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Full text of "Wood in aircraft construction"

GIFT 


OF 




U. S. Department of Agriculture, Forest Service 

FOREST PRODUCTS LABORATORY 

In cooperation with the University of Wisconsin 

MADISON, WISCONSIN 



WOOD IN AIRCRAFT CONSTRUCTION 



Approved Copy Filed 



I* ?f 
~ 



MAU-i LIBRARY-AGRICULTURE DEM". 




AIRCRAFT DESIGN DATA. NOTE NO. 12. 

WOOD IN AIRCRAFT CONSTRUCTION. 

[Prepared by the Forest Products Laboratory, Forest Service, TJ. S. Department of Agriculture.] 






CONTENTS. 

Page. 



Mechanical and physical properties 

Variability of the strength of wood 

Wood nonhomogeneous 

Variation of strength with locality of growth 

Variation of strength with position in tree 

Variation of strength with rate of growth 

Variation of strength with amount of summmvood 

Variation of strength with specific gravity 

Variation of strength with moisture content 

Defects affecting strength 



-Xl/^cUIH.H-H^^l'l^JiSl'ii , ** 

Diagonal and spiral grain 11 

Knots 21 

Pitch pockets 21 

Compression failures and "cross breaks" 22 

Braslmess 22 

Decay 22 

Internal or initial stresses in wood 22 

Wood fibers under stress in the tree 22 

Internal stresses produced during drying 25 

Initial stresses produced in assembling 25 

Working stresses for wood in aircraft construction 25 

Nature of loading " 27 

Tensile strength 27 

Torsional strength 28 

Shrinkage 28 

Suitability of various American woods for aircraft 34 

Construction ." 

Conifers 

Hardwoods 

Storage and kiln drying 40 



Rules for piling lumber 

Kiln drying of wood 

Advantages of kiln drying 

The elimination of moisture from wood 

Three essential qualities of a dry kiln 

Defects due to improper drying 

Case hardening and honeycombing 

Collapse 



40 
41 
41 
41 
42 
42 
42 
44 

Brashness 45 

Methods of testing conditions during drying 45 

Preliminary tests 46 

Current tests 46 

Final tests 49 

Specifications for kiln drying lor aircraft stock. 50 

Treatment of wood after removal from kiln 55 

Changes of moisture in wood with humidity of air 50 

Veneer and plywood 57 

Veneer 57 

Plywood 59 

Properties of wood parallel and perpendicular to the grain . 5<i 

Plywood panels vs. solid panels 61 

Symmetrical construction in plywood 61 

Direction of grain of adjoining plies 62 

Effect of moisture content 62 

Shrinkage of plywood 63 

Effect of varying the number of plies 63 

Effect of varying the ratio of core to total thickness 63 

Species of low density for cores 64 

Plywood test data , 64 

Riveted joints in plywood 70 

Joints in individual plies 80 

Joints extending through the entire thickness of plywood . 81 

Thin plywood 82 

98257 19 No. 12 1 



Page. 

Veneer and plywood Continued. 
Ply wood Continued . 

Woven plywood 83 

Specification for water-resistant veneer panels or plywood. 83 

Glues and gluing 86 

Hide and bone glues 86 

Testing of hide glue 86 

Precautions in using hide glue 90 

Liquid glues 91 

Marine glues 91 

Blood albumen glues 91 

Casein glues 92 

Instructions for use 92 

Equipment 92 

Preparation of glue 92 

I'roportions of dry glue and water 92 

Mixing the glue 92 

Consistency of glue [[ 92 

Application and use of glue 94 

Directions for mixing Certus glue 95 

Directions for mixing Napco glue 95 

' * Directions for mixing Casco glue 95 

Directions for mixing Perkins Waterproof Casein Glue 96 

Aircraft parts 96 

Laminated construction % 

Wing beams 98 

Results of various beam tests 98 

General conclusions 100 

Beam splices 100 

Struts 108 

Methods of test 103 

Tests on standard J-l struts 103 

Tests on rejected J-l struts 104 

Tests on standard de Havilland struts 105 

Tests on rejected de Havilland struts 106 

Tests on standard F5-L struts 106 

Two noninjurious test methods for inspecting struts 106 

Discussion of noninjurious test methods 109 

Comparison of two test methods by actual trials 110 

Miscellaneous strut tests 112 

Tests on struts stream lined with plywood 1 12 

Tests on struts covered with bakelized canvas 112 

Effect of taper on the strength of struts 113 

Design and manufacture of built-up struts 113 

Use of materials of different density 114 

Possibility of using defective material 115 

Possibility of warping or bowing llfi 

Conclusions 117 

Wing ribs 119 

Tests on DH-4 wing ribs 124 

Tests on SE-5 wing ribs 125 

Tests on HS wing ribs 126 

Tests on F5-L wing ribs 128 

Tests on 15-foot wing ribs 13& 

Tests on elevator or aileron spars 1 134 

Tests on aircrat't engine bearers . ^ 136 

Tests on bakelized canvas (micarta) 14C 

Treatments for preventing changes in moisture 140 

Instructions for applying aluminum leaf to aircraft propel- 
lers 112 

Appendix 1*7 

The determination of moisture content In wood . 147 

The determination of specific gravity of wood 147 



415834 



: 'Alkt?BAFT DESIGN DATA. Note 12. 



MECHANICAL AND PHYSICAL PROPERTIES. 

Wood differs from other structural materials in a great many ways, and the maximum 
efficiency in its use demands a thorough knowledge of the properties of wood and of the factors 
which influence these properties. In the following general discussion an attempt is made to 
explain the principal causes for the wide variations found in the strength of wood and to show 
how these variations may be largely eliminated in any group of material by proper specifi- 
cation and inspection. 

VARIABILITY OF THE STRENGTH OF WOOD. 

WOOD NON-HOMOGENEOUS. 

Wood is exceedingly variable as compared with other structural materials. This vari- 
ability is due to a number of factors, heretofore not well understood. For that reason any 
judgment of the strength of a piece was felt to be uncertain. The causes for variations in 
the properties of wood can now be given and their effects anticipated within reasonable limits. 

VARIATION OF STRENGTH WITH LOCALITY OP GROWTH. 

In some cases the locality of growth has an influence on the strength of the timber. For 
example, tests show a marked difference in strength between the Rocky Mountain and coast 
types of Douglas fir in favor of the coast type. 

This influence of locality is usually overestimated. Different stands of the same species 
grown in the same section of the country may show as great differences as stands grown in 
widely separated regions, so that as a rule locality of growth can be neglected. 

VARIATION OF STRENGTH WITH POSITION IN THE TREE. 

In some instances specimens from different parts of the same tree have been found to 
show considerable difference hi strength. In most cases, however, the wood of the highest 
specific gravity has the best mechanical properties regardless of its position in the tree. 
Where this is not the case, the toughest or most shock-resistant material is found near the 
butt. Above a height of 10 or 12 feet variations of mechanical strength correspond to the 
variations of specific gravity. Some variations with position hi cross section or distance from 
the pith of the tree have been found which could not be entirely accounted for by differences 
in specific gravity. 

VARIATION OF STRENGTH WITH RATE OF GROWTH. 

Strength is not definitely proportional to rate of growth, either directly or inversely. 

Timber of any species which has grown with exceptional slowness is usually below the 
average of the species in strength values. 

Among many of the hardwood species, material of very rapid growth is usually above 
the average in strength properties. Notable exceptions to this are found, however, and rapid 
growth is no assurance of excellence of material unless accompanied by relatively high spe- 
cific gravity. This is particularly true of ash. 

In the coniferous species, material of very rapid growth is very likely to be quite brash 
and below the average strength. 



Note 12. AIRCRAFT DESIGN DATA. 



VARIATION OF STRENGTH WITH AMOUNT OF SUMMER WOOD. 



In many species the proportion of summer wood is indicative of the specific gravity, , and; 
different proportions of summer wood are usually accompanied by different specific gravities 
and strength values. However, proportion of summer wood is not a sufficiently accurate 
indicator of strength to permititsuse as the sole criterion for the acceptance or rejection of airplane 
material. After some practice one should be able, through observation of the proportion of 
summer wood, to decide whether any particular piece is considerably below, considerably above, 
or near the required specific gravity. Caution must be observed in applying this to ash, and per- 
haps to other hardwoods, since rapid-growth ash is sometimes very low in specific gravity 
in spite of a large proportion of summer wood. In such cases careful examination will show 
that the summer wood is less dense than usual. 

VARIATION OF STRENGTH WITH SPECIFIC GRAVITY. 

A piece of clear, sound, straight-grained wood of any species is not necessarily a good stick 
of timber. To determine the quality of an individual stick by means of mechanical tests is 
extremely difficult, because the variations in strength of timber due to variations in moisture 
content, temperature, speed of test, etc., are so great. Furthermore, a test for one strength 
property does not always indicate what the other properties of the timber are. Without actual 
and complete tests, the best criterion of the strength properties of any piece of timber is its 
specific gravity or weight per unit volume, weight being taken when the wood is completely 
dry and volume when the wood is at some definite condition of seasoning or moisture content. 
Specific gravity based on oven-dry volume is greater than that based on the volume at any 
other moisture condition in proportion to the shrinkage which takes place as the moisture 
is driven out and the wood is reduced to the oven-dry condition. 

Accurate determinations made on seven species of wood, including both hardwoods and 
conifers, showed a range of only about 4 per cent in the density of the wood substance, or 
material of which the cell walls is composed. Since the density of wood substance is so nearly 
constant, it may be said that the specific gravity of a given piece of wood is a measure of the 
amount of wood substance contained in a unit volume of it. Very careful analyses based on 
a vast amount of data have shown that wood of high specific gravity has greater strength than 
that of low specific gravity. Some fairly definite mathematical relations between specific 
gravity and the various strength properties have been worked out. Some of the strength 
properties (strength in compression parallel to grain and modulus of elasticity) vary directly 
as the first power of the specific gravity; others, however, vary with higher powers of the 
specific gravity, i. e., the strength property changes more rapidly than the specific gravity, a 
10 per cent increase of specific gravity resulting in an increase in the strength properties of 
15 per cent to even 30 per cent. 

The rate of change in strength with changes of specific gravity is usually greater in indi- 
vidual specimens of a single species than in the averages for a number of species. This is 
illustrated by a comparison of figures 1 and 2. Figure 1 indicates that the modulus of rupture 
varies as the 5/4 power of the specific gravity when various species are considered, while figure 
2 indicates that the relation of the crushing strength of individual specimens of white ash varies 
as the 3/2 power of the specific gravity. The modulus of rupture of spruce and of numerous 
other species has been found to vary as the 3/2 power of the specific gravity. Shock-resisting 
ability and other important properties vary as even higher powers of specific gravity. If an 
important airplane part is from wood 10 per cent below the specific gravity given in the speci- 



AIRCRAFT DESIGN DATA. 



Note 12. 



fications, it will not be just 10 per cent but at least 14.5 per cent inferior and perhaps more, 
depending on which particular property is of greatest importance in the part in question. If 
the specific gravity is 20 per cent low, the inferiority will not be less than 28.4 per cent. The 



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Fig. 1. Relation between the modulus of rupture and specific gravity of various American woods. 

lighter pieces of wood are usually exceedingly brash, especially when dry. The importance of 
admitting no material for airplane construction of lower specific gravity than given in the 
specifications is evident. 

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List of species and reference numbers for figure 1. 
HARDWOODS. 



Species. 


Locality. 


Reference 
No. 


Species. 


Locality. 


Reference 
No. 


Alder red 


Washington 


30 


Hickory Continued . 






Ash: 






Pignut 


Pennsylvania 


160 


Biltmore 


Tennessee 


91 


Do 


West Virginia 


161 


Black 


Michigan 


60 


Shagbark 


Mississippi 


140 


Do . ... 


Wisconsin 


70 


Do 


Ohio 


159 


Blue 


Kentucky 


!)() 


Do 


Pennsylvania 


143 


Green 


Louisiana 


93 


Do 


West Virginia 


153 


Do . ... 


Missouri 


100 


Water 


M ississippi 




Pumpkin 


do 


79 


Holly, American 


Tennessee 


Q7 


White 


Arkansas 


106 


Hornbeam 


do 


149 


Do 


New York 


128 


Laurel, mountain 


do 


145 


Do 


West Virginia 


83 


Locust: 






Aspen 


Wisconsin 


23 


Black. 


do 


158 


Largetooth 


..do 


20 


Honey 


Indiana 


169 


Basswood . . . 


Pennsylvania 


12 


Madrona .... 


California 


101 


Do 


Wisconsin 


5 


Do 


Oregon 


128a 


Beech 


Indiana 


110 


Magnolia . 


I ouisiana 


fil> 


Do 


Pennsylvania 


98 


Maple : 






Birclr 






Oregon 


Washington 


58 


Paper 


Wisconsin 


73 


Red 


Pennsylvania 


69 


Sweet 


Pennsylvania 


129 


Do 


Wisconsin 


92 


Yellow 


..do 


107 


Silver 


do 


56 


Do 


Wisconsin 


103 


Sugar 


Indiana ' 


104 


Buckeye yellow 


Tennessee 


9 


Do 


Pennsylvania 


108 


Buckthorn, cascara . . . . 


Oregon 


84a 


Do 


Wisconsin 


124 


Butternut 


Tennessee 


27 


Oak: 






Do 


Wisconsin 


21 


Bur 


do 


125 


Chinquapin, western . . . 


Oregon 


48b 


California black .... 


California 


80 


Cherry- 






Canyon live 


do 


163 


Black 


Pennsylvania 


72 


Chestnut 


Tennessee 


121 


Wild red 


Tennessee 


24 


Cow 


Louisiana 


133 


Chestnut . ... 


M aryland 


46 


Laurel . 


do 


116 


Do 


Tennessee 


40 


Post 


Arkansas 


130 


Cotton wood, black ... . 


Washington 


6 ' 


Do 


Louisiana 


137 


Cucumber tree 


Tennessee 


59 


Red 


Arkansas 


119 


Dogwood : 






Do 


Indiana 


118 


Flowering 


do 


151 


Do 


Louisiana 


117 


Western 


Oregon 


125a 


Do 


Tennessee 


97 


Elder, pale 


. do 


69a 


Highland Spanish . . 


Louisiana 


94 


Elm: 






Lowland Spanish . . . 


. do 


142 


Cork 


Wisconsin, Marathon 


126 


Swamp white 


Indiana 


150 




County. 




Tanbark 


California 


115 


Do 


Wisconsin, Rusk 




Water ... 


Louisiana 


111 




County. 




White 


Arkansas 


132 


Slippery 


Indiana 


102 


Do 


Indiana 


138 


Do 


Wisconsin 


74 


Do 


Louisiana Richland 


136 


White 


Pennsylvania 


55 




Parish 




Do 


Wisconsin 


53 


Do 


Louisiana, Winn Parish 


131 


Greenheart 




165 


Willow 


Louisiana 


109 


Gum: 






Yellow 


Arkansas 


122 


Black 


Tennessee 


68 


Do 


Wisconsin 


105 


Blue (Eucalyptus) 


California 


147 


Osage orange 


Indiana 


164 


Cotton . . 


Louisiana . . . 


76 


Poplar yellow (tulip 


Tennessee 


35 


Red 


Missouri 


54 


tree) 






Hackberry 


Indiana 


90 


Rhododendron, great. 


do 


85 


Do 


Wisconsin 


78 


Sassafras 


do 


51 


Haw, pear 


do 


146 


Serviceberry 


do 


156 


Hickory : 






Silverbell tree 


do 


49 


Big shellbark 


Mississippi 


135 


Sourwood 


do 


89 


Do 


Ohio 


154 


Sumac, staghorn 


Wisconsin 


61 


Butternut 


do 


139 


Sycamore 


Indiana 


63 


Mockernut 


Mississippi 


144 


Do 


Tennessee . ... 


65 


Do 


Pennsylvania . . . 


159 


Umbrella Eraser 


do 


45 


Do 


West Virginia 


155 


Willow 






Nutmeg 


Mississippi 


112 


Black 


Wisconsin 


11 


Pignut 


do 


148 


Western black 


Oregon . ... 


43a 


Do 


Ohio 


157 


Witch hazel 


Tennessee 


114 















AIECEAFT DESIGN DATA. 



Note 12. 



List of species and reference numbers for figure 1 Continued. 

CONIFERS. 



, - - 

Species. 




Locality. 


Reference 
No. 


Species. 


Locality. 


Reference 
No. 


Cedar: 
Incense 


California 


26 


Pine Continued. 
Lodgepole 


Montana, Granite 


41a 


Western red 


Montana . ... 


2 




County. 




Do 


Washington 


10 


Do 


Montana, Jefferson 


40a 


; WJiite 


Wisconsin 


1 




County. 




t 'v press, bald 


Louisiana 


62 


Do 


Wyoming 


34 


Douglas fir 


California 


45a 


Lon ir leaf 


Florida ... 


123 


Do 


Oregon 


67a 


Do 


Ix)uisiana,Lake Charles. 


113 


Do 


Washington, Chelialis 


46a 


Do 


Louisiana, Tangipahoa 


96 


Do 


County. 
Washington, Lewis 


75 


Do 


Parish. 
Mississippi 


95 




County. 




Norway 


Wisconsin 


57 


Do 


Washington and Ore- 


67 


Pitch 


Tennessee . . . 


71 




gon. 




Pond 


Florida 


86 


Do... 


Wyoming 


84 


Shortleaf 


Arkansas 


77 


Fir: 






Sugar 


California 


22 


A Ipine 


Colorado 


4 


Table Mountain 


Tennessee 


82 


Amabilis 


Oregon 


39 


Western white 


Montana 


42 


Do 


Washington 


18 


\Vestern yellow 


Arizona 


19 


Balsam 


Wisconsin 


14 


Do 


California 


37 


Grand 


Montana. . 


36 


Do 


Colorado 


41 


Npble : 


Oregon 


16 


Do 


Montana 


32 


White 


California 


17 


White 


Wisconsin 


25 


Hemlock: 






Redwood 


California, Albion 


28 


Black 


Montana 


47 


Do 


California Korbel 


13 


Eastern 


Tennessee 


52 


Spruce' 






Do 


Wisconsin 


15 


ETigelmann 




g 


Wjestern 


Washington 


50 


Do 


Colorado San Miguel 


3 


Larch,! western 


Montana 


84 








D6 - 


Washington 


64 


Red 




44 


Pine: i 






Do 


Tennessee 


29 


Ciiban 


Florida 


127 


White 




7 


Jack 


Wisconsin 


43 


Do 


Wisconsin 


38 


Jeffrey 


California 


33 


Tamarack 


do 


81 


LdbloUy 


Florida 


88 


Yew, western 




134 


Lqdgepole , 


Colorado 


31 








! Do 


Montana Gallatin 


35a 








: 

1 
_ 1 


County. 











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Note 12. 



AIRCRAFT DESIGN DATA. 



7 



The minimum strength values which may be expected of a particular lot of lumber can 
be raised a good deal by eliminating a relatively small portion of the lighter material. This 
lightweight material can, as a rule, be detected by visual inspection. In order to train the 
visual inspection and to pass judgment on questionable individual pieces, frequent specific 
gravity determinations are necessary. 



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.1 .2 .3 4 .5 .6 .7 .8 .9 1 
SPECIFIC GRAVITY -OVEN DRY 

ASID ON OREEN VOLUME 



Fig. 2. 

A specific gravity determination is relatively simple to make, and it is probably a better 
criterion of all the qualities of the piece than any single mechanical test which is likely to be 
applied; also the specific gravity determinations need no adjustment such as would be neces- 
sary on account of the varied conditions of a mechanical test. 






;l 



AIRCRAFT DESIGN DATA. 



Note 12. 



VARIATION OF STRENGTH WITH MOISTURE CONTENT. 

When a piece of green or wet wood is dried, no change in mechanical properties takes place 
until the fiber-saturation point is reached.* The changes beyond this point for small test 
specimens free from defects and very carefully dried are illusl rated in figures 3 and 4. These 







1,000 



300 



15 20 25 30 35 40 45 50 55 
MOISTURE-PER CENT OF DRY WEJGHT. 



60 



65 70 



Fig. 3. Relation between the stiffness (modulus of elasticity) in binding and moisture content, for three species. 

figures show that the moisture content at the fiber-saturation point differs for different species. 
It will be noted that the influence of moisture is smaller in tests of shearing strength and 
compression perpendicular to the grain than in bending and compression parallel to the grain. 

* The eucalypts and some of the oaks are exceptions to this rule. 



Note 12. 



AIRCRAFT DESIGN DATA. 






9 



Furthermore, there is no definite break at or near the fiber-saturation point in the moisture- 
strength curves for shear and compression perpendicular to the grain. In the case of shear 
this failure to show large increases in strength is probably due to checks which form as the 
material dries. 



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6 5 10 -15 20 25 30 35 40 45 

MOISTURE- PER CENT OF DRY WEIGHT. 

Fig. 4. Comparison of the relation between strength and moisture content for red spruce in various kinds of tests. 
(The lowest curve is for compression at right angles to grain.) 

The moisture content at the fiber-saturation point varies not only with the species but 
with different specimens of the same species. The percentage change of strength which results 
from a given change of moisture also varies with the species and with individual specimens of 
the species. 



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AIRCRAFT DESIGN DATA. 



Note 12. 



The form of the curves shown in figures 3 and 4 applies only to small clear pieces very 
carefully dried and having a practically uniform moisture content throughout. If the mois- 
ture be unequally distributed in the specimen, as is the case of large timbers rapidly dried or 
of "case-hardened" pieces, the outer shelj may be drier than the fiber-saturation point while 
the inside still contains free water. The resulting moisture-strength curve wiJU be higher than 
the curve from carefully dried pieces and wil,l be so rounded off from the driest to the wettest 
condition as to obscure entirely the fiber-saturation point (see fig. 5). 

The increase in strength which takes place in drying wood depends upon the specimen 
and upon the care with which the drying process is carried out. Furthermore, while the strength 
of the fibers is no doubt greatly increased by any reasonable drying process, the increase of the 
strength of a piece of timber taken as a whole may be very much less. Knots are more or less 
loosened, checking takes place, and shakes are further developed. In large bridge and building 
timbers these effects are so great that it is not considered safe to figure on such timbers having 
greater strength when dry than when green. When the pieces are small and practically free 



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i 

k 6.006 

3> 



5.000 



>%. 



10 ft 



30 JS 40 4S' SO SS 60 65 
MOISTURE PERCENT, or DRV WEIOH1. 



70 75 00 86, 90 OVtRSO 



S9 



Fig. 5. Effect of case-hardening upon the form of the moisture-strength curve in bending tests. The upper curve 
is from case-hardened specimens, the lower curve from uniformly dired specimens. 

from defects, as in airplane construction, proper drying with careful control of temperature 
and humidity increases the strength of material very greatly. In whatever way wood is dried, 
upon its being resoaked and brought back to the original green or wet condition it is found to 
be weaker than it was originally. So when it is said that wood has been injured in the drying 
process it must be taken to mean that it is weaker than it should have been after drying and 
while still in a dried condition. 

When a stick of timber dries out below the fiber-saturation point (that is, when it has 
lost all its free moisture and the moisture begins to leave the cell walls), the timber begins to 
shrink and change in its mechanical properties. Also numerous stresses are set up within the 
timber. Under severe or improper drying conditions the stresses may be great enough to 
practically rum the material for purposes where strength is important. Improper drying con- 
ditions, however, do not of necessity mean fast drying conditions. When properly dried, the 
timber gains in its fiber stress at elastic limit, its modulus of rupture, maximum crushing strength, 
etc. It bends farther at the elastic limit when dry than when green, but does not bend so far 
at the maximum load. After having been bent to the maximum load dry timber breaks more 
suddenly than green timber of the same species that is, dry timber is more brash than green, 
although it withstands greater stresses and is stiffer. 



Note 12. 



AIRCRAFT DESIGN DATA. 



11 



DEFECTS AFFECTING STRENGTH. 

DIAGONAL AND SPIRAL GRAIN. 



Diagonal grain is produced when the saw cut is not made parallel to the direction of the 
fibers. It can usually be avoided by careful sawing unless it is caused by crooks in the log. 
Spiral grain, on the other hand, results from a spiral arrangement of the wood fibers in the tree. 










Fig. 6. Spiral grain in Sitka spruce. 

If a log is spiral grained, it is impossible to secure straight-grained material, except in small 
pieces, from the spiral-grained part. The effect of spiral grain is illustrated in figure 6, which 
shows three views of a piece of Sitka spruce. The center part of a log may be straight grained 
and the outer part spiral grained or vice versa. 



12 



AIRCRAFT DESIGN DATA. 



Note 12. 



Figures 7 to 14, inclusive, show the weakening effect of spiral or diagonal grain upon various 
strength properties of Sitka spruce and Douglas fir. The data are based upon about 1,400 
static bending tests, made upon clear specimens, third point loading, 45-inch span. Similar 
impact bending tests have shown similar weakening with increasing slope of grain. 



7OOO 



/00% 




O/-/ess 



OFGRA/N 



o Avenage vaJu.c.s 

Probab/e m/n/~musn 



Fig. 7. The effect of spiral and diagonal grain on the fiber stress at the elastic limit; Sitka spruce. 



- 



-ii 



AIRCRAFT DESIGN DATA. 



13 



/oooo 



9OOO 




/.JO 

SLOPE Of 



/'/O 



/.O 



mtn/mum 
Fig. 8. The effect of spiral and diagonal grain on the modulus of rupture; Sitka spruce. 



14 



AIRCRAFT DESIGN DATA. 



Note 12. 



2000 



/<3OO 



/OO% 




o Menace va/ues 

? X77//7//7? 
Fig. 9. The effect of spiral and diagonal grain on the modulus of elasticity; Sitka spruce. 



Note 12. 



AIRCRAFT DESIGN DATA. 



15 







<$ 

3 



8 * 

Nl 
tt 

^* 



t 



/.'4o 
or /ess 



wa* yes 



7a7, 



^ 



ea,'? 







\ 



^ 




3 



80% 
60% 



20% 



/:3o 



/:2o 



\ L L_L 

Fig. 10. The effect of spiral and diagonal grain on the work to maximum load; Sitka spruce, 



\ 



jr 



16 



AIRCRAFT DESIGN DATA. 



Note 12. 






/oooo 

9OOO 
&OOO 

7000 

6000 




. 

\ 

> S000 



4000 



\ 



3O00 



2000 



/ooo 



/*/: 



/77// 



7/rnu/r? 




era 



X 



\ 



^ 






\ 






% 



/:4-0 /:30 

or /ess 



SLOPE Or r GS?/t//V 



/oo% 

(50% 

60% 



0% 



':o 



Fig. 11. The eflect of spiral and diagonal grain on the fiber stress at the elastic limit; boughs ..*. 



Note 12. 



AIRCRAFT DESIGN DATA. 



17 



S2.OOO 




. 
W L&EMD 

O 



or/ess 



/77/7S77U/7? 

Fig. 12. The effect of spiral and diagonal grain on the modulus of rupture; Douglas fir. 

98257 19 No. 12 2 



18 



AIECEAFT DESIGN DATA. 



Note 12. 



2200 



2OOO 




o/- /ess 



/:30 /.20 

SLOPE 



607* 



40% 



20% 



0% 



^ 



t/a/ues 

'r?//77u/77 i/a/uesr 



Fig. 13. The effect of spiral and diagonal grain on the modulus of elasticity; Douglas fir. 
>n lo witoboin ml) no aims IfinogBib briH Iuiiq to } 



Note 12. 



AIECEAFT DESIGN DATA. 



19 



The tests were made upon seasoned material, but since the moisture content of the indi- 
vidual specimens varied somewhat, it was necessary to reduce such properties as are materially 
affected by changes in moisture content to a uniform basis before comparisons could be made. 
Therefore, the values for fiber stress at the elastic limit, modulus of rupture, and modulus of 
elasticity have been reduced to 11 per cent by means of an empirical exponential formula. 
The work to the maximum load values were not reduced to a uniform moisture basis, since the 
correction would have been very small, and no greater accuracy would have been insured. 




Fig. 14. The effect of spiral and diagonal grain on the work to maximum load; Douglas fir. 

In addition to the curve for average values based on test data, a curve for probable mini- 
muni values (broken line) was calculated and plotted. A third curve was also drawn showing 
the probability of individual values falling below the probable minimum value for straight- 
grained material. This probability is expressed in per cent and, as is to be expected, increases 
greatly as the slope of the grain becomes steeper. 

The rate of falling off in strength increases abruptly at a slope between 1 in 20 and 1 in 15, 
and therefore this slope may be considered to be the critical one. It is to be noted, however, 
that even at slopes at 1 in 20 there is a decided weakening. 



20 



AIRCRAFT DESIGN DATA. 



Note 12. 



As a result of these tests it is recommended that for purposes of design the following values 
for moduli of rupture for spruce at 15 per cent moisture and different slopes of spiral or diagonal 
grain be strictly adhered to: 

From straight to 1 in 25 7,900 pounds per square inch. 

From 1 in 25 to 1 in 20 7,000 pounds per square inch. 

From 1 in 20 to 1 in 15 5,500 pounds per square inch. 

The effect of spiral grain upon the maximum crushing strength is much smaller than upon 
the modulus of rupture. The following stresses for different slopes of grain may be used with 
safety for compression members: 

From straight to 1 in 25.. -! I II 4,300 pounds per square inch. 

From 1 in 25 to 1 in 20 4,200 pounds per square inch. 

From 1 in 20 to 1 in 15 3,800 pounds per square inch. 

When the annual rings run diagonally across the end of a piece the true slope of diagonal 
grain can be obtained as shown by figure 15a. 





Slope of diagonal 
grain. 




D/ L -- Slope 



of spiral 
grain. 









Fig. 15. The measurement of the slope of diagonal and spiral grain. 

The direction of spiral grain is indicated on a tangential (flat sawn) face by the direction 
of the resin ducts. These ducts, however, are often difficult to see. Drops of ink placed on 
tangential faces and allowed to spread are sometimes used to test for spiral grain. The ink 
will tend to follow the angle of the grain. The direction of spiral grain is, howeVer, not given 
correctly by resin ducts or by spreading of ink unless these tests be applied to a truly tangen- 
tial face. In figure 15, for instance, resin ducts or spreading of ink would be practically parallel 
to the edges whether the material was spiral grained or not. In such cases spiral grain can be 
detected only by splitting on a radial line (Fig. 156) or by raising small splinters and observing 
if they have a tendency to tear deeper and deeper. 



Note 12. 



AIRCEAFT DESIGN DATA. 



21 



KNOTS. 

The effect of knots depends upon their location with respect to the stresses to which the 
piece will be subjected, as well as upon their size and character. None but sound knots, 
firmly attached, should be permitted. Obviously, knots of any considerable size can not be 
allowed in any airplane parts because the parts themselves are comparatively small in cross 
section. Since the weakening effect of knots results from their disturbance of normal arrange- 
ment of fibers, their seriousness can best be decided from a consideration of the grain. 

PITCH POCKETS. 

Tests recently completed on 112 solid Douglas fir wing beams, made especially to study 
the effect of pitch pockets upon the strength of beams indicate that this effect may have been 
overrated in previous specifications. The tests were made over a 72-inch span under third- 
point loading. The following conclusions from these tests are presented in the form of speci- 
fications, and are intended to be applied to spruce and fir wing beams: 

(a) In portions of the length where a slope of grain of 1 in 25 is the maximum allowed, 
pitch pockets 1| inches in length and not to exceed one-eighth of an inch in width or depth 
may be allowed in any portion of the section except the outer quarters of the flange. No 
pitch pockets to be allowed in outer quarters of flange. 

(6) Where a slope of spiral grain of 1 in 20 is allowed pitch pockets 2 inches in length 
and not to exceed one-fourth inch in width or depth may occur any place in the section except 
in the outer quarters of the flange. No pitch pockets to be allowed in outer quarters of flange. 

(c) Where a slope of grain of 1 to 15 is allowed pitch pockets 1J inches in length and one- 
fourth inch in width or depth may occur in the outer quarters of the flange, and pitch pockets 
3 inches in length and one-fourth inch in width or depth may occur in any other portion of 
the section. 

(d) Pitch pockets occurring in the web may not be closer together than 20 inches. If 
they are in the same annual ring, they may not be closer together than 40 inches. In other 
portions of the section these distances may be 10 inches and 20 inches, respectively. 

Combining this specification with a knot and spiral-grain specification, the following table 
has been prepared; it is the intention that this table be used in drafting parts specifications 
for spruce and fir wing beams: 

TABLE 1. Size and quantity of defects allowable witli different slopes of grain. 



Allowable slop* in grain not exceeding 


Knots. 


Pitch pockets. 


Maximum 
diameter 
permitted. 


Minimum 

distance 
l>otween 
any two. 


Maximum 
length per- 
mitted. 


Maximum 
width or 
depth per- 
mitted. 


1 inch in 25 


Inches. 
i 

A. 

4 


Inches. 
10 
12 
20 


Inches. 
H 
2 
3 


Inches. 
i 

} 


1 inch in 20 


1 inch in 15 





Supplementing the table are the following clauses: 



1. All knots must be sound. 

2. No defects must fall or cause irregular grain greater in slope than that allowable for cross grain in the outer 
quarter of the upper or lower flange; except that where a slope of 1 in 15 is allowed, pitch pockets 1J inches long ana 
one-fourth inch wide or deep may be permitted. 

3. Pitch pockets occurring in the web may not be closer together than 20 inches. If they are in the same annual 
ring, they may not be closer together than 40 inches. In other portions of the section these distances may be 10 inches 
aud 20 inches, respectively. 

4. The equivalent of the diameters specified may be allowed in a number of smaller knots, provided that they 
are not close together. 



22 AIRCEAFT DESIGN DATA. Note 12. 

COMPRESSION FAILURES AND " CROSS BREAKS." 

All material containing compression failures and "cross breaks" should be eliminated 
from airplane parts where strength is of importance. The cause of certain "cross breaks" 
near the center of large logs such as are quite frequently found in mahogany is not known. 
Compression failures, which are, in fact, of the same nature as "cross breaks," are known 
frequently to be due to injury by storm in the standing trees, to carelessness in felling trees 
across logs, to unloading from a car across a single skid, or to injury during manufacture. 

While some compression failures are so pronounced as to be unmistakable, others are 
difficult to detect. They appear as wrinkles across the face of the piece. Compression fail- 
ures not readily apparent to the eye may seriously reduce the bending strength of wood and 
its shock-resisting ability, complete failure occurring suddenly along the plane of injury. 

Figure 16 shows four samples of African mahogany containing compression failures which 
occurred during growth. These samples were later tested in static bending, and in all cases 
the compression failures developed during te'st followed those originally occurring in the 
samples. This is illustrated in figure 17. 

BRASHNESS. 

The term "brash," frequently used interchangeably with the term "brittle," when used 
to describe wood or failures in wood, indicates a lack of toughness. Brash wood, when tested 
in bending, breaks with a short, sharp fracture instead of developing a splintering failure and 
absorbs a comparatively small amount of work between the elastic limit and final failure. In 
impact tests brash wood fails completely under a comparatively small hammer drop. 

DECAY. 

The first effect of decay is to reduce the shock-resisting ability of the wood. This may 
take place to a serious extent before the decay has sufficiently developed to affect the strength 
under static load or to become evident on visual inspection. Unfortunately there is no method 
of detecting slight decay in wood except with a compound microscope. AH stains and dis- 
colorations should be regarded with suspicion and carefully examined. It must be remembered 
that decay often spreads beyond the discoloration it causes and that pieces adjacent to dis- 
colored areas may already be infected. On the other hand, not all stains and discolorations 
are caused by decay of the wood. The blue sapstain of some hardwoods and of many coniferous 
woods, including spruce, and the brown stain of sugar pine are not caused by decay-producing 
organisms and do not weaken the wood. 

INTERNAL OR INITIAL STRESSES IN WOOD. 

WOOD FIBERS UNDER STRESS IN THE TREE. 

Wood products are quite similar to metal castings as regards internal stresses. It is 
probable that wood fibers are continually under stress of some kind. The fact that freshly 
cut logs of some species split through the center (this frequently happens as the result of heavy 
shocks or jars and without the use of a wedge) is evidence of some tensile stresses in the outer 
portion of the tree and compression in the inner portion. These stresses are independent of 
the stresses due to the weight of the tree and pressure against it. 



Note 12. 



AIRCRAFT DESIGN DATA. 



23 




Fig. 16. Compression failure occurring during growth. African mahogany 



24 



AIRCRAFT DESIGN DATA.| 



Note 12. 




Fig. 17. Influence of compression failure occurring during growth on failures in static bending. African mahogany. 



Note 12. AIECRAFT DESIGN DATA. 25 



INTERNAL STRESSES PRODUCED DURING DRYING. 

The natural stresses may be partially or wholly relieved by sawing the tree into lumber, 
but other stresses are likely to be introduced by subsequent seasoning. Checking, honey- 
combing, warping, twisting, etc., are manifestations of the internal stresses which are produced 
in the drying of wood or whenever any change of moisture content takes place. Presumably 
such stresses are due to unequal distribution of moisture and consequent unequal shrinkage 
combined with more or less inherent lack of homogeneity. 

Air drying for a number of years, which is practiced in some woodworking industries, has 
for its object the equalization of moisture and the relief of stresses induced in the early part 
of the drying. Careful and correct kiln drying followed by a period of seasoning under proper 
and controlled atmospheric conditions should produce results at least equal and probably 
superior to those obtained by long periods of air drying. 

Relieving these internal stresses is important because they amount to an actual weakening 
of the material. If the fibers of a piece of wood are under stress when the piece is free, they 
are just that much less capable of resisting stresses of the same kind produced by exterior forces 
or loads applied to the piece. 

INITIAL STRESSES PRODUCED IN ASSEMBLING. 

When a member of any structure is stressed in assembling the structure and before any 
load is placed on it, it is said to be under initial stress. If the initial stress is of the same char- 
acter as the stress for which the member is designed, it constitutes a weakening, for when the 
structure is loaded the safe working stress of the member will be reached just that much sooner. 
If this initial stress is opposite in character to that for which the member is designed, it amounts 
to a strengthening of the member, for when the structure is loaded the initial stress must be 
overcome before the member takes any of the stress for which it is designed. 

Many of the curved parts of an airplane frame could be simply sprung to place on assembly. 
Were this done, they would be subjected to initial stress and usually of the same sign to which 
the member would later be subjected. In order to avoid initial stress, such parts are steam bent 
before assembly. It is desirable, of course, that this bending be so done as not to injure the 
material and to leave little tendency to spring back from the curves to which it is bent. In 
order that the material may be made sufficiently plastic to accomplish this result, it is essential 
that the steaming and bending be carried out while the wood is at a relatively high moisture 
content. If it is attempted on kiln-dry or thoroughly air-dry material, there is the tendency 
to spring back after the clamps are removed. Bending of such stock can not be compared 
to a considerable part of the bending done in other woodworking industries, where the strength 
of the wood is very greatly damaged by the bending process but without destroying its use- 
fulness for the purpose for which it is intended. Some of the unexpected failures of bent parts 
in airplanes have doubtless been due to the initial stresses set up in the member during the 
bending. 

WORKING STRESSES FOR WOOD IN AIRCRAFT CONSTRUCTION. 

Table 2 gives strength values at 15 per cent moisture (which is probably close to the maxi- 
mum moisture content of wood in a humid atmosphere) for use in airplane design, as well as 
the minimum specific gravity and average density which should be allowed. It is suggested 
that the working stresses for design be obtained by applying factors to the values for static 
load conditions as given in this table. 



26 



AIRCRAFT DESIGN DATA. 



Note 12. 



1C * Tji < 
.r-l^ CO< 



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p OOOCO CM' 



co ic *" co co c 



S t~ CO OO t-- CO 1 



00 Tfri CN OS J> OS CO 



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1C COOS OCNiCt-iCiC (N't 1 
CO r-l t-- CO O OS CO CO i I OS OS 



5; .O 
Q,c?O 
** 



CD CD CD ' 
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i OS CM CO -<f i 



000000 oo 
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ftft&slsg' 

ulilfl 

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g u r-lrH i-l 1-1 .-I I-H r-l 



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COoio'lN C^5CO 
(M r-l i-l iH I-H 



f~lC rHCNOOSrHil 

os'ic lot^ic'cdcdco 



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CD CD CD 

I I CO "t 1 GO 



i rt< co 10 t^ t^ 



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(M 00 CO i I i I Oi iH i 



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t^ t> OS O OO t- OS OO OS r- ( "*! 1C OS OS CD b- 



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Note 12. AIRCRAFT DESIGN DATA. 27 

Since it is impractical to season test specimens to precisely 15 per cent moisture, it was 
necessary to compute the strength values given in table 2 at this moisture from test data 
obtained at slightly different moisture contents. The formulae used in these computations are 
presented here as a matter of record. 



M less than 8, D 15 =' +B 
M 8 to 10, P 



M 10 to 11, 

Mil to 12, . 

D 1S = Strength at 15 per cent, AD = air dry strength value, B = green strength value, M = per 



cent of moisture. 

The factors to be applied, and consequently the exact stress to be used in design, of course, 
will depend largely on the conditions to which it is assumed the machine will be subjected in 
flight. If they are the most severe which the machine is ever expected to sustain while in 
flight, the working stresses can be relatively high. If, on the other hand, the assumed condi- 
tions are only moderately severe, the stresses must be made lower in order to take care of 
exceptional conditions which may occur. It must also be remembered that working stresses 
can not be safely based on average strength figures, but must be lowered to a value which will 
be safe for the weakest piece likely to be accepted. 

to no boftfed y\p. fttftb ymJnom^jnl 

NATURE OF LOADING. 

The time of duration of a stress on a timber is a very great factor in the size of the stress 
which will cause failure. A continuously applied load greater in amount than the fiber stress 
at elastic limit as obtained by the ordinary static bending test will ultimately cause failure. 

The fiber stress at elastic limit in static bending for the dry material is usually somewhat 
more than nine-sixteenths of the modulus of rupture, and in compression parallel to the grain 
the elastic limit is usually more than two-thirds of the maximum crushing strength. Timber 
loaded slightly below the elastic limit will gradually give to loads and ultimately assume greater 
deflections than those computed by using the ordinary modulus of elasticity figures. In impact 
tests where a weight is dropped on the stick and the stress lasts for only a small fraction of a 
second, the stick is found to bend practically twice as far to the elastic limit as in static tests 
where the elastic limit is reached in about two minutes. The elastic stress developed in the 
stick undeY the blow is greater than the maximum stress obtained in the static test. 

TENSILE STRENGTH. 

In general data on the tensile strength of wood are little needed, and consequently there is 
very little data available. The following table presents a few figures on the tensile strength of 
several species tested green. 

no Irtgrni) inoo ioq ait (fount ea moil ^jugim ,ef>oow .trunollii) iltiw ft-n-TUV 

:iom h<hoed.ft boow ^nl> noiiW Jjif v><] & olJJil an <t (eni^xf ^iir/ili -noWi 

iii -bnleirlJi <><: h/uj ,01 ,81 aoiirgi'd .bodrijm ei touoq fio&oi Jfitae ied3 odt Itimr HowaoJ 

Jnioij iioiJinid/:8 Txlit oiiJ bfifl'Jito-Jnoo triirfgiorii o'lax noowJotf gmilowa l>iw o^ihtmifi To 



28 



AIRCRAFT DESIGN DATA. 



Note 12. 






TABLE 3. Strength of various woods in tension parallel to grain. 
[From tests of small clear specimens of green timber.] 



Species. 


Number 
tests 
averaged. 


Number 
trees 
repre- 
sented. 


Moisture 
content. 


Specific 
gravity. 


Tension par- 
allel to 
grain average. 


Probable 
variation of 
individual 
from 
average. 


Mahogany African 


20 
27 

50 
59 
63 
48 
10 
7 
4 
42 
5 
13 


5 
7 
4 
18 
9 
10 
10 
10 
2 
2 
9 
3 
5 


Per cent. 
49.7 
50.1 
47.1 
49.9 
35.0 
24.1 
23.0 
50.0 
39 to 98 
31.0 
41 to 86 
34.5 
40 to 155 


c 0. 457 
c.492 
.550 
c.645 
.399 
c .530 
c.477 
. 369 
.390 
.401 
.351 
.500 
.400 


Lbs. per sg. in. 
15, 110 
16, 400 
14, 900 
14, 012 
11, 730 
16,200 
13. 300 
7,972 
7,716 
9, 760 
9,580 
9, 8SO 
!), GOO 


Lbs. per 
sq. in. 
2, 075 
2,400 


Mahogany Central American 




Oak northern white a . 


2,900 
1,210 
1,735 
2, 050 
1,400 
1, 570 


Cedar Port Orford 


Douglas fir (1) 


Douglas fir (2) 


Fir white 


Hemlock western 




Pine white 


1, 405 




Redwood ' 


1,170 





a Not identified as to species. 

i> Araucaria from Chile, South America. 

Specific gravity based on oven-dry weight and volume. 

(1) Specimens from the 8 feet immediately above stump, 
rom same trees. 



Other specific gravities based on oven-dry weight and volume as tested. 

(2) Specimens from the fifth 8-foot bolt above stump and higher. (1) and (2) 



TORSIONAL STRENGTH. 



Resistance to torsion is important in connection with control surface spars. The following 
fragmentary data are based on only 30 tests in all, 15 of each species: 






TABLE 4. Torsional strength q/ commercial white ash and Sitka spruce. 



Properties. 




White ash. 


Sitka spruce. 


Number of testa 


15 
15.8 
.62 
1, 753 
2, 371 
88, 500 
8.8 
24.0 


15 
15.7 
.39 
1, 090 
1, 654 
72, 300 
4.4 
19.7 


Moisture per cent of oven-dry weight 


Specific gravity (based on oven-dry weight and oven-dry volume) .. 


Shearing strength at elastic limit pounds per square inch 


Shearing strength at maximum load pounds per square inch 


Shearing modulus of elasticity pounds per square inch 


Work to elastic limit inch-pounds per cubic inch . . 


Work to first failure, inch-pounds per cubic inch (1) 



(1) For the spruce and ash tested the first failure oscurred at maximum load in all cases. 

SHRINKAGE. 






Ordinarily when a piece of green lumber is dried no change in dimensions takes place until 
the fiber saturation point is reached. The wood then begins to shrink in cross-sectional area 
until no further moisture can be extracted from the cell walls. It also shrinks longitudinally, 
but in most cases the amount of longitudinal shrinkage is so small as to be negligible. 

The shrinkage in cross-sectional area in drying from the green to the oven-dried condition 
varies with different woods, ranging from as much as 22 per cent (based on the original area 
before drying begins) to as little as 6 per cent. When dry wood absorbs moisture it continues 
to swell until the fiber saturation point is reached. Figures 18, 19, and 20 illustrate the progress 
of shrinkage and swelling between zero moisture con tent' and the fiber saturation point. 



Note 12. 



AIECEAFT DESIGN DATA. 



29 



The shrinkage of wood, like its strength, is very closely related to its specific gravity. 
This illustrated by figure 21. On this curve, "Per cent shrinkage in volume" is the total 
shrinkage from fiber saturation to dryness. It will be noted that*shrinkage, in general, increases 
with specific gravity. This relation in individual specimens of a single species (white ash) is 
shown in figure 22. 

Radial shrinkage, or the shrinkage in width of quarter sawn boards, averages about three- 
fifths as great, as tangential shrinkage, or the shrinkage in width of flat sawn boards. 



/# 
/z 

/o 



^L 



P 
< 

# 

I, 



fr 

i: 

* 

i- 


6 

* 

o 
2 

O 



Fig. 18. Relation between swelling and moisture. Each point is the average of from five to eleven specimens. 
Black dots indicate specimens that were kiln-dried and then allowed to reabsorb moisture. The fiber- 
saturation point is at c. 



fc- 



6 3 SO 5 M /6 J9 2O ZZ Z4 36 Z8 30 3Z 34 36 33 4O 







. iab .Kama lo *oi):>>8 eecr) ^dJ bn Jnalnoa 



arfl 



noUaloH .OS 



30 



AIRCRAFT DESIGN DATA. 



Note 12. 









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19. Relation between the moisture content and the cross section of small, clear pieces of western hemlock. 



K torn 

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ABSORPTION POHV 



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MOSTURC PERCCMT 

Fig. 20. Relation between the moisture content and the cross section of small, clear specimens of western larch. 



Note 12. 



AIRCRAFT DESIGN DATA. 



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Fig. 21. Relation between shrinkage in volume and specific gravity of various American woods. 



.>I-Mif?f 



AIRCRAFT DESIGN DATA. 



Note 12. 



List of species and reference numbers for -figure 21. 
HARDWOODS. 



Species. 


Locality. 


Reference 
No. 


Species. 


Locality. 


Reference 
No. 


Alder, red 


Washington 


30 

91 
60 
70 
99 
93 
100 
79 
106 
128 
83 
23 
20 
12 
5 
110 
98 

73 
129 
107 
103 
9 
84a 
27 
21 
46b 

72 
24 
46 
40 
6 
59 

151 

125a 
69a 

126 

102 
74 
55 
53 
165 

68 
147 
76 
54 
90 
78 
146 

135 

154 
139 
144 
159 
155 
112 
148 
157 


Hickory Continued . 
Pignut 


Pennsylvania 


160 
161 
140 
152 
143 
153 
141 
87 
149 
145 

158 
162 
101 

J^.Sa 
66 

58 
69 
92 
56 
104 
108 
124 

125 
80 
163 
121 
133 
116 
130 
137 
119 
118 
117 
97 
94 
142 
150 
115 
111 
132 
138 
136 

131 
109 
122 
105 
164 
35 

85 
51 
156 
49 
89 
61 
63 
65 
45 

11 
-43a 
114 


Ash: 
Biltmore 




Do 


West Virginia 


Black 


Michigan 


Shagbark 


Mississippi 


Do 


\Viscoiisin 


Do 


Ohio 


Blue 


Xentucky 


Do 


Pennsylvania 


Green 


Louisiana 


Do 


West Virginia 


Do 


Missouri 


Water 


Mississippi 


Pumpkin 


do 


Holly, American 


Tennessee 


White 




Hornbeam 


do 


Do 


New York 


Laurel, mountain 


do 


Do 


West Virginia 


Locust: 
Black 


do 


Aspen 


Wisconsin 


Largetooth 


do 


Honey 


Indiana 


Basswood 


Pennsylvania 


Madrona 


California 


Do 
Beech .... 


Wisconsin 
Indiana .... 


Do 


Oregon .... 


Magnolia 


Louisiana 


Do 


Pennsylvania 


Maple: 
Oregon . . 


Washington 


Birch: 
Paper 


Wisconsin 


Red 


Pennsylvania 


Sweet 


Pennsylvania 


Do 


Wisconsin . . 


Yellow 


do 


Silver 


do 


Do 


Wisconsin 


Sugar 


Indiana . 


Buckeye, yellow 


Tennessee ... 


Do 


Pennsylvania 


Buckthorn, cascara .... 
Butternut 


Oregon 


Do 


Wisconsin 


Tennessee 


Oak: 
Bur 


... .do 


Do 


Wisconsin 


Chinquapin, western . . . 
Cherry: 
Black 


Oregon 


California black 
Canyon live 


California 


Pennsylvania 


do 


Chestnut 


Tennessee 


Wild red 


Tennessee 


Cow 


Louisiana 


Chestnut 


Maryland 


Laurel > 


.. ..do . . 


Do 


Tennessee 


Post 


Arkansas 


Cotton wood, black 


Washington 


Do 


Louisiana 


Cucumber tree 


Tennessee 


Red 


Arkansas 


Dogwood: 
Flowering 


do 


Do 


Indiana .... 


Do 


Louisiana 


Western 


Oregon 


Do 


Tennessee 


Elder, pale 


do 


Highland Spanish . . 
Lowland Spanish. . . 


Louisiana 


Elm: 
Cork 


Wisconsin, Marathon 
County. 
Wisconsin, Rusk 
County. 
Indiana 


do 


Swamp white 


Indiana 


Do 


Tanbark 


California 


Water 


Louisiana 


Slippery... 


White 


Arkansas ... . 


Do 


Indiana 


Do 


Wisconsin 


Do 


Louisiana, Richland 
Parish. 
Louisiana, Winn Parish. 
Louisiana 


White 


Pennsylvania 


Do 


Do 


Wisconsin 


Greenheart 




Willow 


Gum: 
Black 


Tennessee 


Yellow 


Arkansas 


Do 




Blue (Eucalyptus) . 
Cotton 


California 


Osage orange 


Indiana 


Louisiana 


Poplar, yellow (tulip 
tree). 
Rhododendron great 




Red 


Missouri 


do 


Hackberry 


Indiana 


Do 


Wisconsin 


Sassafras 


do 


Haw, pear 


do 


Serviceberry 


do 


Hickory: 
Big shellbark 


Mississippi 


Silverbell tree 


do 


Sourwood 


do 


Do 


Ohio 


Sumac staghorn 




Butternut 


do 


Sycamore 




Mockernut 


Mississippi 


Do 


TGGHGSSGG 


Do 


Pennsylvania 


Umbrella, Fraser 


do 


Do 


West Virginia 


Willow: 
Black 




Nutmeg 


Mississippi 


Pignut 


do.. 






Do 


Ohio 


Witch hazel 













Note 12. 



AIRCRAFT DESIGN DATA. 



33 



List of species and reference numbers for figure 21 Continued. 

CONIFERS. 



Species. 


Locality. 


Reference 
No. 


Species. 


Locality. 


Reference 
No. 


Cedar: 
Incense 


California 


26 
2 
10 
1 
62 
45a 
67a 
46a 

75 
67 
48 

4 
39 
18 
14 
36 
16 
17 

47 
52 
15 
50 
84 
64 

127 
43 
33 
88 ' 
31 
35a 


Pine Continued. 
Lod ^coole 


Montana, Granite 
County. 
Montana, Jefferson 
County. 
Wyoming 


41a 
40a 

34 
123 
113 
96 

95 
57 
71 
86 
77 
22 
82 
42 
19 
37 
41 
32 
25 
28 
13 

8 
3 

44 
29 
7 
38 
81 
134 


Western red 


Montana 


Do 


Do 


Washington 


White 


Wisconsin 


Do 


Cypress, bald 


Louisiana 


Douglas fir 


California 


Longleaf 


Florida 


Do... 


Oregon 


Do 


Louisiana, Lake Charles. 
Louisiana, Tangipahoa 
Parish. 
Mississippi 


Do 


Washington, Chehalis 
County. 
Washington, Lewis 
County. 
Washington and Ore- 
gon. 
Wyoming 


Do 


Do 


Do 


Do 


Norway 


Wisconsin 


Pitch 


Tennessee 


Do... 


Pond 


Florida 


Shortleaf 


Arkansas 


Fir: 
Alpine 


Colorado 


Sugar 


California 


Table Mountain 
Western white 


Tennessee 


Amabilis 


Oregon 


Montana 


Do 


Washington 


Western yellow .... 
Do 


Arizona 


Balsam 


Wisconsin 


California 


Grand 


Montana 


Do 


Colorado 


Noble 


Oregon 


Do 


Montana 


White 


California 


White 


\Visconsin 


Hemlock: 
Black 


Montana 


Redwood 


California, Albion 


Do 


California Korbel 


Eastern 


Tennessee 


Spruce: 
Engelmann 


Colorado,GrandCounty. 
Colorado, San Miguel 
County. 
New Hampshire 


Do 


Wisconsin 


Western 


Washington 


Do 


Larch, western 


Montana 


Red 


Do 


Washington 


Pine: 
Cuban 


Florida 


Do 


Tennessee 


White 


New Hampshire 


Jack 


Wisconsin 


Do 


Wisconsin 


Jeffrey 


California 


Tamarack 


do 


Loblolly 


Florida , 


Yew western 


Washington 


Lodgepole 


Colorado 






Do 


Montana, Gallatin 
County. 





98257 19 No. 12 3 

e. 






SVO-YTIVAfla 












34 



AIRCRAFT DESIGN DATA. 



Note 12. 



SUITABILITY OF VARIOUS AMERICAN WOODS FOR AIRCRAFT CONSTRUCTION. 

The difficulty of securing ample supplies of the woods heretofore considered as the standards 
for aircraft construction has made it necessary to consider the substitution of other species. 
It must must be realized that aircraft can, if necessary, be made from practically any species 
of wood which will furnish material in the required sizes, and progress in laminating and splicing 
has done much to increase the utilization of smaller sized material. It must also be borne in 
mind that the differences in suitability are slight for a number of species and that high-grade 
stock of a species considered to be inferior may actually be better than lower grade stock of 



PERCENT OF SHRINKAGE IN VOLUME 

> O N on 












































































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WHITE ASH 

HILATION OF 

SHRINKAGE TO 
SPECIFIC GRAVITY 

PIRCEUT SMMIMKAai IS 

TOTAL men, 0*1111 TO 

OVIN DRY 
































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.1 .2 .3 .4 .5 .6 .7 .8 3 1.0 

SPECIFIC GRAVITY-OVEN DRY 

ASID OH OVIH DRY VOLUME 

Figure 22. 

the species considered superior. In other words, it may be preferable to change species and 
keep the grade up rather than to lower the grade and use the same species. 

In order to give a general idea of the relative properties of the more common American 
species of timber, with respect to their use in aircraft, a short statement concerning each has 
been prepared. In those cases in which the species might possibly be considered as a substitute 
for spruce its properties are compared with those of spruce. 

CONIFEROUS SPECIES. 

Incense cedar. This species is somewhat lighter than spruce, but lacks considerably in 
stiffness and does not possess the toughness of spruce. It might be substituted for spruce for 
parts which are not highly stressed. 



Note 12. AIRCEAFT DESIGN DATA. 35 

Port Orjord cedar. Port Orford cedar is somewhat heavier than Sitka spruce and equals 
or exceeds it in all its strength properties. Recent data upon this species indicate that it is 
not as strong as originally supposed, but still show it to be equal to spruce, although of slightly 
greater weight. 

Western red cedar. Western red cedar is lighter than spruce and below it in all its strength 
properties. It is more difficult to dry, but could probably be used with success in many parts 
where spruce is now used, but could not be used in parts which are highly stressed. 

White cedar. White cedar is very low in all its strength properties. It is a comparatively 
small tree and could hardly be considered as a possibility for use for the larger members. 

Bald cypress. Bald cypress is slightly heavier than spruce. Its average figures show it 
somewhat superior to spruce when used in the same sizes. The great variability in the wood 
of this species has, however, prevented its recommendation for aircraft construction. Cypress 
is very wet in its green condition and is considered much more difficult to dry and glue than 
many other species. 

Yellow cypress. Data on this species are not very complete. The indications are that it 
is too low in stiffness to be a satisfactory substitute for spruce. 

Douglas fir from the Pacific coast. Douglas fir from the Pacific coast is considerably heavier 
than spruce and all its strength properties are equal to or exceed those of spruce. It is quite 
probable that the bulk of good wing-beam stock will come from second-cut logs and that the 
weight and corresponding strength values will run slightly lower than the average of the species. 
Douglas fir is considerably harder to dry than spruce and more inclined to shakes and to check 
during manufacture and to develop these defects in service. It is inclined to break in long 
splinters and to shatter when hit. The use of Douglas fir in the manufacture of wing beams 
requires considerably more care than is necessary with spruce, but it should give excellent 
results (from the strength standpoint) when substituted for spruce in the same sizes. 

Douglas fir, Rocky Mountain type. The Rocky Mountain type of Douglas fir is much 
smaller than the coast type, is quite knotty and somewhat brash, and probably would not be 
satisfactory as a substitute for spruce. 

Alpine fir. The Alpine fir so far tested was very low in weight and in all its strength prop- 
erties. This material was from small knotty trees and should not be used except to resist low 
stresses. It is quite possible that the wood in more extensive stands of comparatively large 
Alpine fir will be heavier and stronger than that already tested. 

Amabilis fir. The amabilis fir so far tested was slightly heavier than spruce and in most 
of its strength properties it was practically the equal of spruce. Sufficient data are not at 
hand to determine how this material will kiln dry nor to determine its working properties. 
If it can be kiln dried and worked satisfactorily, indications are that it will be a fairly satis- 
factory substitute for spruce in spruce sizes in wing beams, struts, and other highly stressed 
parts. 

Balsam fir. Balsam fir is somewhat lighter than spruce and considerably lower in all its 
strength properties. It does not give promise of being satisfactory in airplane construction. 

Grand fir, nolle fir, and white fir. The grand fir so far as tested was slightly heavier than 
spruce, while the noble and white fir were slightly lighter. In strength properties these species 
compare very favorably with spruce except in the case of the shock-resisting ability of white 
fir, which is a little low. This, however, may be accidental. The statement made concerning 
amabilis fir will apply to these species also. 

Black hemlock. Black hemlock is quite a little heavier than spruce and lacking in stiffness. 

Eastern hemlock. On a basis of strength properties alone eastern hemlock appears to be 
a substitute for spruce, but the lumber is shaky and liable to heart rot, has numerous knots, 
and develops shakes and checks in service. It need not, therefore, be considered. 



36 AIRCRAFT DESIGN DATA. Note 12. 

Western hemlock. Western hemlock is heavier than spruce, but not quite so heavy as 
Douglas fir. It is low in shock-resisting ability, but on a basis of strength alone it might serve 
as a substitute for spruce in spruce sizes. No data are available concerning proper kiln-drying 
methods and the possibility of manufacturing conditions which would cause this species to be 
rejected. 

Western larch. Butts of the western larch tree are very heavy. The material is shaky 
and is hard to dry. It would not seem feasible to use this species for aircraft in view of the 
supply of more suitable species. 

Cuban pine. Cuban pine is entirely too heavy to be considered. 

Jack pine. The jack pine so far tested was 9 per cent heavier than spruce and was lacking 
in stiffness. 

Jeffrey pine. Jeffrey pine is especially lacking in stiffness. 

Loblolly pine. Loblolly pine is quite heavy. It is very variable in its properties and need 
not now be considered. 

Lodgepole pine. Lodgepole pine is somewhat low in its shock-resisting ability and slightly 
low in stiffness. If extensive stands of large trees can be located, there is a possibility that it 
might be found practicable to use some of this species. 

Longleai pine. This material is considered too heavy for use in airplanes without redesign. 

Norway pine. Indications are that Norway pine can be used as a substitute for spruce in 
spruce sizes. More data are needed as to kiln drying and the difficulties which may be met in 
manufacture. 

Pitch and pond pine. Pitch and pond pine are both heavy, and it is not likely that they 
would ever be needed in aircraft work. 

Shortleaf pine. The lighter material from the shortleaf pine could be used for aircraft 
construction, but probably would not be as satisfactory as Douglas fir, since weight for weight 
it shows a lower modulus of rupture and stiffness. 

Sugar pine. Sugar pine is quite low in shock-resisting ability and stiffness and is quite 
variable. It probably would not, therefore, be a suitable substitute for spruce. 

Table mountain pine. Table mountain pine has about the properties of shortleaf pine. It 
probably would not produce clear material satisfactory for aircraft stock. 

Western white pine. Western white pine is slightly heavier than spruce and shows up well 
in all its strength properties except hardness. It is more difficult to dry than the eastern white 
pine, but probably could be substituted for spruce in spruce sizes. 

Western yellow pine. Strength data show the western yellow pine to be lacking in shock- 
resisting ability and stiffness. It is also quite variable. It is not considered a good substitute 
for spruce. 

Eastern white pine. Tests to date show eastern white pine somewhat below spruce in hard- 
ness and rather low in shock-resisting ability. It, however, runs quite uniform in its strength 
properties, is very easily kiln dried without damage, works well, stays in place well, and is rec- 
ommended for aircraft construction as a substitute for spruce in spruce sizes. 

Redwood. The data available on redwood are not comparable to those on other species 
and are too erratic to form a very definite judgment of the species. The indications are that 
the material is quite variable in its properties and likely to be very brash. 

Engelmann spruce. Engelmann spruce is quite light and low in all its strength properties. 

Tamarack. Tamarack is too heavy to be substituted for spruce. It probably would not 
furnish clear material. 

Yew. This wood is very heavy. The tree is small and crooked. 



Note 12. AIRCEAFT DESIGN DATA. 37 

HARDWOODS. 

Red alder. Data on this species are very meager, but it is probably not available in sizes 
sufficiently large to make it of importance. 

Biltmore ash. Biltmore ash should be considered along with white ash and may be used 
for longerons and other work where strength, stiffness, and ability to steam bend are of impor- 
tance. 

Black ash. Black ash is very low in stiffness. It is an exceedingly tough species. It is 
one of the best native species for steam bending. It can not be used, however, where strength 
and stiffness are of great importance, as in places where white ash is used. 

Blue, green, and white ash. These species are known commercially as white ash and are 
very desirable for use in longerons and other places where steam bending, great strength, and 
stiffness are required. 

Oregon ash. Oregon ash appears i,o be about equal to the eastern white ash, although the 
data on this species are somewhat meager. 

Pumpkin ash. Pumpkin ash as a species is somewhat lighter than the white ashes. It is 
considerably less stiff than the white ash. Commercially the term is made to include the weak, 
soft material from all the other species of ash. 

Commercial white ash. Commercial white ash includes the Biltmore, blue, green, and white 
ash already mentioned. 

Aspen. Aspen is quite soft and lacking in stiffness. 

Basswood. Basswood is light in weight and low in practically all its strength properties. 
It is one of the best species to receive nails without splitting and is used extensively for webs, 
veneer cores, and similar work. 

Beech. Beech is quite heavy and has about the strength properties of sweet and yellow 
birch and hard or sugar maple. It might be used to some extent in propellers but not exten- 
sively in other aircraft parts. 

Paper birch. Paper birch is rather low in its stiffness and high in weight. 

Sweet and yellow birch. Sweet and yellow birch are quite heavy, hard, and stiff. They 
have a uniform texture and take a fine finish. On account of their hardness and resistance to 
wear they can be used to face other woods to protect them against abrasion. 

Yellow buckeye.- Yellow buckeye is low in its weight and all its strength properties. 

Cascara buckthorn. Cascara buckthorn is a small tree and need not be considered. 

Butternut. Butternut is lacking in stiffness and probably need not be considered. 

Western chinquapin. Western chinquapin is a small tree and need not be considered. 

Black cherry. Black cherry is a very desirable propeller wood. 

Wild cherry. Wild cherry is a small tree and lacking in stiffness. 

Chestnut. Chestnut is somewhat heavier than spruce and is quite deficient in stiffness. 

Cottonwood. The cottonwood so far tested was slightly heavier than spruce. It is soft, 
low in its strength as a beam or post, and lacks stiffness. It is very tough, however, does not 
split hi nailing, and bends well. Cottonwood can not well be substituted for spruce in wing 
beams and long struts but can be used in minor parts. 

Black cottonwood. Black cottonwood is low in weight and all its strength properties. 

Cucumber tree. The wood of the cucumber tree is somewhat heavier than spruce and shows 
up well in all its strength properties. It is one of the few hardwoods which gives promise of 
being a good substitute for spruce in wing beams and struts. 

Flowering and western dogwood. The dogwood trees are too small to be considered. 

Elder, pale. Elder is too small a tree to be considered. 



AIRCRAFT DESIGN DATA. Note 12. 



Elm, cork (rock elm). Cork elm is slightly heavier than ash. It is low in stiffness and 
very resistant to shocks. It steam bends well and if properly dried can be used for longerons 
as a substitute for ash. Considerably more care is necessary in the drying of elm in order to 
have it remain in shape as it twists and warps badly when not held firmly. 

Slippery elm. Slippery elm is somewhat lighter than cork elm, but when of equal density 
may be used as cork elm. 

White elm. Very dense pieces of white elm have the requisite density and strength to be 
used along with cork elm. Most of the white elm, however, is quite light. It is lacking in 
stiffness, but steam bends well. It could probably be used to excellent advantage in the bent 
work at the ends of the wings, rudders, elevators, etc. Considerable care would be necessary 
in order to hold this material in place while drying, as it warps badly. 

Black gum. Black gum is considerably heavier than spruce and not nearly so stiff. It 
probably will be but little used in aircraft. 

Blue gum (eucalyptus}. Eucalyptus grown in this country is quite heavy. It has large 
internal stresses, swells and shrinks excessively, twists badly in drying, and is very difficult to 
dry. Under present conditions it probably should not be used in aircraft. 

Cotton gum (Tupelo). This species is considerably heavier than spruce, but not nearly so 
stiff. At present it probably should not be considered for aircraft. 

Bed gum. Red gum is considerably heavier than spruce and superior to it in strength 
properties. On account of its locked grain and its tendency to twist, warp, and check it prob- 
ably should not be used in place of spruce. There is some prospect, however, that carefully 
quarter-sawed material of this species can be used in propellers. 

Hackberry. The denser pieces of hackberry might be substituted for ash in longerons. 

Pear haw. Pear haw is a very small tree and of no importance in this connection. 

True hickories, including shellbark, mockernut, pignut, and shagbark. These species are 
heavier than ash and are very tough and strong. They could be substituted for ash in longerons, 
but would probably not give quite as good service for the same weight. 

Pecan hickories, including butternut, nutmeg, pecan, water. These hickories are consider- 
ably inferior to the true hickories, especially in their ability to resist shock, and probably would 
not make satisfactory substitutes for ash. 

American holly. This species is lacking in stiffness and probably is of no importance in 
airplane construction. 

Hornbeam, California laurel, mountain laurel, black locust, honey locust, madrona. The 
laurels, locusts, and madrona are all heavy woods and probably have little use in aircraft 
construction. 

Magnolia. Magnolia has approximately the same properties as cucumber wood, to which 
it is closely related, and could probably be used as a substitute for spruce in wing beams and 
longerons. 

Oregon maple. Oregon maple has about the same properties as silver maple. It is a little 
more stiff and not quite so resistant to shock. There is probably little use for either of these 
species in aircraft. 

Red maple. Red maple is somewhat heavier, stiffer, and stronger than silver maple. Red 
maple might possibly be used in propeller work, but would give much softer propellers than 
sugar maple. 

Sugar maple. Sugar maple is quite heavy, hard, and stiff. It could be used with birch 
in propeller manufacture. It has very uniform texture and takes a fine finish. On account 
of its hardness and resistance to wear it is very often used to face other woods to protect them 
against abrasion. 



Note 12. AIRCRAFT DESIGN DATA. 39 

Silver maple. Silver maple is the lightest and softest of all the maples. It is much too 
soft to be considered as a substitute for sugar maple and lacks the stiffness to make it a satis- 
factory substitute for spruce. 

The oaks. The oaks need not be considered as substitutes for spruce, but they play an 
important part in the manufacture of propellers. The oaks are all quite heavy and hard. 
The oaks, even when a single botanical species is considered, are extremely variable hi their 
strength properties. The differences in the average strength properties of the various eastern 
oaks are not great, and greater differences might readily be found among different logs of any 
one species. The white oaks, as a rule, shrink and swell more slowly with changes in the weather 
than do the red oaks. The radial shrinkage of the oaks is about one-half the tangential shrinkage. 
This accounts for the much greater value of quarter-sawed oak over plain-sawed oak for pro- 
peller construction. The southern-grown oaks are much more difficult to dry than are the 
northern oaks. Experiments are being made in the drying of both northern and southern 
red and white oaks. The northern white oaks when quarter-sawed and carefully dried make 
very satisfactory propellers. It is possible that quarter-sawed northern red oak will also make 
fairly satisfactory propellers but with this disadvantage: It is more subject to defects in the 
living tree, decays more readily, and changes more rapidly with changes in weather conditions. 
To be satisfactory in this work the southern oaks will require exceeding care in drying, as they 
are very difficult to dry without checking, honeycombing, and casehardening. 

Osage orange, persimmon. Osage orange and persimmon have other very important uses 
and are probably of no importance in aircraft construction. 

Yellow poplar. Yellow poplar is but little heavier than spruce, and while rather low in 
shock-resisting ability has good working qualities, retains its shape well, is comparatively free 
from checks, shakes, and such defects. It would probably be a fairly satisfactory substitute 
for spruce in wing beams and struts. It offers no manufacturing difficulties. 

Rhododendron, sassafras, service berry, silverbell, sourwood, sumac. These species probably 
have no place in aircraft construction. 

Sugarberry. This species is closely related to the hackberry and the denser pieces might 
be substituted for ash in longeron construction. 

Sycamore. The trees are very shaky and probably would not furnish material suitable 
for aircraft. 

Eraser umbrella. This species is closely related to the cucumber and magnolia previously 
discussed and has similar properties. The clear stock obtained might be used as a substitute 
for spruce. 

Willow, black and western black, witch hazel. Willow and hazel probably are of no use in 
aircraft construction. 

Walnut, black. Black walnut has many very important uses and need not be considered 
as a substitute for spruce. This species probably makes the best propellers of any of the 
native species. It is somewhat difficult to dry, but stays in place unusually well and is hard 
enough to resist wear. 

SYNOPSIS OF COMMENTS AS TO SUBSTITUTES FOR SPRUCE. 

The following species range in weight from that of spruce to 25 per cent heavier than spruce. 
The data available indicate strongly that these species can be substituted for spruce in highly 
stressed parts using the spruce design: Port Orford cedar, coast type Douglas fir, eastern and 
western white pine, yellow poplar, cucumber tree and magnolia. The following species give 
promise of furnishing substitutes for spruce, but more experiments are needed in order to over- 
come known difficulties before these species can be recommended: Bald cypress, amabilis fir, 



40 



AIRCRAFT DESIGN DATA. Note 12 - 



grand fir, noble fir, white fir, lodgepole pine, Norway pine, and redwood. The following spe- 
cies are lighter than spruce, but could be used in parts where the stresses are relatively 1 
Incense cedar, western red cedar, and Alpine fir. 

As conditions change other species will doubtless come into consideration as subs 

for spruce. 

STORAGE AND KILN DRYING OF LUMBER. 

The proper piling of lumber and timber for air seasoning or as temporary storage previous 
to kiln drying is extremely important. Green or partiaUy dry stock is subject to various forms 
of deterioration, such as staining, decay, severe checking, and (especially in hardwoods) insect 
attack. During warm, humid weather staining may take place in a few days and decay may 
weaken the wood in a few months. 

Proper piling of such stock will tend to reduce the deterioration to a minimum. All lum- 
ber or timber which is to be stored any length of time should be piled on solid foundations 
with stickers between each two courses, and should have some protection from the sun and 
rain. Whenever possible, the stock should be piled in a shed with open sides. If this is not 
practicable, each pile should be covered so as to keep out rain and snow. Green hardwoods, 
especially oak, frequently check severely at the ends. This can be avoided to a large extent 
by coating the ends with linseed-oil paint. 

Stock should be cut up into as small sizes as is practicable before kiln drying. Large 
pieces usually check severely because the outer portion dries and shrinks considerably faster 
than the inner core, which always dries slowly. Timbers which contain the pith and which 
are to be cut into smaller sizes later should at least be cut through the pith once, or, better, 
be quartered before being stored away. This will avoid the large checks which are commonly 
produced in the seasoning of timbers containing the pith by reason of the tangential shrinkage 
being greater than the radial shrinkage. 

RULES FOR PILING LUMBER. 

1. The foundations should be strong, solid, and durable, preferably concrete piers with 
inverted rails or I beams for skids. If this is impracticable, creosoted or naturally durable 
wooden timbers should be used. 

2. Each foundation should be level. 

3. The foundations should not be over 4 feet apart for lumber, but may be farther apart 
for larger timbers. For woods which warp easily or for stock less than 1 inch in thickness 
foundations should not be over 3 feet apart. 

4. If the piles are in the open, they should have a slope from front to rear of 1 inch for 
every foot in length. 

5. The foundations should be sufficiently high to allow the free circulation of air under- 
neath the piles, and weeds or other obstructions to circulation should be removed. 

6. Boards of equal length should be piled together with no free unsupported ends. 

7. A space of about three-fourths of an inch should be left between boards of each layer 
and from 1 to 2 inches between timbers of each layer. 

8. The stickers should be of uniform thickness, preferably seven-eighths of an inch for 1-inch 
lumber and 1 inches for thicker stock. 

9. Stickers should be placed immediately over the foundation beams and kept in vertical 
alignment throughout the piles. Their length should be slightly in excess of the width of the 
pile. 

10. The front and rear stickers should be flush with or protrude slightly beyond the ends of 
the boards. 



Note 12. AIRCRAFT DESIGN DATA. 



KILN DRYING OF WOOD. 

ADVANTAGES OF KILN DRYING. 

The chief objects of kiln-drying airplane stock are (a) to eliminate most of the moisture 
in green or partly dried stock more quickly than can be done in air drying and (&) to reduce 
the moisture content of the wood below that attained in ordinary air drying, so that no more 
drying, with consequent checking, warping, and opening up of seams will occur after the wood 
is in place. Other advantages incident to kiln drying are that a smoother surface can be 

obtained on kiln-dried stock and that glues will hold better. 

. 

THE ELIMINATION OF MOISTURE FROM WOOD. 

Green lumber may contain from about one-third to two and one-half times its oven-dry 
weight of water. Expressed in percentage, this is from 33 J to 250 per cent moisture based 
on the oven-dry weight. The moisture content of green lumber varies with the species, the 
position in the tree, whether heartwood or sapwood, the locality in which the tree grew, and 
the drying which has taken place since the tree was cut. As a rule sapwood contains more 
moisture than heartwood, although in some species, especially in butt logs, the heartwood 
contains as much moisture as the sapwood. Thoroughly air-dried lumber may contain from 
about 10 to 20 per cent moisture for inch stock and more for thicker material. 

Much of the moisture in green wood is contained in the cell cavities (like honey in a comb), 
and the rest is absorbed by the cell walls. When wood is drying the moisture first leaves the 
cell cavities and travels along the cell walls to the surface, where it is evaporated. When the 
cell cavities are empty but the cell walls are still saturated a critical point is reached, known 
as the fiber-saturation point. Wood does not shrink or increase in strength while seasoning 
until it has dried below the fiber-saturation point, which usually ranges between 25 and 30 per 
cent moisture, but may be less or more, and in spruce usually is between 30 and 35 per cent. 
This has an important bearing on the drying operation, since no casehardening, checking, or 
warping can occur so long as the moisture content is above the fiber-saturation point in all parts 
of the stick. 

In practice the stock should be dried to a moisture content slightly less than it will ulti- 
mately have when in use. This may be as low as 6 per cent for interior work and not so low 
for wood to be exposed to weather. 

Two steps are necessary in the drying of lumber (a) the evaporation of moisture from the 
surface, and (6) the passage of moisture from the interior to the surface. Heat hastens both 
these processes. For quick drying as high a temperature should be maintained in the kiln as the 
wood will endure without injury. Some woods (especially coniferous woods) will endure higher 
temperatures than others. The general specifications for kiln-drying airplane stock which follow 
give the temperatures at which a kiln should be operated to prevent injury to lumber to be used 
for airplanes. 

The lumber in a kiln is heated and evaporation is caused by means of hot air passing through 
the piles. To insure proper drying throughout the piles a thorough circulation of air is neces- 
sary. The lumber must be properly piled and the kiln constructed so as to make the necessary 
circulation possible. 

Dry hot air will evaporate the moisture from the surface more rapidly than it can pass 
from the interior to the surface, thus producing uneven drying, with consequent damaging 
results. To prevent excessive evaporation and at the same time keep the lumber heated through, 
the air circulating through the piles must not be too dry; that is, it must have a certain humidity. 
The specifications give the proper humidities at which to operate the kiln for drying airplane 

, i 

stock. 





42 AIRCRAFT DESIGN DATA. Note 12. 

THREE ESSENTIAL QUALITIES OF A DRY KILN. 

The merits of any method of drying airplane woods depend upon the extent to which it affects 
the mechanical properties of the stock and upon the uniformity of the drying. In order that 
complete retention of properties and uniform drying may be guaranteed, it is essential that the 
circulation, temperature, and humidity of the air be adequately controlled. In this connection 
circulation does not-mean the passage of air through flues, ducts, or chimneys, but through the 
piles of lumber, and the terms temperature and humidity control apply to the air within the 
piles of lumber in the kiln. 

Control of air circulation involves rate or speed and uniformity. A uniform passage of air 
through all portions of the piles of lumber is the most essential quality in a kiln. If the cir- 
culation can be made both uniform and rapid, all portions of the pile will dry quickly and at 
the same rate. Furthermore, uniform and rapid circulation of air are necessary before the 
control of temperature and humidity within the piles of lumber is possible. 

When unsaturated air at any given temperature enters a pile of lumber containing moisture, 
it exchanges heat for moisture, is cooled, and rapidly approaches saturation. With green wood 
and a sluggish circulation, the cooling is very rapid. The rate of cooling decreases as the lumber 
dries, and if the circulation is increased the loss of heat in passing through the pile is less. So 
if the air moves rapidly through certain parts of the piles and slowly through others, the differ- 
ent parts of the piles will be at different temperatures. The temperature of the air within the 
lumber can not be maintained at any given value unless the circulation of air is uniform at all 
points in the pile. Even though the air moves at uniform speed from one side of a pile of lumber 
to the other, if the speed is too slow the air loses its heat and approaches saturation rapidly. 
In general a wide variation in the temperature of the lumber in different parts of the kiln is 
proof of very uneven or slow circulation. Inadequate circulation and temperature control 
render the control of humidity and uniform drying impossible. 

Humidity is of prime importance, because the rate of drying and the prevention of checking 
and casehardening are directly dependent thereon. It is generally true that the surface of 
the wood should not dry more rapidly than the moisture transfuses from the center to the 
surface. The rate of evaporation must be controlled, and this can be done by means of the 
relative humidity. Stopping the circulation to obtain a high humidity or increasing the circu- 
lation by opening ventilators to reduce the humidity is not good practice. Humidity should 

be raised, if necessary, to check evaporation without reducing the circulation. 



DEFECTS DUE TO IMPROPER DRYING. 

Casehardening and honeycombing. When the surface of a piece of lumber is dried more 
rapidly than the moisture can pass to it from the ulterior, unequal moisture conditions exist 
in the lumber. The moisture in the outer layers falls below the fiber saturation point. The 
outer layers then tend to shrink but are held from shrinking by the more moist interior, which 
has not yet started to shrink; so the surface either checks or dries in a stretched condition, 
usually both. Later, as the interior dries it also tends to shrink normally, but in turn is held 
by the outside, which has become "set" or " casehardened." Consequently, the interior dries 
under tension, which draws the outer layers together, closing up all checks and producing 
compression. Casehardened lumber, when resawed, will invariably cup toward the inside if 
the interior if the lumber is dry (fig. 23). If the tension in the interior of the wood is severe 
enough, it may produce radial checks which do not extend to the surface. Wood with such 
checks is said to be honeycombed or hollow-horned (fig. 24). Casehardening and honey- 
combing can practically be prevented by regulating the humidity so that the evaporation 
from the surface does not take place too rapidly. 





Fig. 23. Sections of casehardened western larch boards. Nos. 1 and 2 are original sections; Nos. 3 to 8 are resawed 

sections showing cupping; No. 9 is one-side surfaced. 



THE SAME, 

WELL KfLN DRIED IN 
HUMIDITY REGULATED 




don 



~__^^^^_^^^_^___^^_. 

' 

-. -Jloe 

Fig. 24. Oak stock honeycombed by air drying and improper kiln drying. Also similar stock properly dried. 

43 



44 



AIRCRAFT DESIGN DATA. 



Note 12. 



If wood becomes casehardened in kiln drying, it may be brought back to normal con- 
dition by steaming, provided that checks and cracks have not developed. Steaming softens 
the outer fibers and relieves the stresses caused by the contraction of the outer shell. Care 
must be taken not to steam wood which has checked or honeycombed from casehardening 
enought to part the fibers and weaken the piece. Steaming will close up the cracks but will 







Fig. 25. End view of 1-inch boards of western red cedar dried with and without collapse. 

not restore the strength of the piece. It will be much harder to detect cracks and checks due 
to casehardening if they have been closed up again by steaming. 

<Ma^.--Collapse is abnormal shrinkage causing grooves to appear in the surface of the 
lumber qr* genaral distortion of the surface (fig. 25). It is produced when wet lumber 
is dried at too high a temperature. The heat and moisture cause the cell walls to 'become 
soft and plastic. As the water leaves the cell cavities the moist cell walls are drawn together 
if no air. is present. This causes the cells to flatten, and a general reduction in the cross sec- 



Note 12. AIRCRAFT DESIGN DATA. 45 

tion takes place. Collapse occurs especially in such woods as western red cedar, redwood, 
white oak, and others which readily become soft and plastic when hot and moist. It can be 
avoided by not allowing the temperature to rise too high while the wood is still moist (at or 
above the fiber saturation point). 

Brashness. High temperature treatments of all kinds, whether steam or hot air, are 
injurious to lumber, causing it to turn darker and become brash. The injuries thus sustained 
increase with the temperature and length of time the wood is exposed to such severe conditions. 
No definite rule can be laid down as to what conditions of temperature wood will endure with- 
out becoming brash. If the temperatures prescribed in the specifications (see p. 68) are 
not exceeded, no difficulty will be experienced in this respect. 

METHODS OF TESTING CONDITIONS DURING DKYING. 

In drying airplane stock it is advisable to test conditions in the kiln at frequent intervals 
so that the operator will be able to make any changes promptly that the tests indicate are 
necessary to maintain the proper rate of drying and to prevent injury to the lumber. A con- 
tinuous record of proper conditions during kiln drying is a strong assurance of satisfactory 
stock. The following tests will aid the inspector in keeping check on drying conditions. , 

1. Preliminary test: ; . , 

(a) Initial moisture conditions hi the lumber. 
(&) Preparation and placing of samples. 

(c) Initial weights and placing of whole pieces. 

(d) Determination of direction, uniformity, and rate of air circulation. 

(e) Location and calibration of instruments. 

2. Current tests: 

(a) Determination of current temperatures. 
(6) Determination of current humidities. 

(c) Determination of circulation. 

(d) Weighing of samples and determination of current moisture conditions. 

3. Final tests: 

(a) Average kiln-dry moisture condition of samples. 
(6) Distribution of moisture in the kiln-dry samples. 

(c) Determination of casehardening in kiln-dry samples. 

(d) Average kiln-dry moisture condition of whole pieces. 

(e) Calculation of initial moisture condition of whole pieces. 
(/) Distribution of moisture in kiln-dry whole pieces. 

(g) Distribution of casehardening in kiln-dry whole pieces. 

(h) Determining the effect of the process on the toughness and strength of the kiln- 
dry stock. 

In making these tests the following instruments and material will be needed: 
1 sensitive equal arm balance (capacity, 0.1 to 250 grams). 
1 drying oven in which the air can be heated to and held at 212 F. 
1 can of asphalt paint and a brush. 
1 sensitive platform scale (capacity, 0.01 to 250 pounds). 

1 electric flash light (lantern type recommended). 
12 packages of punk sticks. 

3 accurate standardized ordinary glass thermometers (60 to 230 F. by 2 intervals). 

2 accurate standardized glass wet and dry bulb hygrometers with extra wicks 
(60 to 230 F. by 2 intervals). 

Access to a laboratory equipped with machines for making impact, static b.ending, 

hardness, compression parallel to the grain, and other tests. 
Waxed or oiled paper. 



46 AIRCRAFT DESIGN DATA. Note 12. 

1. Preliminary tests. (a) Initial moisture condition: Select at least three representative 
pieces for each 10,000 board feet of stock to be dried. Cut about 2 feet from one end of each. 
Then cut a 1-inch section, a 24-inch sample, and a second 1-inch section in succession. Imme- 
diately weigh the two 1-inch sections to an accuracy of one-tenth of 1 per cent. Mark the 
initial weights on the section and dry them to constant weight in the oven heated to 212 F. 
Keweigh them to the same accuracy and determine the per cent initial moisture content of thfe 
samples from the formulae : 

Initial weight oven-dry weight 
Per cent initial moisture content = oven-dry weight 

(6) Preparation and placing of samples: Immediately after cutting the 24-inch samples 
described under (a) paint the ends of the samples with a heavy coat of asphalt paint. Then 
weigh them separately on the platform to an accuracy of one-tenth of 1 per cent. Mark the 
initial weights on the samples and place them in the piles so as to come under the most severe, 
least severe, and average drying conditions, and so as to be subjected to the same drying con- 
ditions as the adjacent pieces. Where the circulation of air is vertical, place samples near the 
tops, centers, and bottoms of the piles, and where the circulation is lateral place them near 
the sides where the air enters and leaves the piles and near the centers of the piles. 

(c) Initial weights and placing of whole pieces: In addition to the 24-inch samples it is 
desirable to select several representative whole pieces of stock and weigh them to an accuracy 
of one-tenth of 1 per cent on the platform scale. Mark the weights on the pieces and place 
them at various points near the tops, edges, bottoms, and centers of the piles. 

(d) Determination of the direction, uniformity, and rate of air circulation: In order to 
insure correct placing of samples, whole pieces, and instruments it is necessary that the direc- 
tion of the circulating air be known. To determine this light a few punk sticks, take the flash 
light, enter the kiln, close the door, and determine the direction, uniformity, and rate of motion 
of the circulating air in the spaces around the piles and through the piles by observing the smoke 
from the burning punk. 

(e) Location and calibration of instruments: Having determined the direction in which 
the air passes through the piles, place the bulb of the recording thermometer in contact with 
a standardized glass thermometer close to the pile at the center of the side where the air enters 
the pile. If the circulation is up through the piles, place the thermometer bulbs close under 
the bottom center; if it is down through the lumber, place the bulbs close to the top center, and 
if the air moves through the pile laterally, place the bulbs close to the center of the side where 
the air enters the pile. It is also desirable to know the variation of temperature in different 
parts of the piles and kiln. To determine this variation, place several of the standardized 
thermometers in the tops, bottoms, edges, and centers of the piles and at different points in 
the kiln. In order to calibrate a recording thermometer, place the bulb in contact with a 
standardized glass thermometer in the kiln and adjust the stylus until it agrees with the glass 
thermometer. The temperature must not be fluctuating, as is often the case where it is con- 
trolled by a thermostat. It is best to use a steady steam pressure in the heating pipes while 
calibrating instruments. Never attempt to calibrate a recording thermometer out of its place 
in the kiln. 

To determine humidity, place the standardized glass wet and dry bulb hygrometer near 
the bulb of the recording thermometer, so as to indicate the humidity of the air entering the 
piles at the tops, bottoms, or edges, as the case may be. 

2. Current tests. (a) Determination of current temperatures: If any part of a pile is 
exposed to direct radiation from the heating pipes, place a thermometer near the side so exposed. 



Note 12. AIRCRAFT DESIGN DATA. 47 



This will indicate whether or not any part is subject to higher temperature than that indicated 
by the recording instrument. If possible, allow no direct radiation on the lumber. The tem- 
perature of the air entering the piles must be known at all times, preferably by means of recording 
thermometers with extension bulbs which have been calibrated in place, as directed under 1 (e). 

The temperatures in the tops, bottoms, edges, and centers of the piles and at different 
points in the kiln should be determined occasionally by using standardized thermometers located 
as directed under 1 (e). 

(&) Determination of current humidities: Never attempt to determine the relative humidity 
of the air where the bulbs of the hygrometer are exposed to direct radiation. Where direct 
radiation may take place, it is necessary to shield the hygrometer from the heating pipes before 
readings are taken. The relative humidity of the air entering the piles must be indicated at 
all times by means of standardized glass wet and dry bulb hygrometers placed as directed under 
1 (e) . Before reading the hygrometer fan the bulbs briskly for about a minute. An air circu- 
lation of at least 15 feet per second past the wet bulb is necessary for an accurate humidity 
reading. The wick should be of thin silk or linen and it must be free from oil or dirt at all 
times. It should come into close contact with as much of the bulb as possible. Knowing the 
correct wet and dry bulb hygrometer readings, the relative humidity may be determined from 
the humidity diagram, figure 26. 

Relative humidity is shown on the horizontal scale and Fahrenheit temperature on the 
vertical scale. The curves running from the top left to the bottom right part of the chart are 
for various differences in the wet and dry bulb readings. The curves are numbered near the 
center of the chart above the heading " (t t 1 ) degrees Fahrenheit." To get the relative humid- 
ity, follow the curve which is numbered to correspond to the difference of the wet and dry bulb 
readings till it intersects the horizontal line numbered to correspond to the dry bulb reading. 
Directly below this intersection in a vertical line will be found the relative humidity on the 
bottom scale. Example: Dry bulb reading, 120; wet bulb reading, 113; difference, 7. Curve 7 
intersects horizontal line 120 at vertical line 79. Relative humidity is 79 per cent. 

When the humidity is desired in a Tiemann kiln, use the set of curves running from the 
top right to the bottom left part of the chart. Locate the lower of the two thermometer read- 
ings on the scale at the right of the chart. This is the reading of the thermometer in the baffle 
box. Follow along parallel to the nearest curve till the horizontal line is crossed whose number 
is the higher thermometer reading. Vertically below this point of intersection on the lower 
scale will be found the relative humidity. Example: Baffle thermometer reading, 112; flue 
thermometer reading, 120. Start at 112 on right-hand scale, follow parallel to curve 28 till 
horizontal line 120 is crossed. This point falls on vertical line 80. Relative humidity is 80 
per cent. 

(c) Determination of circulation: During each drying operation the circulation of the air 
should be tested several times, as under 1 (d) . As the lumber becomes drier, it has less cooling 
effect on the air, and this may change the circulation in the kiln. If this occurs, correspond- 
ing changes in the location of instruments should be made. 

(d} Weighing of samples and determination of current moisture condition: The 24-inch 
samples, placed as directed under 1 (6), should be weighed daily to an accuracy of one-tenth 
of 1 per cent on the platform scale. From test 1 (a) the initial moisture contents of these 
samples are known. Their initial weights were determined by test 1 (6). Knowing their 
initial moisture contents and weights, their oven-dry weights may be computed from the 
formula: 

~ . , , , initial weight 

Oven-dry weight of sample = 1AA . . . T . -, ^r T r X 100 

' 100 + initial moisture content 



48 



AIRCRAFT DESIGN DATA. 



Note 12. 



O' 10 20 30 40 50 60 70 80 90 100 
40 f 







210 



220 



10 



30 



210 



220 

40 50 60 70 80 90 |QO . 

RELATIVE HUMIDITY-PER CENT 

. 
Fig. 26. 



Note 12. AIRCRAFT DESIGN DATA. 49 



Having the calculated oven-dry weights and daily weights of the samples, their current 
moisture contents may be computed from the formula: 

, current weight oven-dry weight ' 

Current moisture content of sample = - -T - X 100 

oven-dry weight 

Therefore, since the samples were cut from representative stock, the drying rate of the 
material is known currently. 

3. Final tests. -(a) Average kiln-dry moisture condition of samples: When the current 
moisture contents of the samples indicate that the material is dried to the required point, 
three 1-inch sections are cut from the center of each sample. One section from each sample 
is used to determine the average kiln-dry moisture content of each sample by the method of 
test 1 (a) . This test must be made immediately after sawing. 

(6) Distribution of moisture in kiln-dry samples: A thin shell (about one-fourth inch) is 
split from the four outer surfaces of the second 1-inch section cut from each sample. The 
outsides and centers are tested for moisture content separately and immediately after sawing 
by the method of 1 (a). The results of this test show the distribution of moisture in cross 
section of the samples. The difference between the moisture contents of the outer shells and 
the centers shows whether or not the distribution is sufficiently uniform across the sections. 

(c) Determination of casehardening in kiln-dry samples: The first indication of case- 
hardening is surface checking. The next sign of case-hardening is honeycombing or interior 
checking along the medullary rays. This defect can not always be detected by a superficial 
inspection. It is necessary to cut the stock to discover it. Occasionally it is evidenced by 
a bulging of the surface over the honeycombed part. Often neither of these defects is present. 
In this case the third 1-inch section from each sample is resawed two or three times from one 
end down to within about half an inch of the other end (see fig. 23). If the material is case- 
hardened and dry, it will pinch the saw; if it is not dry at the time of sawing, the cupping of 
the outer prongs will increase upon further dcying. If the kiln-dried samples show case- 
hardening, the material should be steamed until the resawed sections do not pinch the saw in 
this test. 

(d) Average kiln-dry moisture condition of the whole pieces: When the kiln is unloaded, 
the whole pieces from different parts of the piles and kiln are weighed and then cut as follows: 
Remove about 2 feet from one end and then cut off three 1-inch sections. The average kiln- 
dry moisture contents of the whole pieces are determined from one section as in test 3 (a). 
The other sections are used as stated in 3 (/) and 3 (</). 

(e) Calculation of initial moisture condition of whole pieces: From the kiln-dry weights 
and kiln-dry moisture contents of the whole pieces, their oven-dry weights may be computed 
from the formula: 

Oven-dry weight of whole pieces = , nn . -, , -: X 100 

100 + kiln-dry moisture content 

Knowing the initial weights and oven-dry weights of the whole pieces, their initial moisture 
contents are computed from the formula : 

T ... , - , e , , initial weight oven-dry weight 

Initial moisture content of whole pieces = r T -r^ X 100 

oven-dry weight 

Therefore the initial and kiln-dry moisture conditions of the samples, whole pieces, and 
the average stock are known. 

(/) Distribution of moisture in kiln-dry whole pieces: This test is a duplicate of test 
3(6). 

98257 19 No. 12 4 



50 AIRCEAFT DESIGN DATA. Note 12. 



(g) Determination of case-hardening in kiln-dry whole pieces: This test is a duplicate of 
test 3 (c). 

(h) To determine the effect of drying on the strength of the stock: It is practically impos- 
sible to determine the effect of the process of drying on the properties of the stock by inspection 
unless some visible defect has developed. This is not usual, and as the inspector can not always 
resort to mechanical tests he should be able to show from his operation records that conditions 
in the kiln have been kept within the specifications recommended as safe for kiln-drying airplane 
stock. 

Detailed instructions for the kiln drying of various airplane woods have been prepared 
and issued in the form of a specification. This specification, which follows, is based upon a 
great many experimental kiln runs and strength tests upon matched specimens. Part of the 
matched specimens were tested while green, part were tested after air drying under shelter, 
and part were kiln dried to the same degree as the air-dried specimens and then tested. In 
this way the effect of kiln drying as compared to air drying was investigated and the conditions 
of kiln drying were determined for most rapid drying without decreasing the strength below 
that obtained in air drying to the same degree. 

SPECIFICATION FOR KILN DRYING FOR AIRCRAFT STOCK. 



GENERAL. 



1. This specification covers general requirements for kiln drying wood for airplane stock. 

2. The kiln-drying operations shall be so conducted that the wood will not lose any strength, 
toughness, or other physical property as compared to wood air dried to the same degree of 
dry ness. 



MATERIAL. 



3. Only one species and approximately one thickness shall constitute a kiln charge. A 
difference of not to exceed one-half inch in the thickness of single pieces will be allowed. 

PILING. 

4. The boards shall be piled so that the horizontal width of the spaces between them will 
be at least 1 inch for each inch of board thickness, but in no case shall the horizontal width of 
such spaces exceed 3 inches. The boards must be held flat and straight while drying. 

5. For stock up to four-quarters (1 inch) in thickness the crossers shall be at least 1 inch 
thick and not over 1| inches wide. 

6. For stock from four to twelve quarters (1 to 3 inches) in thickness the crossers shall be 
at least 1 inches thick and not over 1 inches wide. 

7. For stock over twelve quarters (3 inches) in thickness the thickness of the crossers shall 
be increased in the above proportion but must not exceed 2 inches in any case. 

8. The crossers shall be placed directly over one another and not over 3 feet apart in the 
courses. 

9. The lumber must be so disposed in the kiln as to permit of easy access on both sides 
of the pile and the taking of temperature and humidity readings whenever required by the 
inspector. 

INSTRUMENTS. 

10. At least one recording thermometer or recording hygrometer of approved make shall 
be used m each dry kiln compartment. 

11. Recording thermometers and hygrometers shall be checked at least once every kiln 
run with a standard thermometer or a glass thermometer calibrated to an accuracy of 1 F. 
This comparison shall be made with the thermometers placed so as to record the maximum 
temperature of any portion of the pile. 



Note 12. 



AIRCRAFT DESIGN DATA. 



51 



12. Thermometers. Thermometer bulbs must be shielded from direct radiation from steam 
pipes, wet lumber, cold walls or surfaces, and must receive a free circulation of air. 

13. The inspector may, at his discretion, place other thermometers at any point in the pile. 

14. Hygrometer. Humidity readings shall be made at least three times daily or more often 
as the inspector may desire, according to standard methods approved by the inspector, at the 
same points where the bulbs for the recording thermometers and hygrometers are placed. 

15. The following shall constitute a standard method: Use a glass or recording wet and 
dry bulb hygrometer with distilled water and with the wick changed at least once a week; 
produce a circulation of air past the wet bulb of at least 15 feet per second before reading. 

16. Hygrometer bulbs must be shielded from direct radiation of steam pipes, wet lumber, 
and cold walls or surfaces, and must receive a free circulation of air. 

STEAMING. 

17. At the beginning of the drying operations. Green wood is to be steamed at a" tempera- 
ture not to exceed 15 F. higher than the initial drying temperature specified in tables 5 and 6 
for six hours for each inch of thickness. Humidity during steaming period must be 100 per 
cent, or not below 90 per cent, in every portion of the pile. 

18. Previously air-dried wood is to be steamed at a temperature not to exceed 30 F. 
higher than the initial drying temperature specified in tables 5 and 6 for eight hours for each 
inch of thickness. Humidity during steaming period must be 100 per cent, or not below 90 
per cent, in every portion of the pile. 

19. Near the end of the drying. If on official test the stock shows serious casehardening 
it shall be steamed at a temperature not to exceed 20 F. higher than the final drying tempera- 
ture specified in tables 5 and 6 for not more than three hours. After steaming it shall be 
redried. 

TEMPERATURE AND HUMIDITY. 

20. Operating conditions are specified in tables 5 and 6, but lower temperatures and 
higher humidity conditions are permissible. 

21. The progression from one specified stage to the next must proceed without abrupt 
changes. 

22. Green wood (above 25 per cent moisture} over 8 inches thick. Reduce the temperature 
values given in tables 5 and 6 by 5 F. for each inch increase in thickness. 

23. Air-seasoned wood (below 25 per cent moisture) over 3 inches thick. Reduce the tem- 
perature values given in tables 5 and 6 by 5 F. for each inch increase in thickness. 



TABLE 5. 



srll lo abii'* il iod 




Stage of drying. 


Drying conditions. 


Maximum 
temperature. 


Minimum 
relative 
humidity. 


At the beginning 


120 
125 
128 
138 
142 
145 
145 


Per cent. 
80 
70 
60 
44 
38 
33 
33 


After fiber saturation is passed (25 per cent) 


At 20 per cent moisture 


At 15 per cent moisture 


At 12 per cent moisture 


At 8 per cent moisture... 


Final ... 








. 10 

;te 





lciHurtHj'ti <*i italic; ^\ 



52 



AIRCRAFT DESIGN DATA. 



Note 12. 



24. Table 5 applies to the following woods: 

Ash, white, blue, and Biltmore. 
Birch, yellow. 
Cedar, incense. 
Cedar, northern white. 
Cedar, western red. 
Cedar, Port Orford. 



Cypress. 

Pine, sugar. 

Pine, white (Idaho or eastern), 

Spruce, eastern (red or white) . 

Spruce, Sitka. 

Fir, Douglas. 



TABLE 6. 



Stage of drying. 


Drying conditions. 


Maximum 
temperature. 


Minimum 
relative 
humidity. 


At the beginning 


F 
105 
110 
117 
129 
135 
135 
135 


Per cent. 
85 
73 
62 
46 
42 
40 
40 


After fiber saturation is passed (25 per cent) 


At 20 per cent moisture 


At 15 per cent moisture ; . . . . 


At 12 per cent moisture 


At 8 per cent moisture 


Final 








25. Table 6 applies to the following woods: 

Cherry. 

Mahogany. 

Oak, white and red. 



Walnut, black. 
Maple. 



TESTS DURING DRYING. 



26. Samples shall be inserted in the pile in such manner that they will be subjected to the 
same drying conditions as that portion of the pile where inserted. They shall be so placed 
that they can be removed for periodical weighing in order to ascertain the average moisture 
content of the pile at any time. 

27. Three samples shall be used for each 10,000 board feet or less of material in the pile. 
Each sample is to be 2 feet long and shall not be cut nearer than 2 feet to the end of one of the 
pieces to be dried. 

28. The original moisture content of the samples shall be determined from sections 1 inch 
thick cut from both ends of the sample at the time it is sawed from the stick. This determination 
shall be made as provided in the specifications. (See Appendix, p. 147.) 

29. Before placing them in the pile, the ends of the samples must be given a thorough coating 
of asphaltum varnish to prevent end drying. 

30. The samples shall be weighed to an accuracy of one-tenth of 1 per cent immediately 
after cutting the moisture sections and before placing in the kiln. They shall be weighed at 
least daily when the time of drying is 10 days or less, and at least every other day when the time 
of drying is more than 10 days. 

31. The samples shall be placed in the pile and distributed so that they will be exposed to 
the average, most rapid, and slowest drying, except that they shall not be placed on the top 
or bottom layers. The samples placed in the portion of the pile where drying is most rapid 
shall control the regulation of the temperature and humidity. 

32. After obtaining the dry weight of the samples, the average moisture condition of the 
pile during drying shall be determined after each 



Note 12. AIRCRAFT DESIGN DATA. 53 



33. The following example will illustrate the method employed: 

Original weight of sample = 7. 35 pounds. 

Original moisture per cent (average of the two 1-inch sections) = 47. 

Calculated dry weight of sample=7.35 divided by 1.47 = 5.00 pounds. 

Current weight = 6. 23 pounds. 

Moisture in samples = 6. 23 5.00= 1.23 pounds. 

Current moisture per cent = (1.23 divided by 5.00) X 100 = 24.6. 

34. Continuous and permanent records must be kept of the temperature and humidity 
observations and the percentage of moisture in the lumber in the kiln. 



TESTS AFTER DRYING. 



35. Standard moisture content and case-hardening tests shall be made before the lumber 
is removed from the kiln. Material for these tests shall be taken from four boards for each 
5,000 board feet or less of material in the pile. Pieces selected must fairly represent the dried 
stock and shall be taken from different parts of the pile. At his discretion, the inspector may 
select other pieces for tests. Sections for these tests shall not be cut nearer than 2 feet to the 
ends of the pieces. 

36. Three adjacent sections 1 inch thick shall be cut from the centers of each test piece of 
stock. Each section must be weighed within five minutes to prevent moisture evaporation. 

37. The first section (A, fig. 27) shall be dried whole and the average moisture content 
obtained as provided in specifications. 

38. The second section (B, fig. 27, moisture distribution) shall be cut into an outer 
shell \ inch wide and an inner core inch wide. The moisture content of the outer shell and 
inner core shall be determined. 

39. The third section (C, fig. 27) shall be sawed parallel to the wide faces of the original 
board into tongues or prongs, leaving about ^ inch of solid wood at one end of the section. 
For material less than 2 inches thick two saw cuts shall be made and for material more than 
2 inches thick five saw cuts shall be made. In sections having six prongs the second prong 
from each side shall be broken out, leaving two outer and two central prongs. The center 
prong shall be removed from sections having only three prongs. 

40. The third section shall then be allowed to dry for 24 hours in the drying room and 
any curving of the prongs noted. 

41. If the prongs remain straight, perfect conditions of stress and moisture content are 
indicated. 

42. If the outer prongs bend in, conditions of casehardening are indicated. 

43. Only very slight casehardening is permissible. 

FINAL MOISTURE CONDITIONS. 

44. An average dryness of approximately 8 per cent, unless otherwise specified,* shall be 
required. A moisture content of from 5 to 11 per cent is permissible in individual sticks. 

45. The variation in moisture content between the interior and exterior portions of the 
wood, as shown by the "moisture distribution section" provided for in paragraph 38, must 
not exceed 4 per cent. 

SEASONING. 

46. Before manufacture the wood shall be allowed to remain in a room, with all parts 
under uniform shop conditions, at least two weeks for 3-inch material and other sizes in pro- 
portion. 

*See Note 3. 



54 



AIRCRAFT DESIGN DATA. 



Note 12. 



MOISTURE & CtfSE 



TEST 






SECTION ''8" 

^A^GI* 

0*5 SPLIT 




TH tcx s rock; 3*rtp 



I3 



TEST 
5 To 
OUT. 



C 



SECTION tf 



-7x/. THICKS ess 



/* 



J4 



//V 



\ 


\ 


^ 


\ 


\ 


s 


\ 


s 


\ 


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\ 


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\ 


s 


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\ 


\ 


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\ 








\^ 


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2 


\ 


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s 


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ft 


s 

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\ 




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1 


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r\ 


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r 


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NV 
\\ 

NN 
V 



STOCK 

Fig. 27. Moisture and casehardening test specimens. 



Note 12. AIRCRAFT DESIGN DATA. 55 

STEAMING AND BENDING OP ASH FOR LONGERON CONSTRUCTION. 

47. The ash shall be cut in the form of rough squares sufficiently large to allow for shrinkage 
and finish. 

48. Where it is necessary to bend this material, it shall be steamed in the green condition 
(more than 18 per cent moisture), bent on forms, and then kiln dried, as provided in paragraph 23. 

49. Steaming shall be conducted at a temperature not to exceed 212 F. for a period not 

longer than six hours and the bending shall be accomplished while the material is hot. 

* 

INSPECTION. 

50. At all stages of the process the lumber shall be subjected to inspection by the inspection 
department. 

51. The inspector shall mark all lumber with the official acceptance or rejection symbol. 

52. The inspector shall have free access to every part of the kiln at all times and shall be 
afforded every reasonable opportunity to satisfy himself that this specification is being complied 
with. 

NOTE 1. Steaming. It has been found possible to dry spruce satisfactorily without steaming to relieve casehard- 
ening. A preliminary steaming is given at low temperature, and after the drying has been completed the material 
is held in the kiln for 24 hours, with a humidity of 75 per cent or 80 per cent, at room temperature. 

NOTE 2. Tests during drying. (Paragraph 31.) The most rapid drying sample should not be confused with the 
sample of lowest moisture content. If the original moisture content was practically the same for all samples, then at 
any stage of the run the low sample would be the most rapid drying. However, the original moisture content is not 
likely to be uniform for the whole charge, and with stock of varying moisture content the run should be controlled for 
the stock of high moisture content. Other things being equal, the sample with the highest moisture content will dry 
the most rapidly, so that in such a case the specification would still hold. It would therefore be desirable to place 
the high original sample where it will be the most rapid drying sample. Otherwise it would be necessary to take into 
account the high stock possibly specify following the average of the samples on the entering air side of the pile 
provided the average is not more than 10 per cent below the high sample. 

NOTE 3. Final moisture content. For naval aircraft, it has been found desirable to have the moisture content on 
removal from the kiln about 12 per cent. The maximum individual variation allowed should not be over 3 per cent. 

TREATMENT OF WOOD AFTER REMOVAL FROM THE KILN. 

Lumber should be retained for at least two weeks after removal from the dry kiln in a 
shed or room where the conditions are approximately the same as in the shop where the 
material is to be worked up. The necessity for this will be understood upon consideration 
of the following facts: When lumber is drying in the kiln the outer surface is necessarily some- 
what drier than the interior. In good methods of drying this difference is a minimum and 
in bad methods of drying it is excessive; but it exists to a certain extent in all methods of 
drying. When the lumber has been dried down to a point somewhat below the condition to 
which it will finally come when exposed to the normal shop working conditions, it will gradu- 
ally reabsorb moisture on the outside. Thus, thoroughly kiln-dried lumber, if it has stood in 
an unheated room for some time, will be found to be drier on the inside than it is on the sur- 
face, though the difference is likely to be very small. Since differences in moisture content 
are indicative of internal stresses existing in the wood, it is evidently desirable to have the 
moisture distribution as uniform as possible before the lumber is made up into finished 
products; otherwise the adjustment of stresses, when the lumber has been cut up, will cause 
warping, checking, or other troubles. 

Just how long lumber should remain in the shop air after being kiln-dried will depend, 
of course, upon a great many circumstances. Generally speaking, the longer it remains the 
better it will be, provided the moisture conditions of the room in which it is stored are suitable. 
The same kind of a test as has been explained for casehardening occurring in the dry kiln will 
apply as a test of the lumber after remaining hi storage, to see whether the internal stresses 
have been neutralized. 



56 AIRCRAFT DESIGN DATA. Note 12. 

Even if casehardening has been removed in the dry kiln by resteaming at the end of the 
drying period, there may still exist within the lumber slight differences in moisture content 
which will gradually adjust themselves under proper storage conditions, so that material which 
has been steamed before removal from the kiln is also benefited by being allowed to stand in 
the room before it is manufactured. Recent experiments have shown that the length of time 
required for kiln-dried stock to reach a state of equilibrium under shop conditions after removal 
from the kiln may be reduced very materially by allowing it to remain in the kiln for about 
24 hours, after the drying has been completed, at a humidity of 75 pet cent or 80 per cent and 
shop temperature. 

Ideal conditions for the storage and manufacturing of lumber require regulation of the 
humidity, which should be kept slightly below that of the average conditions to which the 
lumber is to be subjected after it is put into service. The nearer these conditions are actually 
met hi practice the better are the results to be expected, particularly where requirements are 
so exacting as in the construction of airplanes. 

CHANGES OF MOISTURE IN WOOD WITH HUMIDITY OF AIR. 

Wood is a hygroscopic material; that is, it has the property of absorbing moisture from 
the air or surrounding medium. It has already been explained that there are two different 
kinds of moisture found in wood, namely, free water, which occupies the openings in the cell 
structure of the wood, and hygroscopic water, which is actually taken into the cell waUs and 
which upon being removed or added to wood causes shrinkage or swelling. 

There is a definite moisture content to which wood will eventually come if it is held in an 
atmosphere which is at a constant humidity and temperature. The moisture content of wood 
will vary with the average atmospheric conditions, also with the size of the material. Thus, 
ordinary lumber which is stored in the open during the summer months for sufficient time 
will eventually attain a moisture content of from 8 to 15 per cent, and wood stored indoors 
in a heated building will in time fall to about 5 or 6 per cent because of the lower relative 
humidity. If the relative humidity is constant, an increase in temperature decreases the 
moisture-holding power of the wood. However, the moisture content is not appreciably 
affected by temperature within a range of 25 to 30 F. 

Figure 28 shows the relation between the moisture content of wood and the humidity 
conditions of the atmosphere. The data for the curve were obtained by keeping the wood 
at a constant humidity and temperature until no further change in moisture Occurred. This 
curve can be used as an aid in controlling the moisture conditions of wood, the approximate 
atmospheric condition being known, and in determining the proper humidities for storing 
lumber hi order to secure a certain moisture content and give uniform material for use in fine 
wood jointing, propellers, etc. It is of importance to have wood to be used for propellers of 
uniform moisture content. The curve may be used also to prepare wood for use in a given 
locality, such as the border States, where the humidity is usually very low. Propellers for 
use under such conditions should be made up at a low moisture content, in order that there 
may be less tendency for moisture changes to take place when they are put in service. It 
must be remembered that this curve must not be used for dry- kiln work because of the fact 
that the dry-kiln temperatures used are higher than those at which the data were collected 
Furthermore, the curve represents the ultimate moisture content at a given temperature and 
humidity, and in the case of large pieces of wood this moisture content would not be reached 
for a long period of time. Kiln drying tends to reduce the hygroscopic properties of wood, 
hence curves for kiln-dried wood are lower than the one given. For example, wood that had 
been dried to 2 per cent moisture, or less, if subjected to humidities between 30 and 70 per 
cent, would probably show a corresponding moisture content about H to 2$ per cent lower 
than in the curve in figure 28. 



Note 12. 



AIRCRAFT DESIGN DATA. 



57 



VENEER AND PLYWOOD. 

VENEER. 

Veneer may be loosely defined as thin wood. It usually varies in thickness from one- 
hundredth inch to one-eighth inch, though it is commercially possible to cut it thinner, and 
thicker sizes are to be obtained. However, in general, veneer used in aircraft falls within the 
limits stated. 

There are three common methods of manufacturing veneer, as follows: (1) The rotary 
process, (2) the slicing process, (3) the sawing process. 

By far the greater portion of all veneer manufactured is made by the rotary process. 
Veneer made by this process is all slash cut, and the length along the grain is limited by the 
length of the veneer lathe. Rotary veneer longer than 100 inches is more or less uncommon. 



10 

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CO'MPQSlfE! CURVE 

SHOWINQ THE MOISTURE CONTENT OP T 
WOODS AT DIFFERENT HUMIDITIES A 
ORDIMARY "ROOM TEMPERATURES- 

WHITE OAK .OSITKA SPRUC 
OBLACK WALNUT YELLOW BIRC 
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30 



60 



70 



80 



100 



Fig. 28. Composite curve of moisture content of fine woods at different humidities and ordinary room temperature. 

Sliced veneer is usually manufactured only from the finer woods. On account of the fact 
that it is possible to produce quartered veneer on slicing machines, and the waste on account 
of saw kerf is absent, this method of manufacture is preferred where pattern is important 
and the value of the wood is great. The length parallel to the grain of sliced veneer is limited 
by the length of the knife. 

Sawed veneer can be produced in almost any reasonable length and from any kind of stock. 
The material produced may be either quartered or slash. In general, sawed veneer will not be 
specified for aircraft uses, to the exclusion of rotary stock, except where it is necessary to have 
extra long lengths or quartered stock or for some other reason it is impossible to secure the 
stock by rotary cutting. It may happen, for instance, -that the stock from which the veneer 
is to be cut can not be handled to advantage in a rotary lathe on account of its shape. 



58 



AIRCRAFT DESIGN DATA. 



Note 12. 



'S 3 
I I 

5^ 3 



I 



a 


-s 

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CO <> t-~ CO CO i I iO 1C Oi 



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MStfccSPSoice^SPHMccPS 




Note 12. AIRCRAFT DESIGN DATA. 59 

A special series of tests was made to determine the effect of the method of cutting veneer 
on the strength of plywood panels made from it. Detailed results are presented in table 7, 
and the general conclusions drawn follow: 

(a) The effect of the method of cutting veneer on the strength of plywood depends on 
the species cut, although in general, the effect, as shown by the bending and tension tests, 
is not great. 

(b) Of the three methods of cutting, the sawed and sliced material, for the species tested, 
gave the more similar results. The commercial white ash, sugar maple, and yellow poplar 
pannels cut by these methods were slightly superior in bending and tensile strength to the 
rotary-cut panels. 

(c) For birch the panels of rotary-cut veneer were slightly superior in bending and tensile 
strength to panels of either sawed or sliced veneer. 

(d) For the species tested, with the possible exception of the African mahogany, panels 
of sawed veneer twist less than panels of either sliced or rotary-cut veneer. 

(e) With the exception of birch the results show little difference in the twisting of panels 
of sliced or rotary-cut veneer. 

For the convenient calculation of the weight of veneer and plywood, table 8 has been 
prepared. This table presents the weights, per square foot, of veneer of various thicknesses 
and species, at the average air-dry moisture condition shown in the second column. The 
weight of blood albumen glue per square foot and the weight of a typical casein glue (Certus) 
per square foot are also given, so that it is possible to calculate the average weight of any ply- 
wood made up of the species listed and using blood or casein glue. This is done simply by 
adding together the weights of the individual plies and the weight of the glue, which is obtained 
by multiplying the weight of the glue per square foot by the number of glue lines in the ply- 
wood. This number is always one less than the number of plies. 

While it is usually not necessary to know the tensile strength of single-ply veneer as such, 
this figure is very convenient in computing the probable strength in tension of plywood made 
up in various manners. The last column of table 9 presents computed tensile strengths of 
single-ply veneer. Reference to the other data in this table will be found in the text under the 

discussion of plywood. 

PLYWOOD. 

In general plywood consists of a number of layers of wood veneer glued together by some 
suitable glue or adhesive. Occasionally the term is applied to material in which one or more of 
the layers are composed of some other material than wood. 

The weight of plywood has already been discussed in connection with the weight of veneer 
(see table 8). 

Until recently little information was available on the mechanical properties of plywood. 
Within the last year and a half, however, about 50,000 tests have been made and tabulated. 
Since the subject is rather new, a full discussion is presented, followed by tables of strength 
properties. 

PROPERTIES OF WOOD PARALLEL AND PERPENDICULAR TO THE GRAIN. 

Wood, as is well known, is a nonhomegenous material with widely different properties in 
the various directions relative to grain. This difference must be recognized in all wood con- 
struction, and the size and form of parts and placement of wood should be such as to utilize 
to the best advantage the difference in properties along and across the grain. It is the strength 
of -the fibers in the direction of the grain that gives wood its relatively high modulus of rupture, 
and tensile and compressive strength parallel to the grain. Were it a homogenous material, 
such as cast iron, having the same strength properties in all directions that it has parallel to 
the grain, it would be unexcelled for all structural parts where strength with small weight is 
desired. As it is the tensile strength of wood may be 20 times as high parallel to the grain as 
perpendicular to the grain and its modulus of elasticity from 15 to 20 times as high. 



60 



AIRCRAFT DESIGN DATA. 



Note 12. 






J3 

I 

.a 



a 
^ 



I I 














n co oo i- o o> oo oi 



OOlOOOO-<OOOOOSt^C^CO'^*COt^^H 



t-' ^ oci 06 oo t-' o -H H 



<N <N ^ - IN <- -< ^ IN . 



<N IN c ** c c i- N c IN IN M e -< < ^ >-< 






i 






; 






Is 

||=. 

-* 



T3 
C 







rt 



3*8 

"E 60 

o-S . 



0|(3 



o 
X 



o 
X 



X 



** 2 

<H ' 

a 



X -2 



^ 
-d 



6P 



W ^| 

-*-' M 

13 -2 

g g 



'I 



a 



a d 



3 J^'to 



S is- 

QJ *^ 



-a " 
JJ a 
9'S' 

ill 



.2 d 



H 



Note 12. AIECEAFT DESIGN DATA. 



In the case of shear the strength is reversed, the shearing strength perpendicular to the 
grain being much greater than the strength parallel to the grain. The low parallel-to-the-grain 
shearing strength makes the utilization of the tensile strength of wood along the grain difficult 
since failure will usually occur through shear at the fastening before the maximum tensile 
strength of the member is reached. 

The large shrinkage of wood across the grain with changing moisture content may intro- 
duce distortion in a board that decreases its uses where a broad flat surface is desired. The 
shrinkage from the green to the oven-dry condition across the grain for a flat sawn board as 
determined by the average of 150 species is about 8 per cent, and for a quarter-sawed board 
about 4 per cent, while the shrinkage parallel to the grain is practically negligible for most 
species. 

PLYWOOD PANELS V. SOLID PANELS. 

It is not always possible in a given use so to proportion a board or solid panel as to develop 
the necessary strength in every direction and at the same time to utilize the full strength of 
the wood in all directions of the grain. In such cases it is the purpose of plywood to meet this 
deficiency by crossbanding, which results in a redistribution of the material. 

In building up plywood a step is made in obtaining equality of properites in two direc- 
tions parallel and perpendicular to the edge of a board. The greater the number of plies 
used for a given panel thickness, the more nearly homogeneous in properties is the finished panel. 
Thus, in an- airplane engine mounting made of 15-ply veneer the mechanical properties of the 
panel in the direction parallel to the grain of the faces are almost the same as those in the direc- 
tion at right angles to this. However, an increase in such properties as bending strength and 
modulus of elasticity at right angles to the grain of the faces is accompanied by a decrease of 
the values parallel to the grain of the faces with an increase of the number of plies. For a very 
large number of plies (of the same species and thickness) we may assume that the tensile strength 
in the two directions is the same and that it is equal to the average of the parallel-to-the-grain 
and perpendicular-to-the-grain values of an ordinary solid board or panel. This is not always 
exactly true, since the maximum stress of the plies with the grain at right angles to the force 
may not be reached at the same time as the maximum of the plies with the grain parallel to 
the force. Internal stresses due to change of moisture content may also tend to unbalance the 
strength ratio. 

SYMMETRICAL CONSTRUCTION IN PLYWOOD. 

On account of the great difference in shrinkage of wood in the direction parallel to the 
grain and perpendicular to it, a change in moisture content of plywood will inevitably either 
introduce or release internal stresses. Consider, for example, a three-ply construction and 
subject it to low-humidity conditions, so that the moisture content of the plywood is lowered. 
Because the grain of the core is at right angles to the grain of the faces, the core will tend to 
shrink a great deal more than the faces in the direction of the grain of the faces. This shrinkage 
subjects the faces to compression stresses and the core to tensile stresses. If the faces are of 
exactly the same thickness and of like density, the stresses are symmetrically distributed and 
no cupping should ensue. 

Now consider that one face of a three-ply panel has been glued with the grain in the same 
direction as the core and that the moisture content of the panel is reduced. It is obvious that 
the internal stresses are now no longer symmetrically distributed, inasmuch as the compressive 
stress in one face has been removed. This face now shrinks a great deal more than the other 
face in the direction of the grain of the latter. The result is that cupping takes place. Figure 
29a shows the effect of drying on a three-ply construction (unsymmetrical) in which the grain 
of two adjacent plies was parallel. The panel has curled up into a cylindrical surface with the 



AIECEAFT DESIGN DATA. 



Note 12. 



parallel plies on the inner side. By adding another ply at right angles to the core we see that 
symmetry could again be established and that while we would have a four-ply panel in reality 
it gives a three-ply construction with a core of double the face thickness and would be regarded 
as such. 

The necessity for exercising care in sanding the faces of a panel is obvious, inasmuch as 
different thicknesses on the faces would introduce unequal forces with changing moisture content. 

In order to obtain symmetry, it is also necessary that both faces or symmetrical plies be 
of the same species. 

To summarize: A veneer panel must be symmetrically constructed in order to retain its 
form with changes of moisture. Symmetry is obtained by using an odd number of plies. The 




Fig. 29. (a) Cupping resulting from] unsymmetetrical construction in plywood, (b) Twisting resulting from ply- 
wood construction with grain of faces at 45 degrees with grain of core. 

plies should be so arranged that for any ply of a particular thickness there is a parallel ply of 
the same thickness and of the same species on the opposite side of the core and equally removed 
from the core. 

DIRECTION OF THE GRAIN OF ADJOINING PLIES. 

In the discussion of symmetry of construction it was understood that the adjoining plies 
were always glued with the grain either parallel to or exactly at right angles to the core. In care- 
less construction this may not always be the case. An extreme case of this kind is shown in figure 
29b, in which the plies were glued so that the grain of each face of the panel was at 45 degrees 
with the grain of the core and so that the two faces were at 90 degrees with respect to each other. 
Whereas the unsymmetrical construction introduces cupping, a construction involving angles 
other than and 90 degrees introduces twisting. 

In building up a three-ply veneer panel the core should be glued with the grain at 90 
degrees with the faces or as close to this as feasible. 

EFFECT OF MOISTURE CONTENT. 

. 

The previous discussion has brought out the fact that a change in moisture content of a 
panel may introduce cupping and twisting in the panel if the panel is not carefully constructed. 
Hence it is highly desirable that the moisture content of the veneer before gluing be controlled 



Note 12. AIRCRAFT DESIGN DATA. 63 



so as to make the moisture content of the finished panel when it leaves the clamps about the 
same as it will average when in use and that all plies be at the same moisture content before 
gluing. The limits of from 10 to 15 per cent moisture in the finished panel will usually give 
satisfactory results when the panel is in service in the open air. 

SHRINKAGE OF PLYWOOD. 

The shrinkage of plywood will vary with the species, the ratio of ply thickness, the number 
of plies, and the combination of species. The average shrinkage obtained in 54 tests on a variety 
of combinations of species and thicknesses in bringing three-ply wood from the soaked to the 
oven-dry condition was 0.45 per cent parallel to the face grain and 0.67 per cent perpendicular 
to the face grain, with the ranges of from 0.2 to 1 per cent and 0.3 to 1.2 per cent, respectively. 
Other combinations and thicknesses may extend these limits and change the average somewhat. 
The species included in the tests made were mahogany, birch, poplar, basswood, red gum, 
chestnut, cotton gum, elm, and pine. 

EFFECT OF VARYING THE NUMBER OF PLIES. 

The question frequently arises, Should three or more plies be used for a panel of a given 
thickness? The particular use to which the panel is to be put must answer this question. 
Commercial considerations will also enter. Veneer of most species less than 7 V mcn thick can 
not be cut by the rotary process with uniform success, and while a number of species may be 
cut by slicing to -fa inch and less, such material is limited in width. 

In general it may be said that the greater the number of plies the flatter the plywood will 
remain when subjected to moisture variations. 

If the same bending or tensile strength is desired in the two directions, parallel and per- 
pendicular to the grain of the faces, the greater the number of plies the more nearly the desired 
result is obtained. This same result may be obtained by a proper selection of ratio of core to 
total plywood thickness in three-ply construction. It must be borne in mind, however, that 
a plywood with a large number of plies, while stronger at right angles to the grain of the faces, 
can not be as strong parallel to the grain of the faces as three-ply wood, and hence a three-ply 
panel is preferable where greater strength is desired in one direction than in the other. Table 
1 1 gives strength values for three-ply, five-ply, and seven-ply yellow birch plywood. 

Where great resistance to splitting is desired, such as in plywood that is fastened along 
the edges with screws and bolts and is subject to forces through the fastenings, a large number 
of plies affords a better fastening. 

It is a common experience that a glued joint is weakened when two heavy laminations are 
glued with the grain crossed. The same weakness exists in plywood when thick plies are 
glued together. When plywood is subject to moisture changes, stresses in the glued joint 
due to shrinkage are greater for the thick plies than for the thin plies. Hence in plywood 
constructed with many thin plies the glued joints will not be as likely to fail as in plywood 
constructed of a smaller number of thick plies. 

EFFECT OF VARYING THE RATIO OF CORE TO TOTAL THICKNESS. 

At first thought it may seem that the proper selection of the ratio of core to total plywood 
thickness in three-ply construction may enable the designer to get the same strength in both 
directions as is possible with many-plied panels. While this is true in general, it is not true 
that the same ratio will serve for both tension and bending. In birch, for example, a ratio of 
core to total plywood thickness of 5 to 10 gives the same strength in tension in both directions, 



AIRCRAFT DESIGN DATA. Note 12. 



but a ratio of about 7 to 10 gives the same strength in bending. For either ratio the plywood 
is not 'nearly as resistant to splitting as plywood of a greater number of plies totaling the same 
thickness. 

SPECIES OF LOW DENSITY FOR CORES. 

Where column strength and a flat panel are desired, full advantage of a strong species, 
such as birch, in the faces is best attained by using a thick core of a species, such as basswood 
or yellow poplar, rather than a thinner core of the same weight but of a species of geater density. 
A combination of strong faces and a thick light wood core has the advantage of greater separa- 
tion of the faces than when using the thinner core of a heavier species, giving a marked increase 
in the internal resistance to forces that tend to bend the panel and a correspondingly great 
strength in bending with the same weight. 

Consider, for example, that a certain panel contains a core of the same weight but of a 
specific gravity of one-half that of another core. This means that the core of lighter species 
is twice as thick as the core of high density and that the panel faces are spaced twice as far 
apart. In a long column, for instance, this is very desirable, for the maximum load a column 
can carry varies as the cube of the thickness. It is evident that a marked superiority in the 
load sustained might be expected in the low-density core panel over the high-density core panel 
of the same weight when the load is applied parallel to the grain of the faces. 

The same line of reasoning applied to column strength may also be applied to resistance 
to cupping. A panel with a core of low density will cup less than a panel of the same weight 
with a core of high density. The load to produce failure in bending would likewise be greater 
for the former case. 

PLYWOOD TEST DATA. 

The column-bending modulus is obtained by loading a piece of plywood 5 inches by 12 
inches as a column with the 12-inch length vertical. It is computed by the following formula: 

P 6M 

S= A + 5J 2 ' where 

S = Column-bending modulus. 

A A t 

A = Area of cross section. 

P == Load at maximum moment. 

. 
M = Maximum bending moment. 

T TTT- 1 1 t 

o = Width of test piece. 
d = Thickness of test piece. 

Like the modulus of rupture in the standard static bending test, the column-bending 
modulus is not a true stress existing in the fibers at the instant of failure. It is merely a measure 
of the magnitude of the external bending moment that a piece of plywood can withstand before 
it fails. 

If a piece of plywood is subjected to forces that tend to bend it, as would be the case 
either in a long column or in a beam, the designer confronted with the problem of determining 
its proper thickness may use the column-bending modulus in exactly the same way that the 
modulus of rupture is used. It will be noted, of course, that the column-bending modulus 
must be used which applies to the particular plywood construction desired. The total plywood 
thickness is to be used in all equations involving the column-bending modulus. 

The use of the tensile strength data is obvious. The strength values given are based on 
the total plywood thickness, (Table 9.) 



Note 12. 



AIRCRAFT DESIGN DATA. 



65 



TABLE No. 9. Tensile strength of plywood and veneer. 



Number 
of tests. 



Moisture at 

test 
(per cent). 



Specific 
gravity * 
of ply- 
wood. 



Tensile 

strength f of 

3-ply wood 

parallel to 

grain of faces 

(pounds per 

square inch). 



Tensile 
strength 1 of 

single-ply 
veneer, 14 (d) 
(pounds per 
square inch). 



(a) 

Ash, black ................................................ 120 

Ash, commercial white ..................................... 200 

Basswood .................................................. 200 

Beech .................................................... 120 

Birch, yellow .............................................. 200 

Cedar, Spanish ............................................ 115 

Cherry ' ................................................... 115 

Chestnut ................................................. 40 

Cottonwood ................................................ 120 

Cypress, bald ............................................. 35 

Douglas fir ................................................ 174 

Elm, cork ................................................ 65 

Elm, white .......................................... . ____ 160 

Gum, black ............................................... 35 

Gum, cotton ............................................... 80 

Gum, red .................................................. 182 

Hackberry ...................................................... 80 

Hemlock, western .......................................... 119 

Magnolia 2 ................................................. 40 

Mahogany, African 3 ....................................... 20 

Mahogany, Philippine * ................................... 25 

Mahogany, true ........................................... 35 

Maple, soft 5 .............................................. 120 

Maple, sugar ........................ . ..................... 202 

Oak, commercial red ....................................... 115 

Oak, commercial white ..................................... 195 

Pine, white ................................................ 40 

Poplar, yellow ............................................ 165 

Redwood .................................................. 65 

Spruce, Sitka .............................................. 103 

Sycamore ................................................. 163 

Walnut, black ............................................ 110 



(6) 
9. 1 

10. 2 
9. 2 
8. 6 

8. 5 
13. 3 

9. 1 

11. 7 
8.8 

10. 3 

8. 7 

9. 4 

8. 9 
10. 6 
10. 3 

8.7 
10.2 

9. 7 
9. 9 

12. 7 

10. 7 

11. 4 
8. 9 

8. 

9. 3 
9. 5 

10.2 

9. 4 

11. 2 

8. 4 

9. 2 
9. 1 



(c) 

0.49 
.60 
.42 
.67 
.67 
.41 
.56 
.43 
.46 
.47 
.49 
.62 
.52 
.54 
.50 
.54 
.54 
.47 
.59 
.52 
.53 
.48 
.57 
.68 
.59 
.64 
.43 
.50 
.41 
.43 
.56 
.59 



6,180 
6,510 
6,880 

13,000 

13, 200 
5,200 
8,460 
4,430 
7,280 
6,560 
6,230 
8,440 
5,860 
6,960 
6,260 
7,850 
6,920 
6,800 

10,000 
5,370 

10, 670 
6,390 
8,180 

10, 190 
5,480 
6,730 
5,640 
7,390 
5,100 
5,600 
8,030 
8,250 



(4 

9,270 

9,770 
10, 320 
19,500 
19, 800 

7,800 
12, 690 

6,645 
10, 920 

9,840 

9,340 
12, 660 

8,790 
10, 445 

9,390 
11, 775 
10, 380 
10,200 
15,000 

8,060 
16, 010 

9,585 
12, 270 
15, 290 

8,220 
10, 095 

8,460 
11, 080 

7,650 

8,400 
12, 045 
12, 375 



* Specific gravity based on oven-dry weight and volume at test. 

t Based on total cross-sectional area. 

t Based on assumption that center ply carries no load. 

Data based on tests of 3-ply panels with all plies in any one panel same thickness and species. 

' Probably black cherry. Probably evergreen magnolia. Probably khaya sp. Probably tanguile. 



Probably silver maple. 



SAMPLE COMPUTATION. 






To obtain the tensile strength of 3-ply wood consisting of two ^Vincli birch faces and a T Vinch basswood core. 

Parallel to face grain=2X^Xl9,860=l,986 pounds per inch of width. 

Perpendicular to face grain=lXAX9, 450=591 pounds per inch of width. 

This computation neglects the tensile strength of the ply or plies perpendicular to the grain, which is comparatively 
small. The results are therefore slightly in error. 

The resistance to splitting is of considerable importance in panels when these are to be 
fastened with screws or bolts and are subject to forces at the fastenings. The numerical value 
of the work required to split a panel of a given thickness has no direct application in design. 
It is only in comparison with other panels of other species or construction that work in splitting 
has any significance. The work done is, of course, a measure of resistance to splitting. It is 
not entirely a property of the wood, as it depends very largely upon the strength of the glue. 
98257 19 No. 12 5 



66 



AIRCRAFT DESIGN DATA. 



Note 12. 



If - 



.98 



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II 



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111 



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Note 12. 



AIRCRAFT DESIGN DATA. 



67 



The results of strength tests on plywood of various common veneer species are given in 
table 10. Except for birch all tests are on only one shipment of the species, so that the results 
will in all probability be changed somewhat by the addition of future test data. The mahogany 
results are on thin plywood ranging in thickness from ^ inch to -fa inch, while the sizes of the 
plywood for all other species ranged from -^ inch to -f inch. 

In most cases it was found that the column-bending modulus of thin plywood was slightly 
less than the column-bending modulus of the thick plywood. 

TABLE 11. Comparison of strength of 3, 5, and 7 ply yellow birch plywood, all plies of same thick- 
ness in any one panel. 











Column-bending modulus, 
in pounds per square inch. 


Tension, in pounds per 
square inch. 


Average 
splitting 














resistance 


Number 
of plies. 


Average 
specific 
gravity.* 


Average 
per cent 
moisture. 


Number 
of tests. 


Parallel.t 


Perpendicu- 
Tar.f 


Parallel.t 


Perpendicu- 
lar.t 


compared to 
3-ply birch, 
for the 
same ply- 
wood thick- 


















ness, in per 


















cent of 3-ply. 


3 


0.67 


8.5 


195 


16, 000 


3,200 


13, 200 


7,700 


100 


5 


.67 


6.6 


25 


14, 700 


6,800 


13, 100 


8,600 


129 


7 


.70 


7.1 


25 


14,300 


7,900 


12, 900 


9,300 


191 



* Specific gravity, based on oven-dry weight and volume at test. 

t Parallel and perpendicular refer to direction of grain of faces relative to direction of application of force. 

Table 11 shows the decrease in the unit strength of plywood in the direction of the grain 
of the faces when the number of plies is increased, and the increase in the unit strength of ply- 
wood perpendicular to the grain of the faces when the number of plies is increased. 

TABLE 12. Comparison of strength of three-ply wood having a core of high density with similar 
plywood having a core of low density of the same thickness; each ply -fa inch thick. 

Number of tests very limited. Results tabulated will probably be changed by further tests. 



Species. 


Num- 
ber of 
tests. 


Ply- 
wood 
thick- 
ness 


Per 
cent 
mois- 
ture 
at test. 


Specific 
gravity, 
based 
on 
oven- 
dry 
weight 
and 
volume 
at test. 


Column-bending 
modulus in pounds 
per square inch. 


Tension in pounds 
per square inch. 


load in pounds 
per square inch, 
5 by 12 inch 
specimen test- 
ed as a column. 


Face. 


Core. 


Face. 


Parallel.* 


Perpen- 
dicular.* 


Parallel.* 


Perpen- 
dicular.* 


Paral- 
lel.* 


Perpen- 
dicu- 
lar.* 


Birch 


Birch 


Birch 


30 
10 
33 
5 
20 
5 
5 


Inches. 
0.15 
.14 
.15 
.15 
.14 
.15 
.14 


9.4 
8.2 
6.9 
7.0 
9.5 
8.3 
6.5 


0.68 
.61 
.69 
.62 
.55 
.44 
.51 


14,200 
15,200 
16, 100 
17, 700 
9,550 
7,200 
10, 100 


3,170 
1,600 
3,210 
2,600 
2,060 
1,400 


11, 900 
12, 900 
9,910 
12, 000 
8,410 
4,900 
6.200 


7,290 
3,800 
6,540 
3,700 
4,720 
3,000 
4,500 


258 
250 
265 
247 
193 
115 
149 


21 
12 
45 
15 
35 
11 
17 


Do 
Sugar maple 
Do 


Basswood 


...do 


Sugar maple 
Basswood. 
Red gum 


Sugar maple 
...do 
Red gum... 
do 


Red gum... 
Do 


Basswood 


Do 


Yellow poplar.. 


...do 







* Directions refer to direction of application of the force relative to the grain of the faces. 

Table 12 shows that the strength values of plywood parallel to the grain of the faces are 
practically the same for three-ply wood having a core of dense wood as for plywood having a 
core of light wood. The strength values across the grain of the faces are, however, very much 



68 



AIRCRAFT DESIGN DATA. 



Note 12. 



less for the plywood with core of low density. In other words, the strength values of three-ply 
wood parallel to the grain of the faces are almost entirely determined by the strength values 
of the face material, and the strength values across the grain of the faces are very largely deter- 
mined by the strength values of the core species. 

Table 13 gives a number of factors that are of value in selecting the thickness and species 
of the plies for a three-ply panel. 

TABLE 13. Thickness jactors jor veneer. 

Giving: (1) Veneer thickness for the same total bending strength as birch; (2) veneer thickness for the same weight 

as birch. 



Species. 


D. 

Average 
specific 
gravity of 
species * 
based on 
oven-dry 
weight and 
air-dry 
volume. 


Specific 
gravity of 
giued ply- 
wood as 
tested. 


Per cent 
moisture of 
plywood as 
tested. 


S. 

Per cent 
unit bend- 
ing strength 
compared 
with birch.t 


Thickness 
factor for 
the same 
total bend- 
ing strength 
as birch, 

VIoo 
IT 


K,,. 

Thickness 
factor for 
the same 
weight as 
birch, 
0.63 
D ' 


Ash black 


0.50 


49 


9 1 


52 


1 39 


1 26 


Ash, commercial white 


.58 


.60 


10 2 


72 


1. 18 


1 09 


Basswood 


.38 


.42 


9 2 


48 


1 44 


1 66 


Beech 


.63 


.67 


8.6 


94 


1.03 


1.00 


Birch, yellow 


.63 


.67 


8.5 


100 


1 00 


1.00 


Cedar, Spanish 


a.34 


.41 


13 3 


43 


1 52 


1 85 


Cherry & 


.51 


56 


9 1 


80 


1 12 


1 24 


Chestnut 


.44 


.43 


11.7 


34 


1.72 


1.43 


Cottonwood 


.43 


.46 


8 8 


56 


1.34 


1.47 


Cypress, bald 


.44 


.47 


10.3 


53 


1.37 


1.43 


Elm cork 


.66 


62 


9 4 


78 


1 13 


95 


Elm, white 


.51 


.52 


8 9 


58 


1 31 


1.24 


Fir, Douglas 


.52 


.49 
54 


8.7 
10 6 


60 
56 


1.29 
1 34 


1.24 
1 21 


Gum' cotton 


52 


50 


10 3 


48 


1 44 


1 21 


Gum, red . 


.49 


54 


8 7 




1 25 


1 29 


TT T 't 

Hackberry , 


54 


54 


10 2 


55 


1 35 


1 17 


Hemlock, western . . 


42 


47 


9 7 


60 


1 29 


1 50 


Magnolia 


.51 


58 


9 9 


67 


1 22 


1.24 


Mahogany, African 'stic 


a . 46 


52 


12 7 


56 


1 34 


1 37 


Mahogany, Philippine d 


o 57 


53 


10 7 


68 


1 21 


1 10 


Mahogany, true 


a 49 


48 


11 4 


57 


1 32 


1 29 


Maple, soft e 


48 


57 


8 9 


74 


1 16 


1 31 


Maple, sugar 


62 


68 


8 


100 


1 00 


1 02 


Oak, commercial red 


63 


59 


9 3 


59 


1 30 


1 00 


Oak, commercial white 


.69 


.64 


9 5 


69 


1 20 


.91 


Pine, white 


.39 


43 


10 2 


52 


1 38 


1 61 


Poplar, yellow 


41 


50 


9 4 


58 


1 31 


1 54 


Redwood 


a 36 


41 


11 2 


49 


1 43 


1 75 


Sycamore 


.50 


56 


9 2 


71 


1 09 


1.26 


Spruce, Sitka 


38 


43 


8 4 


50 


1 41 


1 66 


Walnut, black 


57 


59 


9 1 


83 


1 10 


1 10 

















* Taken from Bulletin 556 of the U. S. Department of Agriculture. 

' Average of the column-bending moduli parallel and perpendicular to grain compared to birch. 



a Based on subsequent tests. 
fr Probably black cherry. 



Coast type Douglas fir. 
d Probably tanguile. 



Probably silver maple. 



The thickness factor (Ks) is used to obtain the thickness of a ply of any species having 
the same total bending strength as a given ply of birch. It is arrived at as follows: 

The strength of any structural member is determined either by the direct load it can 
sustain or the bending moment it can resist without failure. In plywood the latter factor is 
the better criterion of strength. If we denote the maximum bending moment of a strip of 



Note 12. AIRCRAFT DESIGN DATA. 



three-ply wood 1 inch wide and of thickness d v by M t and the stress at failure by S t (column- 

Ci j n 

bending modulus), then M t = --- 

Similarly, the strength of another strip of a different species will be denoted by M 2 , its 
stress at failure S 2 , and thickness d 2 . By a proper selection of thickness d 2 the second strip 
may be made to withstand the same maximum bending moment, so that M 2 = M x or S 2 d 2 2 = S^ 2 . 

VS 
o* Taking d t as the unit of thickness of a birch ply- 
Oj 

wood strip and expressing the maximum stresses in percentage of birch, we have d 2 = -v a ' or, 

in general, K s = -J-^-> where K s is the thickness of the plywood, whose column-bending 

modulus corresponds to S and whose total bending strength, given by the bending moment, 
is the same as that of birch plywood of thickness unity. 

The same reasoning also applies to single plies, so that K s may be used to get the thickness 
of a single ply, which will give the same total bending strength as a birch ply of thickness unity. 
For example, for yellow poplar K s = 1.46, and a ply of this species, 1.46X^ = 0.091 inch, is 
equivalent in strength in bending to a birch ply yg- mcn thick. 

By way of explanation it must be understood that unit bending strength refers to a maxi- 
mum stress such as the modulus of rupture, or the column-bending modulus, while total bending 
strength refers to the load or bending moment a beam can sustain or the bending moment a 
column can sustain. 

It should be kept in mind that these factors will doubtless be modified, somewhat by further 

LGS vS. 

The thickness factor (K w ) is used to obtain the thickness of a ply of any species equal in 
weight to a ply of yellow birch of given thickness. It is obtained by simply dividing the density 
of birch by the density of the species for which the thickness is desired. The density data 
used in computing K w are the same as that given in United States Department of Agriculture 
Bulletin 556, "Mechanical Properties of Woods Grown in the United States." The weight of 
the glue in the plywood is neglected. 

For yellow poplar, for example, the thickness of a ply equal in weight to a y^-inch ply of 
birch is 1.54 X y^ 0.096 inches. 

The column-bending tests, upon which the data in table 10 are based, were all made on 
specimens of the same lengths, and it was felt desirable to determine what effect, if any, the 
change in length of the column might have upon the maximum unit load, the slenderness ratio 
remaining constant. Special panels of three-ply birch, all plies of the same thickness in each 
panel, were made up from veneer of the following thicknesses: ^, ^, ^r, yV, rV> s> an d test 
columns varying in length from 20 inches to 6 inches were cut from them and tested. The 
conclusion drawn from these tests is that for a given slenderness ratio the length of the column 
has little, if any, effect on the maximum unit load which a three-ply birch column will sustain. 
It is assumed that the same conclusion will apply to panels of other species. 

Table 9, to which reference has already been made, presents data by which it is possible 
to calculate the strength in tension of plywood composed of various kinds of veneer. Column 
(d) of this table is identical with the corresponding column in table 10. Column (e) is to be 
used in calculating the strength in tension of plywood made up of different species. The 
method of calculation is based upon the fact that the tensile strength of wood in a direction 
perpendicular to the grain is very small in comparison with that parallel to the grain and 



70 



AIRCRAFT DESIGN DATA. 



Note 12. 



may, therefore, for purposes of approximation, be neglected. To obtain the tensile strength 
in any direction, simply add together the tensile strength, parallel to the grain, of the indi- 
vidual plies the grain of which lies parallel to the direction in which the strength is desired. 
The sample computation will make this entirely clear. 

The shearing strength of plywood is of importance in connection with the design of box 
beams having plywood cheek pieces and for similar construction. Several series of tests are 
under way to determine the shearing strength of plywood of various thicknesses when unsup- 
ported for various distances. While these tests are not as yet completed, it is evident that 
it will not be possible to use a shearing strength in calculating these members much greater 
than that of solid wood of the same species. There is much more residual strength in ply- 
wood after the first failure than in solid wood, and for this reason a somewhat higher working 
stress would be justified. Until more data are available the shear allowed in plywood should 
not be over 25 per cent greater than that allowed in solid wood of the same species. This 
assumes that in the cheeks of horizontal beams the face plies will be vertical, a condition 
dictated by experience to be best practice. 

RIVETED JOINTS IN PLYWOOD. 

The matter of joints in plywood is of the greatest importance in connection with the 
construction of various types of built-up structures such as fuselages, boat hulls, pontoons, 



')([) imibi: 


-< 


m 


jd 


s 

- 4 


3" 

... 


\ 


Mrsy/iv 




. 3' \ 





& 


Off// 


? f 


O O O O O O O < 


) C 


) O 


) 
O O O O O Q O C 


) ( 


) O 


, 







s \ 

Fig. 30. (a) Test specimen for single-rivet tests, (b) Test specimen for multiple-rivet tests. 

and beams and girders. Several series of tests have been made to determine the efficiency 
of various types of joint for different kinds of loading. 

The first series of tests was made upon riveted joints designed for tension and compres- 
sion. The tests were all made in tension; both solid and hollow rivets were used. Two types 
of test were run; most of the tests were made on specimens only wide enough to accommo- 
date one rivet (fig. 30a), and later enough wide specimens were tested (fig. 30b) to verify the 
assumption that the data on the narrow specimens could be applied without correction to 
wider ones. 

In general, most of the tests were made on butt joints, with straps on each side. In some 
cases the straps were of plywood and in others of galvanized sheet metal about 0.02 inches 
thick. The nomenclature used will become clear upon examination of figure 30. 









Note 12. 



AIRCRAFT DESIGN DATA. 



71 



The first tests were made upon red gum plywood composed of three plies of -^ material, 
riveted with solid copper rivets through sheet-metal cover plates. The grain of the face plies 
was perpendicular to the seam. Figure 31 shows the strength of the joint with varying mar- 
gins and spacing. It is apparent that the best conditions are obtained with a 1-inch margin 
and a one-half inch spacing. 



6OO 



"';>>.' 




/ / . a + . / // 

"3f. ~2 * ' '& '* 






Fig. 31. Single-riveted butt joints in plywood. Relations among strength, margin, and spacing: Red gum ply- 
wood, plies 1/16 by 1/16 by 1/16 inch; solid copper rivets, 0.15 inch diameter; sheet-metal cover plates; grain 
of faces perpendicular to seam; moisture, 7.4 per cent. 



AIRCRAFT DESIGN DATA. 



Note 12. 



Figure 32 shows the variation of strength when using a constant spacing of one-half inch 
and margins varying from one-quarter inch to 2 inches. This figure shows very clearly that 
no appreciable additional strength can be obtained by increasing the margin above 1 inch. 



600 




O.2S O.SO 



/.OO 

/A/ /MCHES 



/.7S 



2.0O 



Fig. 32. Single-riveted butt joints in plywood. Relation between strength and margin: Spacing 1/2 inch; red 
gum plywood, plies 1/16 by 1/16 by 1/16 inch; solid copper rivets, 0.15 inch diameter; sheet-metal cover 
plates; grain on faces perpendicular to seam; moisture, 7.4 per cent. 

In fact, it was found that in case the grain of the face plies was parallel to the seam, the margin 
could be reduced to three-quarters inch without sacrificing an appreciable amount of strength. 
Similar tests made on three-ply birch, each ply one-sixteenth inch, gave similar results, 
as shown in figures 33 and 34. With a margin of 1^ inches, the maximum strength was 
secured with a spacing of one-half inch. 



Note 12. 



AIRCRAFT DESIGN DATA. 



73 



TOO 



6OO 



k 






300 



I 



/00 






# 





O.2S O. 

SP/JC//VG 




>v 



% 



O.7S 



/.oo 



Fig. 33. Single-riveted butt joints in plywood. Relation between strength and spacing: Margin, 1 1/2 inches; 
birch plywood, plies 1/16 by 1/16 by 1/16 inch; solid copper rivets, 0.15 inch diameter; sheet-metal cover 
plates; moisture, 6.6 per cent. 



74 



AIRCRAFT DESIGN DATA. 



Note 12. 




Fig. 3 



O.Z5 a SO O.7S 



/.OO /.2S /.SO /.7S 

X/V //VC//S 



4. Single-riveted butt joints in plywood. Relation between strength and margin; spacing, 1/2 inch; birch 
plywood, plies 1/16 by 1/16 by 1/16 inch; solid copper rivets, 0.15 inch diameter; sheet- metal cover plates; 
moisture, 6.6 per cent. 

f ,ni$iBM :snfcwjqa im* msirtroa nwied 
iBtem-Jooria ,-THmi;il> rfani 3I.O',8J9vh wqqo-) biloa ;ri)ni ;>t ! iaiid 



Note 12. 



AIRCRAFT DESIGN DATA. 



75 



The margin could have been reduced to 1 inch or even less without a great falling off in 
efficiency. Figure 35 indicates that a spacing of one-half inch is the best with thinner birch 
(each ply -fa inch). 



ST/?N6Tff /N POUAfaS Pft /MCH OF JO/A/r 

1 8 8 t 8 8 1 
























































/ 


*~~ > 


^ 
S 


b 


















/ 




1 


t 

s 


















1 

1 






1 


r 
) 

[4 

V d 
















' ? 


fOil ' 

x^^ 

t 


^< 

^ 


^ 


s 
















/ 

/ 




< 


^*s 
) 


^ \ 


^^wl 












I 


/ 

/ 










s 


^\ 


^ 








,' 


/ 












^. 


^ 






/ 


























p 

// 
























J' 
























I 
























t 

( 























/// /MCHS 

Fig. 35. Multiple-riveted butt joints in plywood; relation between strength and spacing; test Joint, 5 to 5 1/2 Inches 
wide; margin, 1 inch; birch plywood, plies 1/20 by 1/20 by 1/20 Inch; solid copper rivets, 0.15 inch diameter; 
sheet-metal cover plates; moisture, 5.6 per cent. 



ni noiiai/boi 



76 



AIRCRAFT DESIGN DATA. 



Note 12. 



Figures 36 and 37 show the strength of joints made in three-ply birch (each ply one-twen- 
tieth of an inch) with five-eighths-inch hollow aluminum rivets and plywood cover plates. 
A spacing of \\ inches gave the best efficiency with a margin of 2 inches. It is possible that 



700 













O.S-0 



/.OO 



2.00 



3.00 



/.so 
//v 

Fig. 36. Single -rive ted butt joints in plywood; relation between strength and margin; spacing, 1.25 inches; birch 
plywood, plies 1/20 by 1/20 by 1/20 inch; hollow aluminum rivets, 5/8 inch outside diameter; plywood cover 
plates; moisture, 5.6 per cent. 

greater strength could have been secured in the case of the specimens with the grain of the 
faces perpendicular to the seam had a greater margin than 2 inches been used. In the case 
of the specimens with the grain of the faces parallel to the seam a margin of \\ inches could 
have been used without any great reduction in strength. 



Note 12. 



AIRCRAFT DESIGN DATA. 



77 



7OO 




/.SO 



2.00 






O.SO S-OO 

boow{iti naJamflib ^blnJuo rfani \l 
Fig. 37. Single-riveted butt joints in plywood; relation between strength and spacing; margin, 2 inches; birch 
plywood, plies 1/20 by 1/20 by 1/20 inch; hollow aluminum rivets, 5/8 inch outside diameter; plywood cover 
plates; moisture, 5.6 per cent. 



78 



AIRCRAFT DESIGN DATA. 



Note 12. 



The results of tests upon three-ply birch (each ply one-twentieth inch) with plywood 
cover plates and one-half inch and three-eighths inch hollow aluminum rivets, respectively, 
are plotted in figures 38 and 39. These tests were made with margins of 2 inches. However, 
smaller margins could no doubt have been used without appreciable loss in strength. 



600 




/.SO 



^-j^ C^ V . 

Fig. 38. Single-riveted butt joints in plywood; relation between strength and spacing; margin, 2 inches; birch 
plywood, plies 1/20 by 1/20 by 1/20 inch; hollow aluminum rivets, 1/2 inch outside diameter; plywood cover 
plates; moisture, 5.6 per cent. 

_ 



Note 12. 



AIRCRAFT DESIGN DATA. 



79 



700 




SPA&W&7N Wtf5 



/.3& 



Fig. 39. Single-riveted butt joints in plywood; relation between strength and spacing; margin, 2 inches; birch 
plywood, plies 1/20 by 1/20 by 1/20 inch; hollow aluminum rivets, 3/8 inch outside diameter; plywood cover 
plates; moisture, 5.6 per cent. 



80 



AIRCRAFT DESIGN DATA. 



Note 12. 



When the most efficient spacing and margin are used, there is practically no difference in 
strength for the different sizes of rivets investigated. However, the smaller rivets require a 
smaller spacing and therefore more labor in manufacture. On the other hand, the margin 
required is less than in the case of the larger rivets, and this may in some cases be a decided 
advantage. 

Cover plates may be of metal or plywood, as preferred. If of metal, aluminum sheet 
about three-sixty-fourths inch or one-sixteenth inch thick is recommended for the thicknesses 
of plywood investigated. 

The efficiency of the joints was determined by testing a number of samples of the ply- 
wood, both parallel and perpendicular to the face plies, and it was determined that under the 
best conditions the efficiency of the joints with the face plies perpendicular to the seam was 
about 30 per cent, while with the face plies parallel to the seam the maximum efficiency was a 
little over 50 per cent. 

While riveted joints may be satisfactory under certain circumstances, they can not be used 
where an efficiency much over 50 per cent is required. In these cases it is necessary to use 
glued joints, of which there are several different types. 








I 

s 

! 

I 

S/sn/3/e Scarfs/a//?/- ticyona/ Scarf Jo/rtf //nf>Je uff~/o//7/- D/ayona/ fiutf-Jbtfit Satv-Joerfih SuffJoM 

Fig. 40. Joints in the face veneer of three-ply wood. 
JOINTS IN INDIVIDUAL PLIES. 

Joints in individual plies may be made in a variety of ways. Figure 40 shows several 
possible methods for joining pieces of veneer. A considerable number of strength tests upon 
several of these joints have been made. The simple scarf joint has been tested for a long range 
of slopes of scarf. The diagonal scarf joint, as well as the diogonal butt joint, have been tested 
for various slopes of the diagonal. The saw-tooth butt joint has been tested for various angles 
of the saw tooth. 

In balancing up the various factors of strength, ease of manufacture, and efficiency it was 
decided that the simple scarf joint is the most desirable of the group. The simple butt joint 
should not be used where strength is important. The edge joint is satisfactory if carefully 
made. The slope of the scarf in the simple scarf joint should be within the range of from 1 to 
20 to 1 to 30. 

In comparison with the use of rivets, joints in individual plies are probably more practical. 
They have an advantage, too, in that the joints in the plies of a given panel may be staggered, 
so that any defect that may occur in any particular joint only partially weakens the entire 
panel. The time and labor involved in the preparation of this type of joint, while probably 
less than the time and labor involved in the preparation of riveted joints, is greater than that 
in preparing the scarf joint extending through the entire thickness of the panel. 



) 



Note 12. 



AIRCRAFT DESIGN DATA. 



JOINTS EXTENDING THROUGH THE ENTIRE THICKNESS OF PLYWOOD. 

Many tests hare been made upon scarf joints extending through the entire thickness of a 
panel. Such joints were prepared by various manufacturers using different glues, different 
combinations of veneer thicknesses and species, and various slopes of scarf. Two types of 
scarf joints extending through the entire plywood thickness have been tested and are here 
described as the straight scarf joint and the Albatros scarf joint. The two types are shown in 
figure 41. The tests indicate quite conclusively that the straight scarf joint is the superior 
joint of the two. An examination of the Albatros joint will show that the face ply of the one 
panel does not meet the face ply of the second panel or only partially meets it. In place of 
being glued to wood that has the grain running in the same direction, the face ply of one panel 
is glued to the core of the second panel, in which the grain runs at right angles to the grain in 
the face. Joints in which the grain of the two pieces joined is at right angles are not as strong 
as joints in which the grain of the two pieces is parallel. 



/ Sr? 2O -fa / //? 3O 




/7// / r> x< 

/J/bafros ocarf 

Fig. 41. Joints in plywood extending through the entire thickness. 

Tension tests on the straight scarf joint show that an efficiency of over 90 per cent may 
be obtained with this type of joint for a slope of scarf as low as 1 in 10. On account of the 
variations hi the effectiveness of the gluing by different manufacturers, it is recommended that 
a slope of scarf greater than this be used. A slope in the neighborhood of 1 in 25, with a range 
of from 1 in 20 to 1 in 30, is recommended. 

Severe weakening of scarf joints is often due to sanding of the face plies at the joint. Obser- 
vations on joints of this kind that were sanded showed that at times more than hah* of the face 
ply is ground away. Inasmuch as the strength of a panel lies almost entirely in the face plies 
(in case of three-ply panels parallel to the direction of the grain of the faces), it is obvious that a 
reduction in the thickness of the face plies will materially affect the strength of a panel. Con- 
sequently it is recommended that if the scarf joint is sanded at all that it be only lightly sanded 
by hand, so as not to decrease the thickness of the face veneer. 

Figure 42 shows the method used for cutting the scarf and for gluing the two pieces of 
plywood together. The board above the panel should be relatively massive and flat so as to 
distribute the pressure from the screws. Two or three layers of blotting paper furnish sufficient 
padding to accommodate irregularities in the surface. 

98257 19 No. 12 6 - G . 

gniiaoJ ni flonil rlriw vMs-iova! beiaqnioo ^eifT .gaomiguoJ m wol fn* 



82 



AIRCRAFT DESIGN DATA. 



Note 12 



THIN PLYWOOD. 

In an effort to develop a substitute for linen for wing covering which could be used on 
present types of wing framework, several different kinds of thin plywood have been developed. 
Among these are plywoods composed of three plies of wood, each ply as thin as one one-hundred- 
and-tenth inch, plywoods with veneer faces and fabric cores, plywoods with veneer faces and 
metal wire core, plywoods with veneer core and cloth faces, and several other types. A method 
was developed which made it commercially possible to glue up very thin plywood without 
undue loss, although the losses in making thin plywood are naturally much greater than in mak- 
ing comparatively thick plywood on account of the fragile nature of the thin sheets and their 
tendency to warp and twist when glue is applied to them. It was not found possible to produce 
a plywood having all the requisite properties which was as light as doped linen. The genera} 
conclusions drawn from the investigation follow: 

1. Spanish cedar, mahogany, birch, sugar maple, red gum, yellow poplar, black walnut, 
and basswood may be cut into veneer sufficiently thin for consideration in plywood air-plane 
wing covering as substitutes for linen. 




METHOD OF CL/rr/MG SC/lffF METHOD OfP/?5S/MG GLUED JO/NT 

Fig. 42. Method of making plywood joints extending through entire thickness. 

2. These species may be glued satisfactorily by the method of introducing the glue between 
the plies by means of tissue paper previously coated with glue. 

3. It does not seem that plywood sheets of the same weight per square foot as doped linen 
can be prepared on a practical scale. 

4. Covering made either of veneer or of a combination of veneer with fabric, such as linen, 
cotton, wire screening, or kraft paper, in order to be both practical from the point of view of 
manufacture and satisfactory in mechanical properties as shown by test, weighs from two to 
three times as much as doped linen. 

5. Plywood that might be considered practical from the point of view of manufacture 
possesses from two to three times the tensile strength of doped linen. 

6. The thinnest ply-wood that can be manufactured at present with any degree of facility 
(3 plies of one one-hundred-and-tenth inch Spanish cedar) lacks toughness and tearing strength. 

7. In general the tearing strength of a practical thin plywood covering is considerably 
higher than that of doped linen, while its resistance to blows as indicated by the toughness test 
is lower. 

8. In order to obtain the requisite degree of toughness, it is necessary to introduce a cloth 
fabric into the construction. Grade A cotton now in use in airplane construction is satisfactory 
for this purpose. 

9. Combinations of veneer with kraft paper developed satisfactory tensile strength, but 
are low in toughness. They compared favorably with linen in tearing resistance. 



Note 12. AIRCRAFT DESIGN DATA. 



10. Combinations of veneer with light wire screening, thus far tested, are heavy and unsatis- 
factory from the point of view of tensile strength per unit weight. Their toughness and tearing 
resistance are not superior to cloth when used in combination with veneer. 

11. Thin plywood or a combination of veneer with cloth is more rigid than linen. 

12. Thin plywood unprotected by a finish changes moisture content rapidly and shrinks 
or expands with a change in atmospheric humidity to the extent of either showing an appre- 
ciable loosening or assuming a drum-head tightness when fastened along the edges. A finish 
of three coats of spar varnish very largely eliminates rapid change in moisture content. 

WOVEN PLYWOOD. 

f 

Tests have been conducted upon plywood made up with basket-weave faces and corru- 
gated core. The faces are woven out of splints of spruce veneer 1-^ inches wide and 0.017 
inch thick, while the core is made of spruce If inches wide and 0.018 inch thick. The total 
thickness over all is almost 0.2 inch. 

The following conclusion is drawn from the tests : The high rigidity at low loads, the high 
tearing strength, stability under varying humidities, and comparatively high toughness indicate 
that the woven plywood tested may be a very desirable material for construction in airplanes. 

Data concerning glues for ply-wood will be found in the text under the general heading 
"Glues." 

The following specification for waterproof plywood is based upon the strength tests just 
described and upon the glue tests presented farther on. 

SPECIFICATION FOR WATER-RESISTANT VENEER PANELS *OR PLYWOOD. 

r c 



GENERAL. 

' 



1. General specifications for inspection of material, issued by the Bureau of Construction 
and Repair, in effect at date of opening of bids, shall form part of these specifications. 

2. This specification covers the requirements for veneer panels for use in aircraft where 
a water-resistant ply-wood is specified. 

MATERIALS. 

3. The following species of wood may be used in plywood construction: 

^mlKioqab ^a$> ISKJ 
Basswood. Mahogany (true and African). Walnut. 

Beech. Maple (hard and soft.) Western hemlock. 

Birch. Redwood. White elm. 

Cherry. Spanish cedar. White pine. 

Fir (grand, noble, or silver). Spruce. Yellow poplar. 

4. Other species of wood shall not be used without the written approval of the Bureau of 
Construction and Repair. 

5. Veneer. The veneer must be sound, clear, smooth, well-manufactured stock, of uniform 
thickness and free from injurious defects. Sap streaks and sound pin knots will not be con- 
sidered defects. Discoloration will be allowed. 

6. The veneer may be rotary cut, sliced, or sawed. 

7. Thickness. Unless otherwise specified, no single ply of veneer shall be thicker than 
^ inch. In three-ply stock the thickness of the core must be between 40 and 75 per cent 
of the total thickness of the plywood, except for panels one-sixteenth inch or less in thickness. 

8. Glue and cement. Any glue or cement may be used which will meet the tests specified 
in paragraphs 20 and 21. 



84 AIRCRAFT DESIGN DATA. Note 12. 



MANUFACTURE. 



9. Grain. The grain in each ply shall run at right angles to the grain in the adjacent 
plies unless otherwise stated in the order. 

10. Manufacture. The plywood must have a core of soft or low-density wood and faces 
of hard or high-density wood unless otherwise specifically stated in the order. The core may 
be made of several plies, in which case the grain of the adjacent plies must be perpendicular. 
The plies must be securely glued together, after which the plywood must remain flat and free 
from blisters, wrinkles, lapping, checks, and other defects. Plywood manufactured with cold 
glue must remain in the press or retaining clamps not less than three hours. 

11. Joints. Plywood 10 inches wide or less shall have faces made of one-piece stock. In 
order to conserve the narrow widths of veneer, accurately made edge joints will be allowed in 
the faces and cores of wider stock, but the number of joints permitted in any ply shall not 
exceed the width of the panel, in inches, divided by eight. Edge joints are joints running 
parallel to the grain of the plies joined. All plywood built of jointed stock must be so con- 
structed that all joints are staggered at least 1 inch. 

12. In panels over 8 feet long scarf joints will be permitted; the smaller angle of the scarf 
shall have a slope of less than 1 in 25. Scarf joints in adjacent plies must be staggered. Scarf 
joints are joints in which the seam runs across the ply at right angles to the grain. 

13. Butt joints will not be permitted. 

14. In case the core or crossbanding is taped at joints only unsized perforated cloth tape 
or open-mesh unsized cloth tape applied with waterproof glue or cement shall be used. 

15. Moisture content. --The finished plywood shall be dried to a moisture content of 9 
to 1 1 per cent, with a tolerance of plus or minus 2 per cent, before it is shipped from the manu- 
facturer's plant. The equalization of moisture shall be effected by kiln drying, followed by 
conditioning. 

16. Kiln drying. The panels must be piled and placed in dry kilns as soon as possible 
after being released from the press. The method of piling must be approved by the Bureau 
of Construction and Repair. After the stacking is completed the panels shall be properly 
weighted to prevent warping during the drying process. The best results in the kiln are 
obtained with a temperature of from 95 to 115 F. and a humidity ranging from 50 to 60 
per cent, depending upon the thickness of plywood and number of plies. The circulation 
must be maintained at all times. 

17. Conditioning. All panels must be conditioned before fabrication or shipment. The 
conditioning shall be done indoors under temperature and humidity conditions existing in 
the factory for a period of not less than 24 hours for three-ply panels one-eighth inch thick 
and proportionately longer for thicker stock. The piling and weighting shall be the same as 
specified for dry-kiln stacks. 

18. Cutting. Cutting for length and width shall be full and true. The veneer shall be 
cut to the thickness desired in the finished plywood and any overallowance on this thickness 
for the sanding operation is very undesirable. 

19. Finish. In all cases the tape must be removed from the faces of the panel, and, 
unless otherwise specified in the order, the plywood shall be lightly sanded to a smooth finish 
free from defects. 

TESTS. 

20. Submission of samples for test. The manufacturer shall submit to the Bureau of 
Construction and Repair for test 20 samples, each 1 foot square, of the plywood which he 
proposes to furnish to airplane manufacturers. 



Note 12. 



AIRCRAFT DESIGN DATA. 



85 



21. Boiling or soaking test. The waterproof quality of the glue shall be tested either by 
boiling in water for a period of eight hours or by soaking in water at room temperature for 
a period of 10 days. After boiling or soaking the samples shall be dried at a temperature 
not exceeding 150 F. to a 10 per cent moisture content. The plies must not separate when 
the sample panels are subjected to this test. 

22. Shear test. The strength of the glue shall be tested in five test specimens cut from 
a sample panel. The form of the test specimen is shown in figure 43. The ends of the speci- 
men shall be gripped in the jaws of a tension- testing machine and the load applied at a speed 
of less than one-half inch per minute. The glued surface must not fail at a load of less than 
150 pounds per square inch. 

23. Approved list. Manufacturers whose plywood does not comply with these specifi- 
cations will not be considered in awarding of contracts. The list of manufacturers whose 
product has satisfactorily passed the tests outlined in paragraphs 20 and 21 may be procured 
from the Bureau of Construction and Repair, Navy Department, Washington, D. C. 



- 


JL 


*- >* ' 








i I 
1 
1 
1 

r 
i 
i i 

li - . 


I 



Plywood GJue S/jeor TesT 
Fig. 43. Plywood glue shear test specimen. 

INSPECTION. 



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ii4 ni feoul^ ebifi lo 

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25. Unless otherwise stated, all veneer and plywood shall be inspected at the plywood 
manufacturer's plant. 

26. The inspector shall make the tests specified in paragraphs 21 and 22 on at least one 
sample panel from each press for each eight-hours' run. 

27. In case the plywood fails to meet the soaking and shear tests it shall be rejected. If 
the glue fails to meet one of these tests but passes the other, the test in which it fails must 
be repeated on not less than twice the original number of specimens selected taken from two 
or more panels. If the glue fails to pass the second test, the plywood represented by the 
samples must be rejected. 

28. In case of consistent failure or lack of uniformity in product, the manufacturer wiU 
be required to submit a detailed written statement giving the following information : 

(a) The composition of the glue and the correct practice in mixing it. 
(&) The maximum time between mixing and applying the glue. 

(c) The exact procedure in applying the glue and in pressing and curing the plywoot 
and such other details as the Inspection Department may direct. 
The inspector shall see that thereafter this schedule is observed. 



AIRCRAFT DESIGN DATA. Note 12. 



29. The inspector shall have free access to all parts of the plants where the plywood is 
being manufactured and shall be afforded every reasonable facility for inspecting the materials 
used, the methods of manufacture, and the finished plywood. 

PACKING AND SHIPPING. 

30. Plywood which has passed inspection shall be packed in crates which will protect 
all edges and surfaces from injury during shipment. 

ORDERING. 

31. To facilitate the execution of contracts the order will state any special requirements 
which this material must meet. The order shall state the number of pieces, the width across 
the grain in inches, the length with the grain in inches, the thickness of the plywood and the 
individual plies, the number of plies, and the species of wood to be used for faces (to be 
marked " Faces"), for core (to be marked "Core"), and for cross-banding (to be marked 
"Crossband"). Sizes given shall be finished sizes and shall conform to commercial sizes when 
practicable. The order shall also bear the specification number. 

GLUES AND GLUING. 

There are a number of distinct kinds of glue commonly used in aircraft manufacture. The 
more important of these are as follows: 

1. Hide and bone glues. 

2. Liquid glues. 

3. Marine glues. 

4. Blood albumen glues. 

5. Casein glues. 

In addition to these there are many kinds of glue and cement used in the arts which are 
not well adapted to aircraft uses and which, consequently, need not be mentioned here. 

HIDE AND BONE GLUES. 

In general only the better grades of these glues are used in aircraft, and these are made 
from hides and are known simply as hide glues. Occasionally nonwater-resistant plywood 
panels made up with bone glue are used in unimportant parts of aircraft. The principal uses 
of hide glues in aircraft have been in laminated and spliced construction of various kinds, prin- 
cipally in propeller manufacture. Hide glue is still the standard propeller glue, though it has 
been replaced to an important extent in other laminated work. 

In order to secure a very good grade of glue for propeller and similar work, suitable methods 
of testing were developed and certain specifications prepared. The Bureau of Aircraft Pro- 
duction regularly inspects lots of glue at the request of manufacturers, and glue passing the 
required tests is sealed and certified. It is then made available for purchase by aircraft manu- 
facturers, who are thus assured of uniform glue of proper quality. The methods of test devel- 
oped and used are given in detail in the following statement. The shearing test forms the 
basis for the certification of casein glue also. 

TESTING OF HIDE GLUE. 

Chemical analysis has been found practically useless as a means of testing glues because 
of the lack of knowledge of their chemical composition. Physical tests must, therefore, be 
relied upon. A considerable number of physical tests have been devised, some of which are 
important for one class of work and some for another. For judging the suitability of glue for 
high-grade joint work the tests considered most important are strength, adhesiveness, vis- 
cosity, jelly strength, odor, keeping qualities, grease, foam, and reaction to litmus. In the 
subsequent discussion of these tests their application to joint glue will be especially kept in mind. 

| , X i . . 



Note 12. 



AIRCRAFT DESIGN DATA. 



87 



Strength tests are made by gluing together two or more pieces of wood and noting the 
pressure or pull required to break them apart. Many different methods of making the test 
specimens and breaking them have been devised. These depend to a certain extent upon the 
character of work expected of the glue and the nature of the testing apparatus available. The 
simplest and most convenient strength test is to glue two blocks together, as shown in figures 
44 and 45b, and shear them apart in a timber-testing machine (see fig. 45 a and c). It will 



MfTHOO Of PttP/trt/MQ 




Fig. 44. Method of preparing specimens for glue-strength tests. 

usually be found that there is considerable difference in the values obtained for the individual 
specimens. The amount of difference, however, can be kept at a minimum by using care to 
see that the specimens are selected, prepared, and tested under as nearly the same conditions 
as possible. In making strength tests the selection of the wood is a very important factor. 
The species selected should be the one upon which it is proposed to use the glue or one fully as 
strong. Care should be taken also that the wood is above the average strength of the species, 
in order that there may be less opportunity for the wood to fail before the glue. If the wood is 
too weak, the full strength of the glue is not determined. 



AIRCRAFT DESIGN DATA. 



Note 12. 



ot & 



hji . 







Note 12. AIRCRAFT DESIGN DATA. 



No block should fail below 2,200 pounds per square inch, and the average shearing strength 
for a propeller glue should be at least 2,400 pounds per square inch. 

The viscosity of a glue is determined by allowing a specified amount at a given temperature 
to flow through an orifice. The time required is a measure of the viscosity. The time required 
for water to flow through is taken as the standard. In general it is found that a glue with high 
viscosity is stronger than one with a low viscostiy and will absorb more water, although there 
are exceptions. Hide glues, as a rule, have higher viscosities than bone glues. 

A number of different shaped viscosimeters have been devised. In the glue manufacturer's 
laboratory, where many tests must be made each day, an instrument must be used which will 
give results quickly. This can be done with a pipette cut off at one end or with a straight 
glass tube contracted at one end. These instruments are not always arranged so the tempera- 
ture of the glue within them can be controlled, and for a number of other reasons they are not 
entirely accurate. Better control of temperature and greater accuracy can be had with the 
Engler viscosimeter. This is more complicated and more expensive than the glass tubes and 
also slower to operate, but it has the advantage, in addition to greater accuracy, of being an 
instrument which is in general use for testing many kinds of materials. The values obtained 
by its use are readily understood by laboratory men and can be readily checked. The instru- 
ment can be purchased standardized and ready for use. 

The term "jelly strength'' refers to the firmness or strength of the jelly formed by a glue 
solution of specified strength upon cooling. Strong glues usually have high jelly strength. 
There is no standard instrument for determining jelly strength and no standard unit for expessing 
it. In some laboratories the pressure required to break the surface of the jelly is measured. 
In others the depth to which a weight of special shape will sink is observed. Sometimes the 
jelly is cast in a conical shape, and the weight required to press the point of the cone a certain 
distance is taken. More common, however, is the finger test, in which the relative strength 
of two or more jellies is compared by pressing the jelly witn the fingers. In making this test 
with any apparatus it is important that the conditions be very carefully controlled in order 
that comparative results may be obtained. The temperature of the jelly when tested is par- 
ticularly important, as the relative strength of a number of jellies is not the same at different 
temperatures. In other words, the jelly strength of the different glues is not affected to the 
same extent by changes in temperature. The ideal condition is to cool and test the jellies in 
a room constantly maintained at the proper temperature. This is seldom practicable, how- 
ever, and the jellies must be cooled in a refrigerator and tested in a warmer room. When this 
is done it is important that the test be made as quickly as possible after removing the jelly from 
the refrigerator, so that the temperature will be practically the same as it was in the refrigerator. 
The strength of the glue solution must always be the same once a standard is adopted. For 
high-strength glues weaker solutions can be used than for low-strength glues. 

The odor of a glue is determined by smelling a hot solution and gives some indication of 
its source or its condition. Glue which has an offensive odor is not considered of the highest 
grade. The bad odor may be due to the fact that partly decomposed stock was used or that 
the glue itself is decaying. For high-grade work it is usually specified that the glue be sweet; 
that is. it must not have an offensive odor. The odor of different glues varies considerably, 
and it is difficult or impossible to express the different "shades." It is usually not difficult, 
however, to determine whether or not the odor is clean, or, as it is commonly called, sweet. 
The temperature and strength of solution are not usually specified. 

The keeping quality of a glue is determined by allowing the jelly left from the jelly-strength 
test to stand in the laboratory at room temperature for a number of days. The odor and con- 



90 AIRCRAFT DESIGN DATA. Note 12. 

dition of the glue are noted at intervals. Glues with good keeping qualities will stand several 
days without developing an offensive odor or showing any appearance of decomposition. 

For joint work a small amount of grease in glue is not a serious objection. Too much 
grease, however, is objectionable, as grease has no adhesive properties. The grease can be 
determined by chemical means, if desired, but this is not necessary unless the exact amount 
of grease must be determined. The common method of testing for grease is to mix a little 
dye with the glue solution and paint it upon a piece of unsized white paper. If grease is present, 
the painted streak will have a mottled or spotted appearance. If there is no grease present, 
the streak will have a uniform appearance. 

Glue which foams badly is objectionable because air bubbles are apt to get into the joint 
and thus reduce the area over which the glue is in contact with both faces. Foamy glue is 
especially undesirable for use in gluing machines, as in them the glue is agitated much more 
than when it is used by hand, and the danger of incorporating air bubbles is greater. The amount 
of foam is tested by beating the glue solution for a specified time with an egg beater or similar 
instrument and then noting the height to which the foam rises and the quickness with which 
it siibsides. Different laboratories do not make the test in exactly the same way, but in any 
laboratory after a method is once adopted it should be strictly adhered to thereafter. It is 
common to determine the foam on the solution used in the viscosity test. 

.oul" Py its reaction to litmus a glue shows whether it is acid, alkaline, or neutral. The test is 
made by dipping strips of red and blue litmus paper in the glue solution remaining after the 
viscosity test or some other test and noting the color change. An acid glue turns blue litmus 
red, an alkaline glue turns red litmus blue, and a neutral glue will not change the color of either 
red or blue litmus. A glue containing a slight amount of acid is slightly preferable to one which 
is neutral or alkaline, because it is not quite so favorable a medium for the growth of the organ- 
isms which cause the decay of glue. 

From the above description of the various glue tests it is apparent that, for the most part, 
they give comparative rather than absolute results. It is rather difficult to compare the results 
of tests made by one laboratory with those of another, as the strength of solution, temperature, 
and manipulation are often different. For this reason the most satisfactory method of pur- 
chasing glues is to specify that they must be equal to a standard sample which is furnished the 
bidder to test in any way he sees fit. The bidder should also be informed as to the methods the 
purchaser proposes to use in testing a glue submitted to him as equal to the standard sample. 
*i((j iiorfcW ./HOOT 101 >n 

PRECAUTIONS IN USING HIDE GLUE. 

moll yJi'H wU iMiivoir 

In using hide glue there are a number of precautions that must be observed to obtain satis- 
factory results. If improperly used, a very high-grade glue may give poor joints. It is impor- 
tant, first, to find out the right proportion of glue and water to get the best results. This is 
largely a matter of experience, but it can also be determined by strength tests. When the right 
proportions are decided upon, they should be strictly adhered to thereafter, and the glue and 
water should be weighed out when making up a new batch of glue rather than measured or 
guessed at. Clean cold water should be put on the glue, which should be allowed to stand in 
a cool place until it is thoroughly water soaked and softened. This may take only an hour or 
it may take all night, depending upon the size of the glue particles. When the glue is soft, it 
should be melted over a water bath and the temperature not allowed to go higher than about 
150 F. High temperatures and long-continued heating reduce the strength of the glue solu- 
tion and are to be avoided. 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. 



Note 12. AIRCRAFT DESIGN DATA. 91 

The room in which the glue is used should be as warm as possible without causing too 
much discomfort to the workmen, and it should be free from drafts. In a cold, drafty room 
the glue cools too quickly and is apt to set before the joint has been put into the' clamps. This 
results in weak joints. It is also considered good practice to warm the wood before applying 
the glue. Wood should never be glued when it is cold, and of course only thoroughly seasoned 
wood should be used. Since high-strength animal glues set so quickly on cooling, they should 
be applied and the joints clamped as quickly as consistent with good workmanship. 

In clamping glued joints the pressure should be evenly distributed over the joint, so that 
the faces will be in contact at all points. The amount of pressure which will give the best 
results is a question which has never been definitely settled. One experimenter found that a 
pressure of about 30 pounds per square inch gave better results on end joints than higher or 
lower pressures. Apparently no tests have yet been made to show the best pressure to use on 
edge or flat grain joints. In gluing veneers it is necessary to use high pressure in order to flatten 
out the irregularities of the laminations. Pressures as high as 150 or 200 pounds per square 
inch are sometimes used. 

Strict cleanliness of glue pots and apparatus and of the floors and tables of the glue room 
should be observed. Old glue soon becomes foul and affords a breeding place for the bacteria 
which decompose glue. The fresh glue is therefore in constant danger of becoming contami- 
nated. Glue pots should be washed after every day's run in hot weather and two or three times 
a week in cooler weather. Only enough glue for a day's run should be mixed at a time, so that 
mixed glue will not have to be held over from one day to another. If these sanitary precau- 
tions are not observed, poor joints are apt to be the result. 

LIQUID GLUES. 

Liquid glues, frequently known as fish glues, have been used to quite an extent for the 
smaller work such as gluing cap strips, tape, blocks, moldings, etc. They are being replaced 
gradually by casein glues, which have the advantage of water resistance. In general liquid 
glues are not as strong as certified hide glue, although the shearing strength of several which 
have been tested has been as high as 2,400 pounds per square inch. 

MARINE GLUES. 

These glues are used mainly to apply muslin between the inner and outer skins of floats 
and flying boat hulls. They are required to be of a sticky, viscous nature and relatively non- 
drying and elastic. They are usually composed of pine tar, rosin, manila resin, and alcohol. 
On account of their nondrying nature, these glues have comparatively low strength. They 
are readily soluble in gasoline, and it is necessary, therefore, to make provision to prevent 
gasoline from getting into the bilge water. In general, marine glues are not used to make joints 
in wood construction where high strength is required. 

BLOOD ALBUMEN GLUES. 

These glues, which are made from blood albumen secured from packing houses, are the 
strongest and most water resistant of all so-called ''waterproof glues" in common use to-day. 
In general, it is necessary to use heat (about 225 F.) to set them, and consequently their use- 
fulness is limited largely to plywood and similar thin material, although it is possible to glue 
thicker material in cases where the proper heat can be applied successfully. Practically all 
plywood glued with blood glues is glued between steam-heated plates, which furnish a con- 
venient source of heat. 

on I hTvrnijfife ?. :-}ib9Tni 

tttt \d bedailqOTCHXNi od Y- 8 " 1 Sffixira eMT . <.[ edi <xt 



92 AIRCRAFT DESIGN DATA. Note 12. 

Properly manufactured blood albumen plywood will pass all the tests prescribed in the 
plywood specification without difficulty. Not only does the shearing strength average far 
above that required, but the resistance to boiling and soaking is generally much greater than 
the specification requires. Further, the residual strength of the glue after boiling and soaking 
is, in general, decidedly superior to that of casein glues. 

A method has recently been developed for the gluing of very thin plywood, in which fine 
tissue paper is impregnated with blood albumen glue and then dried. This tissue is then used 
just as ordinary mending tissue. A sheet is placed between the layers of veneer to be glued 
and the whole put under pressure between steam-heated plates. Since the process is a,dry one, 
the troubles due to swelling and warping are eliminated. 

In general, it is anticipated that the use of blood albumen glues will be confined to manu- 
facturers of plywood for some time to come and that the only contact which the aircraft manu- 
facturer will have with it will be in the plywood which he purchases. 

CASEIN GLUES. 

The major ingredient of these glues is casein, a product secured from the souring of milk. 
Until a year ago casein glues were hardly known in this country, but they have been developed 
commercially by several concerns, and their use in aircraft has increased rapidly. They have 
the advantage that they may be used quite cold and that no heat is used either in mixing or 
in setting them. Further, they set up quickly but have the disadvantage of taking a compara- 
tively long time to develop their maximum strength. 

Casein glue is widely used in making water-resistant plywood and its use in laminated 
construction (except propellers) is steadily increasing. It is also being used in places where 
formerly fish glues were mostly used. 

The best grades of casein glue are fully as strong as certified hide glue in shear, and their 
resistance to high humidities and to soaking is much greater. Tests now under way indicate 
that the shock resistance of casein glues is as great as that of certified hide glue. The technical 
use of casein glue is very simple, but it is necessary to follow instructions carefully in order to 
secure best results. The instructions which follow represent the best practice and are based 
upon experience both in laboratory and in the shop. 

INSTRUCTIONS FOR USE. 

Equipment. In using waterproof casein glues the mixers used ordinarily for animal glue 
and vegetable glue are generally not very successful, as a more rapid and thorough stirring 
than these mixers give is usually necessary. It is possible that some types of ordinary glue 
mixers can be speeded up enough to give good results with casein glues, but they have additional 
disadvantages in being rather difficult to keep clean. The most successful mixer so far found 
for these glues is the power cake mixer, such as is used by bakers, or machines constructed on 
a similar plan. These machines have several speeds and mix the glue in a detachable kettle 
which is easily cleaned. They can also mix relatively small quantities, so that no batch of 
glue needs to stand very long before being used up. Copper, brass, or aluminum vessels should 
not be used for mixing casein glues, as the alkali in the glues attacks these metals. It is advisable 
also to equip the glue pot with a metal hood fitted with a feed hopper in order to prevent spat- 
tering outside of the glue pot during the course of mixing. 

Preparation of glue. It is advisable, in all cases, to thoroughly mix the contents of a 
freshly opened barrel of prepared glue, and preferably several barrels should be mixed at once 
before any of the dry powder is withdrawn for use, in order to counteract the segregation of 
ingredients of varying specific gravities which may have occurred during shipment from the 
factory to the point of consumption. This mixing may be accomplished by transferring the 



Note 12. AIRCRAFT DESIGN DATA. 93 

contents of the barrels to a box of suitable size in which the dry glue is turned over a sufficient 
number of times and thoroughly mixed with a clean shovel. 

It is necessary to caution against the practice observed in some plants of sifting the pow- 
dered glue and discarding from it the coarse matter which remains upon the screen. This 
may remove from the glue an essential ingredient and thus defeat the purpose for which the 
glue is intended. 

Proportions of dry glue and water. The proportion of water to mix with the dry glue 
should be as directed by the glue manufacturer. It is to be borne in mind, however, that 
fixed proportions, satisfactory for each and every barrel of glue received, can not be speci- 
fied because of a slight lack of uniformity which may exist in the product. Hence, only 
average proportions can be stipulated by the manufacturer, and the operator, in order to 
obtain satisfactory consistencies, may find it necessary at times to vary from the average 
proportions specified. It has been found in some cases that using exactly the same propor- 
tions of glue and water, the glue from one barrel may be thinner than that from another. It 
is hoped that this difficulty will be overcome before long by improved manufacturing methods, 
but until it is much will have to depend upon the judgment of the operator. It should also 
be remembered that some classes of work require thicker glue than others. 

Mixing the glue. The correct quantity of water is placed in the glue pot and the mixing 
blade is brought into action at proper speed. A high speed is necessary at first, especially 
if the glue is not added to the water very slowly, in order to avoid the formation of lumps in 
the glue. There is a considerable range of speed, however, which will give satisfactory results. 
In some cases a speed of 140 revolutions per minute of the shaft which carries the mixing blade 
(about 350 revolutions per minute of the blade itself) is used satisfactorily. By adding the 
glue carefully, however, a speed as low as 80 revolutions per minute of the vertical shaft (180 
revolutions per minute of the blade) can be successfully used. The powdered glue is now 
slowly introduced through the feed hopper and the agitation is allowed to continue for about 
five minutes and then stopped. 

The sides of the glue pot should now be scraped in order to direct any of the spattered 
material into the mixture, whereupon the blade is again brought into action at reduced speed 
(60 to 90 revolutions per minute) for a period of at least ten minutes. The object of reducing 
the speed after the first stage of mixing is to prevent the incorporation of an excess of air. At 
the end of this stirring period the glue is ready for use, provided all the fine casein particles 
are dissolved and no appreciable amount of air has been whipped in. If the glue still con- 
tains fine particles of undissolved casein and has the appearance of "cream of wheat" mush, 
however, the mixture should be continued. It was formerly considered necessary to allow 
the glue to stand without stirring for a short period before using it. The object of this was 
to allow all the casein to dissolve. It has now been found, however, that it is better practice 
to accomplish this solution by continued mixing than by standing. If, however, it is found 
that air bubbles have been whipped into the glue during mixing, it is desirable to let it stand 
awhile so the air can separate. 

In mixing casein glues which may require the addition of different ingredients singly 
the above practice should be varied from to conform with the directions of the manufacturer. 

Consistency of glue. It may be found that the proportions used do not always give exactly 
the same consistency. So long as the glue is neither too thick nor too thin to spread well, 
however, slight differences in consistency between individual batches or shipments of glue 
need not be considered serious. Good results may be expected if the glue spreads properly. 
Other things being equal, thick mixtures develop higher strength than thin mixtures, and 
when great strength is desired it is desirable to use the thickest mixtures practicable. 



AIRCRAFT DESIGN DATA. Note 12. 



If in mixing up a batch of glue from a new barrel or shipment of some kinds of glue it is 
found that the proper consistency is not obtained, it is possible to alter it if attended to imme- 
diately and before the glue has been removed from the mixing pot. This should not be 
attempted on important work unless the operator fully understands his glue, and it should be 
entirely avoided if possible. 

If the glue mixture obtained is seen, before it is taken from the mixing pot, to be too thick 
to spread properly, it can be thinned by adding an extra part or two of water, as may be required, 
and stirring at slow speed until the water is thoroughly incorporated. This holds for any 
casein glue. Under no circumstances, however, should water be added to glue which has 
thickened on standing or after being used awhile. 

If the glue mixture is found, before removing from the mixing pot, to be too thin, it may 
be thickened by carefully adding a proper amount of dry glue with continued stirring. This 
is practicable only for glue in which all the ingredients are mixed together dry, and is not suit- 
able for glues in which the various ingredients are added separately. The stirring should then 
be continued long enough to dissolve all the casein of the added glue. Another method which 
might be used is to mix a thicker batch of glue and then mix the two batches together. It 
is far preferable to avoid using either method, and with proper care it should seldom be found 
necessary. 

Application and use of glue. The glue in any batch should be used up completely before 
it begins to thicken materially. The length of time during which the mixed glue can be success- 
fully used may vary with different shipments. The operator must judge whether or not the 
glue is fit to use at any time by its consistency. Tests have shown that good results may be 
expected from a normal glue at any time during its working life up to the time when it becomes 
too thick to spread properly. 

In spreading the glue it is important that enough be applied to coat all the surface of both 
faces of the joint. An appreciable amount of glue should squeeze out of the joints when pres- 
sure is applied. As little time as possible should elapse between the spreading of the glue and 
the pressing. The exact time which can safely elapse will vary with the kind of wood being 
used, the consistency of the glue, the amount of glue applied, the temperature, and other factors. 
In making veneer panels it is considered best practice to get the stack under pressure within 
ten minutes or less from the time the first ply is spread. 

The minimum time the joints must be left under pressure is not known. It is considered 
safest and best practice, howerer, to leave the joints in the press or in retaining clamps for at 
least three hours. After the glued material is taken from the press it should be dried either 
artificially or naturally to remove the moisture added by the glue. It is best also to allow the 
material to stand a week or two to develop the full strength and water resistance of the glue. 
The panels should, of course, be piled properly during the drying period to prevent warping. 

The above discussion is applicable in general to casein glues, whether of the prepared type, 
such as Certus, Napco, Casco, or Perkins waterproof glue, or of the type which is mixed by 
the user directly from the raw materials. 

The following points should be kept in mind in preparing and using casein glues: 

(1) Thoroughly mix each barrel of glue before using. 

(2) Weigh the glue and water; do not measure it. 

(3) Avoid lumpy mixtures. 

(4) Avoid mixtures which are too thick or too thin. 

(5) Mix until all the fine particles dissolve and a smooth mixture is obtained. 

(6) Do not use glue after it becomes too thick to spread properly. 

(7) Do not attempt to thin or thicken glue after it leaves the mixer. 



Note 12. AIRCRAFT DESIGN DATA. 



DIRECTIONS FOR MIXING CERTUS GLUE. 

In general use about 10 parts of glue and 17 to 20 parts of water. Both water and glue 
should be weighed, not measured. With the water in the mixing can, start the mixing blade 
at high speed (80 to 140 revolutions per minute of the vertical shaft is about right) and add 
the dry glue rather slowly. Continue this rapid stirring for about 3 to 5 minutes after the last 
dry glue is added; then stop the mixer, scrape down the sides of the can, and start mixing at 
slow speed (40 to 60 revolutions per minute of the vertical shaft is about right). After 10 to 
15 minutes at slow speed the glue should be ready for use. If it has a granular appearance 
at the end of this time, however, the casein is not all dissolved, and mixing should be continued 
long enough to get casein particles into solution. The glue^ is then ready to use. 

DIRECTIONS FOR MIXING NAPCO GLUE. 

In general use about 10 parts of glue and 17 to 20 parts of water. Both water and glue 
should be weighed, not measured. With the water in the mixing can, start the mixing blade 
at high speed (80 to 140 revolutions per minute of the vertical shaft is about right) and add the 
dry glue rather slowly. Continue this rapid stirring for about 3 to 5 minutes after the last 
dry glue is added, then stop the mixer, scrape down the sides of the can, and start mixing at 
slow speed (40 to 60 revolutions per minute of the vertical shaft is about right). After about 
30 minutes at slow speed the glue should be ready for use. If it has a granular appearance at 
the end of this time, however, the casein is not all dissolved, and mixing should be continued 
long enough to get the casein particles into solution. The glue is then ready to use. 

DIRECTIONS FOR MIXING CASCO GLUE. 

Before starting any mixing weigh out all ingredients, using the following proportions: 

. ( Water, 22 parts. 

I Prepared Casco casein, 10 parts. 
R J Water, 1 part. 

1 Caustic soda, part. 
p( Water, 5 parts. 

IHydrated lime, 5 parts. 

With the water of A in the mixer and paddle operating at an intermediate speed (in the 
neighborhood of 60 to 90 revolutions per minute of the vertical shaft of a cake and dough mixer) 
slowly add the casein and continue stirring till the mass is free from lumps. This should require 
about 3 or 4 minutes. 

Now slowly add the one-half part of caustic soda which has been previously completely 
dissolved in the 1 part of water, and continue stirring for about 3 minutes. 

Next add the 5 parts of hydrated lime which has previously been worked into a smooth 
paste with the 5 parts of water, and continue stirring until a smooth mixture free from lumps 
and undissolved particles of casein is obtained. This should require about 15 minutes, possibly 
a little longer. The glue is now ready for use. If it is found that any appreciable quantity of 
air has been incorporated into the glue by the stirring, the glue should be allowed to stand 10 
to 20 minutes before using to allow the air to escape. 

Glue mixed according to the above procedure is ordinarily considered satisfactory for 
gluing veneer one- twelfth inch thick or thinner. If the glue appears too thin, however, it can 
be made thicker by using less water, as suggested below. For joint work or thicker veneer 
also a somewhat thicker consistency is desirable. This can be obtained by using 17 to 20 parts 
of water under A instead of 22$ parts. 



AIRCRAFT DESIGN DATA. Note 12. 



DIRECTIONS FOR MIXING PERKINS WATERPROOF CASEIN GLUE. 
(As recommended by the manufacturer September, 1918.) 

When the paddle itself is running about 400 revolutions per minute, the following method 
is highly satisfactory for making up "P. W. G." into finished glue: 

Dissolve 1 pound of 76 per cent caustic soda in 30 pounds of water contained in the large 
bowl. Add 14 pounds of "P. W. G." slowly to the caustic solution with thorough and brisk 
agitation. Continue agitation for about 5 minutes. Allow the glue to stand 20 to 30 minutes 
after mixing before using. 

When the speed of the paddle itself is less than 400 revolutions per minute the following 
method will give a smooth, fine flowing batch: 

Add 14 pounds of "P. W. G."*to 27 pounds of water. Agitate to smooth consistency. 
Continue agitation and add in small portions a solution made by dissolving 1 pound of caustic 
soda in 3 pounds of water. Continue agitation for about 5 minutes after ingredients are all in. 
Allow to stand 20 or 30 minutes after mixing before using. 

Neither casein nor blood albumen glues seem to be affected by gasoline in the slightest 
degree. A number of panels made up by representative manufacturers were soaked for a long 
period (several months) in gasoline without any sign of deterioration. Similar panels were also 
soaked for a like period in gas engine oil (Polarine) without any apparent deterioration. These 
tests indicate that both blood albumen and casein plywoods can be used around the engine 
without fear of damage by gasoline and oil. 

Frequently it becomes desirable to fill, shellac, or varnish parts which are later to be glued. 
Tests made to determine the strength of joints made on wood treated in this manner show 
that they are very weak and absolutely unreliable. No joints in aircraft work should be made 
except with the bare wood. ,rfo ^ 

AIRCRAFT PARTS. 

On account of the impossibility of computing, with any degree of accuracy, the strength 
of many aircraft parts and assemblies, it has been found necessary to supplement the designs 
and calculations with actual test to destruction. The tests have frequently shown unexpected 
weak points, which have been strengthened and the parts retested. Through development of 
this character some very remarkable results have been achieved, and the way has been opened 
for similar work along allied lines. 

LAMINATED CONSTRUCTION. 

One of the first problems to come up in this connection was a study of laminated wood 
construction. Opinion concerning the merits of this type of construction have been divided 
for a long time, and designers have allowed their fancy free reign in devising widely varying 
styles of built-up members. Until about a year ago designers were allowed to use either solid 
or built-up construction, in accordance with their individual needs or desires, but during the 
present year there has been a very decided trend toward official insistence upon laminated 
construction in preference to solid, especially in the case of wing spars. There are several 
reasons back of this trend, not the least important of which is the increasing difficulty of securing 
large sizes in the desired grades. In the case of propellers lamination has been practically uni- 
versal for many years. 

While lamination undoubtedly does promote the use of smaller and shorter material, with 
the consequent better utilization of lumber and does insure the elimination of large hidden 
defects, it requires the exercise of a great deal of care to insure satisfactory results. The prin- 
cipal difficulties encountered lie in the warping and twisting of the finished part. The relations 



Note 12. AIRCRAFT DESIGN DATA. 97 

existing between shrinkage and moisture, density, and, direction of grain have already been 
discussed in detail. Let it suffice to say that unequal shrinkage, with consequent twisting or 
warping, will result in a laminated structure if the various laminations differ materially from 
each other in any of the three factors mentioned, namely, moisture, density, and direction 
of grain. 

Propellers probably need as much care in their manufacture as any aircraft parts in order to 
insure permanence of pitch, balance, etc. The following rules for the selection of wood for 
laminated construction have been prepared especially for propeller manufacture, though they 
apply in general to all laminated construction: 

(1) All material should be quarter-sawed if possible. 

(2) Quarter and flat-sawed laminae should not.be used in the same propeller. 

(3) All laminae should be brought to the same moisture content before gluing up. 

(4) All laminae in the same propeller should have approximately the same specific 

gravity. 

(5) All laminae in the same propeller should be of the same species. 

Dry wood when exposed to very humid air absorbs moisture and swells. Wood dried in a 
normally dry atmosphere till its moisture content becomes practically constant loses moisture, 
and shrinks when exposed to extremely dry conditions. Two pieces of wood when exposed con- 
tinuously to the same environment will eventually come to practically the same moisture con- 
tent, irrespective of their relative moisture contents when first exposed to this environment. 

Individual pieces of wood, even those of the same species, vary greatly in their rate of 
drying. Quarter-sawed pieces have a different drying rate from plain-sawed pieces. Dense 
pieces dry more slowly than those which are less dense. 

Suppose that a flat-sawed board is glued between two quarter-sawed boards, all three 
having the same moisture content, say, 15 per cent, when glued up; or, suppose, that under 
similar conditions a very dense piece is glued between two pieces which are less dense; or, sup- 
pose that a board containing 15 per cent moisture is glued between two others, each containing 
10 per cent but all three being of the same density and cut in the same manner. Then suppose 
the finished product to be dried to, say, 8 per cent moisture. Every piece will shrink, but in 
each instance the center piece will tend to shrink more than the outside ones. The glued joint 
will be under a shearing stress, since the center piece has a tendency to move with respect to 
those on the outside. Under this condition the glued joint may give way entirely, it may 
partially hold, or it may hold perfectly. In either of the latter cases the center piece will be 
under stress in tension across the grain, and consequently will have a tendency to split. This 
tendency may become localized and result in visible splitting or it may remain distributed and 
cause a lessening of the cohesion between the wood fibers, but without visible effect. 

If a combination of these three cases occurs, it may be much more serious in its effect than 
any one alone. For instance, suppose that in a propeller alternate laminations are of flat- 
sawed, dense boards, glued at a relatively high moisture content, while the others are quarter- 
sawed, less dense, and at a much lower moisture when glued. The tendency of the flat-sawed 
laminations to shrink will be very much greater than that of the others, with the result that 
internal stresses of considerable magnitude will be set up. 

It is not difficult to see how these internal stresses may combine with the stresses from 
external causes and with the continual vibration to produce failure under external loads which 
are considerably smaller than the propeller would safely resist if manufactured with proper care. 

In the case of laminated struts and beams the laminations should be matched as to direc- 
tion of annual rings, as they appear on the end section, to balance shrinkage as much as possible. 

98257 19 No. 12 7 

.no: i Iliv/ ,!>'n!;j Ynsqo'Kf 



AIRCRAFT DESIGN DATA. 



Note 12. 



Quarter-sawed and flat-sawed material should never be used in the same member. Neither 
should either quartered or flat stock be used with stock cut at intermediate angles. In lami- 
nating together pieces cut with the annual rings at an angle of about 45 degrees with the 
faces the rings in the adjacent laminations should be approximately perpendicular to each 
other instead of approximately parallel to each other. 

WING BEAMS. 

In order to determine the general principles underlying the design of built-up wing beams, 
to develop the best forms from the standpoints of efficiency, utilization of low-grade stock, and 
ease of manufacture, and to study problems connected with manufacture, a series of 300 beams 
of various types and designs were built and tested. These types included only those which 
gave promise of strength efficiency combined with utilization of smaller material than that 
needed for the manufacture of solid beams, since the problem at the time was primarily one 
of shortage of material. The types selected besides the solid ones used for comparison are 
shown in figure 46. 










- .* 


,i. 


=H 








r 




^d 


s 

M 




^ 
* 


* 

4 





a** 
To 




i 




,f 








i 


\ 


r 











' * 








V- 


?* Jrt 


'/" 


< , 


g x 






J 






LJ 





Veneer cheeks, birch 



'/3 andlfo" /aminafions faces and ' 



Po/o/ar veneer 
'/ie "/am/nof/ori'S 





hms 



Fig. 46. Cross sections of built-up test beams. 



While it is impossible at the present time to present detailed analyses of these tests, the 
general conclusions drawn from them are given in the following statements: 

The tests were divided into various series for ease in reference, each series representing 
different conditions from the others. The conclusions from each series are first given, with 
the general conclusions at the end. 

RESULTS OF VARIOUS BEAM TESTS. 

Series 1 and 12 (fig. 46e and f): These consisted of one-piece spruce beams of acceptable 
material compared with three-piece beams of similar matched material. The results compared 
favorably, although with the built-up beam without filleted joints (Fig. 46f), the work to the 
maximum load was approximately two-thirds of that of the single-piece beam. On the other 
hand, the work to maximum work in the other series (fig. 46e) was 20 per cent higher than for 
the single-piece beams. Consideration of the results as a whole indicate that this type of beam, 
properly glued, will compare favorably with the single-piece construction. 



Note 12. AIRCRAFT DESIGN DATA. 99 

Series 2 and 13 (fig. 46i) : This series included single-piece spruce beams made from rejected 
material compared with laminated spruce beams made from similar matched material. The 
laminated material gave 5 to 10 per cent lower values in modulus of rupture and 5 to 10 per 
cent greater values in work to maximum load. The results show that not only will the glue 
hold satisfactorily but that higher values would not be secured by laminating defective material 
than by using it in solid form. 

Series 3 and 10 (fig. 46b) : Series 3 is made from one-eighth-inch poplar with the grain 
of all plies longitudinal compared to similar material and construction with the grain of the 
center ply vertical. 

Series 10 consists of beams of one-sixteenth-inch poplar laminations with vertical joints. 

Three types were made up as follows: (a) The grain of the center ply vertical; the grain 
of all other plies horizontal. (6) The grain of all plies having a slope of one in five from the 
horizontal, the slope in adjacent plies being in opposite directions, (c) The grain of the six 
center plies having a slope of one in five from the horizontal, the slope in adjacent plies being 
in opposite directions; the grain of all other plies (namely the flange plies) being horizontal. 

The tests showed (1) a 5 to 10 per cent reduction in the mechanical properties where the 
grain of the center ply was vertical, with no reduction made in the thickness of the web due 
to using this form of construction; (2) a reduction of approximately 20 per cent in total load 
and stiffness where a slope of one in five was used in alternate directions in adjacent lami- 
nations throughout the whole beam; (3) a reduction midway between the foregoing where 
a slope of one in five in alternate directions was used only in the web. 

The conclusions to be drawn from this series are that if cross-grained material must be 
used, better results would be secured by laminating and placing the grain of adjacent lamina- 
tions in opposite directions than to use solid beams of similar material, but that it would not 
be possible to secure a strength equivalent to beams of satisfactory grain throughout. 

Series 4 (fig. 46g) : A plywood web with Douglas fir flanges was used in this series, and 
included beams with the grain in the outside plies of the web longitudinal, vertical, and at 
45 degrees. Hide glue was used in making the beams, and failures of the glued joints developed 
in the tests presumably due to faulty control in the application of the glue. These tests are 
being repeated, using casein glue. 

Series 5 and 6 (figs. 46g and h) : These series included spruce flanges with plywood webs. 
The face plies were one- thirty-second inch with vertical gram, while the thickness of the core 
was varied from one-eighth to one-sixteenth inch. The results indicate the desirability of 
making a web of this construction somewhat thicker than required for shear stresses only. 

Series 7 (fig. 46c) : An acceptable grade of spruce with plywood sides was used in these 
tests. Four thicknesses of plywood were used, as follows: 
Outside plies -^ inch, core J inch. 
Outside plies ^ inch, core i inch. 
Outside plies -^ inch, core y& inch. 
Outside plies y-J^ inch, core ^ inch. 

This type of beam gave very satisfactory results, but the very thin plywood proved entirely 
inadequate. The results indicate that plywood with a one-sixteenth-inch core and one-thirty- 
second-inch faces would be suficient and that possibly a lighter construction would prove 
satisfactory. 

Series 18 (fig. 46d) : This series included beams made up of one-sixteenth-inch poplar 
veneer with the center ply of the web vertical. The results were satisfactory and showed 
that the glue held sufficiently to develop the strength of the section. This type of beam, how- 
ever, would probably be increased in weight about 10 per cent above that of a solid beam of 
similar material due to the large quantity of glue which would be required. 



100 AIRCRAFT DESIGN DATA. Note 12. 

Series 11 and 19 (fig. 46j): This series used white pine, with one-half of each beam of 
quarter-sawed and the other half of plain-sawed material and with moisture content 5 per 
cent higher in one-half of the beam than in the other. It is planned to subject different beams 
from this series to varying conditions of humidity in order to determine the effect of such con- 
ditions where the grain of the two faces of the beam are of a different character and in differ- 
ent directions. The greater part of this series has not yet been run, but the variations in 
results not due to the gluing indicate that greater defects can not be allowed in either piece 
than are now allowed in solid beams. 

GENERAL CONCLUSIONS. 

In general, practically all types of beams so far tested have given values commensurate 
with what might be expected of the section under test. In other words, the tests have shown 
that waterproof glue properly applied enables the full value of the section to be developed. 

Since the success of the laminated type of construction is primarily dependent upon the 
efficiency of the glue, it is of the utmost importance that means be provided to insure the 
satisfactory supervision of the technique of gluing. 

The types of beam illustrated in figure 46e and f and in figure 46c seems to offer the most 
immediate opportunity for effectively increasing production from the class of material now on 
hand and being received by the airplane manufacturers. Since in the types indicated in figure 
46e and f spiral grain material can be used in the webs, these types would have the particular 
advantage of permitting utilization of material now rejected. The tests thus far made indicate 
that these beams properly made are no more variable in their strength properties than solid 
beams. 

All of the beams of the foregoing series were made under laboratory conditions. In order 
to determine just what might be expected under factory conditions, several hundred of the 
types shown in figure 46c and e were ordered from various aircraft manufacturers and tested. 
The results of these tests, while not yet completely analyzed, show that, with proper super- 
vision, it is possible for the average aircraft manufacturer to produce satisfactory built-up 
beams. They also show, however, that the need for thorough, intelligent supervision is 
imperative. 

In addition to these series, numerous miscellaneous types of beams have been tested. 
Several of these types were similar to the types which have become more or less standard, 
while others may be considered freak designs. So far none of these freak designs have shown 
up satisfactorily. Several of the designs had some form of plywood in the flanges. In no 
instance have beams of this type proven as strong as beams with solid flanges or flanges in which 
all the grain was parallel to the longitudinal axis. Figure 47 shows various types of wing beam 
construction which have been used in machines or approved for use. 

BEAM SPLICES. 

Until the present year the matter of beam splices had not received a great deal of attention. 
There were in use, and embodied in specifications, many different kinds of splices, some of 
which were obviously very inefficient. The growing shortage of full-length material made the 
matter of increasing importance, and several series of tests were run both in this country and 
in Great Britain. 

The following report is based on tests on about 150 spliced beams and 150 unspliced beams, 
each spliced beam being matched to an unspliced one by being cut alongside of it out of the 
same plank. The beams were all Douglas fir, kiln dried, and of good quality, If by 2f inches 
in cross section, and the splices were made up by hand, using certified hide glue. The dowels 



Note 12. 



AIRCRAFT DESIGN DATA 



101 




Si approved 
appro wet 
Army approved 





Sritisri approved. 






Slrmy 






5.Y.A 
(A/frnafe rou f 





CUV TUNNEL GOTM 





22 



m 



<^>SS? 



w 



G&f /frets* 

(O/d). 




SOPW/T 

( Upper on/y) 




S.C.5. 
HALBE'/rsrADT 



1 



Wide. &h 
( IV//A 








3r/S/3rt approved 
Outer /ami'ncLf-/'bns 
faof f/afije. fhrcJrrtesi 




Fig. 47. Typical built-up wing spars. 




102 



AIRCRAFT DESIGN DATA. 



Note 12. 



were also of Douglas fir. In no cases were clamps or tape used to reinforce the splices ; neither 
were any of the splices bolted. The beams were all tested over a 60-inch span under third- 
point loading, thus producing uniform bending moment, without shear, in the central third 
of the span, in which the splices were all located. In order to eliminate as many variables as 
possible, the efficiency of each splice was calculated in per cent of the strength of the unspliced 
beam matched with it. The efficiencies thus obtained were then averaged for each type of 
splice. 

Table 14 presents in condensed form the data secured and shows the average, maximum, 
and minimum efficiencies of each of the nine types tested. A number of these types were 
selected for test, not because it was thought that they would develop high efficiencies but 
because they had already been used or included in some specification. 



TABLE 14. Strength of wing learn splices spars, If by 

in diameter. 



inches in cross section; dowels, % inch 




Wing beam splice No 


i 


2 


3 


4 




6 


7 


g 


g 






















Length of splice, inches 


16 25 


11 00 


8 125 


5 50 


8 50 


16 25 


11 


8 125 


16 25 


Slope of splice 


1 in 10 


1 in 4 


1 in 10 


1 in 4 




1 in 10 


1 in 4 


1 in 10 


1 in 10 


Glued area, square inches . . . . 


44 8 


18 43 


22 4 


9 22 


*23 40 


44 80 


18 43 


22 40 


85 60 


Minimum efficiency . ... 


49 5 


17 5 


77 7 


39 1 


74 1 


74 9 


19 6 


72 


50 


Maximum efficiency 


88.0 


53 2 


105 5 


86 5 


91 


107 


61 4 


123 5 


100 7 


Average efficiency 


73 


34 


90 


66 7 


81 


86 4 


38 7 


100 


75 9 























* Does not include two end areas, 2x (2.75X0.406), 2.23 square inches. 

The conclusions drawn from the tests are as follows: 

(1) A laminated beam spliced in one lamination is stronger than a solid beam spliced with 
the same type and slope. 

(2) Dowels add to the strength of splices from 10 to 20 per cent on the average for the 
spliced beams tested. 

(3) Plain scarf joints, with the plane of the scarf vertical, are the most satisfactory from 
all points of view. Dowels or bolts provide a great deal of residual strength in case of glue 
failure, while adding to the maximum strength as well. 

(4) In general, a slope of scarf of one in ten will provide a satisfactory joint in either solid 
or laminated beams. 

It is very interesting to note that the British have arrived at practically the same conclu- 
sions and that the standard British splice has a slope of one in nine, with dowels or bolts and 
dowels. 



Note 12. AIRCRAFT DESIGN DATA. 103 



STRUTS. 

The discussion and conclusions presented in the following paragraphs are based upon 
strength tests conducted on about 400 struts of various types, some of which were made of 
accepted material and others of material rejected by airplane inspectors for one reason or another. 
Among the principal objects of these tests are the following: 

(a ) To check the individual designs and the factors of safety developed. 

(&) To determine the variability of the material. 

(c) To study the effect of spiral grain and other defects upon the properties of the 

finished struts and to develop methods of inspection. 
Tests have been made upon the following kinds of struts: 
Standard J-l inners, accepted and rejected, spruce. 
Standard J-l outers, accepted and rejected, spruce. 
Standard J-l center, accepted and rejected, spruce. 
DH-4 inners, accepted and rejected, spruce and fir. 
DH-4 outers, accepted and rejected, spruce and fir. 
F5-L outers, accepted, spruce (laminated, 1\ by 6f inches). 

All of these except the F5-L struts were solid. The F5-L struts are laminateji, with 
three laminations, of which the center one is lightened by means of two oblong lightening holes. 

METHODS OF TEST. 

The following kinds of test were made: 

(1) Standard-screw testing machine, used for making column tests on struts with the 
regular end fittings supplied by the manufacturer. Slow, uniform speed of compression. A 
number of these struts were tested up to the maximum load repeatedly without any injury. 

(2) Standard-screw testing machine, used for making column tests on struts, with special 
knife-edge and fittings, which provided practically perfect "pin ends." Slow, uniform speed of 
compression. Many of the struts tested repeatedly to maximum load without injury. 

(3) Dead-load tests on struts carried nearly to the maximum load. 

(4) Special tests in hand machines designed for use in the inspection of struts. These 
machines show the maximum load direct or allow it to be calculated from the stiffness in bending. 

The results secured are presented according to groups of struts as tested, and the conclu- 
sions drawn are presented at the end of the discussion for each group. 

TESTS ON STANDARD J-l STRUTS. 

The first series tested consisted of 60 J-l struts, outers, inners, and centers, all spruce. 
These were accepted stock and were tested principally to check the designs and determine the 
quality of the spruce. The following general conclusions were drawn: 

(1) The quality of the spruce was satisfactory, except that 10 struts had a specific gravity 
less than 0.36. 

(2) The struts were all slender enough to enable the maximum load to be determined with- 
out injury to the strut. In fact it was found possible to load the struts repeatedly to maximum 
load without injury. 

(3) It was found that the ball-and-socket joints provided by the manufacturer offered some 
resistance to the free deflection of the struts. This resistance would probably not be present 
in actual flight, due to vibration. The knife-edge fittings were found to obviate this source 
of error and were adopted as the standard fitting for future tests. 



104 



AIRCRAFT DESIGN DATA. 



Note 12. 



The loads sustained by the various classes are as follows: 





Minimum. 


Maximum. 


Average. 


Front outers 


935 


1,510 


1,203 


Front inners 


1,620 


2,980 


2,325 


.Rear outers. 


830 


1,505 


1,148 


Rear inners 


1,610 


2,965 


2,067 











The average moisture content for the outers was 8.2 per cent and for the inners, 8.3 per 
cent. 

In order to form a basis for comparing the variations in the individual struts with normal 
variations in the spruce itself, an analysis of the stiffness of 500 specimens of spruce was made, 
and it was found that the average variation of the individual moduli of elasticity from the 
average of them all was 15 per cent. This average variation was secured as follows: The dif- 
ference between each individual modulus of elasticity and the average modulus was expressed 
in per cent of the latter, and these percentage differences were then averaged to secure the 
average variation. 

The average variation from the average strengths for the struts compares favorably with 
this figure of 15 per cent and is tabulated by strut classes: 

Per cent. 

Outers. 13 

Inners 16 

Centers , 12 

fill I nil'// 

The individual variation of the maximum and minimum from the average strengths is 
as follows, again by classes of struts : 

Outers: Per cent. 

Minimum 30 

Maximum 30 

Inners: 

Minimum..... '/IVi 1 ./! 1 :. 27 

Maximum f I a kp.W. A!; 40 

Centers: 

Minimum 34 

Maximum . . 14 

In general, the struts followed Euler's law as well as could be expected, except that the 
ideal load deflection curve, OABC, figure 48, was modified in the actual tests to a curve more 
nearly represented by ODBC. This was in all probability due to unavoidable eccentricity of 
fittings and loading. According to the Euler theory the elastic curve of a slender column is 
a sine curve. The actual curve, as determined by direct measurement, approaches very near 
to the theoretical curve of Euler. 

TESTS ON KEJECTED J-l STRUTS. 

This group of struts, spruce outers and inners, was rejected by Government inspectors, 
and tested primarily to determine the effect of defects upon the strength of the struts and to 
study means of inspection. Standard methods of test were followed. The general conclu- 
sions drawn are as follows : 

(1) As a group, these struts were not as good as the 40 accepted struts previously tested. 
A larger portion of the rejects broke suddenly and a larger proportion broke without prelimi- 
nary compression failure. 



Note 12. 



AIRCRAFT DESIGN DATA. 



105 



(2) Forty-one of the rejected struts appeared to the laboratory staff making the tests 
as satisfactory regarding both direction of grain and specific gravity. With one exception, 
the weakest of these 41 was as strong as the weakest of the 40 accepted struts previously 
tested. Further, the average of these 41 rejected struts was nearly as good as that of the 40 
accepted ones. 

(3) There were 18 struts whose diagonal or spiral grain was between 1 in 15 and 1 in 20. 
Of these, 16 compared favorably with the 41 discussed in the two preceding paragraphs. 

(4) A total of 57 (16 plus 41) of the 100 rejected struts compared favorably with the 40 
accepted struts previously tested. 

(5) It is possible to segregate the acceptable struts from lots of rejected struts by means 
of simple strength tests if the passing values are appropriately chosen from preceding labo- 
ratory tests on struts like those in question. 

(6) The limiting grain may safely be reduced to 1 in 15 without causing a reduction in 
the factor of safety, provided that strength tests and appropriate passing values are imposed. 
Such a plan of inspection by test would undoubtedly increase the quality and percentage of 
acceptance. 



Load 



D 



O 



Fig. 48 Load deflection curves for slender struts. 
TESTS ON STANDARD DE HAVILLAND STRUTS. 

The purpose of the tests was, in general, to check the design calculations and afford a 
direct comparison between spruce and Douglas fir when used as struts. Half of the struts 
were tested in the machine in the usual manner and the other half were tested in a special 
dead-load apparatus. A summary of the results follows: 

(1) With the exception of one strut, a spruce stick notably below specification both as to 
spiral grain and density, all the struts developed maximum loads greater than that for which 
they were designed. 

(2) The weakest of the fir struts was notably low in density, but still it was considerably 
stronger than the calculated load. 

(3) There was practically no difference in the average strengths of the spruce and the fir 
struts; but there was wider variation between the minimum and maximum values for spruce 
than for fir. Without exception, the spruce struts were lighter than the lightest fir strut. 
For unit weight (of strut) the spruce struts were 17| per cent stronger on the average than 
the fir. 



[tape sdi bfifl ,7io-)DJ8l ad$ ni alaai noiioaqeai Qfiiii/oi o) aid ism fttoH 



106 AIRCRAFT DESIGN DATA. Note 12. 

(4) In the dead-weight test all of the struts, with one exception, were stable; that is, if 
deflected by a side push (when under the weight of 3,200 pounds chosen for the test, which 
was just under the crippling load for the weakest strut), they would come back upon removal 
of the push. The exception was the weakest strut, which was unstable at a dead-weight of 
3,030 pounds. 

(5) Notwithstanding the general low specific gravity of the fir struts, the maximum loads 
which they sustained were high, and it would seem safe to reduce the limit from 0.47 to 0.45 
for struts of the same size as spruce and to use fir interchangeably with spruce. 

TESTS ON REJECTED DE HAVILLAND STRUTS. 

These tests were primarily made in connection with the development of strut- testing 
machines and inspection by actual test. There were 70 spruce and 70 Douglas fir struts, all 
rejected by Government inspectors for one reason or another. One hundred had been rejected 
for spiral grain and the other 40 for miscellaneous defects, which, under actual test, did not 
influence the failures at all. The results of these tests confirmed the conclusions drawn from 
previous tests, both as to the need and practicability of a strength specification and test, and 
the limits of slope of grain and specific gravity for Douglas fir already mentioned. In addition, 
careful study was made of the variation of spiral grain along the length of the strut and its 
effect upon the maximum load, and as a result of this study the conclusion has been reached 
that for struts of uniform cross section, like the D-H struts, the most severe requirements 
for straightness of grain should be limited to the middle third and to the tapered ends and that 
the requirements for the balance of the strut can be more lenient. 

The final recommendation concerning the slope of grain is that, assuming the determination 
of the maximum load for each strut and no reduction in the factor of safety, the steepest slope 
allowed in the center third and in the tapered ends be 1 in 15 and that the passing load for 
struts with a slope between 1 in 15 and 1 in 20 be set higher than for straighter-grained struts. 
Struts with a slope between 1 in 15 and 1 in 20 at the center third and at the tapered ends 
and showing the larger load specified for them are to be allowed a slope of 1 in 12 for the remainder 
of the strut; also, struts with straighter grain than 1 in 20, which also show the larger load 
specified for struts with steeper slope, may have a slope of 1 in 12 outside the middle third 
and the tapered ends; but struts having a grain straighter than 1 in 20 in the middle third 
and in the tapered ends and which meet the lower load requirements specified for them, but 
do not meet the higher load specified for the. struts with the steeper slope, may be allowed to 
have grain with a slope of 1 in 15 or straighter in the remainder. The requirement for greater 
load in the case of the steeper slopes is put in to insure against possible greater variability in 
shock resistance of this material. 

TESTS ON STANDARD F5-L STRUTS. 

The main purpose of these tests was to determine whether or not they fall in the class of 
slender struts and can be loaded to their maximum loads without injury. These struts are 
built up of three laminations each, the center laminations being lightened by two oblong light- 
ening holes. They are 2| by 6| by 102 inches. It was found that they could be tested up to 
their maximum load without injury, and it was also found possible to calculate the maximum 
loads by means of stiffness determinations based upon simple bending tests. The details of 
these methods will be described in the following paragraphs : 

TWO NONINJURIOUS TEST METHODS FOR INSPECTING STRUTS. 

Two noninjurious methods of test for determining the ultimate strength of interplane struts 
have been developed as a result of the series of tests which have been described in the preceding 
pages. Both methods are applicable to routine inspection tests in the factory, and the equip- 



Note 12. 



AIRCRAFT DESIGN DATA. 



107 



ment needed is simple and cheap. Both methods are applicable to slender struts (all the struts 
so far have fallen in this class). 

In order to determine the limiting slenderness ratio, - , governing the use of these two 

r 

methods for solid spruce and Douglas fir struts, tests were made upon three spruce and three 

fir struts, as follows: They were first tested full length, - about 165, and were then successively 

r 

shortened to ratios of 140, 120, 100, 90, and 80, and tested at each length by both methods. 

As a result of these tests the conclusion is reached that for spruce the limiting slenderness ratio 
is about 100 and for Douglas fir about 90. 

Three types of machine have been built and tried out satisfactorily. In the first two types 
the strut is actually loaded up to the maximum load. In the third type the modulus of elas- 
ticity is determined by means of a simple beam test well within the elastic limit and the maximum 
load calculated by a simple conversion formula. 




Fig. 49. Homemade strut-testing machine, first design. 

The first machine (fig. 49) employs the lever principle and is especially suitable for larger 
strut loads, say over 5,000 pounds. A (fig. 49) is a strut in place for testing; B is a base rigidly 
fastened to the top of table C; it affords support for one end of the strut and also for the pulling 
screw D. E is a lever, by means of which the pull (multiplied) is brought to bear on the strut 
as strut load. F is a knife-edge fulcrum; and G a spring dynamometer. H and I are supports 
for pulling rod and fulcrum rod, respectively. J-J are the stops at either side of the middle 
of the strut to limit excessive deflection of the strut through careless operation. The dial K is 
not a part of the machine for making the proposed acceptance tests. It was used for measuring 
strut deflections in another investigation. The dynamometer (John Chatillon & Sons, of New 



AIRCRAFT DESIGN DATA. 



Note 12. 



York) is of 1,500 pounds capacity. It is graduated in 25-pound intervals, and 5 pounds can be 
estimated easily. The pulling rig is an ordinary carpenter bench vise screw, handle, etc. ; the 

screw has eight threads to the inch. 



The second machine (fig. 50) is of the direct-pull type without multiplying lever, especially 
suitable for the smaller strut loads, say under 5,000 pounds. It consists of a long shallow box, 




Fig. 50. Homemade strut-testing machine, second design. 

into one end of which a rigid and strong frame is built; AB is the frame mentioned; C is a strut 
in place for test. The load is brought upon the strut by the headpiece D. The load is applied 
by means of the handwheel E on the screw F; it is applied through the spring dynamometer G 
and pulling rods H to the headpiece D. The rods extend freely through the piece B. They are 
supported at their ends by the headpiece D and the part I, both on castors which track on the 
floor of the box when the machine is used in horizontal position. J is wood block encircling the 




D 



Fig. 51. Beam machine for'strut testing. 

pulling nut. It prevents the nut from turning and affords attachment for the dynamometer. 
Adjustment for different strut lengths is afforded by the turnbuckles K and the distance rods L. 
The third machine (fig. 51) is a "beam machine" for the second method of determining 
strut strength. A is the strut in place for testing; BB are I beams forming the base of the 
entire appliance; they support the weighing scale C, the loading screw D, and one end of the 
strut. The middle deflections of the strut are measured by means of the usual device, a thread 
stretched between two points on the strut just over the supports and a suitable vertical scale 
just behind this thread and fixed to the strut or to the loading block E. 



Note 12. AIRCRAFT DESIGN DATA. 



Discussion of noninjurious test methods. Keference has been made to a simple formula 
used to calculate the maximum load of a strut from a smaller load and the corresponding deflec- 
tion in a bending test (the method illustrated in fig. 51). The following discussion will show 
how this formula is developed. 

Euler's column formula seems to be in most common use for calculating the maximum 
strength of interplane struts, and the method under discussion is based mainly on that formula. 
It is: 

_C/7r 2 EI Q\ 

^ L 2 

Where Q = Total crushing strength of column in pounds. 

C = A coefficient depending on the character of the end bearings (free or fixed). 

E = Modulus of elasticity in pounds per square inch. 

I = Moment of inertia. 

L = Length of column between bearings in inches. 

The deflection in a strut supported flatwise near its ends and then subjected to a cross- 
bending load, such that the strut (as a beam) is not overstrained, is given by the formula: 

,-^PF (2) 

El 

Where d = Deflection at center in inches. 

K = A coefficient depending on loading and manner of support of the strut as a beam. 
P = Any moderate (beam) load, not overstraining the beam, in pounds. 
Z = Span in the beam test in inches. 
E = Modulus of elasticity in pounds per square inch. 
I = Moment of inertia. 

For any given strut equations (1) and (2) may be equated by solving for El in both cases, 
thus: 



-"0?" d 

Solving for Q gives the formula: 



For struts on knife-edges supports C=l. Struts (on ball-and-socket supports, pin sup- 
ports, and the like) in flying airplanes are subjected to vibration which breaks down the friction 
at the supports and makes the supports equivalent to knife-edges. Hence it seems wise, as in 
practice, to calculate the ultimate strength of airplane struts as though knife-edge supported; 
that is, with = 1. In regard to the most suitable kind of loading of the strut as a beam, only 
center and third point were considered; others were regarded as impractical. By actual trial 
of 12 struts it was found, contrary to expectation, that center loading gave the better results; 
accordingly, that loading was finally decided upon. For such loading and simple nonrestraining 
supports, K = 1/48. Hence equation (3) becomes 

.206? P ( } 

^~ L 2 * d 

* 

which is the final form. It will be noted that P and d (or their ratio) are the only quantities 

p 
for which test must be made in order to furnish the value of Q for any particular strut. -^ is 

the center load per inch of deflection; it is therefore a measure of the stiffness of the strut. 



110 



AIRCRAFT DESIGN DATA. 



Note 12. 



For struts not uniform in cross section or composition the Euler (column) formula and the 
beam deflection formula still hold. Appropriate mean or average values of E and of I must, 
of course, be used in each, but whether or not these average values in the column formula are 
respectively equal to those in the deflection formula, thus permitting their cancellation or elimi- 
nation, can not be answered positively for all nonuniform struts. It is believed that the answer 
is affirmative. There is affirmative evidence from tests of 20 tapered solid struts (10 outer 
and 10 inner struts for the J-l airplane), also from tests on 5 built-up struts (5 pieces, plywood 
covered); that is to say, the second method of test, based on formula (4), was applied to these 
struts and very good results were obtained. 

Comparison of two test methods by actual trials. Thirty-five struts were tested by the beam 
method and for comparison by the column method also. The tests by the beam method were 
made with the struts on knife-edge supports. The results are recorded in the columns marked 
Qj (table 15). The results by the column method are recorded in columns marked Q 2 (table 15). 
The per cent differences between Q t and Q 2 appear in the following columns. They are decidedly 
small, and the test verification of the theory of this second method is highly satisfactory. The 
table includes solid struts of spruce and Douglas fir, both of uniform and tapered section, and 
struts of uniform section built up of spruce and birch. 

TABLE 15. Maximum or crippling loads for certain struts determined by measurement in column 

tests and by calculation from cross-bending tests. 
(a) Solid struts uniform in section. 



No. 


Species. 


Qi 


Q, 


Qi-Qj 


Average grain. 


Qi 


Z=52 inches. 


1=60 inches. 


1=52 inches. 


Z=60 inches. 


Spiral. 


Diagonal. 


DH-4 inners 

Gr-41 


Spruce 


Pounds. 
5,175 
6,350 
5,125 
4,375 
3,445 

2,075 
2,240 
2,560 
2,020 
2,460 

2,540 
1,800 
1,975 
1,950 
2,170 

1,450 
1,235 
1,060 
1,415 
1,390 


5,380 
6,420 
5,270 
4,310 
3,640 

2,040 
2,200 
2,520 
2,035 

2,485 

2,570 
1,750 
1,920 
1,920 
2,220 

1, 425 
1,195 
1,010 
1,355 
1,385 


5,390 
6,530 
5,530 
4,575 
3,645 

2,080 
2,180 
2,570 
2,060 
2,510 

2,510 
1,820 
1,945 
2,030 
2,200 

1,430 
1,200 
1,030 
1,385 
1,360 


Per cent. 
-4.0 
-1.1 
-2.8 
+1.5 
-5.6 

+1.7 
+1.8 
+1.6 
-0.7 
-1.0 

-1.2 
+2.8 
+2.8 
+1.5 
-2.3 

+1.7 
+3.2 

+4.7 
+4.2 
+0.4 


Per cent. 
-4.1 
-2.8 
-7.9 
-4.6 
-5.8 

-0.2 
+2.7 
-0.4 
-2.0 
-2.0 

+ 1.2 
-1.1 
+1.5 
-4.1 
-1.4 

+1.4 
+2.8 
+2.8 
+2.1 
+2.2 


65 
65 
80 
14 
25 

30 
15 
39 
18 
16 


95 
50 
80 
60 
100 

80 
95 
21 
80 
95 


G-42 


do 


G-56 . . .. 


.... do 


G-57 . 


do 


G-64 


do 


DH-4 outers: 
G-70 


Fir... 


G-74 


. . do 


G-76 


do 


G-79 


. ...do 


G-80 


do 


J-l inners: 
D-l.. 


Spruce 


D-13 . . 


do 






D-14 


do 






D-17 


do 






D-2 


Fir 






J-l outers: 
D-19 


Spruce 






D-20 


.... do 






D-21 


do 






D-7 


do 






D-8 


. . do 






Average 








2.4 


2.7 

















Q t =Max. load as measured in column-bending test. 
Q 2 =Max. load as calculated from cross-bending test. 
n _ 2PZ 3 



D=Deflection at load P in cross bending. 
Z=Span in cross bending. 
L=Effective length in column bending. 






Note 12. 



AIRCRAFT DESIGN DATA. 



Ill 



TABLE 15. Maximum or crippling loads for certain struts determined by measurement in column 

tests and by calculation from cross-bending tests Continued. 

(6) Solid struts tapered (Span=Z=64 inches). 





No. 


Species. 


Qi 


Q 2 


Qi-Q s 












Qi 


J-l 


inners : 
D 1 


Spruce 


Pounds. 
2 275 


Pounds. 
2 450 


Per cent. 
7 1 




D-13 


... do 


1 700 


1 720 


-1 2 




D-14 


... do 


1 790 


] 835 


2 5 




D-17 . 


. . do 


1 775 


1 750 


4-1 4 




D-19 


do 


1 400 


1 430 


2 1 




D-2 


Fir 


2 030 


2 120 


4 4 


J-l 


outers: 
D-20 


Spruce ... 


1,165 


1,210 


3 9 




D-21 


... do 


1 000 


1 040 


-4 




D-7 


Fir 


1 300 


1 330 


-2 3 




D-8 


do 


1 315 


1 350 


2 6 
















Average 








3 2 















(c) Built-up struts,* uniform in section (span=Z=60 inches). 



No. 


Species. 




Qi 


Q, 


Qi-Qs 












Qi 


J-14 




( 


Pounds. 
4 250 


Pound*. 
4 160 


Per cent. 
4-2 1 


.1-15 












J-16 






4,815 


4,710 


4-2 2 


j 17 


All spruce and birch 




3 760 


3 600 


4-4 2 


J-18 






3 500 


3 540 


1 1 


J-19 






3 425 


3 440 


4 














Average ' 










2 















* The core was a double box made of spruce; it was covered or stream lined with two-ply spruce; the inner ply was longitudinal, about one- 
eighth inch thick, the outer circumferential, about one-thirty-second inch thick. Other dimensions were as for DH-4 inners. 

It will be noted that many of the struts were tested on two spans. One span was practi- 
cally the maximum which the strut afforded. The two spans were tried out to ascertain whether 
choice of span is important. As expected, the choice was unimportant with struts of uniform 
cross sections, but with tapered struts the longest span gave best results. Several struts were 

tested twice on the same span. The second time turned over that is, the side which was the 

p 

upper in the first test was the lower in the second. The values of -v in the two tests were prac- 
tically alike in each case. 

A high degree of skill is not necessary in using the cross-bending test for inspecting struts, 
but for good results care should be taken about details. Both ends of the strut should be sup- 
ported in such a way that bending can occur without the ends slipping on the supports. The 
supports should be such that there is no doubt where the points of support are, because the 
exact value of span is required in formula (4). The bending load P is relatively small com- 
pared with the maximum (100 to 400 pounds for struts so far tested). Hence, a weighing appa- 
ratus correct to 1 or 2 pounds should be provided. The deflection should be read with reference 

to points on the strut immediately over the support and not on the machine. For best results 
p 

a single value of -7 should not be relied upon. Good practice is to read loads and deflections 
a 

. 



112 AIRCRAFT DESIGN DATA. Note 12. 



-|-v 

for a load deflection graph. The mean straight line gives the best value of -r for use in the 

formula. Of course, the loadings should not be carried to the elastic limit. In the tests of J-l 
and DH-4 struts deflections up to one-half inch were used. This was really more than necessary. 

p 
All that is needed is enough of the (straight) load deflection graph to be certain of its slope, -5 

MISCELLANEOUS STEUT TESTS. 

Tests of struts stream lined with plywood. Seven struts of two distinct designs were tested 
as square-ended columns and compared directly with solid spruce struts of the same gross area 
and solid spruce struts of the same weight, also tested as square-ended columns. The sections 
of the built-up struts are shown in figure 52a and b. The test length was 5 feet. As was to 
be expected, the design shown in figure 52a did not develop satisfactory strength, and after 
testing four struts the design shown in figure 52b was developed and three struts made up, 
using, respectively, birch, soft maple, and red gum plywood. These struts developed about 
double the strength of the other type, and appear to be rather well balanced (as square-ended 
columns), since one of them failed by shearing of the spruce web. 

The plywood struts were naturally larger than solid spruce struts of the same strength 
and shape, although lighter, and consequently would create greater wind resistance or drift. 

(7) W 





Fig. 52. Spruce and plywood struts. 

In order to reach an equitable basis of comparison it was necessary to consider both weight 
and drift. Assuming an air speed of 80 feet per second and that 1 pound of resistance is 
equivalent to 6 pounds of weight, the equivalent weight of the plywood struts was calculated 
to be 91 per cent of that of the solid struts (of the same strength and shape) at this speed. 

Naturally at higher speeds the advantage of the plywood struts is correspondingly less, 
disappearing entirely long before present maximum speeds are reached. The average weight 
of the plywood struts was 3.91 pounds and the average actual load sustained was 9,700 pounds 
(as square-ended columns). 

Tests on struts covered with bakelized canvas. Tests were made on 24 spruce struts, more 
or less cross grained, and covered with bakelized canvas (micarta). The external dimen- 
sions of all the struts were alike, but half were covered with two layers of canvas and the other 
half with four layers; the former having, therefore, more wood in them than the latter. All 
the struts were tested in column bending for maximum load without injuring them. All were 
subsequently tested to failure, 16 with the canvas partially or wholly removed. 

Since the modulus of elasticity of bakelized canvas is lower and its specific gravity much 
higher than that of spruce, one would expect this material to be a poor substitute for spruce 
in struts, so far as total strength and strength per unit weight of strut is concerned. All tests 
made verify this expectation, but the canvas covering improved the quality of the defective 
spruce struts in one respect, namely, the capacity of the strut to withstand severe shock. This 
conclusion is based on the fact that the deflection at failure for eight covered struts was con- 
siderably greater than for four struts stripped. 



Note 12. 



AIRCRAFT DESIGN DATA. 



113 



The struts covered with two layers of canvas were stronger than those with four because 
there was more wood in them and they were much stronger per unit of weight than the latter. 

Struts covered with canvas were but little stronger than the same struts stripped of canvas. 
The covered ones were weaker than the stripped ones per unit weight of strut. Further, it 
is computed that the canvas-covered struts were weaker than spruce struts of the same size 
would have been. 

Comparisons with 40 J-l struts previously tested show that the covered struts were not 
as high in total strength or strength per unit weight as the plain struts. 

Several struts had the outer layer of canvas removed for some distance from the ends, 
and these struts so stripped were to all intents and purposes as strong as they were originally. 

Effect of taper on the strength of struts. Tests were made on 40 solid struts to determine 
the effect of taper. These struts were of spruce and Douglas fir. Some were of the sizes and 
shapes corresponding to DH-4 inners and outers and the others of the sizes and shapes cor- 
responding to the central sections of Standard J-l inners and outers. (It will be remembered 
that the J-l struts have a central section about 0.46 the length of the strut, which is of uni- 
form section, the taper starting at the ends of this section and running in a smooth curve to 
the ends.) These 40 struts were all first tested for maximum load while of uniform section. 
They were all tapered to the geometrical form of the J-l taper and again tested for maximum 
load. Finally the DH-4 struts were given a very pronounced taper and tested a third time 
for maximum load. The results of the series of tests are presented in condensed form in 
table 16. 

TABLE 16. Effect of taper on the strength and weight of struts. 







Change due to first taper. 


Change due to second taper. 


Lot No. 


Number 
of struts. 






















A. 


B. 


c. 


A. 


B. 


C. 






Per cent. 


Per cent. 


Per cent. 


Per cent. 


Per cent. 


Per cent. 


1 


3 


-4.9 


+0.6 


+5.7 


-22.2 


-32.3 


-6.6 


2 


3 


- 4.4 


0.0 


+4.4 


-26.7 


-23.5 


-3.9 


3 


5 


- 3.7 


-1.5 


+2.1 


-17.8 


-20.3 


-3.6 


4 


5 


- 5.1 


-2.0 


+3.1 


-18.8 


-22.1 


-4.6 


5 


7 


8 1 


3 1 


+4 8 








6 


5 


9 


3 5 


+6 2 








7 


7 


9 7 


2 4 


4-7 9 








8 


5 


-11.2 


-3.1 


+9.0 















A represents change in weight due to taper. 

B represents change in maximum load due to taper. 

C represents change in maximum load per unit weight of strut, due to taper. 

Lots 1 to 4 were DH-4 struts, and lots 5 to 8 of J-l size. 

The maximum load per unit weight was increased by the first taper from a minimum of 
2.1 per cent for lot 3 to a maximum of 9 per cent for lot 8. The weighted average increase was 
5.5 per cent. 

It will be noted that the second taper reduced the strength weight ratio as well as the 
maximum load. 

DESIGN AND MANUFACTURE OF BUILT-UP STRUTS. 

The following general discussion is based upon the results of several hundred thousand tests 
on wood in various forms, as well as upon the experience gained in the design, manufacture, 
and test of struts of various types. While much of the discussion is quite obvious, it is 
believed to be pertinent. 

98257 19 No. 12 8 



114 AIRCRAFT DESIGN DATA. Note 12. 

Built-up struts possess a number of advantages and disadvantages as compared to the 
solid one-piece construction, some of which are as follows: 
Advantages : 

Use of small pieces of material. 

More effective distribution of material. 
(a) By routing. 
(6) By using materials of different density. 

Possibility of using defective material. 

Complete failure may not occur with failure of one lamina. 
Disadvantages: 

Greater warping or bowing if pieces are not rightly selected and well manufactured. 

Greater difficulty in manufacture. 

Greater time required for manufacture. 

One of the mam advantages of built-up struts is the possible use of smaller dimens'on 
material with its corresponding lower cost and greater availability. It is further a matter of 
common observation that many of the larger pieces which contain defects such as to make them 
unsatisfactory for use as a single unit would yield smaller pieces free from defects and suitable 
for built-up construction. The material near the center of a solid strut contributes but little 
in proportion to its weight to the maximum load the strut will carry. Struts lightened by routing 
at the center, therefore, have the advantage of a greater strength-weight ratio than a solid 
strut. Enough material at the major axis of symmetry is, of course, necessary to carry the 
shear, which is gfeatest along this axis and near the ends of the strut. A built-up strut lends 
itself readily to routing or lightening at the center. 

The taper of solid struts is likewise meant to accomplish a reduction in weight. Weight 
reduction with a minimum reduction in strength, however, can probably be most effectively 
obtained through routing in built-up construction. This, however, is more feasible with struts 
of larger dimension, and probably, all things considered, should not be undertaken on struts 
whose minor axis is less than If inches. It is common practice in built-up struts lightened in 
this manner to discontinue the routing at regular intervals, thus leaving a solid cross section 
at these given points. 

Use of materials of different density. It may be shown that a metal column with proper 
distribution of material will theoretically withstand a load two or three times greater than a 
solid wooden section of the same total weight, length, and section boundary. This is based 
on the assumption that no local buckling takes place. With thin metal walls this assumption 
would, of course, not be strictly true, as buckling actually does occur. The conclusion is valid, 
however, that the denser material, with its greater stiffness, may be desirable for struts and is 
most effective when distributed at the greatest possible distance from the neutral axis. This 
points to the possible advantages of a combined wood and metal strut and demonstrates in 
built-up wooden struts, especially the larger sizes, that the use of denser species for the outer 
portions, with a lighter species for a core, would furnish a possible efficient combination. The 
use of a combination of species of wood of different density, however, would not be desirable 
in solid built-up struts of small size, and if used in the larger sizes would require special con- 
struction to distribute stresses resulting from unequal changes in dimension and unequal stiff- 
ness, as will be considered later. 

Tests on combined metal and wood struts are now under investigation, and while very 
encouraging results have been obtained additional work along this line will be necessary before 
definite recommendations can be made for production consideration. 



Note 12. AIRCRAFT DESIGN DATA. 115 

Possibility of using defective material. But little data is available on the effect of defects 
such as spiral or diagonal grain in the individual pieces on the strength of built-up struts. In 
connection with the use of spiral grain material for struts, however, it may be noted that the 
modulus of elasticity is not as greatly reduced by this defect as are the other mechanical prop- 
erties, and therefore the maximum load in struts which is largely dependent on the stiffness 
may not be greatly reduced with slopes of grain as great as 1 in 15. In built-up struts con- 
taining but one glued surface parallel to the major axis the limitations of defective material 
should be maintained up to the standard required for one-piece construction. Large struts, 
however, may be composed of three (or more) sections, as shown in figure 54. The center section, 
containing the major axis of symmetry, receives little other than shear stress. It is probable 
that a greater tolerance of grain could be permitted here than in the outer laminations or in 
one-piece construction. Tests to secure information on this point are necessary and are under 
consideration. 

Possibility of warping or bowing. One difficulty frequently encountered on the manu- 
facture of built-up struts is the tendency to warp or bow. Practically all wood contains inter- 
nal stresses to a greater or lesser extent, and failure to take into consideration the factors which 
influence these stresses contributes largely to the trouble mentioned. As is well known, wood 
changes dimensions at right angles to the grain to a considerable extent with change in 
moistuie content. Unequal changes in the widths of various laminations causes severe stress 
in the glued joints and may even cause failure. Among the important factors which cause 
unequal changes in dimensions in the different laminations are : 

(a) The use of plain-sawed and quarter-sawed laminations in the same strut. 

(6) The use of laminations that differ in density. 

(c) The use of laminations that differ in moisture content. 

(a) In connection with the use of plain-sawed and quarter-sawed material it may be 
noted that the shrinkage of Sitka spruce in a radial direction is only about jix- tenths of that 
in a tangential direction. For a given change in moisture, it will theiefore be seen that a 
plain-sawed board would normally undergo a greater change in dimension than would quarter- 
sawed material. In built-up construction the best results would therefore be expected with 
quai ter-sawed material, as shown in sections 1-a and 1-b, figures 53 and 54. The use of both 
plain and quarter sawed material in the same built-up part should be avoided. 

(6) Another factor which may influence the warping of built-up struts is the density of 
material in adjacent laminations. It has been shown that in general the shrinkage of wood 
varies directly as the density, and light pieces would therefore, as a rule, retain their shape 
better than denser ones. The adjacent laminations should be made of pieces of approxi- 
mately the same density to give the best results, as otherwise considerable stress may be intro- 
duced along the glued joints, due to the tendency of the various laminations to change 
dimensions unequally. 

(c) Differences in the moisture content of the various laminations at the time of manu- 
facture may also contribute to the warping of built-up struts or other parts. Since wood 
shrinks with change of moisture content and since all material stored or used under similar 
conditions will ultimately assume approximately the same moisture content, it follows that 
differences in moisture content at the time of gluing will cause unequal changes in dimen- 
sions which introduce stresses in the glued surface. The fact that all material used in a given 
laminated member comes from the same stock does not necessarily insure against differences 
in moisture content between individual pieces. The wide range in the rate of drying of indi- 
vidual pieces, the difference in drying between quarter-sawed and plain-sawed lumber, as well 
q siii Tk iooBs 6ib oj as oMisiiav/* *i t rY9woji <,*{> .-iia goibnod oi 



116 



AIRCRAFT DESIGN DATA. 



Note 12. 



as the fact that heavy pieces usually dry more slowly than lighter ones, contribute to the dif- 
ferences of moisture content which may be found at any time in a given stock. The position 
of material in a pile while air seasoning or in the kiln while being dried may also influence the 
rate of drying and consequently the difference in moisture content between individual pieces 
at a given time. 

The manufacture of built-up struts with proper attention to the various factors which 
may affect the quality of the product as outlined in the preceding discussion would be more 
difficult than the manufacture of single-piece members. The time required for inspection 








2a 3a. -a. /6 

Fig. 53. Sections of built-up struts, two and four piece construction. 






/a. 



20. 




A 






46 



Fig. 54. Sections of built-up struts, three and six piece construction. 



would be increased on account of the greater number of pieces involved and because of the 
matching required. The gluing would also be an additional item to be considered in 
manufacture. 

The additional work involved in the proper manufacture of laminated struts would prob- 
ably have a tendency to reduce production, or at least would require greater facilities and 
more labor for a given output particularly for struts of smaller sizes. These considerations 
would tend to offset the lower cost resulting from the more complete utilization of the small 
pieces. 

Static and impact bending tests made on Sitka spruce and a few other species have shown 
that the position of the growth rings with respect to the faces of the test pieces does not influ- 
ence the bending strength. No data, however, is available as to the effect of the position of 



Note 12. AIRCRAFT DESIGN DATA. 117 

growth rings on the strength of struts, although it is expeceted that some data along this line 
will be secured in the near future. From data available at present the position of growth 
rings in a built-up strut would be expected to affect physical properties, such as the ability 
to retain shape rather than strength. It is desirable in built-up members that the construc- 
tion be such as to reduce the stresses to a minimum. This involves the use of material of 
approximately the same rate of growth, density, moisture content, and direction of growth 
rings in the cross section. 

CONCLUSIONS. 

1. The manufacture of built-up struts with a minor axis of If inches or less is not recom- 
mended. 

2. (a) To secure the best results, the laminations of built-up strut should be approximately 
of the same moisture content, density, rate of growth, and, in general, except in cases of special 
design, of the same species. 

(6) The construction of stream-line struts should be symmetrical about the major axis. 
It may be noted that symmetry and consequent balance of internal stresses can in some cases 
be secured without conformity to the exact requirements under (a} above. 

3. Figures 53 and 54 show recommended sections of built-up struts. 

(a) Sections 1-a and 1-b in both figures 53 and 54 would be expected to give the greatest 
freedom from internal stresses and consequent warping. 

(6) In figure 53 but little difference in ability to retain shape would be expected between 
sections 2-a, 3-a, and 4-a, and also between 2-b, 3-b, and 4-b. 

(cZ) There are a great number of possible combinations of material with different combina- 
tions of growth rings, and it is quite possible that other combinations giving modification of 
types shown should also prove satisfactory. 

4. In types such as 1-b, 2-b, 3-b, and 4-b in both figures 53 and 54 it is desirable but 
not essential that the edge joints come under the end fittings. 

5. The edge joints as shown in types 1-b, 2-b, 3-b, and 4-b in figures 53 and 54 should be 
staggered, preferably about 1 inch. 

6. The taping of built-up struts hides the glued surfaces from inspection and, as it does 
not add to the strength, seems unnecessary. 

7. The use of waterproof glue for built-up struts is recommended. 

8. There is reason to believe that the construction of solid and routed built-up struts can 
be improved over present practice and over that here shown so as to more effectively relieve 
the internal stresses which tend to produce warping. It should be remembered, therefore, that 
while the information here presented is based on the most complete data now available on 
built-up struts the subject is one which has been but little studied and great improvements 
may consequently be expected. 






118 



AIECRAFT DESIGN DATA. 



Note 12 



Figure 55 shows various types of strut construction which have been used in machines or 
proposed for use. 




atnemavcnqmi 1 




Fig. 55. Typical built-up strut sections. 






Note 12. 



AIRCEAFT DESIGN DATA. 



119 



WING RIBS. 

The construction and loading of wing ribs is of such a nature that it is practically impos- 
sible to calculate, with any reasonable degree of accuracy, the actual strength of any particular 
design. Further, it is quite impossible to determine without actual test the relative efficiency 
and strength of the various elements of the rib. As a result of these conditions it has been 
found necessary to develop a number of types through test. Some of the types which have 
been used or proposed for use are shown in figure 56. A number of these types have been 
tested, and several of them were developed as a result of the experiments. 




7. P/ywooct we> ; osa./ //t 




<3. Wwooct we>; ova/ 

Fig. 56. Typical wing-rib designs. 



120 



AIRCRAFT DESIGN DATA. 



Note 12. 





CD dD^ 



/O 







/S Se/n/- /russ (S. ~. 




/6. Se/n/- Truss (/lancf/ey 

Fig. 56. Typical wing-rib designs 



OOC 



. s 

; 



Note 12. 



AIRCRAFT DESIGN DATA. 



121 



//-uss a// wooct La.f/-/c.e. 




Ill 




23. 



rru.ss ' wood aS7c( /nera./ (Mowe fy^e-) 





Detat/ of mocftf teat/on of 
A/o. 2&/fr tv/i/c/i encf/ess basias 
arc u..stt tfi jo/&de of wft 

or/oops 





Fig. 56. Typical wing-rib designs. 

The two outstanding conclusions from the tests are: (1) The type of rib most suitable for 
small and medium chords, from the standpoint of the strength weight ratio combined with 
manufacturing ease, is the plywood web type, with oval and circular openings (fig. 56, case 8) 
and with vertical grain in the outer plies of the web. 

(2) The type of rib most suitable for large chords is the full truss type. This has the 
greatest strength- weight ratio of all types, and the manufacturing difficulties are not over- 
whelmingly large in the case of large ribs. 



122 



AIRCRAFT DESIGN DATA. 



Note 12. 



Minor conclusions will be found in the discussions of the development of the various 
individual types. 

The method of test is briefly as follows : 

The ribs are mounted in a testing machine specially equipped to apply the load to the 
ribs at a number of points and the testing head is run down at a slow uniform speed until failure 




i i ... ^^^^^^^^^i^^^^^^MMa^^^^K . 

Fig. 57. Apparatus for testing small wing ribs 



occurs. In the case of small ribs the load is applied at 8 points, as shown in figure 57. With 
the larger ribs 16-point loading is used (fig. 58). During the test the travel of the testing head 
is recorded at the various loads, and for some of the ribs the deformation at a number of points 
along the rib is measured. Figure 59 shows the relation between the total load in pounds and 







Fig. 58. Apparatus for testing large wing ribs. 






the travel of the testing head in inches. The strengthening and stiffening accomplished by 
judicious reinforcement are clearly shown. 

The load distribution used in the first series of tests is shown in figure 60. Later a triangular 
distribution was adopted, in which the apex of the triangle is one-fourth of the chord from the 



leading edge (fig. 68). 









Note 12. 



AIRCRAFT DESIGN DATA. 



123 



25O 




Fig. 59. Wing rib load deformation curves: DH-4 ribs. 










Fig. 60. Low-speed load distribution used in wing rib tests. 






AIRCRAFT DESIGN DATA. 



Note 12. 



TESTS ON DH-4 WING KIBS. 



The first ribs upon which development work was undertaken were some DH-4 ribs sub- 
mitted by one of the manufacturers. The original design is No. 1, figure 61. It was found 
that this rib, which has a plywood web, was lightened out too much near the spars, and the 




Fig. 61. Tests on DH-4 wing ribs. 



Rib. 
No. 


Designation. 


Description. 


Number 
of tests. 


Net weight 
of rib, 
ounces. 
W. 


Average 
total load 
sustained, 
pounds, 
P. 


Ratio of 
strength 
to weight, 
P 
W 


Faces. 


Core. 


1 

2 

3 

4 
5 


Dayton- Wright 


^-u^h birch 


T^-inch yellow pop- 
lar. 
do 


4 

2 
3 

3 
2 

3 
5 

- 3 
5 

4 

4 

3 
3 
3 

2 


7.71 

5.23 

5.58 

5.26 
5.59 

5.85 
5.06 

5.64 
6.37 
6.12 
5.46 

5.20 
5.61 
5.52 

5.40 


136 

232 
243 

274 
232 

243 

253 

266 
297 
300 

274 

288 
325 
337 

346 


17.7 

44.4 
43.5 

52.1 
41.5 

41.5 
50.0 

47.2 
46.6 
49.0 
50.2 

55.4 
57.9 
61.0 

64.0 


Improved original 


j-^g-inch maple 


^V-inch yellow pop- 
lar, 
j^-inch Spanish ce- 
dar, 
yfopinch maple 

yfr-inch basswood . . . 


do 

jVinch Spanish ce- 
dar, 
^-inch yellow pop- 
lar. 
do 


Complete truss 


^Vinch Spanish ce- 
dar. 
T^-inch birch 


TV-inch Spanish ce- 
dar. 
TVinch yellow pop- 
lar. 
iV-inch yellow pop- 
lar. 
jJj-inch yellow pop- 
lar, 
t^-inch yellow pop- 
lar. 
.....do 
do 


Semitruss 
Circular opening 


q^-inch basswood *. . 
....do 

^Vinch yellow pop- 
lar. 
.. v do 

^-inch basswood . . . 


TTj-inch birch 


jJj-inch yellow pop- 
lar. 
iVinch Spanish ce- 
dar. 


^y-inch Spanish ce- 
dar. 





* Core and face grain run parallel and perpendicular to diagonal members. 



Note 12. 



AIECEAFT DESIGN DATA. 



125 



first improvement consisted in changing the shape and size of the lightening holes and inci- 
dentally reducing the weight by making the face veneer much lighter. The improvement in 
strength is shown in the last column of the table. Further development work led through the 
semitruss and full truss (plywood) designs to the design which was finally decided upon as the 
best obtainable (No. 5). This rib is shown drawn to scale in figure 62. 

Several other types of DH-4 ribs were submitted for test, among them being several 
similar to case 13, figure 56. These were found to be very weak indeed, but stiffening and 
strengthening by means of wires, case 24, figure 56, produced a marked improvement. In fact, 
one rib developed as much as 42 pounds per ounce of weight. 

Conclusions drawn from these tests, which included 150 ribs, are as follows: 

(1) Plywood webs are superior to single-piece webs in strength, even if the latter are 
reinforced with vertical strips glued and nailed in position. 

(2) Plywood webs with the face grain vertical are superior to plywood webs having the 
face grain longitudinal. 

(3) Nails in the cap strips are practically useless in so far as contributing to the strength 
of the rib is concerned. 

(4) Cap strips should be fastened rigidly to the spars. 

(5) The circular-opening type of rib is superior to the other types tested. 

(6) For the size of rib tested a core of one-sixteenth yellow poplar or Spanish cedar veneer 
with longitudinal grain is satisfactory. If high-density wood, like birch, is used for face veneer, 
the thickness should be from one-sixtieth to one-seventieth inch, while if low-density face veneer, 
such as yellow poplar, is to be used, a thickness of one-fortieth to one-fiftieth inch is required. 

(7) Low-density face veneer is superior from the standpoint of manufacture of the ply- 
wood, and also gives somewhat greater stiffness for the same weight. 

(8) Spruce cap strips re by YJ mcn are satisfactory. They should be grooved and well glued. 

TESTS ON SE-5 WING RIBS. 

Table 17 presents the test data on a number of SE-5 ribs of the original design and 
of the design developed at the laboratory. The original ribs submitted for test were similar to 
case 15, figure 56, and consisted of 22 pieces. Under low-speed loading, figure 60, these ribs 
developed a strength of 25.3 pounds per ounce of weight, and under high-speed loading, figure 68, 
the average strength was 28.1 pounds per ounce of weight. 

TABLE 17. Tests on SE-5. wing ribs. 





Rib number. 


Type of rib. 


Web construction. 


Load distribu- 
tion. 


Net 
weight 
of rib. 
oz., W. 


Total 
load 
sus- 
tained. 
Ibs., P. 


P 
W 


Faces. 


Core. 


Average of 1, 2, 6, 7 ... 
Average of 11, 12, 13, 14 


Original 


Spruce braces and g 
do 


truts '. . 


Low speed.. . 
High speed . . 
Low speed 
High speed . . 
Low speed . . . 

High speed . . 
Low speed 
High speed . . 


6.67 
6.59 
6.17 
5.89 
4.61 

4. 60 
4.21 
4.23 


169 
185 
315 
291 
270 

249 
246 
275 
:;i aiL 


25.3 
28.1 
51.0 
49.4 
58.5 

54.2 
58.5 
65.0 


do 




Plywood No. 1 
do 


^5-inch birch 


^-inch basswood . . 
do 


A v^rao-A nf 8 Q 10 


do 


Average of 19, 20, 21, 22 

Average of 15, 16, 17, 18 
Average of 23, 24, 25, 26 
Average of 27, 28, 29, 30 


Plywood No. 2 
do '. . 


^Vinch Spanish ce- 
dar. 
.....do 


Yf-inch Spanish ce- 
dar. 
do 


do 
do 


do 
do 


.....do 
do 









-viq fit 
,78 bint , 



Ribs No. 1 to 22, inclusive, loose in spars, bound by wire wound around rib at spars. 

Ribs No. 23 to 30, inclusive, were glued to spars. ; ( HJ{W _ Q i y n l 

In all plywood web ribs the face grain was vertical. 

Cap strips for ribs 1, 2, 6, 7, 11, 12, 13, and 14 were A by * inch spruce. 

Cap strips for ribs 3, 4, 5, 8, 9, and 10 were & by & inch spruce. 

Cap strips for ribs 15 to 30, inclusive, were J by & inch spruce. ] . ] 



126 



AIECEAFT DESIGN DATA. 



Note 12. 



The design finally developed is shown in detail in figure 63. Several types were made 
up, using different species and thicknesses of veneer in the web plywood. Of these the ribs 
having webs composed of one-fortieth-inch Spanish cedar faces and one-sixteenth-inch Spanish 
cedar core proved to be the strongest per unit of weight. The strength under low-speed loading 
was 58.5 pounds per ounce of weight, and under high-speed loading a strength of 65 pounds 
per ounce of weight was developed. 

Besides being much stronger and lighter than the original ribs, the final design is decidedly 
stiff er. 

TESTS ON HS WING RIBS. 

The original HS ribs have a pine web, and are of the general type shown in case 4, figure 
56. The final design is of the plywood web, oval and circular opening type, and is shown in 
detail in figure 64. Detailed results of the tests are presented in table 18. Attention is 
directed to the cap strips, which are patterned after the design used by Fokker in his recent 
biplane. Better cap-strip fastening is secured by this method when the web is thin. The 
basswood faces on the plywood web of the final design appear to be somewhat light, and it 
is anticipated that better results would be secured by the use of slightly heavier veneer. 

TABLE 18. Tests on HS-1L wing ribs. 



Type of construction. 


Load distribution. 


Net weight 
of rib, 
ounces, 
W. 


Total load 
sustained, 
pounds, 
P. 


Ratio of 
strength 
to weight. 
P 

w' 


Remarks. 


Present construction single-ply pine 


Hih speed 


16.80 


410 




-inch, single-ply pine web; ^ by 


web. 
Do 


do 


17.28 


440 




f inch spruce cap strips; 1 by 
^ inch stiff eners on each side of 


Do 


do 


16.31 


350 




web between openings. 














Average 


do 


16.80 


400 


24 
















Present construction single-ply pine 


Low speed 


16. 15 


458 




Construction same as above. 


web. 
Do .... 


do . ... 


16.31 


530 








do 


16.80 


590 






t <^<) ^ 












Average 


do 


16.42 


526 


32 
















Circular opening plywood web 


High speed 


10 96 


385 




Ply -wood web; Tj-inch basswood 


Do 


. do 


10.81 


365 




faces; J-inch basswood core; -& 


Do 


do 


10 81 


310 




by J inch spruce cap strip on 












each side of web; grain of faces 
of web vertical. 


Average 


...do 


10 86 


353 


32 
















Circular opening plywood web 


Low speed 


10 68 


490 




Construction same as above. 


Do .' 


do 


10 75 


600 






Do 


do .. 


10 90 


500 


















i Ml . 
Average 


do 


10.78 


530 


49 

















TESTS ON F5-L WING RIBS. 

The original F5-L ribs were of the general type of the HS ribs, case 4, figure 56. In 
developing the new ribs, it was thought that the use of a full truss type rib might be justified 
and, therefore, a rib of this type was designed and tested. Further, a truss type with ply- 
wood web was included in the series. The three designs are shown in figures 65, 66, and 67, 
and the results of the tests upon the three types with high-speed loading and low-speed load- 
ing are shown in table 19. Data on the strength of the original design are also included. 



Note 12. 



AIRCEAFT DESIGN DATA. 



127 







'Q, 

0, 



V 



128 



AIRCRAFT DESIGN DATA. 



Note 12. 



j 

D, 



f oi 



.1 






. i 



Note 12. 



AIRCRAFT DESIGN DATA. 



129 



TABLE 19. Tests of F5-L wing ribs . 



Design of rib. 

. 


Load distribution. 


Net weight 
of rib, 
ounces. 
W. 


Total 
load 
sustained, 
pounds. 
P. 


Ratio of 
strength 
to Weight. 
P 
W 


TJl J ' 1 

Plywood, circular opening 


High speed 


15.5 


540 




...'..do . . . . 


do 


15 5 


485 




do 


do 


15.7 


400 














Average 1,2, and 3 




15.6 


475 


31 












Plywood, circular opening 


Low speed 


15.5 


592 




do , 


do 


15.7 


498 




do 


do 


15.5 


642 














, 
Average 4, 5, and 6 




15.6 


577 


37 












Plywood truss 


High speed . . 


22.4 


508 




do 


. . do 


23.4 


533 




... do 


.. do 


26.4 


670 














Average 7,8, and 9 




24.0 


570 


23. 7 












Plvwood, truss 


Low speed 


23.0 


610 




do 


do 


23.8 


578 




do 


do 


23.0 


683 














Average 10 11, and 12 




23.3 


624 


26 7 












Truss 


High speed 


12.5 


580 




do 


do 


12.5 


505 




do 


do 


12 3 


520 














Average 13 14 and 15 




12 4 


535 


43 












Truss 


Low speed 


12.5 


665 




do 


do 


12.9 


710 




do .-...- 


do 


12 6 


610 














Average 16 17, and 18 




12 7 


662 


52 












Original design - 


High speed 


22 1 


485 




do 


do 


22 1 


405 




do 


do 


21.0 


400 




do 


do 


21. 8 


435 














Average 19 20 21 and 22 




21 7 


431 


20 












Original design 


Low speed 


22 8 


593 




r\(\ 


do 


23 4 


585 






. do 


23 5 


550 




a 


do 


24.2 


590 














\verage 23 24 25, and 26 




23 5 


579 


25 













It will be seen from the data presented that the full truss type, figure 67, developed very 
much greater strength per unit weight than either of the other types and that the plywood 
truss type, figure 66, was by far the weakest of the three. Final choice between the full truss 
type and the plywood web type must be determined by the relative importance of weight 
saving and cost of production. 
98257 19 No. 12 9 



130 



AIRCKAFT DESIGN DATA. 



Note 12. 



TESTS ON 15-FOOT WING BIBS. 

The largest ribs so far tested have a 15-foot chord and were designed for a machine under 
contemplation but not yet built. Three general types of rib were first tested, a plywood web 
circular opening type, a semitruss type with reinforced plywood web, and a full truss type 
with vertical compression members and diagonal tension members (Pratt type). A glance at 
table 20 shows that the full truss was far superior to the other types in strength-weight ratio. 
The low-speed load distribution used is shown in figure 60 and the high-speed distribution in 
figure 68. The full truss design is shown in detail in figure 69. The stiffness of this design is 
illustrated in figure 70, which shows the relation between the travel of the testing head and 
the toal load in pounds. The uniformity in the properties of the three ribs is noteworthy. 
The need for thorough fastening of the cap strips and the verticals to the spars is emphasized. 

TABLE 20. Tests on 15-foot wing ribs. 



No. 
of 
rib. 


Type of rib. 


Species of web. 


Load distribution. 


Cap strips. 


Net 
weight 
of rib, 
pounds, 
W. 


Total 
load 
sus- 
tained, 
pounds, 
P. 


P 
\V 
w= 

Weight 
in 
ounces. 


Faces. 


Core. 


1 
2 

7 
8 

10 

9 
11 
12 
13 
14 


Circular opening . . . 
do 


^s-inch birch., 
^y-inch birch.. 


^5-inch Spanish 
cedar, 
do 


Low speed 
. . do 


J by J inch 
spruce. 
. do 


2.42 
2.28 


251 
318 


6.5 

8.7 


Average values 








2.35 


285 


7.6 


Semitruss 


yVinch birch.. 
^Vmch birch.. 


T^-inch Spanish 
cedar, 
do 


High speed . . . 
do 


} by I inch 
spruce. 
do. 


2.92 

2.68 


286 
175 


6.1 
4.1 


do 


Average values 








2.80 


231 


5.1 


Truss 


Spruce compre 
web. 
do 


ssion members and 


High speed . . . 
..do... 


& by f inch 
spruce. 
do 


2.49 

2.41 
2.42 
2.48 
2.49 
2.44 


565 

672 
710 
707 
721 
690 


14.2 

17.4 
18.3 
17.8 
18.1 

17.7 


do 


do 


do 


do 


do 


do 


do 


do 


do 


do 


..do 


do 


do 


do 


do 


do 


do 


Average values* 








2.45 


700 


17.9 











^^_ * (Rib No. 10 culled and omitted.) 

After this series of tests was completed it .was thought desirable to develop a truss type of 
rib which did not depend so largely upon glue for the security of the fastenings, and so a rib 
of the Warren type was designed and three built and tested. The design is shown in figure 71 
and table 21 presents the results of the tests and also the results of the previous tests on the 
Pratt type for comparison. While the objects aimed at were attained, it was at the sacrifice 
of considerable weight, as will be seen from an inspection of the table. Tests have just been 
completed upon a number of modified ribs of the Warren type. These ribs showed a greater 
strength-weight ratio than any other 15-foot ribs tested at the laboratory. 



Note 12. 



AIRCRAFT DESIGN DATA. 



131 




132 



AIRCRAFTgDESIGN DATA. 



Note 12. 



700 



600 



v 

\ 

^ 



00 



/OO 









O.2 



ae 



/.ff 



Fig. 70. Wing rib load-deformation curves: Pratt truss type ribs for 15-foot chord machine. 






TABLE 21. Tests on 15-foot wing ribs. 
Pratt truss and Warren truss type. 



No. 
of 
rib. 


Type ol rib. 


Construction. 


Cap strips. 


Net weight 
of rib 
pounds. 
W. 


Total load 
sustained, 
pounds. 
P. 


P 
W 

W= weight 
in ounces. 


q 


Pratt truss 


Spruce compression members 


^ by | inch spruce. . , 


2.41 


672 


17.4 


11 


do 


and birch veneer tension 
members, 
do 


..do.. 


2.42 


710 


18.3 


1? 


do 


do 


do 


2.48 


707 


17.8 


IS 


do 


do 


do 


2.49 


721 


18.1 


14 


do 


.. do t... 


do 


2.44 


690 


17.7 


















Average values 






2.45 


700 


17.9 
















15 


Warren truss 


Plywood members 


Spruce channel see sketch .... 


3.72 


770 


12.9 


16 


do 


do 


do 


3.54 


855 


15. 1 


17 


do 


do 


do 


3.63 


830 


14.3 


















Average values 






3.63 


850 


14. 1 

















Ribs tested with high speed load distribution. 



Note 12. 



AIRCRAFT DESIGN DATA. 



133 






















134 



AIRCRAFT DESIGN DATA. 



Note 12. 



In addition to these types of 15-foot rib experiments were made upon three other types, 
as follows: 

1. Full truss type, with vertical compression members and diagonal tension members 
running in both directions. These diagonal members consisted of two birch veneer bands 
wrapped continuously around the whole rib from end to end, one to the right and the other to 
the left. These bands passed around the caps at the panel points. These ribs developed a 
strength of 13 pounds per ounce of weight. 

2. Full truss type, similar to 1, except that the veneer bands, instead of passing around 
the caps, passed between the caps and ends of the verticals, being given a twist at this point. 
The strength developed was 9 pounds per ounce of weight. 

3. Full truss type, similar to 1, except that the veneer bands, instead of passing around 
the caps, were cut at these points and glued to the sides of the caps, which were of channel 
section. This type developed a strength of 13 pounds per ounce of weight and has the advantage 
of greater ease of assembly than types 1 and 2. 

It is to be noted that none of these types developed as great strength as either the Pratt 

or Warren types. 

TESTS ON ELEVATOR OR AILERON SPARS. 

Comparatively little is known about the behavior of wood under torsion. This has not 
been of particular importance in the past, but the proper design of control surface spars demands 
such knowledge. Mention has been made, under Mechanical and Physical Properties of Wood, 
of a few torsion tests made on solid specimens of spruce and ash. A few tests have also been 
made on hollow dummy control spars of Sitka spruce. The individual results of the tests are 
given in table 22, and a comparison between these results and those on the solid specimens 
previously mentioned is shown in table 23. Details of the test specimens will be found in 
figure 72. 

TABLE 22. Individual results of torsion tests on 15 hollow Sitka spruce elevator spars. 



Specimen No. 


Moisture, 
per cent of 
oven-dry 
weight 


Specific 
gravity 
(oven-dry 
weight 
and 
oven-dry 
volume). 


Shearing 
stress at 
elastic 
limit 
(pounds 
per square 
inch). 


Shearing 
stress at 
maximum 
load 
(pounds 
per square 
inch). 


Shearing 
modulus of 
elasticity 
(pounds per 
square inch). 


Work to 
elastic limit 
(inch pounds 
per cubic 
inch). 


Work to 
maximum 
load (inch 
pounds 
per cubic 
inch). 


1 . 


12.6 


0.44 


500 


1,000 


92, 100 


1.12 


7.1 


2 


15.0 


.48 


820 


1,370 


83, 300 


3.38 


15.5 


3 


13.5 


.38 


950 


1,000 


79, 500 


4.75 


6.4 


4 


12.8 


.48 


610 


780 


88, 900 


1.72 




5 


15.0 


.45 


930 


1,260 


76,100 


4.74 


11.4 


6 


14.2 


.51 


820 


1,170 


77, 700 


3.62 


10.5 


7 


14.6 


.34 


710 


940 


55, 300 


3.84 


9.1 


8 


13.2 


:43 


910 


1,270 


75,900 


4.54 


13.8 


9 


14.8 


.50 


820 


1,070 


77, 800 


3.62 


8.0 


10 


13.6 


.47 ' 


820 


1,030 


83, 400 


3.38 


6.9 


11 


12.0 


.52 


740 


1,040 


73, 800 


3.06 


7.3 


13 . 


13.4 


.48 




1,400 








14 


15.2 


.37 


910 


1,350 


80,600 


4.27 


15.8 


15 


13.4 


.43 


840 


1,080 


71,900 


4.13 


9.5 


16 


14 3 


49 




970 
























Average 


13.8 


.455 


800 


1,110 


78, 200 


3.55 


10.11 



















Note 12. 



AIRCRAFT DESIGN DATA. 



TABLE 23. Summary of results of torsion tests on hollow Sitlca spruce elevator spars and tests 

on solid circular specimens. 



.;?- . :<';<; y. 

: : i! ,',('> V- Ml !. 


Tests on 15 
hollow elevator 
spars, Sltka 
spruce (1). 


Tests on 15 
solid circular 
specimens 
Sitka spruce, 
(2). 


Ratio of (1) 
to (2) in per 
cent. 


! .! ;; '.ft., 


Moisture, per cent of oven-dry weight. 1 


13.8 
0.46 
1 800 
1,110 
1 78, 200 
1 3.6 
1 10.1 


15.7 
0. 39 
1,090 
1,650 
72, 300 
4.4 
19.7 


88 
118 
73 
67 
108 
82 
51 


Specific gravity, based on oven-dry weight and oven-dry volume 


Shearing stress at elastic limit (pounds per square inch) 


Shearing stress at maximum load (pounds per square inch) 


Shearing modulus of elasticity (pounds per square inch) 


Work to elastic limit (inch pounds per cubic inch) 


Work to maximum load (inch pounds per cubic inch) 





1 Based on 13 tests. 








OT'" 










*'J! 

L '?" . 




















r 








. 








A '"' , 




/<?" 




^" 


T4 








** 1 



Tesf spec/me/? 
are 3/?ot+ f s? g/u<sa '//? e/7c/s. 



s/oGC/mef? 



cross-secf/pr? of e/e 
S/OG/~ /-esfec/ //? fors/o/? 




/?ff /77gc/?//?e 




Defa/SofP/ag 



co/7/7ec 
Fig. 72. Torsion test specimen. 

It is to be noted that 80 per cent of the specimens failed at or near the spline joint, indi- 
cating that the joint was a source of weakness in the specimens. 

The relation between specific gravity and strength in shear is not definite enough to be 
used as a basis for selection of material to withstand shearing stresses. 

These tests, as well as torsion tests in general, are subject to large variations. These varia- 
tions are probably more pronounced in hollow spliced construction and will therefore neces- 
sitate using very large safety factors in order to obtain safe working stresses. 

In addition to the tests already mentioned, a few tests have been made upon hollow spars 
with a hollow wooden core, around which veneer is wrapped in right and left spirals. The 
indications are that both the ultimate strength in torsion and torsional stiffness can be doubled 
by this method of construction. 



136 



AIRCRAFT DESIGN DATA. 



Note 12. 



TESTS ON AIRCRAFT ENGINE BEARERS. 

A short series of tests was made to determine the relative merits of engine bearers built 
of all veneer and those built with a spruce filler. A preliminary series indicated the desira- 
bility of making a few modifications in the arrangement of the material which were embodied 
in the bearers here reported. The details of the veneer and spruce filler types are shown in 
figures 75 and 76, respectively, and the methods used in thrust loading and in vertical loading 
are illustrated in figures 73 and 74, respectively. The results of the tests are shown in table 24: 

TABLE 24. Tests on modified engine bearers (second series}. 



Engine 
bearers 
No. 


Type. 


Weight, 
pounds. 


Moisture con- 
tent at test. 


Deflection 
at maximum 
thrust load, 
in inches. 


Maximum 
thrust load, 
in pounds. 


Deforma- 
tion at 
maximum 
vertical 
load, in 
inches. 


Maximum ver- 
tical load, in 
pounds. 


1 

2 
3 


1 All- veneer (grain of faces horizontal) . . . 
_ T ' I 


f 6.88 
7.27 
I 7.30 


13.0 
13.4 
13.2 


2.81 
1.75 
2.25 


1,430 
1,360 
1,580 


0.63 
.61 
.49 


11, 560 
12,260 
11,800 




Average 


7. 15 


13.2 


2.27 


1,457 


.58 


11, 873 


















4 
5 


> All- veneer (grain of faces vertical) .... 


f 7.54 
I 7. 11 


12.8 
11.8 


2.12 

2.42 


1,940 
1.850 


.40 

.56 


11, 360 
11, 540 


6 




I 7.40 


13.6 


1.65 


2.000 


.45 


10, 500 




Average 


7.35 


12.7 


2.06 


1,930 


.47 


11,133 


















7 
8 
9 


[Plywood with spruce filler (grain of 
faces horizontal). 


f 7.10 
7.14 
I 7.26 


12.3 
12.5 
12.2 


2.04 

1.82 
1.84 


1,850 
1,720 
1,690 


.38 
.48 
.50 


12,500 
17,000 
16,000 




Average 


7.17 


12.3 


1.90 


1,753 


.45 


15, 167 


















10 
11 

12 


[Plvwood with spruce filler (grain of 
faces vertical). 

^x 


f 7.12 
7.20 
I 7.02 


11.6 
11.7 
12.1 


1.88 
1.62 
2.45 


1,960 
1,910 
2,150 


.60 
.49 
.67 


16,500 
16,500 
15, 430 


/ 


Averaee 


7. 11 


11.8 


1.98 


2,007 


.59 


16, 143 




' 
















31 


> *-~- f- 


/ 














ifqe 9< 

'. 



bflB fli- 



vi.Kf]% wolf of i no<f '>!)rii nosd ovarf ftte^) ' 
orfT .sl/rjiqa ttei bn trfgh m I>qqmw 
baidBoh ud jin gasftftite buioierml form noi^ 





Note 12. 



AIRCRAFT DESIGN DATA. 





Fig. 73. Strength tests of engine bearers : Method of testing for thrust loading. 






138 



AIRCRAFT DESIGN DATA. 



Note 12. 




Fig. 74. Strength tests of engine bearers: Method of testing for vertical loading. 



Note 12. 



AIRCRAFT DESIGN DATA. 



139 




. y,.J, Graft? of face. 
r7or/z.onfa/ 




Wos. 4-, 3~; 6; Gra/rr of face 
p//es [/es-f/co./ 



Fig. 75. Engine bearers, all-veneer type. 
f/V/er 







l/err/ca/ 



Fig. 76. Engine bearers, spruce-filler type. 



; - Spruce 
Fitter 



l/eneer 
Each P/y 

//7C/" 



It is to be noted that the spruce-filler type was somewhat superior to the other in thrust 
loading and much superior in vertical loading ; also that the bearers of the former type with 
the face grain of the plywood vertical were superior to those with the face grain horizontal. 
It may be mentioned that these particular bearers were designed to take vertical loading only. 



140 AIRCRAFT DESIGN DATA. Note 12. 

TESTS ON BAKELIZED CANVAS (MICARTA). 

In connection with tests on substitutes for wood in aircraft construction several series of 
tests were made upon micarta members. Reference has already been made to tests upon 
spruce struts covered with micarta. Tests were also made on hollow longerons of micarta and 
on a few samples of micarta wing spars. 

Several tubes of micarta were submitted for test as substitutes for wood in longerons. 
These tubes were hollow, 36 inches long, 1 inch square outside, and made up of 6 plies of canvas , 
the walls being about one-eighth inch thick. For comparison a number of spruce sticks 1 inch 
square and 36 inches long were cut from a plank selected at random and comparative tests 
made upon the tubes and sticks. The tests show the following properties, compared to spruce 
(moisture about 10 per cent): 

1 . Modulus of elasticity about two-thirds that of spruce. 

2. Fiber stress at elastic limit in bending about four-fifths that of spruce. 

3. Tensile strength half that of spruce. 

4. Specific gravity three times that of spruce. 

5. Compression parallel to the grain, elastic limit, one-half that of spruce. 

6. Compression parallel to the grain, maximum load, three times that of spruce. 
Impact tests upon several longerons and impact tests made upon several samples of I-beam 

and hollow section wing spars- of micarta indicate that this material is superior to average 
spruce (about 10 per cent moisture) in the following properties: (a) Fiber stress at the elastic 
limit in impact; (6) elastic resilience in impact (much superior). 

The cause of the much greater elastic resilience lies not only in the higher fiber stress at 
the elastic limit but also in the lower modulus of elasticity in impact. 

In general, micarta can not be considered as a substitute for spruce in aircraft construction. 

TREATMENTS FOR PREVENTING CHANGES IN MOISTURE. 

Several long series of tests have been conducted in the hope of finding some means of 
preventing changes of moisture in finished parts with changing weather conditions. 

The first series had to do principally with varnishes of the so-called waterproof type. 
Yellow birch blocks were given a coat of silex filler and then three coats of the varnish under 
test. In some cases the varnish was applied with a brush and in others the blocks were dipped . 
Some of the specimens were dried in the air between coats and others were baked. After the 
final coat had set the blocks were hung in a humidity chamber in which the relative humidity 
was 95 per cent and the temperature between 75 and 80 degrees F. The blocks were weighed 
at intervals to determine the absorption of moisture. It was found that the absorption varied 
widely among the different varnishes and that baking improved some varnishes while increasing 
the rate of moisture transmission through others. The absorption in 17 days varied from a 
minimum of 4.36 grams per square foot of surface to a maximum of 26.8 grams per square foot 
of surface. The specimen showing the least absorption happened to be one which had been 
dipped and air dried, while the one showing the greatest absorption happened to be brushed 
and baked. The tests showed not only the great variability in moisture resistance among 
good varnishes but showed also that the moisture resistance was in all cases increased by increas- 
ing the number of coats of varnish applied. Table 25 shows the absorption of water by speci- 
mens given various miscellaneous treatments. The absorptions at 17 days are comparable with 
the figures just quoted. None of the treatments furnished the desired water resistance. 



p 
.v;Lno ^nibjsol Uoitaav odn) <>J Iwrrgie'*!) -WN - 



Note 12. 



AIRCEAFT DESIGN DATA. 



141 



TABLE 25. Humidity tests of miscellaneous treatments. 

Wood, yellow birch: Average thickness, 0.6 inch; average width, 4 inches; average length, 8 inches; average surface 
area, 0.54 square foot; average weight, 0.49 pound air dry; average volume, 0.011 cubic foot. 



Treatment. 


Number 
of speci- 
mens 
aver- 
aged. 


Average absorption in grams per 
square foot of surface in 


3 days. 


10 days. 


17 days. 


1. Muslin glued with Le Page's Cold Glue, 4 coats of airplane dope, and 2 
coats of airplane gray enamel 


2 
2 

2 

2 
2 
5 
2 
2 

2 
2 
1 

2 
2 

2 
1 
10 


1.85 
1.68 

1.80 
1.43 
1.55 
2.60 
1.97 
2.22 

4.20 
3.56 

4.86 

6.04 
6.55 

14.0 
21.6 
20.5 


4.36 
4.49 

4.79 
5.36 
5.11 
6.21 
6.75 
5.79 

10.31 
11.90 
13.40 

16.72 
23.9 

36.7 


7.39 

7.98 

8.23 
8.30 
8.35 
10. 23 
10.36 
9.93 

15.44 
17.88 
20.7 
25.4 


2. Three brush coats of orange shellac 


3. Paste filler (silex), 1 coat airplane gray undercoat, 3 coats airplane gray 
enamel (Adams & Elting Co ) 


4. Paste filler (silex), 2 brush coats orange shellac, 2 brush coats Lowe Bros. 
Finishing Varnish V 801 


5. Paste filler (silex), 2 coats of Hampden's W. P. Varnish No. 1 and 1 coat 
of Lowe Bros. Marine Spar 


6. Paste filler, 2 coats of white lead, linseed oil, and lampblack, 1 coat of 
rubbing varnish , 4 coats of spar varnish 


7. Two brush coats of orange shellac and 2 brush coats of Lowe Bros. Finish- 
ing Varnish V 801 


8. Two brush coats of orange shellac and 3 brush coats of Lowe Bros. Finish- 
ing Varnish V 801 


9. Wood dyed alternating the two following solutions: No. 1 100 gr. aniline 
hydrochloride, 40 gr. ammonium chloride, 650 gr. water. No. 2 100 
gr. copper sulphate, 50 gr. potassium chlorate, 615 gr. water; washed 
with soap and water and thoroughly rubbed with vaseline; 3 coats of 
Lowe Bros. Marine Spar Varnish were added 


10. Four brush coats of Toch Bros. 1017 Marine Varnish thinned with turpen- 
tine 


11. One-half hour vacuum and 1 hour atmospheric pressure (Special Varnish, 
Adams & Elting Co ) 


12. One-half hour vacuum and 1 hour atmospheric pressure (Toch Bros. No. 
1017 M. S. Preservative 


13 Hot and cold treatment with paraffin dissolved in gasoline 


14. Five applications of hot boiled linseed oil and 2 coats of prepared wax, 
each coat applied at intervals of not less than 4 hours and each thor- 
oughly rubbed 


47.6 


15 Same as 9 except that no varnish was applied 


16 Plain yellow birch panels no treatment . ... 


42.5 


51.9 





Some conclusions not already mentioned follow: 

(1) A more effective coating may be secured by dipping than by hand brushing. 

(2) Cellulose varnishes are not as durable as oil varnishes. 

(3) Linseed oil and wax treatments are not effective in keeping out moisture. 

(4) All the varnishes tested were somewhat affected by water, ^including those that do 
not turn white as well as those which do. 

(5) Very resistant coatings may be secured by using certain rubbing varnishes followed 
by top coats of spar vainish as a protection, also by using certain linseed oil varnishes, covered 
by a more durable China wood oil varnish. 

Tests were also conducted on electroplated metal coatings and on vulcanized rubber coat- 
ings. Both of these types of coating are extremely resistant to the penetration of moisture, 
so long as they remain intact. The metal coating in particular, however, is rather delicate 
and does not adhere to the wood. The vulcanized rubber coatings were about an eighth of an 
inch thick and would probably be quite satisfactory from the standpoint of durability. 

Of all the coatings upon which experiments were made, an aluminum leaf coating appears 
to be the most satisfactory from the standpoint of resistance to moisture penetration com- 
bined with general feasibility. This coating consists, in effect, of aluminum leaf laid over the 



AIRCRAFT DESIGN DATA. 



Note 12. 



surface between layers of varnish, just as sign painters lay on leaf over size. The leaf itself 
has no wearing strength and the coating has just the durability and wear resistance of the 
coats of varnish and enamel placed over the leaf. The resistance of the leaf coating to the 
passage of moisture is very remarkable indeed, as will be seen from a study of table 26 and 
figure 81, which present comparable data on several kinds of aluminum leaf coatings and several 
common kinds of finish. 

TABLE 26. Humidity tests of metal leaf coatings. 



Treatment. 


Number 
of speci- 
mens av- 
eraged. 1 


Average absorption in grams per square foot of surface for 


3 days. 10 days. 


17 days. 


24 days. 


31 days. 


Silex filler, gold size, aluminum leaf, and 3 coats of 
E. P. black lacquer 


2 
3 
1 
10 

2 
10 


-0. 210 
0.218 

1.28 

14.0 
20.5 


0.084 
0.445 
0.252 
4.56 

36.7 
42.5 


0.168 
0.805 
0.420 
7.29 

47.6 
51.9 


-0.042 


0. 461 


Silex filler, 1 coat of rubbing varnish, gold size, imita- 
tation gold leaf 1 coat Valspar 


Silex filler, 1 coat of rubbing varnish, gold size, alu- 
minum leaf 1 coat Valspar . 




Silex filler, 3 brush coats Hampden's waterproof var- 
nish No. 2 






5 applications of linseed oil applied hot and 2 coats of 
wax 






No treatment 











1 Average data on yellow birch panels. 

Thickness inch . . 0. 60 

Width inches. . 3. 960 

Length do. . - 8. 000 

Surface square feet . . . 540 

Weight (air-dry) pound . . .490 

Volume cubic feet . . .011 

It has been found, in actual practice, that the process is entirely workable, and very good 
results have already been secured from its use. 

The following instructions explain in detail the method of applying aluminum leaf to 
propellers. The same method could be used in coating other aircraft parts if it were found 
desirable to do so. 

INSTRUCTIONS FOR APPLYING ALUMINUM LEAF TO AIRCRAFT PROPELLERS. 

The leaf used in this process is exceedingly thin and light, there being probably 12,000 
to 15,000 leaves per inch which makes it appear difficult to handle. If the instructions are 
carefully followed, however, the leaf may be easily and thoroughly applied. 

Preparation. It is important to provide a perfectly smooth surface over which to apply 
the coating. The surface should be sanded perfectly smooth and be free from all tool marks 
or other imperfections. The bolt holes at the hub should be plugged with corks which should 
be cut off flush and finished in the same manner as the rest of the surface. 

Filling. For open-grained woods a coat of filler consisting of 83 per cent liquid and 17 
per cent silex should be used. The liquid should consist of 77 per cent airplane spar varnish 
and 23 per cent turpentine. The silex should pass a 200-mesh sieve. 

The filler should be applied to the wood and allowed to flatten, after which it should be 
rubbed off across the grain so as to thoroughly fill the pores. The filler should dry at least 
24 hours, after which it should be sanded lightly. 

TJIJ . <-" V 

aril -mo bial la^f mifaiaufU to .*>*& ni ,etet 



Note 12. AIRCRAFT DESIGN DATA. 143 



Shellac varnish undercoating. The shellac varnish should consist of four and one-half 
pounds of orange shellac gum in one gallon of clean, neutral, denatured alcohol. 

This varnish should be applied evenly over the surface of the propeller and allowed to dry 
three or four hours, after which it should be sanded lightly. 

Size. The size should consist of 75 per cent airplane spar varnish and 25 per cent turpen- 
tine. It is suggested that a small amount of Prussian blue in Japan be added to the varnish 
to give it a color, so that spots subsequently left uncovered by the leaf will be readily visible. 

This size should be brushed evenly over the surface as sparingly as possible and allowed 
to dry until a tack is reached, which will permit the handling of the propeller immediately 
after the application of the leaf. The time will vary with the varnish arid the kind of a day. 
The varnish should probably dry an hour and a half on a light dry day or in a heated building 
in the winter time, but a longer time may be required on cloudy or damp days. This is a very 
important point and should be carefully considered as the coating hardens very slowly after 
the leaf is applied. 

Care should be exercised so as not to produce fatty edges or runs in applying the size. 
If they occur, the leaf will be easily rubbed from the surf ace in handling the blade. 

It has been found convenient to size one side of the blade at a time; that is, the front or 
back of the blade. This is a convenience in applying the leaf later. 

Aluminum leaf. After the size has reached the right tack the leaf should be applied very 
rapidly over the surface, and after the sized surface has been entirely covered the leaf should 
be patted down with the palm of the hand or with a pad of cotton, after which the rough edges 
should be rubbed away (see fig. 79b). Any points not covered with leaf should be coated by 
applying a small piece of leaf to the spot with the fingers. The coating should be rubbed well 
with a piece of cotton which has been dipped in aluminum powder. This will insure the leaf 
sticking securely over the entire surface and will fill any small holes not already filled. 

Aluminum leaf comes in packs containing 500 leaves. The pack is divided up into 10 or 
20 books containing 50 or 25 leaves, respectively. The metal leaf is placed between the pages 
of these books and comes in 4-inch, 4^-inch, 5-inch, or 5^-inch squares. 

It has been found best to apply the leaf directly from the book by turning back the first 
page of the book halfway, holding the same between the first and second fingers of the right 
hand (see fig. 78a) . The book itself should be held between the thumb and fingers and in 
such a way that the back of the hand will be toward the work when the leaf is applied, the 
book being given a slight bend to prevent the corners of the leaf from drooping. The end of 
the leaf exposed by turning back the first page of the book should be placed against the surface 
to be coated and held securely in place by the left hand (see fig. 78b). The sheet held between 
the first and second fingers should be drawn back so as to allow the whole leaf to come in contact 
with the surface (see fig. 79a). The next sheet should be applied in a like manner, lapping 
edges with the first, and so on. The best results will be obtained if the gilder works in one 
direction with. each row of leaf; that is, from left to right. If this be done, it will aid considerably 
in completing and smoothing off the surface. 

It is suggested that in turning the pages of the books the back of the book be held between 
the first two fingers of the left hand (see fig. 77a). The leaves from which the leaf has been 
removed should be turned back and held between the thumb and first finger of the left hand. 
The next sheet of paper may then be turned back exposing one-half of the next leaf. The 
operation of changing the book from left to right hand is shown in figure 77b. 

Large hub hole. The large hub hole should receive the same treatment as the rest of the 
propeller. In applying the leaf to the hub hole it has been found convenient to cut the books 



144 



AIRCRAFT DESIGN DATA. 



Note 12. 




Fig. 77. Aluminum leaf coating, (a) Method used in turningjpage of book, (b) Transferring book from left to 

right hand. 




Fig. 78. Aluminum leaf coating, (a) Method of holding book when applying leaf, (b) First operation in laying leaf. 




Fig. 79. Aluminum leaf coating, (a) Second operation in laying leaf, (b) Smoothing off surface after application 

of leaf. 




Fig. 80. Aluminum leaf coating, (a) Applying leaf to large hub hole, (b) Smoothing off leaf in large hub hole. 



Note 12. 



AIRCRAFT DESIGN DATA. 



145 



of leaf up into about 1-inch strips of leaf and paper and drop them vertically into the opening 
and bring into contact with the size (see fig. 80a). After the entire surface of the hole has 
been covered the leaf should be patted into place with a wad of cotton attached to the end of a 
stick (see fig. 80b). 

Small hub holes. These holes should be simply corked up with ordinary corks, the tops of 

which should be cut off flush with the surface of the propeller and covered with the regular finish. 

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95 to 100 per cent. 

Shellac color varnish. After the application of the leaf two coats of shellac color varnish 
should be applied. This varnish should be made as described under the heading of "Shellac 
varnish undercoat," except that enough color should be added to produce a mahogany color. 
Four or 5 per cent of Bismark brown in the shellac varnish gives about the right color. The 
amount of this material to get the best results should be determined by trial. The varnish 
should dry three or four hours before rubbing or recoating. 
98257 19 No. 12 10 



146 AIRCRAFT DESIGN DATA. Note 12. 

Each coat of shellac should be rubbed down lightly between coats without the use of oil. 

Finishing varnish. A final flowing coat of airplane spar varnish should be applied and 
allowed to dry about 48 hours. This coating should not be rubbed or sanded. 

Estimated time required to coat a propeller. The time required to apply the leaf to a propeller 
should not be more than 40 or 50 minutes. This time could be reduced after the finisher becomes 
more experienced. The estimated time required for applying the complete finish described 
in the foregoing paragraphs would be in the neighborhood of 8 or 10 hours, and the total 
time required for drying the various coats about 90 hours. The total time required for the total 
operation would probably be in the neighborhood of 100 hours. 

Modification of aluminum leaf spirit varnish process. It might be desirable in some cases 
to use oil varnishes or enamels in lieu of the shellac described above. This may be done and 
satisfactory results obtained. In case oil varnishes are substituted, it is possible that a more 
durable coating will be obtained. It requires a much longer time to apply the finish because 
of the greater time required for the oil varnishes to dry. Each coat of varnish should dry at 
least 72 hours before recoating. 








,ATA<I HOiasa-TiAflOXIA 



APPENDIX. 

For convenient reference, specifications for the determination of the moisture content in 
wood and for the determination of the specific gravity of wood are embodied in the appendix. 

THE DETERMINATION OF MOISTURE CONTENT IN WOOD. 

SELECTION OF TEST SPECIMENS. 

1. Short pieces of wood dry out much more rapidly than longer ones. In order to reduce 
the time required for drying, the length of the test specimen in the direction of the grain should 
usually be about 1 inch or not more than enough to give a volume of from 5 to 25 cubic inches. 

TESTS. 

2. Having selected a representative piece of material for a test specimen, the procedure 
for determining the moisture content is as follows: 

3. Immediately after sawing remove all loose splinters and weigh the test specimen. It 
is important that the weight be taken immediately after sawing, since the wood is subject to 
moisture changes on exposure to the air. The degree and rapidity of change are dependent 
on the moisture content of the piece and the conditions of the air to which it is exposed. 

4. Put the test specimen into a drying oven and dry at approximately 212 F. (100 C.) 
to constant weight. This usually requires three to five days. Specimens placed in the oven for 
drying must be open piled to allow free access of air to all parts of each piece. 

5. Weigh the test specimens immediately after removing from the oven. 

6. The loss in weight expressed in per cent of the dry weight is the percentage moisture 
content of the wood from which the test specimen was cut. 

CW" D) 
Percentage moisture = r\ X 100 

\V = original weight as found under paragraph 3. 

D=i oven-dry weight as found under paragraph 5. 

I -^ 11 

ACCURACY. 



7. In order to insure good results, the weight should be correct to within at least one-half 

of 1 per cent. 

THE DETERMINATION OF SPECIFIC GRAVITY OF WOOD. 

GENERAL. 
.H< 

1 . The specific gravity (or density) of all woods used in aircraft construction shall be deter- 
mined, when required, in accordance with this specification. Method A shall be used whenever 
possible. 

SELECTION OF TEST SPECIMENS. 

2. Short pieces of wood dry out much more rapidly than longer ones. In order to reduce 
the time required for drying, the length of the test specimen in the direction of the grain should 
usually be about 3 centimeters. 

147 

.Klft! 



148 



AIRCRAFT DESIGN DATA. 



Note 12. 



METHOD A. 

3. Having selected a representative piece of material for a test specimen, the procedure is 
as follows: 

4. Immediately after sawing remove all loose splinters and put the test specimen into a 
drying oven and dry at about 212 F. (100 C.) to constant weight. This usually requires 
three to five days. Specimens placed in the oven for drying must be open piled to allow free 
access of air to all parts of each piece. 

5. Weigh the test specimen. 

6. Determine the volume of the oven-dry specimen preferably by the method described in 
paragraphs 9 to 12. 

7. Specific gravity = y 

M = oven-dry weight in grams as determined under paragraphs 4 and 5. 

V = oven-dry volume in cubic centimeters as determined under paragraph 6. 



S( l* A* 
o^ tr>9i<fw8 e II 

Jnabneqeb n II 
;x 



.0 OOI) .1 c vbi 







Fig. 82. Determination of specific gravity of wood. 



REDUCTION FACTORS. 

!*>)':> L ~ . . . . . 

8. Onemch = 2.54 centimeters; 1 ounce = 28.4 grams; 1 cubic mch = 16.4 cubic centimeters; 

1 pound = 454 grams. 



DETERMINATION OF VOLUME. 



9. After the oven-dry weight has been obtained dip the test specimen in hot paraffin and 
allow it to cool. Scrape off any surplus paraffin which adheres to the specimen. 

10. The volume of the test specimen is found by determining the weight of water it dis- 
places when immersed, as shown in figure 82. This weight in grams is numerically equal to 
the volume of the specimen in cubic centimeters. 



Note 12. AIRCRAFT DESIGN DATA. 149 

11. It is important that the determination of the volume by weighing be made as quickly 
as possible after the immersion of the specimen, since any absorption of water by the specimen 
directly influences the accuracy of the result. By estimating the volume of the specimen and 
placing approximately the required weights on the plan before the specimen is immersed the 
time necessary for balancing may be reduced to a minimum. 

12. To determine the volume, a container holding sufficient water for the complete sub- 
mergence of the specimen is placed on one pan of a balance scale. The container and water are 
then balanced with weights added to the other scale pan. By means of a sharp-pointed rod, 
shown in figure 82, the specimen is held completely submerged and not touching the container 
while the scales are again balanced. The weight required to balance is the weight of water 
displaced by the specimen, and, if in grams, is numerically equal to the volume of the specimen 
in cubic centimeters. 

13. The sharp-pointed rod, by means of which the specimen is held in position, should be 
of as small diameter as possible. Care should be taken not to lower the specimen into the 
water to a much greater depth than required to completely submerge it; otherwise the weight 
of water displaced by the rod will affect the accuracy of the result. 

ACCURACY. 

14. In order to insure good results, the weights and volumes should be correct to within 

at least one-half of 1 per cent. 

METHOD B. 

15. The following method of determining the specific gravity may be used when the appa- 
ratus required by test A is not available. 

16. Select the test specimen as in paragraph 2. 

17. Dry the specimen as in paragraph 4. 

18. Cut the oven-dried specimen while hot to a standard volume of not less than 80 cubic 
centimeters so that its volume may be accurately determined by measurement. 

19. Weigh the oven-dried specimen while hot and record its weight in grams. This weight 
must be accurate to within one-half of 1 per cent. 

20. Determine the volume in cubic centimeters of the oven-dried specimen while hot by 
measuring each edge in centimeters and taking measurements to the nearest one-half millimeter. 

. . M weight in grams 

20. Specific gravity-- 



WASHINGTON : GOVERNMENT PRINTING OFFICE : 1919 



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