch group to be obtained
Follow instructions for "Drill"
2. Praetiee No. n, gronping
Range, 25^ard8
Three magazines each with ten rounds
Three six-inch groups to be obtained
Aim respectively at flrst, second, and third marhs from left of
target
3. Practice No. m, application
Range, 25 yards
Three magazines; one with ten rounds, two with twenty rounds
each
Five six-inch groups to be obtained
Fire as follows
1. Ten rounds in flrst magazine at flrst mark from left of
target
2. Ten rounds from second magazine at second mark from
left of target
3. Ten rounds from second magazine at third mark from
left .of target
4. Ten rounds from third magazine at fourth mark from
left of target
5. Ten rounds from third magazine at flf th mark from left of
target
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OUNNEBY 423
B. RANGE DISCIPLINE
1. Duties of cadeti
Number I cadet is in charge of the squad and is responsible for
the actions of each cadet in the squad. He transmits all instructions
from the officer in charge to his squad, and assumes the duties
of each cadet in the squad when the latter is firing.
Number n cadet records the number of the gun and number of
rounds fired by each cadet, scores all shots, and notes all stoppages.
Number m cadet superintends the filling of magazines and in-
spection of ammunition. He sees that all maga2dnes together with
any live rounds, are returned to the munitions room. He sorts any
live rounds from the empty cases, and puts the latter into a proper
receptacle.
Number IV cadet takes charge of the gun.
Number V cadet takes charge of the tai^et.
Number VI cadet prepares the cleaning materials for use and
superintends the cleaning of the gun. He runs an oily rag through the
barrel of the gun at the immediate close of range practice before re-
turning to the cleaning room.
2. Procedure
The procedure of range practice shall be as follows:
The targets having been set, the guns are mounted on the tripods,
muzzles down. When the first whistle is blown by the officer in charge
each gunner takes his seat and performs his practice, all the other men
of each squad remaining behind their gun. At the conclusion of the
practice, each gunner tmloads and leaves his gun with muzzle down.
At the second whistle all gunners leave their seats. At the third
whistle the targets may be examined.
No disobedience of these orders will be tolerated.
9. NOMXNOLATUBE OF MABLIN AIBORAFT OUN
A.
LOOK CONTAINER
1.
2.
8.
Lock container screws
Hammer screw
Hammer
a. Sear and trigger notch
4.
5.
6.
7.
8.
Hammer spring
Hammer spring guide
Sear and trigger pin
Trigger
Sear with sear spring
B.
TRIGGER SPRING
C.
RECEIVER PLUG
D.
BOLT PIN RETAINER
E.
BOLT PIN
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OUNNEBY 425
P. BOLT
1. Cam slot
2. Firing pin
3. Firing pin spring
4« Firing pin retaining pin
6. Shell extractor
6. Shell extractor spring
7. Shell extractor retaining pin
8. Triangolar fin
G. SIDE PLATU REAR SCREW
H, SIDE PLATE FRONT SCREWS (2)
I, LEFT SIDE PLATE
1. Feedway
2. Bnllet guide
J. TBIP
L Nose
2. Ann
8. Head
K. RIGHT SIDE PLATE
1. Charging slide and handle
2. Charging slide retaining lug
3. Char^g slide guide
4. Charging slide spring
5. Inspection opening
6. Beltway
7. Inspection opening cover
8. Inspection opening cover cartridge guide
9. Inspection opening cover retaining spring
10. Feed lever
11. Ratchet lever
12. Ratchet lever pawl
18. Feed throw-off
L. BOTTOM PLATE
1. Feed wheel
2. Feed wheel supports (2)
3. Cartridge stop
4. Feed wheel dog
5. Feed wheel dog spring
6. Feed wheel shafts (2)
7. Feed wheel shafts spring
8. Belt support
9. Cartridge extractor cam
10. Cartridge extractor cam seating
11. Cartridge extractor cam spring
12. Carrier stop
a. Carrier stop screw
b. Carrier stop spring
13. Rear mounting bracket
J
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426 aUNNEBY
M. CARKIERPIN
N. CARRIER
1. Carrier dog
2. Carrier dog pin
3. Carrier dog plunger
4. Carrier dog spring
5. Carrier dog spring guide
0. GAS CYLINDER
1. Gas cylinder bracket
2. Gas cylinder block
a. Positioning lugs
8. Gas adjuster
4. Channd
P. BARREL
1. Gas port
2. Shell extractor clearance slot i
3. Bullet guide
Q. SLIDE
1. Cartridge extractor
2. Cartridge guides (right and left)
3. Cartridge extractor spring
4. Carrier cam notch
5. Slide cam groove
6. Feed lugs (front and rear)
7. Piston rod lock pin and spring
R. ACTION SPRING GUIDE AND BELT GUIDE
S. ACTION SPRING
T. PISTON ROD
1. Piston
2. Balance block
U. BUFFER BLOCK AND SPRINGS
1. Buffer block pin
V. RECEIVER
. 1. Receiver plug lock-latches (2)
2. Receiver plug lock-latch spring jcatch
3. Ejection opening
4. Hammer way
5. Trip way
6. Barrel lock pin and spring
7. Bolt way
a. Locking recess
b. Ejector
c Ejector spring
d. Ejector plunger
e. Cartridge retainer
f. Chamber guide
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ODNNEST 427
W. TOOLS
1. Gas adjuster wrench
2. Combination barrel and action spring tool
10. STBIPPINO AND ASSEHBUNG— MABUN AIBORAFT GUN
A. STRIPPING
1.
Bemove lock container
2.
Bemove trigger spring
3.
Remove receiver plug
4.
Draw back charging slide
5.
Remove bolt pin retainer, bolt pin, and bolt.
6.
Release charging slide
7.
Remove left side plate
8.
Remove trip
9.
Remove right side plate
10.
Remove bottom plate
11.
Remove carrier pin and carrier
12.
Remove gas cylinder
13.
Remove barrel
14.
Remove slide
15.
Remove action spring goide and action spring
16.
Remove piston rod from slide
17.
Remove buffer block
B. ASSEMBLING
The gun is assembled by reversing the operations given above for stripping.
C. STBIPPING AND ASSEMBLING GROUP PARTS OP THE MARLIN
AIRCRAFT GUN
1. To strip the lock container
a. Release hammer hj pressing trigger
b. Remove hammer screw, holding f^gers on hamn^er
c Remove hammer, hammer spring, and hammer spring guide
d. Remove sear and trigger pin
e. .Remove sear and trigger
2. To assemble the lock container
a. Replace sear and trigger
b. Replace sear and trigger pin
c. Put hammer spring on hammer spring guide; start hammer
spring guide into position ; place hammer against hammer spring guide
(sear and trigger notch to rear) ; push hammer into position; insert
hammer screw
d. Screw hammer screw into position
e. Press trigger to raise hammer spring guide into position; cock
hammer
8. To remove feed wheel from bottom plate
a. With drift, push in on front feed wheel shaft, lifting front
end of feed wheel slightly
b. With drift, push in on rear feed wheel shaft, lifting rear end
of feed wheel slightly
The feed wheel shafts are now held in against the action of the
feed wheel shafts spring by the feed wheel supports
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428 C^UNNEBY
c. Lift up feed wheel with right hand keeping both ^nds of feed
wheel covered with left hand so as to catch feed wheel shafts as
th^r are forced out.
d. Bemove feed wheel shafts and feed wheel shafts spring
4. To assemble feed wheel to bottom plate
a. Insert feed wheel shafts spring and feed wheel shafts into
feed wheel
b. Place feed wheel between feed wheel supports, ratchet end to
front
c. Compress feed wheel shafts until shoulders engage between
the feed wheel supports
d. With drift push in on front feed wheel shaft, pushing down
front end of feed wheel slightly until this shaft is engaged behind
front feed wheel support
e. With drift push in on rear feed wheel shaft, pushing down
rear end of feed wheel slightly until this shaft is engaged in front
of the rear feed wheel support
f . Push down on both ends of feed wheel evenly until feed wheel
shafts spring into place
6. To remove firing pin from bolt
a. Holding bolt in left hand, press in on rear end of firing pin
with index finger
b. With drift remove firing pin retaining pin
c. Let firing pin spring extend gently
6. To assemble firing pin to bolt
a. Holding bolt in left hand with rear fin up, press firing pin and
firing pin spring into position, notch on firing pin up.
b. Replace firing pin retaining pin from right
7. To remove shell extractor from bolt
a. Holding bolt in left hand with shell extractor up, face of bolt
to front, press down on rear of shell extractor with thumb
b. With drift remove shell extractor retaining pin by pushing
f roiji right
c. Shell extractor and shell extractor spring will fall ofiF
8. To assemble shell extractor to bolt
a. Holding bolt in left hand with extractor seating up, face of
bolt to front, press shell extractor spring and shell extractor into
position
b. Replace shell extractor retaining pin from left
9. To remove action spring from slide
a. With slide vertical, piston resting on table, press down with
left thumb on belt guide and push belt guide clear of cartridge extractor
b. Grasp belt guide with right hand, thumb on top, and allow
action spring to extend gently
c. Withdraw action spring
10. To assemble action spring to slide
a. Hold slide vertical in left hand, cartridge extractor to left,
piston pressing on table or fioor
b. Insert action spring and action spring guide into piston from
right
c. Place combination barrel and action spring tool on belt guide
d. Guiding action spring with left hand, compress action spring
with right hand until belt guide comes into position
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GUNNERY
429
I
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IL To r«moye pifton rod from dide
a. Remove action spring from slide
b. Holding back pistoti rod lock pin with screw-driver, unscrew
piston rod from slide
13. To assemble piston rod to slide
a. Screw piston rod into slide, holding back piston rod lock pin
with screw-driver. Screw piston rod tight and then unscrew until
piston rod lock pin engages in first notdu
D. POINTS TO BE CAREFULLY OBSERVED WHEN STRIPPING OB
ASSEMBLING THE MARLIN AIRCRAFT GUN
1. Always cock the lock before removing it from gun.
2. When removing the lock container screws remove the front screws
first because of the action of the trigger spring in forcing the rear of the
lock container up
3. Always remove action spring guide and action spring from slide
as soon as slide is removed from receiver
4. When assembling, never hammer or force a screw that does not
start properly
5. When placing slide into receiver care must be exercised not to
allow the belt guide to be too low to engage in the dove-tailed slot in re-
ceiver, as this will prevent cartridges from being fed properly against the
cartridge stop by the feed wheel
6. When assembling right side plate to receiver, be sure that feed
lever is at extreme forward position so that it lies between the front and
rear feed lugs on the slide
7. Be very careful when screwing barrel into receiver to insure proper
alignment of tiireads. Barrel should be screwed fully home, and locked.
8. When replacing lock container, hammer should be in cocked
pocdtion
9. Replace rear lock container screws first
10. After assembling gun, always pull charging slide to rear and
release it.
11. HECHANISH OF THE MARLIN AntGRAFT GUN
A. BACKWARD MOVEMENT
1. Firing the First Shot
When the trigger is depressed it is disengaged from the sear and
trigger notch of the hammer. The hammer spring guide, actuated
by the hammer spring and bearing against a grooved notch in the
hammer, forces it forward. The hammer strikes the rear end of the
firing pin, forcing the front end through the striker way, priming the
cartridge. (See plate 10)
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432 OTJNNEBY
2. Action of the Gasei
When the bullet passes the gas port, a portion of the powder gases
escape through the gas port down the gas channel into the gas cylinder
and impinge against the head of the piston, forcing it with the slide
to the rear, compressing the action spring.
3. Backward Action of the Slide and Bolt
As the slide moves to the rear, the bolt pin, acting in the cam
slot in the rear guide fin of the bolt, raises the rear end of the bolt
out of the locking recess in the receiver. The nose of the trip, under
pressure of the sear spring at its head, rides up into the slide cam
groov§i thus thenose of the sear is lowered ready to engage in the sear
and trigger notch of the hammer. The bolt moving to the rear pushes
b^ck-the hammer and rides beneath it, compressing the hammer spring.
As the slide passes to the rear, the rear end of the slide flanges strikes
the front end of the carrier dog, forces it back compressing the carrier
dog spring, and rides above it. The carrier dog spring then forces the
carrier dog into the carrier cam notch of the slide.
4.\BackwiRrd Action of the Shell Extractor and the Cartridge
Extractor
As the bolt moves to the rear, a shell is . withdrawn 'from the
chamber by the shell extractor and is carried! to the rear until its
base strikes the stud on the ejector, when it is thrown out through the
ejection opening. The ejector spring and plunger cushion the blow on
the ejector. In the meantime the cartridge extractor has withdrawn
the next cartridge from the bdt and carried it back on the carrier,
guided by the cartridge guides and the inspection opening cover
cartridge guide. The backward motion of the slide is arrested by the
buffer blodc and springs.
5. First Action of the Feed Lever
As the slide moves backward the front feed lug forces thik feed
lever to the rear. The ratchet lever is thereby raised until the ratchet
lever pawl rides over the next tooth of the feed wheel and engages
behind it. The feed wheel dog prevents counter-clockwise rotation of
the feed wheel. j
B. FORWARD MOVEMENT
1. Action of tiie Carrier
The force of the gases now being expended, the action spring begins
to extend, forcing the slide forward. As the slide moves forward, the
carrier dog riding in the carrier cam notch on the bottom of the slide
flanges, and acting as an integral part of the carrier, depresses the rear
end, and it being pivoted, elevates the front end of the carrier.
Thus the carrier cSsengages the cartridge from the cartridge extractor
and carries it up against the guide shoulders below the boltway. The
upward motion is limited by the carrier stop upon which the rear end
of the carrier strikes. The carrier stop spring acting on the rear end
of the carrier, forces the front end of the carrier down as soon as
the slide moves clear of the carrier dog. The cartridge, being pushed
forward by the bolt, is then supported at its rear end by the cartridge
retainer and the sides of the receiver. (See plate 11)
2. Second Action of the Feed Lever
As the slide moves forward, the rear feed lug forces the feed
lever forward. The ratchet lever is thereby forced down, rotating the
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feed wheel one tooth clockwise (from gunner) thus bringing the next
cartridge into position in the feedway against the cartridge stop. At
the same time the feed wheel dog rides over the next tooth of the feed
wheel and engages behind it.
8. Forward Aetton of the Slide and Bolt
As the bolt moves forward, the hammer is forced down by the
hammer spring guide, actuated by the hammer spring, until the nose
of the sear engages in the sear and trigger notch in the hammer.
The triangular fin, on the front of the bolt, strikes the rim of
the cartridge, supported by the cartridge retainer in front of the
bolt, and forces it into the chamber; the nose of the bullet is guided
into the chamber by the chamber gi^ide.
After the bolt reaches its forward position, the bolt pin, riding
in the cam slot of the rear guide fin of the bolt, draws the rear end
of the bolt down into the locking recess in the receiver, thus locking
the breech. After the breech is locked, the nose of the trip rides out
of the dide cam groove. This causes the head of the trip to move for-
ward, and working against the arm of the sear, disengages the nose
of the sear from the sear and trigger notch in the hammer. There-
upon the hammer moves slightly forward and is held back only by
the nose of the trigger which tiien engages in the sear and trigger
notch in the hammer.
4. Forward Action of the Shell Extractor and the Cartridge
Extractor
As the bolt reaches its forward position, the shell extractor springs
over and grips the rim of the cartridge in the chamber.
As the slide nears its forward position, the cartridge extractor
strikes the base of the next cartridge in the belt, and is forced down
against the cartridge extractor cam thus causing the cartridge ex-
tractor claw to grip the rim of the cartridge.
The forward motion of the slide is luooited by the piston rod bal-
ance block striking the rear end of the gas cylinder block.
13. DBni^KABUN AIBOBAFT GUN
Drill consists in
A. Loading
B. Firing
C. Unloading
A. LOADING
Insert belt in the feedway from left until first cartridge is engaged in
feed wheel. Draw charging slide to rear and release. Draw charging dide
again to rear and release. The gun is then ready for automatic fire.
B. FIMNG
The gun is fired only by synchronized gear trigger trippers which actuate,
according to the type of gear, either the tail end of the trigger or the vertical
projection to be seen above the lock container. After the first shot the gun
is automatically fed.
C. UNLOADING
Draw charging slide to full cock, ejecting live round from chamber. Push
forward on feed throw off and withdraw the belt, then allow charging slide
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434 OUNNEBT
to go forward quickly. The live round in the chamber may then either be fired
by depressing the trigger or ejected by drawing the charging slide to rear.
As a safety precaution this action is repeated.
13. STOPPAGES AND JAMS— MABUN AIBORAFT OUIT
If the Marlin Aircraft gun is properly cleaned, oiled, inspected, assembled,
and adjusted many defects in operation may be prevented.
A. DEFECTS PREVENTED BY PROPER CLEANING AND OILING
1. Excessive* friction caused by too little oil or too much oil (con-
gealing)
2. Clogged gas port
B. DEFECTS PREVENTED BY PROPER INSPECTION OF PARTS
1. Excessive friction due to burrs
2. Worn or broken parts
C. DEFECTS PREVENTED BY PROPER ASSEMBLY OF GUN
1. Belt guide too low to engage properly in dove-tailed slot in re-
ceiver, which will prevent cartridge from being fed properly against car-
tridge stop.
2. Barrel improperly assembled to receiver, resulting in lack of align-
ment between shell extractor and shell extractor clearance slot; also be-
tween gas port and gas channel.
3. Feed lever not between feed lugs, which will be detected when
applying ''Immediate Action" after assembly.
4. Improperly assembled lock container which will prevent gun from
firing.
D. DEFECTS PREVENTED BY PROPER ADJUSTMENT OF GUN
1. Defective trip : To test the trip
a. Use the no irvp gauge to determine if the sear is engaged be-
fore the breech is locked. Draw charging slide to the rear, hold the
no trip gauge against the gas cylinder block (fillet to rear), and re-
lease charging slide quickly. The trigger should not be engaji:ed. If
it is, it means there would be a premature priming of the cartridge. If
it is engaged, the head and nose of the trip are too close and must
be separated a short distance.
b. Use the trip gauge to determine if the sear is disengaged
by the trip after the breech is locked. Draw charging slide to center
notch, hold the trip gauge against the gas cylinder block (fillet to rear),
and release charging slide quickly. The trigger should now be en-
gaged, otherwise the gun would not fire. In case the trigger is not
engaged, the head and nose of the trip are too far apart and must be
bent closer.
c. After any change is made in the trip, the no trip gauge must
be used followed by the trip gauge, until both fulfill their purpose.
2. Gas adjuster unscrewed too far.
E. DEFECTIVE AMMUNITION ELIMINATED BY TESTING AND IN-
SPECTING
1. Deep set caps, high primers, split cases, thick rims (eliminated by
inspection).
2. Bulged rounds, (eliminated by dropping rounds into a new gun
barrel).
F. DEFECTS PREVENTED BY PROPER LOADING OF BELTS
1. Empty space in belt
2. Rounds out of line in belt
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G. STOPPAGES AND JAMS
The principal causes of the Marlin Aircraft Gun stopping when in action
are divided into three positions.
1. Poittion I Bolt forward and breech locked, or charging slide
is held by slide shonlder.
''Immediate Action '^ Draw charging slide to rear, release, and
carry on.
a. If automatic fire occurs, the stoppage was due to an insuf-
ficient charge resulting in very little backward action.
b. If one shot is fired and gun stops, examine for
1. Excessive friction
2. Incorrectly adjusted gnn
c. If gun does not fire after applying ''Immediate Action'
at least twice, examine for
1. Defective trip mechanism
2. Defective lock container mechanism
3. Defective feed wheel mechanism
4. Defective firing pin ; bolt must be changed. To change
bolt
1. Bemove belt
2. Bemove receiver plug
3. Draw charging slide to rear
4. Bemove bolt pin retainer and bolt pin
5. Insert drift from front end of lock container under
hammer spring guide. This will hold hammer up while
changing bolt.
6. Bemove bolt
7. Beplace new bolt
8. Bemove drift
9. Beplace bolt pin and bolt pin retainer
10. Belease charging slide
11. Beplace receiver plug"
12. Apply "Immediate Action"
2. Position II Bolt not more than one-third of the way back, or
charging slide is free for a distance of about 1% inches. Apply "Imme-
diate Action".
a. If automatic fire occurs the stoppage was due to an insuf-
ficient charge resulting in enough backward action to withdraw car-
tridge from belt but not enough to operate carrier.
b. If automatic fire does not occur, examine for
1. Cartridge in feed wheel jammed against cartridge stop.
2. Defective cartridge extractor mechanism
3. Obstruction in chamber such as a primer or a piece of
a shell.
3. Position m Bolt in back of Position II, or charging slide is free
for a distance greater than 1^ inches.
Apply "Immediate Action".
a. If automatic fire does not occur, examine the ejection opening.
1. If there is an empty case in chamber, the jam is due
to a broken shell extractor or broken shell extractor spring.
2. If there is an empty case in the boltway and a cartridge
jammed beneath it, the jam is due to a broken ejector stud.
3. If there is a cartridge on the carrier which is just vis-
ible in the ejection opening, the jam is due to a solid cartridge
retainer.
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436 31INNEBT
4. If there is a cartridge in the boltway projecting par-
tially into the chamber, the jam is due to a ruptured case re-
maining in the chamber.
5. If the slide cannot be moved, the bolt pin is partially out.
6. Examine also for
a. Defective carrier dog mechanism
b. Defective chamber cartridge guide.
H. UNCONTROLLED AUTOMATIC FIRE
Pull back charging slide and lock it.
1. Cause: defective trigger mechanism
I. REPORTS OP STOPPAGES AND JAMS
Whenever the gun is used in the air, gunner must report all stoppages
and jams, position of bolt, ''Immediate Action" applied, and whether the
trouble was remedied and how.
14. OABEOFTHEHABLINAIBOBAFTOnN
A. CLEANING
The gun muts be completely stripped and thoroughly cleaned after
every firing in the same manner as the Lewis Gun. All working parts should
be slightly oiled except the face of the bolt and the piston which should
not be oiled. The slide should be well oiled.
B. POINTS TO BE OBSERVED BEFORE PLIGHT
1. The gun must be completely stripped, cleaned, inspected, oiled,
and assembled. Remove the oil from the barrel bore and see that there
is no obstruction in it, that it is dry and that the chamber is dry.
2. Screw gas adjuster fully home.
3. See that mounting pins and bolts are securely fastened
4. Securely fasten feed box and deflector box
5. Securely fasten trigger motor
6. Pill and "time** C. C. gear
7. Harmonize ring sights
8. Fire a burst of twenty
9. Unload gun, then reload so that there is a cartridge gripped by
the cartridge extractor but none in the chamber. Finish loading when
out of the airdrome.
C. POINTS TO BE OBSERVED DURING PLIGHT
1. Reservoir handle must be kept up.
2. On cold days or at high altitudes, fire burst of ten every ten
minutes.
3. After each burst, determine if breech is closed by pulling on
charging slide.
D. POINTS TO BE OBSERVED AFTER FLIGHT
1. Unload gun
2. Have gun taken to gun shop and thoroughly cleaned and in-
spected.
a. Replace any damaged part, and ''time" it with the mech-
anism.
b. Leave a thin film of oil in bore of barrel
c. If gun is not used regularly, barrel should be cleaned every
day for ten days.
3. Test the trip
4. Give a detailed account of guns' behavior in the air to armorer.
5. Test ammunition as directed for the Lewis Gun, except that
rounds should not be run through the Marlin Aircraft Gun.
6. Fill belts.
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a. Belts shooMbe dean and reasonably stiff; do not use belts
after the pockets have become so large that the cartridges fit into
them loosely.
b. When filling belts, nse the belt loading machine and ex-
amine for
1. Empty space in belt
2. Rounds out of line in belt
15. AERIAL SIGHTS
A. PURPOSE
The purpose of special sights for aerial gunnery is to compensate auto-
maticalfy for gunner's speed and for enemy's speed so that gunner sights di-
rectly on target (which usually is the enemy pilot).
B. TYPES
L For a fixed gun— The Marlin Aircraft Oun.
a. Ring foresight with fixed bead back sight
' b. Ring backsight with fixed bead foresight
i* Aldis optical sight
2. For a movable gun— The Lewis Aerial Qun.
a. Ring backsight with a Norman Wind-Vane foresight
b. Al^ optical sight with special mountings and gears
C. RING SIGHTS
1. Description Two concentric rings (the outer ring being of defi-
nite diameter) mounted on a post and sometimes connected by four radial
wires (See Fig. 401)
2. Purpose To compensate for the enemy's speed. (See Fig. 402)
3. Factors determining the diameter of ring
a. The enemy's speed
b. The distance between gunner's eye and ring sight
c Velocity of bullet
4. Common sizes of ring sights
a. For a fixed gun toifk fixed head hackgigkt
1. Enemy's speed, 100 miles per hour
2. Distance from gunner's eye to ring sight, 38 in.
3. Diameter of ring, 5 inches
h. For a movable gun unth Norman Wind-Vane foresight
1. Enemy's speed, 80 miles per hour, 1% in. diameter
2. Enemy's speed, 100 miles per hour, 2 5/16 in. diameter
3. For both of these sizes, distance from gunner's eye to ring
sight is 19 inches i
4. Enemy's speed 110 miles per hour, 2% inches; distance
from gunner's eye, 18 inches
5. The independence of range
D. PRINCIPLES GOVERNING USE OF RING SIGHTS ON A FIXED
GUN WHEN USED EITHER
1. As a backsight with fixed bead foresight
2. As a foresigKt with fixed bead backsight
a. Align the eye so that the bead appears to be in the center
of the ring sight.
b. The eye must be held at the distance in back of the ring sight
for which the ring sight was designed.
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Line Ot'
F//ghf
/fed Bead
3i<fht
E.
Fig. 401. — Aerial Ring Sight
c. The enemy airplane must be placed in the ring so that it is
flying towards the center of the ring. (See Fig. 403)
d. An enemy airplane flying at right angles to the line of fire,
at the speed for which the ring is designed, must be placed on the
outer ring with the pilot cutting the ring.
e. An enemy airplane approaching the line of fire obliquely must
be placed in a position suggested by its foreshortening. The more the
body of the airplane is foreshortened, the nearer it must be placed to
the vertical diameter of the ring sight.
USE OF RING FORESIGHT WITH BEAD BACKSIGHT ON A FIXED
GUN
1. Under conditions for which the ring sight is designed.
2. At variable speeds.
3. As a range finder.
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GUNNBBY 439
P. THE NORMAN WIND-VANE FORESIGHT
1. Deicription A bead sight connected by levers to a wind vane
which, when the airplane is moving, is blown badk in opposite direction to
the line of flight thus causing b^ sight to place itsdf ahead of gan«
(See Fig. 404)
2. Purpose of Wind-Vane foresight To compensate for gunner's
speed (See Fig. 405)
3. Factors determining the dimensions of Wind-Vane foresight
a. Speed of Gunner
b. Distance between the wind-vane foresight and ring badosaght
c. Velocity of bullet ,
4. Oommon sixes of Wind- Vane foresights
a. Speed of gunner, 80 miles per hour
b. Speed of gunner, 100 miles per hour
c Both of these foresights are to be set 18 inches ahead of ring
backsights
G. USE OF NORMAN WIND-VANE FORESIGHT WITH RING BACK-
SIGHT ON A MOVABLE GUN
1. Wind- Vane foresight compensates for gunner's speed. (See
Fig. 406)
2. Ring backsight compensates for enemy's speed
3. Gun is placed according to instructions regarding ring sights
(See par. D above)
H. COMPENSATION FOR DEPRESSION AND ELEVATION OF MOV-
ABLE GUN
1. ' If gun is depressed, bead on Norman Wind-Vane sight will rise
through wind pressing on upper surface of the Vane B, Fig. 404.
2. If gun is elevated, beiad on Norman Wind-Vane sight will fall
3. (1) and (2) apply when gun is pointed in direction of flight
When gun is pointed in opposite direction to flight, reverse is true.
I. PRACTICE WITH THE MODEL AIMING AIRPLANE, USING A
DUMMY GUN EQUIPPED WITH RING BACKSIGHT AND
FIXED BEAD FORESIGHT
1. Aim by lining up the center of the ring backsight, the bead fore-
sight, and the ball on the aiming rod of the airplane. Note the position
of the airplane in the ring back sight
2. With the aiming rod removed from the Model Aiming Airplane,
aim the gun by following the instructions for aiming with the ring and bead
sight, (See par. D above). Check the aim by inserting the aiming rod
into the Model Aiming Airplane, and noting the distance the ball of the
aiming rod departs from an imaginary line extending from the center of
the ring through the bead of the foresight. This distance indicates the
error.
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440
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C-D-Path of Aeroplane
r- fiitf^ rores/9^r
X-DcotifSacA Sight
Fig. 403.-DIAQBAM of SioHnNO
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442
GI7NNEBT
U-s"
Fig. 404. — ^Norman Wind- Vane Fqresioht
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CHAPTER V.
INSTRUMENTS
OUTLINE
1. Instruments for Determining Direction
A. Gyroscopic Compass
B. Magnetic Compass
1. Description
2. Variation
3. Deviation
4. Computation of Compass Course
5. Errors of Compass
C. Drift Meter
2. Instruments for Determining Altitude
A. Altimeter
1. Description
2. Errors and Corrections
B. Barograph
3. Instruments for Determining Speed Through the Air
A. Air Speed Indicator
B. Tachometer
4. Instruments for indicating Stability
A. Inclinometer
B. Angle of Incidence Meter
C. Bai^dng Indicator
5. Instruments for Determining Ground Speed
A. First Method
B. Second Method i
C. Third Method
6. Special Instruments used in Testing
A. Climbing Meter
B. Statoscope
7. Miscellaneous Instruments and their Specifications
Oil Gauge, oil-pressure gauge, gasoline supply gauge, gasoline-flow in-
dicator, and radiator temperature indicator.
£01
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CHAPTER V.
INSTRUMENTS
1. INSTBUHENTS FOB DBTERMININO DIBEOTION
A. GYROSCOPIC COMPASS
The gyroscopic compass works on the principle of the gyroscope. It con-
sists of a comparatively heavy fly-wheel whose axis is mounted as the diameter
of a circular rim, which in turn is mounted in gimbals so that when the wheel
is rotated rapidly by electrical energy the axis will always point in tlie same
direction regardless of the movements of the airplane in which the compass
is mounted. The axis of the wheel acts as the reference line, always constant
in direction, which is the essential feature of any direction instrument. There
is also a circular scale by means of which the direction of the airplane with
respect to the reference line is determined.
The direction of the axis of the rapidly rotating wheel will not be changed
by any motion of the base, but there is a force which will slowly pull the wheel
into a position of rotation parallel to the earth's rotation. Thus for use as a
compass, the axis at starting should lie in a north and south vertical plane.
The gyroscopic compass is the most satisfactory compass yet devised, but
owing to its weight, bulk, and cost is not used to any extent.
B. MAGNETIC COMPASS
1. Deioription The magnetic compass consists of: a glass-covered
hemispherical bowl ; a circular card with a central jeweled cap resting lightly
on an iridium-pointed pin fixed perpendicular to the bottom of the bowl;
magnetic needles fixed parallel to each other on the lower surface of the
card ; a pointer or lvi)ber line ; and a corrugated expansion chamber at the
bottom of the bowl which allows for changes in the volume of the colorless
liquid which fills up the bowl. The liquid is a mixture of alcohol and
water. The card is of mica, and its upper surface is marked in degrees in
a clockwise direction. The plane of the card remaining always horizontal,
its point of support should be so low that its edge will not touch the glass
top when the compass is tilted through a considerable angle, otherwise it
will be necessary to mount the entire instrument in gimbals to keep it level.
The lubber line is simply a pointer on the bowl adjacent to the edge of
the card. The compass is mounted so that a line joining the center of card
to the lubber line either coincides with or is parallel to the longitudinal
axis of the airplane.
2. Variation True north is the direction along the arc of a great
circle towards the north pole of the earth. Magnetic north is the direction
of the pointing of the magnetic needles when mounted as described (the
needles do not point towards the north magnetic pole). The angular dif-
ference between true north and magnetic north is called variation. The
variation at a place can be read from a map showing isogonic lines, i.e.,
lines passing through the points of equal variation. In the United States
the variation ranges from about 25' E to about 20** W. In Belgium and
France the variation is from 11' W. to 14' W.
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INSTRUMENTS 503
8. Deviation When a mass of soft iron is brought near the magnetic
compass, the card is observed to move from its normal position. This angu-
lar change is called deviation. The engine and other metal fittings of an
airplane cause disturbances of this kind The amount of the deviation
will depend on the position- of the engine, etc., with respect to the compass.
It varies for different pointings of the airplane. To determine the amount
of the deviation, the airplane is placed with its axis pointing in a number
of directions of magnetic azimuth (0'', 45"*, 90*", etc.) and the deviations
recorded in a deviation table. This operation is known as ''swinging
the ship'\
4. Ckmipiitation of the compass course The order of procedure in
computing a compass course is as follows :
a. Find the map course On the map draw a line AN from the
starting point A towards true north, and lUso the intended line of
flight AB. The angle measured (with a protractor) clockwise from
AN to AB is the map course or azimuth of AB, or azimuth of B as
seen from A.
h. Correct for variation Find the magnetic variation at the
starting point from an isogenic chart. If the variation is west, add
it to the map course ; if east, subtract it. This gives the magnetic course,
c. Correct for drift due to unnd Draw a line AW (wind from
A to W) on a convenient scale to represent the estimated velocity of
the wind at the ^jing level. With W as the center and a radius equal
to the intended airplane speed, using the same scale as before, strike
an arc cutting the line AB at point C. Through A draw a line AD
parallel to WC. Then CAD is the angle of drift; the machine will
be headed always parallel to AD; AC represents in magnitude and
direction the ground speed of the airplane ; and the angle NAD is the
corrected magnetic course.
d. Correct for deviation The amount of the deviation is found
by interpolating from a table of deviations. Add the deviation if it
is west, and subtract if it is east. The result is the Compass Course.
It is essential that the deviation correction be applied last. The above
steps may be put in the following formula :
Compass Course — Map Course ±i Variation Hh Drift db Devi-
ation
As a rule the return course does not differ from the outward course
by just 180', therefore it must be computed independently.
6. Errors of magnetic compass When flying in a straight line in
fair weather, the compass can be depended on to give the proper direction ;
but on the other hand, if the machine turns about quickly, the compass
may become very unreliable. Especially, if the machine while flying ftt)m
south to north turns to the right, the compass card will almost invariably
turn faster than the airplane. That is, the card will turn to the right,
and will tell the pilot that he is turning to the left, which is not true.
If, starting from that point, he turns to the left, the card will not seem
to turn at all, and after a turn of 90"*, even though the machine at that
moment is directed toward the west, the compass will report it pointing
towards the north. The principal cause of this error is the inclination
the compass card assumes under the influence of centrifugal force or inertia
on turning. This error is called '*the northerly-tum error''.
At a certain speed of the engine, the vibrations are sufficient to pro-
duce turns in the compass card up to QO"". This speed should be known and
avoided when using the compass.
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504 INSTRUMENTS
a DRIFT METER
The drift meter in its simplest form is an open tube mounted rertically
in the airplane and free to move about its vertical axis. Inside the tube as
a diameter there is a wire and on the outside a pointer which moves around a
graduated arc. When the wire is in the longitudinal axis of the airplane, the
pointer indicates zero on the arc.
The instrument is used by turning the tube until the wire is parallel to
the ''stream lines" of objects seen on the ground. The reading of the scale then
gives the drift angle due to the wind. The drift meter and the compass can
be {synchronized so that as the wire of the former is turned through the neces-
sary angle, the lubber line of the latter will be turned through the same angle.
In this way drift is eliminated from the calculations.
2. INSTRUHENTS FOB DETERMININO ALTITUDB
The barometer is an instrument for measuring the pressure of the air.
The atmospheric pressure as indicated by the barometer is a measure of the alti-
tude of the instrument above sea level. Barometers of two kinds are used
in aviation work, namely, the Altimeter and the Barograph.
A. ALTIMETER
1. Description The altimeter consists of a stout metal case con-
taining a small metal box with a thin corrugated top from which the
air has been partially exhausted, a series of levers for magnifying the
motion of the top of the box, and a pointer at the end of the lever sys-
tem which moves around a dial on the outside face of the case. The vary-
ing pressure of the atmosphere acting upon the top of the vacuum box causes
a movement which is transmitted through the levers to the pointer which
indicates on the dial the altitude in feet. The dial plate can be rotated
until the zero is opposite the pointer, as required at the beginning of a flight.
AH altimeters should register up to at least 8000 meters or 26,300 ft
2. Corrections There are four sources of inaccuracy in the readings
of an altimeter. These are (1) lag, (2) temperature, (3) change in ground
elevation, and (4) clymge in surface pressure.
By ^'lag'' is meant the slowness of the instrument in responding to
rapid changes in altitude. An altimeter may be regarded as fairly sat-
isfactory if, after an ascent and descent of 20,000 feet at the rate of 1000
feet per minute, the error on account of lag does not exceed 150 feet. The
extent of this defect depends on the quality of the metal used in the con-
struction of the vacuum box. By the use of hard steel boxes, lag may be
practically eliminated.
The readings of an altimeter may be affected by its own temperature.
The error due to this cause is slight. In an instrument tested recently,
the error was found to be 2.18 feet per 1* F. change in temperature. The
principal source of inaccuracy in the altimeter is the variability of the tem-
perature of the air. The difference in pressure between any two levels
depends on the temperature of the intervening layer of air. The scale of
an altimeter is usually graduated for a uniform temperature of 50"* P., and
when the average temperature of the layer-of air between the ground and the
altimeter is other than 50' F. a correction must be made. For a temperature
of -27* F. at the level of the instrument, the correction to be subtracted
varies from 15% at 1000 ft. to 10% at 20,000 ft. ; for -9' F., the correction
varies from 11% to 7% at the same levels ; for 9' F., from 8% to 3% ; and
for 27** F., from 4% to 0%.
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INSTRUMENTS . 605
The change which takes place in the elevation of the surface may be
obtained from a map and the necessary correction made.
On returning from a long flight, the aviator may find the reading of
his altimeter to differ from the initial reading due to changes which are
constantly taking place in the pressure of the atmosphere. However, ordi-
narily the error due to this cause will rarely exceed 100 feet and may be
neglected.
B. BAROGRAPH
A barograph is a recording altimeter. The pointer of the altimeter is re-
placed by a pen arm which records altitudes on a chart wound round a drum
actuated by clockwork. In this way a record of the altitude at every instant
of flight is obtained which may be studied later on the ground.
3. XNSTBUMBNTS FOB DETEftlONINO SPEBD THBOUOH THE AIB
A. AIR SPEED INDICATOR
The air speed indicator, or buoyancy meter, is a box with two chambers
separated by a diaphragm whose motions are magnified by levers and com-
municated to a pointer which moves around a dial. A tube from one chamber
opens into the wind (dynamic opening), and one from the other chamber ofpexm
into stiU air (static opening). Both openings are contained in a cone-shaped
head which is fastened to one of the forward struts with the dynamic opening
parallel to the longitudinal axis of the airplane. This instrument measures
wind pressures, so that as the density of the air decreases with altitude, the
scale reading is too low, due to the fact that the dial is calibrated at a density
which normally exists comparatively close to the ground. The true air speed may
be obtained from the formula.
True air speed =
. Indicated speed
V Density
The valaes of the densily to be used
. in the above formula are as follows
Altitade above sea level
Percentage of standard density
800 Feet
1.000
3000
.932
5000
.874
8000
.792
10000
.740
13000
.673
15000
.630
18000
.671
20000
.538
TACHOHfETBB
B.
The engine tachometer or revolution counter is of two types. The type
ordinarily used is similar to the spedometer on an automobile and may indi-
cate either revolutions or velocity. The recording instrument on the instru-
ment board is connected with the engine by means of a flexible shaft
The second type makes use of the principle of the electric generator and
ammeter.
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806 . INSTRUMENTS
4. INSTBUMBNTS FOB INDICATINO STABUJTT
A. INCLINOMETER
The inclinometer is either simply a spirit level, or a wheel weighted like
a locomotive driver and having its motions damped by paddles immersed in a
liquid. In either case inclinations with respect to gravity are measured. There
may be two inclinometers in an airplane, one mounted longitudinally and the
other laterally. Accelerations of the airplane cause errors in inclinometers of
these types.
B. ANGLE OF INCIDENCE METER
The angle of incidence meter consists of a cylindrical dial box with a cen-
tral shaft, and, fixed perpendicular to the shaft, a very light tail piece with
supporting fin. This tail piece can move up and down. For velocities neces-
sary to flight, it lies along the direction in which the air is moving. The ratio
between the angular motions of the tail and the dial hand is the same as that
between the angular motions of the hour and the minute hands of a watch. As
the instrument is mounted in a position away from propeller influence and
consequently at a distance from the pilot, the dial numbers (0 to 15) must
be large.
This instrument is independent of gravity and gives the pilot the true angle
between the chord of the planes and the direction of the air in which he is
flying.
C. BANKING INDICATOR
The banking indicator is simply an inclinometer mounted laterally. If of
the wheel type, an arrangement for causing an electric contact and the lighting
of a colored light can be provided for the purpose of showing dangerous angles.
6. XNSTRUMBNTS FOR DETERBONINO OROXTND SPEED
A. FIRST METHOD
If the aviator is familiar with the ground over which he is fljdn^ and knows
the distance between two well defined objects, he can note from his dock the
time it takes him to fly the known distance, and can compute his ground speed.
B. SECOND METHOD
The aviator flrst determines his elevation above the ground using his alti-
meter and the contours on his map, and then flying horizontally as indicated
by his inclinometer he sights along a line of sight fastened to the side of his
plane at a fixed angle with the horizontal axis of the plane at a definite object
on the ground ahead and notes the time it takes to fly directly over the object.
If the Une of sight makes an angle of 45 *" with the horizontal, the distance
traversed is the same as the elevation; and for other angles there is a definite
relation between the distance and the elevation. Knowing the distance and
the time, the ground speed is easily computed.
C. THIRD METHOD
Lieut. Crocco of the French Aviation Corps has recently invented a stadia
device which, used in connection with the altimeter and clock, makes the com-
putation of ground speed a very simple matter. For a detailed description
and illustrations of this instrument, the student is referred to the Jan. 9, 1918,
issue of Aeronautics.
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INSTBUMENTS 507
6. SPBOZAL INSTBUHENTS USED IN TE8TIN0
A. CLIMBING METER
The climbing meter consists of a bottle with a tube of small bore opening
into the air, and a U-tnbe partly filled with liquid and opening into the air and
into the bottle. There is a reading scale on the arm of the U-tube which opens
into the air. While climbing the density and pressure of the air inside the
bottle is greater than that on the outside, and this is shown by the elevation of
the liquid in the outer arm. When the liquid stands highest in the outer arm,
the best climbing angle is being used.
B. STATOSCOPE
The statoscope is an instrument used in testing airplanes to indicate to
the pilot changes in altitude which are too small to be read on the altimeter.
It is very similar to the climbing meter and differs from it only in the
shape and position of the glass tube attached to the outside of the botUe. The
tube is bent to the arc of a circle of large radius and is mounted horizontally
on the bottle with the convex side down — just opposite to the mounting of a
spirit level tube. Inside the tube there is a drop of liquid, and one end of
the tube enters the bottle and gives bottie-pressure and the other end is open
to the air and gives air pressure. For horizontal flight, gravity holdn the drop
of liquid to the bottom of the tube, but for slight changes of altitude the drop
is forced to one side or the other depending on whether the bottie-pressure is
greater or less than air-pressure.
7. HISOELLANEOnS INSTBUMENTS AND THEIB SPEOIFIOATIONS
The oU gauge should indicate definitely the amount of oil in the crank case,
and the oUrpressure gauge should indicate the pressure in the oil system and
that the fiow is undisturbed.
The gasoUne-supply indicator should preferably be of the mechanical type,
the gasoline-flow indicator should show that gasoline is being supplied to the
service tanks, and the gasoline feed system pressure indicator must be reliable
under vibrations and changes of temperature.
The radiator temperature indicator should clearly indicate the best operating
temperatures, and be readily inserted in the radiator.
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CHAPTER VL
MAP READING
OUTLINB
1. Classification
A. Plane Maps
B. Topographic Maps
2. Scales
A. Beading Scales
1. lUsed on U. S. Maps
2. Used on French Maps
3. Units of Measurement
B. Working Scales
1. ^nds of Working Scales
2. Constmction of Working Scales
3. Peace Maps and War Maps
4. Sketches
A. Line Sketches
B. Area Sketches
1. Place Sketch
2. Position Sketch
3. Outpost Sketch
5. Orientation
6. Map Symbols and Abbreviations
7. Contours
A. Definition
B. Method of Showing
C. Characteristics
D. Slope Board and Slope Scale
B. Profiles from Contour Maps
8. Landscape Sketches.
9. Photographic Maps
10. The ''Squared" Map
A. English Map
B. French Map
11. Map Beading Proper
12. Practical Applications of Map Reading
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CHAPTER VL
MAP READING
The following notes prepared for the course in Map Beading are largely
presented as map making since a thorough knowledge of the methods of making
maps and sketches is essential for a proper study of maps. .
1. CLASSIFICATION
A. PLANE MAPS
A plane map is one that shows all the ground features in their true rela-
tion to each other to some scale but does not distinguish variations in elevation.
Plane maps for war purposes are made usually from sketches or from photo-
graphs.
B. TOPOGBAPHIC MAPS
A topographic map has all the information that is shown on a plane map
and in addition shows relative elevations. A topographic map is made from
data obtained with surveying instruments, so that all distances, directions, and
elevations are quite accurate. In war times information of a military nature
obtained from sketches or from photographs is added to the topographic maps
made in times of peace.
2. SCALES
A. BEADING SCALES
Beading scales are shown on a completed map in one of three ways:
(1) by the Bepresentative Fraction (B. P.) where the numerator indicates the
number of imits on the map which corresponds to the number of like units on
the ground as indicated by the denominator; (2) by words and figures, for
example, 3 inches=l mile; and (3) graphically, that is a line of definite length
is drawn at the bottom of the map and divided into the miles and fractions
thereof that it represents.
1. Scales used on U. S. maps The scales in use in the United States
for topographic maps are 1/250,000 with an associated contour interval
of 20 ft., 1/125,000 with a contour interval of 20 ft., and 1/62,500 with a
contour interval of 10 ft. The last is frequently spoken of as a one-inch-
to-the-mile map because of its dose approximation.
2. Scales used on French maps The scales of the maps used in
France and the use to which each is put are as follows :
SCALE NAME
1/600,000
General Reference Map
1/250,000
Emergency Map
1/200,000
Trip Map
1/100,000
Strategical Map
1/80,000
Traffic Map
1/50,000
Zone Map
1/40,000
Tactical Map
1/20,000
Artillery Map
1/20,000
Trench Map
1/5,000
Detail Map
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MAP READING 603
The intention of the American Forces is to eliminate the 1/250,000
and the 1/100,000 and to use the 1/200,000 and the 1/80,000, respectively,
in their stead. Also the 1/50,000 map is to be gradually replaced by the
1/40,000.
The General Reference map is for ofSce purposes, and because of its
small scale enables a large area to be shown on a small map.
The Emergency map on account of the small scale to which it is
drawn is the aviator's reference map and is his personal property. The
enemy's trench lines should be drawn on this map, and the location and
relative a^uracy of all anti-aircraft guns should be indicated. Pilots and
observers should compare their maps frequently to make sure that they
contain all the available information.
The Strategical map gives more detail than the Emergency map as
it is made to a little more than twice the scale of the latter. One map sheet
covers a distance of about 25 miles back of the enemy's lines and most
reconnaissances do not extend beyond this distance.
The Traffic map is used to show traffic routes, railways, bridges, roads,
canals, water supply, communication lines, etc.
The Zone map is used for artillery work and is divided into the zones
effectively covered by each battery. The zones are colored to agree with
a small designated circle indicating battery headquarters.
The Tactical map has more detail than the Strategical map. All houses
of any size, roads including the fourth class, small orclumis, and small
streams are shown. One map sheet covers a distance of about 13 miles,
so that one map sheet would be sufficient for all ordinary tactical recon-
naissances.
The Artillery map shows considerable detail, such as small groups of
two or three trees, so that gun emplacements, etc., can be located accurately.
One map sheet covers a distance of about six miles so that a machine on
artUlery reconnaissance would ordinarily have to use only one map of this
scale. On the Artillery map the entire trench system of the enemy should
be drawn.
The Trench map is a larger scale map and upon it are shown the enemy's
defenses in the intrenched area.
The Detail map is used to reproduce to scale battery targets and small
portions of the enemy's trench system for accurate work.
3. Units of measurement In order to have uniformity in calcula-
tions the following approximate conversion factors between the English and
the Metric systems of measurements are given as being sufficiently accurate
for all practical purposes :
1 inch = 2.54 centimeters (cm.) 1 centimeter (cm.) = 0.4 inch
1 yard = 0.9 meter (m.) 1 meter (m.) = 1.1 yards
1 mile = 1.6 kilometers (km.) 1 kilometer (km.) = 0.62 miles
B. WORKING SCALES
Working scales are the scales actually used in the construction of military
sketches. However, working scales never appear upon the completed map.
1. Kinds of working scales Working scales are of three kinds:
(a) a stride scale, if the sketcher is walking; (b) a time scale, if the sketcher
is on horseback or in an airplane; and (c) a revolution scale, if the sketcher
is riding in a vehicle where the revolutions of the wheels are counted.
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604 MAP BEADING
2. Oonstraetion of worldng scales
a. EoMJLmple of stride scale Given: — sketcher's stride = 60 ins.
and scale of sketch -» 3 ins. to the mile.
Then 1056 strides -= 1 mi. on ground — 3 ins. on sketch
And 1000 '' — 2.84 ins. on sketch
A line 2.84 ins. is laid off and divided into five equal divisions each
representing 200 strides. The first of these divisions may be further
subdivided.
6. Example of time scale Given : — speed of airplane — 150 km.
per hour and scale of map = 1/200,000.
Then 60 min. of time = 150 km. on ground = 75 cm. on map
And 20 '' '' " = 50 km. '' '' = 25 cm. '' ''
A line 25 cm. is laid off and marked 20 minutes and subdivided to show
map distances for shorter intervals of time.
The revolution scale is calculated and constructed in a similar manner.
3. PEACE MAPS AND WAR MAPS
When any section of a country becomes the scene of war operations, it is
necessary for army commanders to have war map of this area. These maps
differ from peace maps in that they have additional information of a purely
military nature, such as the cultivation of fields along the roads and the location
and character of all fences so that a commander can determine what resistance
he will have to moving the various arms of his army outside of the road. For
constructing war maps, accurate topographic maps of the territory which have
been made in times of peace are used as a foundation, and information of a mili-
tary nature is secured by means of sketches and photographs and added to them.
4. SKETCHES
A. LINE SKETCHES
The Road Sketch is the only subdivision of the Line Sketch. The United
States Regulations prescribe that road sketches shall be on a scale of 3 inches to
the mile, with a contour interval of 20 ft. A road sketch is one of some particular
route or direction over which the sketcher travels obtaining his direction by
means of a pocket compass or simply by means of deflection angles obtained by
sighting ahead along the road with the map oriented. Distances along the road
are laid off on the sketch by the use of a working scale. All information to the
right or left of the road for a distance of three or four hundred yards is shown.
Contours are drawn in to show differences of elevation by the use of a slope
board and slope scale in a manner to be described later.
B. AREA SKETCHES
The United States Regulations prescribe that area sketches shall be made to
a scale of six inches to the mile with a contour interval of 10 feet. Area
sketches are of three kinds, namely, Place, Position, and Outpost.
1. Place sketch A place sketch is one made by the sketcher located
at a single point of observation and as dose to the enemy as possible. Dis-
tances are estimated and directions are taken by sighting along a pencil or
other improvised straight-edge to locate the position of desired details.
This map is made under the most adverse conditions and requires much
knowledge and sHU in handling the sketching outfit. The average man
gains such skill and knowledge only by practice.
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MAP READING 605
2. Poiition sketch A position sketch is one made by a sketcher
who travels over or has access to all the area to be placed on the sketch. It
shows all the details that would be given on several road sketches combined
and any military information concerning good positions for guns and troops
in camp.
8. Ontpost sketch An outpost sketch is one made of t^ie ground
along the finng line pr friendly outpost and extends as close to the hostile
line as possible. The sketch, by necessity, is made from observation in the
rear along the line. Points are located as far as possible by schemes of in-
tersection, or by estimation of the distance away along a line of sight.
When area sketches are made of fortifications, a scale of twelve inches
to the mile is used with a contour interval of 5 feet.
6. ORIENTATION
By orientation is meant the placing of the map in its true relation to the
ground it represents so that this area, if compressed to the size of the map, would
be represented i>oint by point as shown on the map. In short, this is obtaining
the north and south direction of the map and placing it parallel to the same
direction on the ground. In general, maps have true north and south lines
printed on them. These lines are called meridians. In some instances, sketches
also have the true meridian given, but more often only the magnetic meridian is
shown. The sketch is oriented by turning the sketch until the magnetic merid-
ian is parallel to the needle of the compass.
A second method of orienting a map or sketch is to locate on the map the
position of the observer and also the position of some point which can be seen.
When a line connecting these two points coincides in direction with the line on
the ground between these points, the map is oriented.
Resection is the opposite of intersection and the most important use made
of it is in locating one's self on a map. Orient the map by means of the compass
and then choose two landmarks which can be identified on the map. Through
these points on the map draw lines parallel to the lines of sight to the objects on
the ground. The observer is at the intersection of the two iLies.
6. KAP SYMBOLS AND ABBREVIATIONS
The fifymbols and abbreviations included in these notes are more or less
standard, and a knowledge of them will be of assistance in reading any map.
Any symbols in use other than those shown can be readily understood if it is re-
membered that a fifymbol is usually a profile or a plan of the object it represents.
The student should study these symbols and abbreviations carefully. A com-
parison of the symbols used by the British, French, and Americans should be
made. The French erymbols should be memorized as they are the ones that, will
appear on the maps used by the Americans in France. (The British and French
symbols may be found on the back cover of book.)
7. CONTOURS
A. DEFINITION
Differences in elevation are represented on a map by means of lines called
Contours. These lines are obtained by cutting the surface by several horizontal
planes spaced an equal distance apart. In other words, contours are light lines
drawn through the plotted position of points that have the same elevation, at reg-
ular vertical intervals, to show differences of elevation.
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MAP READING
/iogimentQl Headquorters. i8sl
Brigacfe Meodcfuorfers. ^ 4o*3c
Division Meadquorfers. ^dwc
CoqTS ifeoa/quorfers , ^K^
Acf/ufiont GeneraL _ -^
'Quarfer-masten^ J — ®
Commissary, - ({
J^edico/ Corps - PQ
Ordnance O
Signa/ Corps, F^
fhgineer Corp^ ffiS
Gun Softery. .,_ _. ^ff^
ii^orfor ffaffery. -"^^oo^
fbrf, [Tnj€ plan io be\ _ — — ^
/^isdoubtJ^^iiTOwn //' /rnotrA .__
Camp. _ AAAA
Oattle >!?
Trench _ <s^«^''*^
OdSTAClCS
NoM: When oohr m usmaf €Mcuf9 fhese in ncf
Abaffis.,^ "^ Hf- HJ^ Hj'
Win fnfang^ement. MMIBWt
/hiisades ....ttttttlrttttt
Contact i^ines. _— oVo^
Contro/fed Mines^,, '^^V^)
Demo/irions..... ^^'J?^
ABBREVlAnONS
Arroyo — A.
AbufrrTent. abut.
Biocirsmifh 3hop. ._ B.5,
Bottom, _ bot
Branch, ^ Br
Bridge.. _ br
Cape C.
Cemetery.,.. __ cem,
Concmte. con.
Covered. — coy.
Creeir...... _ Cr
Cuirert. 1 - cul
Deep. - — d.
last, ^ ...£
estuary, — ^
fbndahle f.
/brf, Ft,
Seneraf Store,, 65.
Girder, gir
GristmilL - M
Iron : /
isiand.. - /
Junction — ofc
ffing-post 1(^
lake , L
Latitude, — lot
landing. /fl^
Li/e-saving Bfotion, U^
Lighthouse _ Ui
Ard) -Ar.
Bnck. A
Longitude, Iprg.
iirfountain - ^ ^/
Hffountains. _ - Mis.
iVorth. -N.
Not /bnJabfe. nf
Pier.. R
Pfanir. pir.
PbstOffice. PO.
Pbint, Pt.
Queen-post .qp
River. R.
Roundhouse, R^-
/^iiroad. , RJ^
South 5.
Steei. s
Schoolhouse. SJi.
SoivmiiL S.li
Station. 3to.
Stone. .St.
Stream.. , str
Tollgate.. _ .7S
Trestle _ Tres
Truss.. ^ _ tr
WaterTonA. , m
WateryHortrs..... WM
West. __ M
Wood.,.. ^J9it.z!i^y,^^^^^^
Wide M
MAP BEADING
607
' Single Thick :
Double Track \ <
fhUmcKfs \ Tlffoff^t/roacfs '.■.■.■.'.■■'.'■■■.■.■■*-rr"
Urban or
Suburban
WogonPooab
*■■>•■■
IstChss, Meki/ed-
Znof.Chs9, CouMr^JHoacf- w ;;*,:
'5P9Qp /nc/ine ^
Trail or fhfh
Road Crossings -
Grade
Above Grade
delofvGrade
II \\
-7^
.^
Oifc/)-.
Sfneorrys .
GeneroL
InAuTntffent > --• i:^^---''^-*^:
falls
Rapids.^.
Direction of Current -*-
Cona/ with lock. j
Truss flltoocf-krjSMei-s)
Foot
Drot/f.
Bridges \ Su^enshn.
Anch.
fbntoon....!
Boat,
Ferries \ iflope orTiolL
3team :—
■^^
'NlKlll:lflll\)l'
^mmwr
fords
tnftintrycQf^..
Cdn^aijyi
^ WagonSArtflery....
Dam.
Springs
Uahesonel Pi>nds^.
^-
Telegraph Une.
Tunnei.
T T T T T T T
Medjge^,. i^c^'c^ecNSic^'eocj^sHftfk^^
S^one fence.
Worm i^inoe...
Wire Fence,...
Board fence.
fresh Marsii,
Berbrnd
n — ^ji — m — fl
Saitkhrsh.
Tidal flats.
Woods.
Orcharcls.
'h u' J* 'f ^ ' ^ • *^ •* •-..««* —
Cultitrated lands.^
isolated Tf-eeSroups.
Com. AteadonrlOTTd.
Cemetery.. fiark.
Ffailm^ fmbankment
Plaiii¥ay Cutting.
Windmill. - ^ .¥
Waterworks _■■
factoty(silafe charactery. em
tk>use. Wk
Churd}.
Sehod House..
.A
se
City, Tbwn or \tiihge
Contour System.
Depression Contours.
c^O
aim.
Sand, Grarei.,
* ' «
-)r.-^*===<!-
608 MAP BEADING
B. METHOD OF SHOWING ,
On the ground the contours are imaginary lines. This may be best illus-
trated by considering an island in the center of a body of water (See Figs. 601
and 602). Imagine the surface of the water to be raised a distance of ten feet
The shore line thus formed would represent a contour drawn through all points
whose elevation was ten feet above the elevation of the points on the contour
represented by the first shore line. Successive contours representing equal in-
creases in elevation can be secured in a similar manner. These contours when
projected down onto a single plane would represent a contour map (See Fig.
603). The vertical distance between the successive elevations is known as the
contour or vertical interval, which is abbreviated to V. I. The distance measured
on the map between two successive contours is called the map distance, which is
abbreviated to M. D.
C. CHARACTERISTICS
1. All points on any contour have the same elevation above datum.
2. Contours always dose or run off the map.
3. A contour line never splits.
4. Contours of different elevations do not cross or run into each other, ex-
cept in the case of overhanging diffs.
5. Contours run up valleys and cross at right angles in a Y shape. (See
Fig. 607)
6. Contours run down ridges and cross at right angles in a U shape. (See
Fig. 607)
7. On a uniform slope, contours are equally spaced.
8. The spacing of contours indicates the slope of the ground.
9. A hilltop is represented by several concentric closed contours. (See
Fig. 604)
10. Contours do not cross a road or railroad, but break and continue on the
other side.
11. Usually every fourth or fifth contour is numbered according to the ver-
tical interval used.
D. SLOPE BOARD AND SLOPE SCALE
A simple device for measuring the degree of slopes is constructed by fasten-
ing a string to the back of the sketching board near one edge and attaching to
it a weight so as to form a plumb line. An arc drawn as large as the board will
permit, with the point of suspension as the center, is marked in degrees. The
zero of the scale is immediately under the line formed by the string when it
hangs perpendicular to the top edge of the board. By sighting along the top
edge of the board parallel to any slope, the slope of the ground will be measured
by the arc through which the plumb line swings.
Knowing the slope of the ground, the contours can be spaced by the use of a
slope scale. This scale is constructed by computing the map distance between
contours for the scale to which the map is being made and the vertical interval
used. These computations are based upon the mathematical fact that a slope of
l"" gives an increase in elevation of 1 ft. in a horizontal distance of 57.3 ft. If a
vertical interval of 20 ft. is to be used, this increase in devation is secured when
a horizontal distance of 1146 ft. is traveled. To a scale of 3 inches to the mile,
this distance would be represented by a line .65 of an inch in length. A 2'
dope would reach an increase in elevation of 20 ft. in one half this distance, and
would be represented to the same scale by a line .33 of an inch in length. These
values are laid off on a strip of cardboard and marked with the corresponding
degrees of slope, thus making a slope scale.
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609
Pig. 601
Fig. 602
'/53^^
i90
Fig. 604
Pig. 603
Fig. 605
Fig. 606
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610 MAP EEADING
A formula developed for computing the spacing of contours for any slope is
V. I. X 57.3
G. D. (ground distance) «=
Degree of Slope
The formula can be used with either the English or the Metric units if it is
remembered that the ground distance obtained is in the same units as the vertical
interval. After the ground distance is obtained, the map distance between suc-
cessive contours is secured by introducing the scale of the map.
E. PROFILES FROM CONTOUR MAPS
In military work it is often desired to secure a profile of the ground between
two points. This is done by drawing a line across the contour map between the
two points, and projecting the intersections between this line and the contours
onto successive horizontal lines spaced to the same scale as the scale of the map.
(See Figs. 605 and 606) Profiles constructed in this manner are useful for
visibility problems, which will be discussed later.
8. LANDSCAPE SKETCHES
At the Fort Sill school of artillery, a method of sketching targets by land-
scape sketches is being taught. For this purpose a standard sketch pad 814^%
inches is divided by vertical lines into ten %-inch strips the long way of the pad.
In the center of the pad, horizontal lines are drawn between which all the sketch-
ing is done. At the top of the sheet, three horizontal lines are drawn and marked
T, RN, and DEF, respectively. Upon these lines target, range, and deflection
are given.
The sketch is made from one position as in place sketching, and fre-
quently an observation balloon is used for this purpose. The position of an object
on the sketch is located in direction by the angular deflection measured in mils
from the reference point, and the distance is estimated. A mil is 1/6400 of a
circle, or approximately 3 minutes of arc. The reference point selected must
be clearly dedhied. The %-inch strips give arcs of 50 mils if the pad is held 15
inches from the eye when sighting. This can be done by knotting a string 15
inches from the pad and holding it in the teeth at the knot. By lining up the
reference point with a vertical line near the center of the pad and sighting along
the top edge of the pad to some other object, the angle of deflection is graphically
measured. The sketch is oriented when completed by turning it so that the
reference line points to the reference point and drawing a magnetic meridian
parallel to the compass needle.
9. PHOTOOSAPHIC HAPS
The use of photography for securing information for constructing war maps
is quite general in the present war. The majority of the information secured in
this manner is used in making plane maps, but it can be superimposed upon a
topographical map of the same area made in peace times to make a war map.
It is practically impossible for the camera to be perpendicular to the earth's
surface when every picture is taken. For that reason considerable distortion
results and before a photograph can be transferred detail by detail to a map it
is necessary to correct this distortion. This is done by taking a photograph of
the completed negative with the camera placed at the same angle to the negative
that the camera was to the ground when the first picture was taken. A special
instrument has been constructed which will transfer, item by item, all informa-
tion on a photograph to any given scale map when the photograph shows every-
thing in its true relation. A camera can be used also to construct topographical
maps. A special Theodolite camera is used for this purpose.
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10. THE ''SQUASED" HAP
A. ENGLISH MAP
For the purpose of reporting quickly and accurately the location of any
thing of military importance, the artillery map is divided into squared sections
and the position of objects is given by co-ordinates. The area of operations is
divided into standard areas, 36,000 yards from east to west and 22,000 yards from
north to south. This is divided into six strips, 6,000 yards wide, from west to
east and into four strips from north to south. The first and last of these four
are 5,000 yards wide and the second and third are 6,000 yards wide. Beginning
at the northwest subdivision, they are lettered from capital A to capital X from
west to east. These lettered areas are divided into thirty or thirty-six squares
1,000 yards on a side, depending upon whether the lettered area is 6,000 by 5,000
yards or is 6,000 yards square. These squares are numbered from 1 to 30 or 36,
beginning at the northwest square and numbering from west to east. Each
numbered square is divided into four equal parts and lettered small li, b, c, and
d, beginning in the northwest comer and lettering from west to east. To locate
any target in these smallest areas an origin is taken at the southwest comer and
the number of tenths to the east is given first and those to the north from this
origin are given next. Thus a point in one of the smallest squares would be
described as' G19c54.
B. FRENCH MAP
The French system of designating i)oints on their maps is by metric co- '
ordinates. An initial point was chosen to the south and west of the battle
area. Through this point are drawn a north and south line and an east and west
line. These lines are the axes of the co-ordinate system. The area is then
divided into squares one kilometer on a side by two series of parallel lines ; one
series is parallel to the north and south axis, and the other series is parallel to
the east and west axis. These lines are numbered from the initial point to the
east and to the north. Only the ten-kilometer lines are marked with the full
number; the intermediate or kilometer lines are numbered from 1 to 9. Each
kilometer square is subdivided by imaginary lines into squares 100 meters on a
side, thus making 100 meters the unit of measurement of the system. A point
on the map is designated by the coordinates of the nearest comer of the 100-meter
square in which the point is situated. The west to east coordinate is given first.
For example, the designation 2165 — ^2943 locates a point which is 2165 100-meter
units east of the initial point and 2943 100-meter units north of that point.
11. MAP BEADmO PROPER
A map is a more or less complete picture of a large area drawn on a small
scale and its purpose is to convey to one who has not seen the mapped area an
idea as to the relative positions of the works of man, such as towns, highways,
railways, etc., and the natural objects such as streams, hills, ridges, and general
relief. To read a map is to familiarize one's self with the data shown and to
form a mental picture of the area in question. In forming a mental picture of
the area covered on any map the first thing to study is the location and appear-
ance of the villages, towns and cities. Each of these will cover a certain relative
area and have a certain shape on the map. By the number of roads and railroads
running into each and the different shapes that they form they can be distin-
guished readily from each other. If a proper study is made it should be im-
possible for an aviator to fly over any of these towns without knowing which one
it was. A study of the lakes and rivers is important as they form good land-
marks on dull days, as they will shine up to you when other features on the
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612 MAP BEADING
ground can not be distinguished. Streams look black from the air and are some-
times very useful to follow. Flooded areas should be shaded in blue on your
maps as they serve the same purpose as lakes. In the case of tactical work and
forced landings^ a knowle^^ of the relief is essential. The relief disappears
almost entirely from seven to eight thousand feet except at sunrise and sunset
when shadows are cast by the hiUs. The sun shadows thrown by objects such as
trees or buildings are of great assistance in judging the configuration of the
ground as these vary in length in proportion to the slope of the ground. Thus if
you see a long shadow thrown by a tree and a short one by another tree of equal
size you will know at once that the ground where the long shadow falls is sloping
more than where the short one falls. Saddle hills form important landmarks and
can be picked out on your map by the peculiar contour construction. Woods
and forests are also good landmarks and are, represented by standard symbols
on the maps.
12. PSAOTIOAL APPLICATIONS OF MAP BEADINO
The ability to readily form a mental picture of any area from a study of the
map is valuable for all types of reconnaissance. Especially in tactical recon-
naissance, landmarks are used almost entirely to fly by, and gun emplac^nents,
troop movements, etc., are located on the map by finding the features on the
ground as represented on the map. One should be able to pick out good landing
places from a study of the map. This can be done by first determining if the
slope is suitable from a study of the contours. Then see if there is any cultiva-
tion of the area which would interfere with landing. An apparently good land-
ing place, picked out from the map, will often be found to be ditched when in-
vestigated and therefore unsuitable for this purpose. More landmarks such as
a group of two or three trees are shown on a larger scale artillery map and are
very useful in locating gun emplacements in artillery observation and in follow-
ing an attack in cooperation with infantry.
The construction of profiles and a study of them for visibility problems is
necessary for a commander of troops before any troop movement is undertaken.
This will enable a conunander to determine locations of likely ambush and posi-
tions where the enemy would be likely to unlimber their batteries, and how much
of the area to either side of a road can be seen from the road.
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CHAPTER Vn.
METEOROLOGY
OUTLINE
1. The Atmosphere
A. Composition
B. Extent
C. Weight or Pressure
D. Density
E. Effect of Temperature
F. Effect of Pressure
2. Heat
A. Source of Heat
B. Effect of Sun's Heat
C. Effect of Altitude on the Temperature of the Air
3. Wind
A. Cause of Wind
B. Major Wind System
C. Minor Winds
D. Cydonio Storms
E. Weather Maps
1. Lows or Cyclones
2. Highs or Anti-eydones ,
3. V-shaped Depressions
4. Wedges
F. Wind Velocity
G. Variation in Direction and Velocity of Wind above the Surface
4. Clouds 1
A. Formation
B. Types of Clouds
C. Velocity of Clouds
6. Air Currents
A. Ascending Currents
B. Descending Currents
C. Surface Contour Currents
D. Gusts
E. Wind Eddies
F. Thunder Storm Currents
G. Line Squalls
701
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CHAPTER Vn.
METEOROLOGY
Meteorology may be defined as the study of atmospheric phenomena. It
treats of the condition of the atmosphere, its changes of condition, and the causes
of these changes.
1. THE ATMOSPHERE
A. COMPOSITION
The atmosphere is a gaseous mantle which envelops the earth and moves
along with it. It is a mixture of dry air; water vapor, and dust. Dry air is
composed of nitrogen, 78%; oxygen, 21%; and several minor gases, 1%. The
oxygen is the part of the atmosphere which sustains life.
B. EXTENT
The atmosphere extends from 50 to 200 miles above the surface of the earth.
However, that part of it which is dense enough or contains oxygen enough to
support life is limited to about five or six miles.
C. WEIGHT OB PRESSURE
Although it is light and invisible, the air has perceptible weight, and at
sea level at a temperature of 60"" F. a column of air one square inch in section
weighs 14.7 lbs., which is the equivalent of a column of mercury 30 inches high.
This column of air weighing 14.7 lbs. will exert a pressure of 14.7 lbs. per square
inch on whatever surface it may be resting. The column of air resting on the
top of a mountain is less in height than a similar column at sea level, and,
therefore, is of such diminished weight that the atmospheric pressure on the
mountain top is less than it is at sea level. The pressure decreases 1 lb. per
square inch for approximately each 2500 feet of ascenjt.
D. DENSITY
Pressure pushes the particles of the atmosphere together so that the air
is denser or more compact near the sea than it is on mountain tops. At a height
of about 3.6 ^niles there is as much air by weight above- as there is bdow. As
shown in Fig. 701, the density of the air at this point is ^, that is, the air at
this level contains just i^ as much oxygen, etc., as the air at sea level, and hence
it is difficult for one to breathe. The change in density will also affect the
working of the airplane's engine. From Fig. 701 it is also seen that % of the
atmosphere, by weight, is within seven miles of sea level. At an altitude of
ten miles the density of the air is just ^ of its density at sea level. As the
density of the air decreases, the supporting power of the air is reduced in the
same ratio ; and, since the resistance to propulsion through- the air against head
winds increases faster than their density diminishes, it follows that an airplane
can not be operated efficiently higher than is necessary to overcome the irregvlar
conditions at the earth's surface.
E. EFFECT OF TEMPEEATUBE
A rise in the temperature expands the air and decreases its weight per unit
volume. Warm air will rise in cooler air just as a cork held under water will
rise when it is released.
702
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704 MBTEOBOLOOT
F. EFFECT OF PRESSURE
When air is compressed it becomes warmer and when it is expanded it
becomes cooler. Thus ascending air is expanded and becomes cooler; and de-
scending air is compressed and becomes warmer and drier.
2. HEAT
A. SOURCE OF HEAT
Practically all of the earth's heat is received from the sun. The rays of
the sun in passing through the ether of space do not give off any heat due to
the fact that there is nothing there to heat. It is only when the heat vibrations
are interfered with by dust particles, water vapor, or the earth itself that any
heat is absorbed. For this reason the ether of space above our atmosphere is
extremely cold, and the upper layers of the atmosphere itself are so tldn that
very little heat is absorbed by them.
B. EFFECT OF SUN'S HEAT
When the heat vibrations from the sun strike any object, part of the heat
is absorbed and part is reflected back into space. Some bodies reflect little heat
and absorb nearly all the sun's rays which strike them. This is especially true
of black objects. The blacker the soil, the more heat will it absorb; hence, a
field dark from recent plowing will become warmer than an adjoining stubble
field, and ground covered with vegetation will warm up less rapidly than bare
ground. Water reflects about 40% of the heat waves which strike its surface.
The aviator can make use of this knowledge in making a forced landing. The
cooler the ground, the safer the landing.
The heat which is thus absorbed by the earth is then given off by radiation
and conduction, and the air near the surface is heated. Then by convection
this heated surface layer is mixed with the air above. So then the atmosphere
is really heated at the bottom instead of at the top.
C. EFFECT OF ALTITUDE ON THE TEMPERATURE OF THE AIR
The average temperature at the surface of the earth is about 50"* F. As
we ascend into the air, there is a decrease in temperature which amounts to
about l"" F. for each 300 feet of ascent The temperature of the air decreases
at this rate until a temperature of -67*" F. is reached at an altitude of about
seven miles where the temperature becomes stationary and even increases slightly.
This warmer portion of the upper air is called the stratosphere. Near the
ground the temperature may vary considerably in the course of a day, but
the upper air alters its temperature much more slowly. At a height of six miles
above the earth, a temperature much below zero constantly prevails, while
at ten miles, -80' F. has been recorded in a sounding balloon. The dotted line
in Fig. 702 shows the temperature of the air at different heights as determined
by ol^ervations at St. Louis.
Owing to the decrease in temperature with altitude, the aviator must dress
himself to withstand the cold experienced at high altitudes. Also the low
temperature will affect to some extent the operation of his engine and ma-
chine gun.
3. WIND
A. CAUSE OF WIND
Wind is simply air in motion, usually in a horizontal direction. It is due
primarily to the unequal heating of the earth's surface by the sun. The air
at one place is heated more than it is at another, and consequently becomes
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lighter. The lighter air is forced to riae by the pull of gravity on the cooler
heavier air nearby, which rushes in to take the place of the lighter air, and
in this way a circulation is set-up which we call tuind.
B. MAJOR WIND SYSTEM
The circulation of the air around the earth results in the formation of seven
belts or areas of varying atmospheric pressure. Three of these belts are areas
of calms. The equatorial belt is called the ' ' doldrums" and is a belt of low pres-
sure in which air rises. In about latitude SS"" in both hemispheres we have other
belts of calms called the ''horse latitudes". These are belts of high pressure
in which the air is descending. When the air in these descending columns
strikes the earth it divides into two parts: one of these is drawn toward the
equator and forms the winds known as the "trade winds"; the other part is
drawn toward the poles by the area of low pressure there and forms the belt
of winds known as the ** prevailing westerlies". The latter are the surface winds
with which we have to deal in tMs latitude.
C. MINOR WINDS
Along the sea coast and the shores of large lakes there are minor wind
circulations known as land and sea breezes. During the day the breeze is from
the sea, and during the night from the land. These breezes are caused by the
unequal absorption of heat by land and by water, and by the difference in the
rate at which each gives off its heat.
In mountainous regions, the hilltops and slopes at night are cooled rapidly
by radiation, and the cool heavy air flows down into the valleys, causing moun-
tain tuinds which may attain considerable force during the night. The opposite
circulation takes place in the day time.
D. CYCLONIC STORMS
If the surface of the earth were uniform throughout, so that all parts were
capable of absorbing the same amount of heat, in our latitude we would have
a surface wind from the southwest every day in the year ; and at an altitude
of less than a mile we would have a wind from the northwest, and at about three
miles a wind from the southwest. However, owing to the non-uniformity of the
surface composition, there are numerous irregularities and even reversals of
wind direction. These are due to the so-called cyclonic storms which are indi-
cated on our weather maps as ''Lows".
E. WEATHER MAPS
The accompanying weather* maps with explanations should be carefully
studied before proceeding further.
A study of the weather maps will enable one to forecast with considerable
certainty the changes that will take place in the direction and velocity of the
wind and in the clearness of the air with respect to clouds for the ensuing
24 hours.
Pressure is the basis upon which the weather is predicted. The pressure
is indicated on the weather map by isobars, which are found in four formations,
namely, Lows, Highs, V-shaped Depressions, and Wedges.
1. Lows The Lows charted on the weather map are technically
known as cyclones, because they are areas of low pressure toward which the
wind blows from all directions and in which rain frequently falls. A
cyclone should be distinguished from that type of violent wind of small
area known as a tornado. In a cyclonic area the wind blows inward and
in an anti-clockwise direction around the area of low pressure which is itself
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METEOROLOGY
707
moving. When the diameter of an area of low pressure becomes unosoally
small, the pressure gradient is said to be ''steep" and the weather changes
are sudden and violent. In general, the closer the isobars, tJie higher
the velocity of the wind.
2. Highs The Highs or anti-cyclones are the opposite of the lows.
In them the wind is blowing spirally outward in a clockwise direction.
While a high is passing over, the weather is usually excellent and the winds
are never high. High pressure areas are erratic in behavior, moving slowly,
sometimes standing for days, and follow no rule for direction.
3. V-shaped depressions Sometimes the isobars are shaped like the
letter ' ' V " with the low pressure inside the ' * V ". This formation is known
as a V-shaped Depression, and moves along with the area of low pressure. If
the foot of the ''V'' points south, tiie wind is southerly in front and north-
erly behind the depression. The weather in front is very rainy. As the
center passes there are squalls and heavy showers which are followed by
a quick improvement in the weather.
4. Wedges Sometimes the isobars inclose an area of high pressure
In the shape of a ''V", usually with the foot of the " V to the north. This
formation is called a Wedge. The winds are light, northerly in front and
southerly behind. While a wedge is passing over, a short period of ex-
tremely fine weather is experienced.
F. WINDVELOCITT
To assist in estimating the velocity of winds, the following table known as
the Beaufort Scale is given :
BXAUrOBT
Gensbal
SPBCinGATIONS OF SCALE
Vklooptt im
No.
Dbsgbiption
Milks fib hb.
Calm
Calm, smoke riaes vertieallj
-1
Light Air
Direction of wind shown bj smoke drift, bat
not by wind vanes.
W
SUght Breeze
4-7
Gentle "
Wind extends light flag
S-18
Moderate "
Baises dust and loose paper
13-18
Predi "
Small trees in leaf begin to swaj
19-24
Strong "
Whistling heard in telegraph wires.
Umbrellas used with difficoltj
25-31
High Wind
Whole trees in motion; ineonvenienoe felt
%
when walking against wind
32-38
Gale
Breaks twigs off trees; generally impedes
progress
3i^-46
Strong Gale
Chimney pots and slates removed
47-54
Whole "
Seldom experienced inland; trees uprooted
55-^
Stonn
Very rarely experienced. Accompanied by
widespread damage
64-75
12
Hurricane
75H1P
The average wind velocity in the U. S. throughout the year at a height of 50
ft. above the ground is 11 miles per hour. Over the oceans the velocity of the
wind is slightly greater than this.
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708 METEOROLOGY
G. VARIATION IN THE DIRECTION AND VELOCITY OF THE WIND
ABOVE THE SURFACE
With a high barometric pressure at the ground and in clear weather, the
wind velocity increases slowly with altitude ; but with a low barometer and in
cloudy weather, its velocity increases rapidly with altitude, especially near the
strata of lower clouds.
The Blue Hill Meteorological Observatory at Milton, Mass., gives the follow-
ing results of its observations of the wind velocity at different heights.
Height in Miles
0.5
1.2
» 2.5
4.7
5.0
ociTT IN Summer (nupJi.)
16
18
2a
81
61
ociTT in Wintbe (m.p.h.)
19
32
47
108
119
Near the ground the wind is retarded owing to the friction of the air with the
surface. The effect of friction does not extend beyond an altitude of about 2,000
feet above land, and from 300 to 500 feet above water. As a rule for winds other
than north, a doubling\)f the wind velocity may be expected between the surface
and 2,000 feet, excepting during a hot sunny day when a general mixing of the
air by convection currents tends to equalize the different strata, and the velocity
at the surface will not be much less than that of the wind above.
Not only does l^e wind change in velocity but it also changes in direction.
With increased altitude the wind is veered on the surface wind — ^that is, more
from the right as you face the wind. As a rule the average change in the wind
direction from the surface to a height of 2,000 feet amounts to about 22*.
Above 2,000 feet, there may be five types of wind changes.
1. After the first increase up to 2,000 feet, the wind remains constant
in velocity and direction up to great heights ; this condition is frequent in
easterly winds and is occasionally found in westerly unnds, but is rare untJt
winds from north or south.
2. After the first increase up to 2,000 feet, the wind falls off in velocity
and is calm to great heights. This distribution is associated entirely with
easterly winds, with an anti-cydone to the north and a cyclone to the south.
3. After the initial increase, the wind falls off as in the last case, but
above the calm there is a reversal of wind direction and the upper wind
usually increases slowly with height. This happens with easterly winds
when there is a low pressure area off the southwest.
4. The wind may continue to increase but at a slower rate than near the
surface. The increase may continue to great heights, or after an increase
for 8,000 or 10,000 feet, there may be a slight decrease followed again by an
increase. This happens in the westerly mnds on the south side of a Low,
and may be found with winds from any direction, though perhaps less often
with easterly winds.
5. After the initial increase, the wind direction changes very widdy
from that at the surface. This happens in the southwest unnds of an ap-
proaching area of low pressure. The upper winds blow away from the
center of the area of low pressure and may be veered 90* on the surface wind.
The changes of direction in types 3 and 5 may take place at any height
up to 15,000 feet or so. In type 5, the change may be gradual or quick.
When there is a quick change of direction with height, there is always a
calm at the height at which the change occurs.
4. CLOUDS
The douds may be used as drifting aerial buoys indicating to the observer
on the ground the direction and velocity of the air currents at thdr respective
heights.
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METEOROLOGY
709
A. FORMATION
The water vapor in the air is supplied by evaporation from the surface of
oceans, lakes, rivers, etc. The air is said to be saturated when it contains all the
water vapor it can possibly hold. If saturated air is warmed, it is no longer
saturated, because its capacity for moisture increases as the temperature in-
creases. On the other hand, if saturated air is cooled, it can no longer hold all
its moisture, and some of the water vapor will condense into fog or doud.
Fog is foiled when the air over damp plains is chilled to the point of satu-
ration. Fog consists of particles of water of such minute size that they float.
Fia. 703. — Common Cirrus
Olayd^n
During the summer many clouds are caused by the rise of damp warm air
to such altitudes that the air is cooled to the point of saturation. The formation
of clouds is chiefly due to the cooling which accompanies the expansion of the
rising air. Another cause of cloud formation is the blowing of damp air over
cold surfaces, such as the top of a mountain. Still another cause is the coming
in contact of a warm and a cold current of air, one above the other. Clouds of
this kind are common on days when the warm air is also very damp.
B. TYPES OF CLOUDS
The highest clouds are the Cirrus and the Cirro-stratus, which are at an
average height of six miles and are composed of particles of ice and snow. Cirrus
clouds are delicate, fibrous, and hair-like. Fig. 703 illustrates the most persistent
and probably the most frequent form of the cirrus cloud, which is called common
cirrus. It occurs in detached masses which have very variable forms but are
wholly fibrous. This cloud when moving slowly indicates settled weather condi-
tions and fine weather. Cirrus clouds moving rapidly from the southwest indi-
cate a fall in the temperature, and moving rapidly from the northwest indicate
a decided rise in the temperature within the next 24 hours.
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710
MBTEOEOLOGT
When the cirrus douds appear in a sheet or layer formation they are called
Cirr(hstratus. In the lower part of Fig. 704 are seen fluffy cirrus douds which are
several thousand feet above the drro-stratus douds in the upper part of the
picture. This cloud is an indication of condensation in a calm atmosphere, and
usually means the end of a period of good weather. About 80% of the time this
doud is followed by rain within 24 hours.
Pia. 704. — CiRBO-STRATns
Olayd^n
The douds of intermediate height, that is from three to five miles, are called
CirrO'Cumvlus, Alto-cumulus, and Alto-stratus.
In Fig. 705 cirrus douds are in the upper part of the picture and cirro-
cumulus in the lower part. This doud forms at an altitude of above five miles
during the hottest months of the year when the air is still and the evaporation is
great — ^the same conditions which cause thunder storms.
The alto-cumulus form occurs at an altitude of from three to four miles.
Fig. 706 illustrates this type. These clouds may either dose up forming a uniform
layer which is then called alto-stratus, or they may break up and gradually dis-
appear. This cloud is usually seen in the afternoon. The alto-cumulus is char-
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MBTEOBOLOQY
711
Pia. 705. — Cirro-cumulus
Olapdtn
FiQ. 706.— Ai/ro-cuMULUB
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712
METEOROLOGY
Pig. 707. — Common Stratus
Olayd«»
Pia. 708.— Cumulus
CUtffdm
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METEOROLOGY 718
acteristic of thunder weather, and is followed by rain within 24 hours, three times
out of four.
The lowest of all the clouds, the Stratus, which are shown in Fig. 707, form
at an altitude of from 1,500 to 2,500 feet. This doud is without structure of any
kind, and is the typical overcast-weather cloud. They often come so low as to
lie upon the tops of hills in the form of mist or fog.
The lower clouds formed daily by ascending air currents are called Cumvlus,
Cumvlo-nimhus, and Nimbus, Cumulus is the lumpy, piled-up, white doud
which looks like exploded cotton bales. They are detached masses of clouds with
flat bases and rounded, dome-like summits. Fig. 708 shows this type of cloud.
The average thickness of the cumulus clouds is about one mile; but when the
uprush of air is very strong, as in thunder storms, their tops reach to the level
of the cirrus clouds. They are then called Cumulo-nimbus. This doud forma-
tion is well illustrated in Fig. 702. Cumulus clouds are convection clouds and are
at the tops of rising columns of air. When entering and leaving such a column
of air the aviator must be prepared for a change in the condition of the air and
adjust his controls accordingly.
On hot oppressive mornings in summer, when the air is full of moisture,
cumulus clouds form in all parts of the sky. In the afternoons of such days,
these clouds grow larger and darker and devdop into cumulo-nimbus or thunder
clouds, which rise in rolling surging masses to great heights. These are always
followed by rain, lightning, and thunder, and sometimes by hail. They are very
dangerous douds to enter as the air is full of whirls and eddies. The air currents
here are frequently of sufficient force to carry rain drops at a low level upward
to such a height that they freeze and become hail. Fig. 709 is a picture of a
thunder doud forming. Thunder storms may cover very large areas, but are
sometimes only a mile across. They travd eastward in the prevailing westerly
winds at the rate of from 20 to 50 iniles an hour.
A nimbus cloud is any cloud from which rain is falling. It is a very low
doud, but may have a thickness of two miles. Bain seldom falls from douds
less than one-half mile thick.
Of all the cloud forms, the dark sheet of stratus and clouds of lenticular
shape are most frequently followed by rain. In general, flat and flaky clouds,
douds forming and disappearing rapidly, and douds changing to form at a
higher levd precede dry and colder weather.
C. VELOCITY OF CLOUDS
The average velocity of the cirriform clouds is 90 miles an hour in winter
and 60 miles an hour in summer; but occasionally in winter drms clouds have
been observed to have the enormous vdocity of 230 miles an hour. On the
average, the velocity of the air currents increase, from the lowest to the highest
douds, at the rate of about 3 miles an hour for each 1,000 feet of height ; but near
the ground the increase with height is greater.
6. AIR OXTBRENTS
A. ASCENDING CURRENTS
Since the air is heated by the radiation of the heat which the earth receives
from the sun, it follows that ascending currents of air are more numerous and
more vigorous in warm clear weather. These currents of air may rise with a
velocity as high as 25 feet per second. On entering a column of ascending air
the supporting power of the air is increased by the upward current and the air-
plane will start to climb. On leaving this column of air the conditions are re-
versed, the supporting power of the air is decreased, and the machine will glide
downward till the velocity of flight is increased. Rising currents are more fre-
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714
MBTBOROLOGT
Fig. 709.S— Thundeei-clouds Forming
Olayden
quent during the middle of the day than in the morning and evening, and are
rarely experienced during the night or in the daytime when the sky is entirely
covered with clouds. Usually a cumulus doud will be at the top of the ascending
current, and the height to wUch the doud extends is an indication of the velocity
of the current of air beneath.
B. DESCENDING CURRENTS
The effect of a descending current upon an airplane is just the reverse of
that of an ascending current. Such currents may be found over water during
the day and over large areas of ground covered with trees or vegetation. Clear
patches between clouds usually indicate descending currents.
C. SURFACE CONTOUR CURRENTS
In hilly country the wind follows somewhat closely the contour of the sur-
face. The swift downward sweep of the air on the leeward side of a hill when the
wind is strong may carry the airplane with it. Such currents of air should be
entirdy harmless so long as the aviator keeps his machine wdl above the surface
and out of the treacherous ed^es near the ground.
D. GUSTS
What the aviator most wants is steady air conditions, particularly so in a
high wind. The condition of flight in still air and in steady moving air are nearly
identical, the only difference being the motion of the airplane with respect to the
ground. Owing to the irregularities in the surface of the earth, the wind at the
surface is not at all uniform in velocity nor constant in direction, but moves in
gusts which are troublesome. Wind gusts may be either from the front or the
rear. If from the front, the result is an increase in the supporting power of the
air and the machine will rise. On the other hand, a rear gust will decrease the
supporting power of the air, and the machine will fall. A rear gust of 20 ft. per
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METEOROLOGY 715
second will cause an nnconstrained machine to drop 80 feet in 15 seconds.
However, gn^ists rapidly disappear with altitude; hence, in general, the windier
it is the higher the ayiator should fly.
E. WIND EDDIES
In an eddy the air is moving in a loop, and hence the direction of the air
at the top and at the bottom of an eddy are in opposite directions. The direction
at the top is with the wind higher up, and at the bottom it is against it; and the
stronger the wind the more rapid is the rotation of the eddy. Eddies are most
pronounced on the leeward side of cliffs and steep mountains, but also occur
to a lesser extent on the windward side of such places. For this reason aviators
should avoid landing on the leeward side of hills or of large buildings.
F. THUNDER STORM CURRENTS
Thunder storm conditions are accompanied by rolling, dashing, and choppy
winds. They are often of such violence, up, down, and sideways that an airplane
in their grasp is in a very dangerous situation. These are without doubt the
most dangerous currents that the aviator can encounter.
G. LINE SQUALLS
The most dangerous wind met with in England and France is the ''line
squall", so called because it advances with a line front like a tidal wave, usually
lying southwest and northeast, and advancing from northwest to southeast. The
squfdl is accompanied by thunder and lightning. It probably extends to consid-
erable heights and it is not advisable for the aviator to try to dimb above the
storm. The signs of this wind are:
1. A sudden fall in the temperature.
2. A veer of the wind.
3. A rise of pressure.
4. A squall of rain, hail, or snow.
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CHAPTER VUI.
SIGNALLING
OUTLINE
1. Ai'rplane Radio Equipment
A. Oeneral
B. Description of Apparatus
1. Transmitter
2. Storage Battery
3. Sending Keys
4. Safety Switch
5. Fairlead
6. Aerial System
2. Use of Radio in Artillery Observation
A. Handling of the Radio Set
1. Preparatory to Flight
2. During Flight
B. Methods of Preventing Interference
1. Wave-lengths
2. Tuned Sparks
3. Directional Sending
4. Synchronized Watches
801
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CHAPTER Vm.
SIGNALLING
1. AIRPLANE RADIO EQUIPMENT
A. GENERAL
The radio equipment of an airplane should possess the following properties:
(1) simplicity; (2) ruggedness; (3) selectivity. The transmitting set used by
the British possesses the first two qualities in a marked degree, this, however, at
the expense of the third. The American and French types possess ruggedness
and selectivity, but are not so simple, and therefore require more knowledge on
the part of the pilot. An American set modeled after the British Sterling trans-
mitter is being made and used for training purposes, hence the following dis-
cussion will apply particularly to this set, although the remarks on the handling
of the set and on the prevention of interference will apply more or less rigidly
to all aerial transmitter sets now in use.
The Signid Corps transmitting set, known as type SCR-65, consists of the
following units: transmitter; storage battery; sending keys; safety switch; fadr-
lead; aerial wire with reel and weight; connecting wires.
B. DESCRIPTION OF APPARATUS
1. Transmitter The transmitter consists of a spark-coil having an
adjustable vibrator, a condenser, a spark gap, and a flat spiral sending
helix, or oscillation transformer. The spark-coU, condenser, and spark gap
are enclosed in a box 7 7/16 inches long, 6% inches wide, and 3^ indies
deep, outside measurement. The top of the box is of hard rubber and forms
a panel upon the outer side of which are mounted the sending helix and four
binding posts which are marked respectively ''antenna", ''counterpoise",
"key", and "battery". The outer turn of the sending helix is connected
permanently to the "counterpoise" binding post. What is known as the
"closed circuit" of the radio set is formed by connecting in series the spark
gap, the condenser, and several turns of the helix. The connections to
the helix are made by leading a heavily insulated wire from one elec-
trode of the spark gap to the "counterpoise" binding post, connection
being made on the under side of the panel, and by passing another
heavily insulated conductor leading from one terminal of the condenser
through an opening in the center of the panel, the outer end of the con-
ductor being attached to some point on the helix by means of a clip. Hard
rubber markers bearing numbers from 100 to 300 are placed at certain points
on the heUx and indicate the proper point of attachment of the closed circuit
clip for the corresponding wave-length. The helix also forms a part of the
open, or "radiating", circuit of the set. This circuit is formed by connect-
ing the "counterpoise" binding post to the bonded wire stays of the ma-
chine, which constitute the "ground", and by connecting the "antenna"
binding post to the fairlead by means of a heavily insulated wire. The
"antenna" binding post is connected to the helix by means of a flexible in-
sulated cord which ends in a clip. The position of this clip is determined
by experiment, and after once being determined remains fixed. The pur-
pose of the spark-coil is to change the low pressure direct current from the
storage battery into high pressure pulsations in order to provide successive
802
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SIONALLINO 808
re-charges of the condenser, which discharges through the helix and spark
gap when the pressure becomes sufficiently great To attain this, the sec-
ondary terminals of the spark-coil are connected to the spark gap electrodes.
The primary terminals are connected to the ''key'' and ''battery" binding
poets. The transmitter and the storage battery are mounted on a tray which
is placed in position on a shelf just back of the rear cockpit This shelf is
made accessible simply by moving the turtle-back a short distance to the rear.
2. Storage battery The storage battery differs from any four-cell,
eight-volt automobile storage battery only in that the plates are smaller^ and
less current can be obtained. The battery will provide current for about
five hours of continuous sending before a re-charge is necessary. When in-
stalled in a plane, the battery rests inside a deep box which is fastened to the
tray upon which the transmitter is mounted,
8. Bending keys Three sending keys are provided in a two-seater
machine, one in the forward cockpit and two in the rear cockpit. These
keys are larger and much more rugged than the ordinary telegraph key, the
heavy lever and large knob making sending with the gloved hand less diffi-
cult. All three keys are connected in parallel, so that the pilot may send
with either hand, or that the observer may also send any necessary signal.
4. Safety switch The safety switch is located in the rear cockpit
within easy reach of the pilot's right hand, and is for the purpose of com-
pletely disconnecting the battery from the transmitter. Two leads from the
battery are connected to one side of the switch. From the other side, one
lead goes directly to the spark-coil primary and a second lead goes to the
right hand key in the rear cockpit, and from the key a lead goes to the spark-
coil primary, thus completing the battery circuit
5. Falrlead The fairlead is a metal bushing set into a bushing of
ebonite, and is located directly below the aerial rcNcL The fairlead serves
as a runway for the aerial wire, makes the electrical connection between the
aerial and the transmitter, and also insulates the aerial from the machine.
It is clamped to the lower longeron on the right hand side of the maohine,
and outside of the forward part of the pilot's cockpit.
6. Aerial system The aerial consists of a stranded bare wire. Ex-
cept when the machine is in flight the aerial is wound on a fibre reel, the free
end passing down through the fairlead and being attached to a lead weight
of about two pounds. The weight is necessary for the proper action of the
aerial during the process of reeling in and out, and also during flight The
reel is equipped with a brake in order to control the speed at which the aerial
runs out. The length of the aerial depends upon the wave-length to be used.
For example, for a 200-meter wave-length, ike radiating circuit should be
two hundred feet long. The wiring in the machine as far as the fairlead
supplies a part of this and the aerial supplies the remainder. The aerial
reel is fastened to the cowl at the right hand of the forward part of the
pilot's cockpit, so that its position is directly above the fairlead.
3. USB or RADIO IN ABTILLKRT OBSntVATIOH
A. HANDLING OF THE RADIO SET
1. Preparatory to flight The responsibility for the proper working
condition of the racUo set rests with the Radio Officer of the squadron. It
is his duty to see that the plane contains a transmitter and properly charged
battery with all connections made, to adjust the vibrator of the spark-coil,
make the proper settings of the helix dips, and to adjust the spark gap, se-
curely tightening all lock nuts or screws as he does so. Also, he provides the
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804 SIGNALLING
machine with a reel carrying an aerial of the proper leng^ to correspond
to the wave-length adjustment of the helix.
Before beginning a flight, the pilot should assure himself as to the
proper condition of the radio equipment by proceeding as follows: (1)
see that the plane contains a transmitter; (2) examine ^e aerial system to
see that the reel works freely, that the wire passes readily through the fair-
lead, that the weight is securely fastened to the end of the aerial and is
pulled up dose to the f airlead, and that there are no broken strands in the
aerial ; (3) adjust sending keys both as to spring tension and length of beat,
tightening lock nuts securely; (4) close safety switch, press the key and
listen for the sound of the spark; (5) open safety switch. If any broken
strands are noticed in the aerial, a new aerial should be asked for.
2. During flight When the machine has reached an elevation of
five hundred feet or more, the aerial may be let out. It should not be allowed
to run out rapidly and stop with a jerk, as the sudden stop will cause the
wire to break. When the weight is lost, the aerial should be reeled in at
once. The reeling in will be less difficult if the machine is flown in a zig-zag
course in order to keep the wire stretched out behind the machine. When
the aerial has been let out to its full length, the safety switch may be closed,
and the set is ready for use. The central radio station of the squadron
should be called first, and upon receipt of his acknowledgment, the battery
station should be called.
In calling up ground stations in order that they may "tune in*' your
signals, the machine should not approach within two miles of the ground
station while sending. When two stations are very close together, the tun-
ing in becomes ineffective because of the intensity of the signals, and then
when the two stations become separated by the proper distance, the receiving
station must repeat the tuning in process before any signals can be heard.
When the tuning has been completed and both the central station and
the battery station have answered their calls, the pilot may take his position
above the target, ready to report the results of his observations. In report-
ing to the battery station, the sender should take pains to send "as plain
as print '\ The signals should be spaced as carefully as possible, as they
represent words in themselves, otherwise they would have no meaning. The
receiving operator hears many signals in his receivers, and can follow any
one sender only when the sending is firm, steady, and properly spaced.
Care should be taken never to send while turning, but only when flying
directiy toward the receiving station. When the observations have been
completed and the last signal has been sent, the safety switch should be
opened and the aerial reeled in.
B. METHODS OP PREVENTING INTERFERENCE
The confusion of signals, commonly spoken of as interference, or " jambing'',
results from the large number of airplanes which often operate within a given
region of the front. Four methods are used in the attempt to prevent this
"jambing" at receiving stations: (1) use of different wave-lengths; (2) use of
variously tuned sparks ; (3) directional sending; (4) synchronized chronometers.
1. Wave-lengths The use of different wave-lengths, when tuning
is carefully done, serves to eliminate jambing, provided the wave-lengths
can be varied over a sufficiently wide range. Since the shortest practical
wave-length for an airplane set is about one hundred meters, and the longest
about three hundred meters, this method does not eliminate interference
completely, and other means must be employed.
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SIGNALLING 805
2. Tuned sparks Although the receiving operator often hears sig-
nals from a number of machines simnltaneouEdy, he is able to follow the
signals from one machine and to disregard the others if the spark notes from
the various machines differ in pitch. The tone of the spark depends upon
the rapidity of vibration of the spark-coil vibrator. Hence the tone of the
spark for each machine is fixed by the Radio Officer through a proper ad-
justment of the vibrator. From the standpoint of the receiving operator,
the situation becomes that of reading signals from an open buzzer in a room
where a number of other buzzers are in operation at the same time, except
that the signals are much less intense and more outside interference is
present
8. Directional sending Directional sending is very important in
the reduction of interference. Not only is it necessary that the sending
operator avoid sending any unnecessary signals, that all information be
transmitted in code as far as possible, and that the spacing of the letter
groups be so distinct as to leave no doubt in the mind of the receiving oper-
ator as to what the signals are, but also it is very important that the machine
be flying directly toward the receiving station when the sending is done.
The strongest radiation from an aerial occurs in a direction perpendicular
to the aerial. Hence on account of the backward drift of the aerial, an ap-
proadiing machine presents the broadside of its aerial to the receiving sta-
tion, and the signals will be of maximum strength for the given distance.
Sending should never be done while the machme is turning, because the
swinging of the aerial will cause the direction of the strong signals to be
changing rapidly for a time. Such signals would be extremdy difficult for
a receiving operator to follow on account of their continual change in in-
tensity. An additional reason for flying toward the receiving station when
sending is that the battery station also possesses a ''directional aerial".
This aeritf is erected in such a position that it responds to signals coming
from the direction of the target much more strongly than to signals of the
same strength coming in from other directions.
4. Synchronized watches Under exceptional circumstances, when
'* jambing" is very bad, pilots are supplied with synchronized chronometers,
or very accurate watches^ and each machine has a definite time assigned in
which its sending must be done. This is an emergency measure, as it renders
observation very slow on account of the necessity that machines take turns in
sending down the results of their observations.
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DATE DUE
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UNIVERSITY OF MICHIGAN
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