12

AERONAUTICS IN THEORY AND EXPERIMENT [CH I

§ 13. Experience has shown that the instruments at our disposal
for wind channel tests enable the forces and moments to be measured
to an accuracy of approximately 1 %. This is extremely fortunate
in view of the fact that frequently the effects of very delicate
changes in relative positions of parts and modifications in shape
require to be investigated. In consequence great care must be
taken to ensure that the models themselves be made with extreme
accuracy. It has been remarked, and the real significance will be
brought out in the next chapter, that the nature of the air flow
in the vicinity of any body determines the forces and pressures
exerted by the wind upon it. If a strut, whose shape is such, as
to allow the air to stream easily past it, be not quite truly formed,
the discrepancy from the perfect shape will continually act as a
source of disturbance to the air by altering the direction of the
flow in its neighbourhood. Energy will thus be uselessly expended
and the resistance of the strut consequently increased. Certain
portions of the strut—and in fact of most bodies entering into
aeronautical work—may be more sensitive than others to such
discrepancies in shape. It is the object of the next chapter to
study the laws of air flow and the principles that must be used in
the determination of the shapes of bodies most suitable for the
functions demanded of them. The importance of this aspect of the
subject cannot be too strongly emphasised.