Variable Pitch Propellers Page: 3 of 16
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N.A.C.A. Technical Memorandum No. 459
forward speed is reduced to V1. The relative wind velocity
becomes Vw. and the angle of attack a.
The modern conception of propeller theory is rather more
elaborate than the foregoing, allowance being made for inflow
velocity and slip stream rotation which modify our Vw both
in direction and magnitude but for the present purpose we can
neglect these refinements.
Reverting to Figure 1 we see that the angle of attack a
depends on the forward speed V and speed of rotation n. Fur-
ther, if V and n were reduced in the same ratio a would
not be affected.
During a climb the engine R.P.M. are lower than at forward
top speed. The airplane speed also falls but more rapidly than
the R.P.M. Hence it is clear that the propeller operates at a
greater angle of attack during a climb than at top speed.
The efficiency of the propeller depends to a great extent
on a and there is a certain critical value of the angle of
attack which, other things being equal, will give maximum effici-
ency. It is this angle which the designer contrives to attain
in designing the propeller. Hence it is obvious that a propeller
designed for top speed in level flight will not give: the best
possible results in climbing or at any speed other than the de-
signed condition.
The consideration of take-off is important. When an air-
plane commences its run, due to the low forward velocity, the2
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Milner, H. L. Variable Pitch Propellers, report, April 1928; (https://digital.library.unt.edu/ark:/67531/metadc65188/m1/3/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.