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National Advisory Committee for Aeronautics, Technical Notes - Calculation of the Effect of Thrust Axis Inclination on Propeller Disk Loading and Comparison with Flight Measurements

An analysis based on simple geometry has been made of the effect
of thrust-axis inclination on propeller disk loading. Calculations are
in excellent agreement with available flight measurements, so that the ,
analysis is indicated to be adequate for predicting the primary effects
of inclination. For accuracy in the general case, however, a more
complete treatment may be necessary. Fuselage and wing interference
effects may be large and, for accuracy, surveys for determining flow
angles are suggested.

Consideration of factors involved indicates that, for a given blade
design, the simplest means for reducing the fluctuating blade stresses
due to inclination is suitable thrust-axis setting and/or restriction of
airplane angle—of-attack range through use of flaps.

Published papers (references 1 and 2) concerning propeller flight
tests have shown that inclination of the thrust axis to the air stream
causes large variations in propeller disk loading. With the trend
toward high—solidity, large—diameter propellers for high—speed, long-
range operation, this effect is now becoming important in the structural
design of propeller blades. Consequently, it is desirable to be able
to predict the magnitude of this effect.‘

In this paper a simple analysis is made of the effect of angle
of attack on thrust distribution. Calculations based on the analysis
are compared with some unpublished data from previous flight tests in
which the thrust distributions on the right and left sides of the prop—
eller disk and the thrust-axis angle of attack were measured.

The analysis, as presented, is developed for determining the minim
effects of angle of attack. At a positive angle of attack, the down-
traveling blade will experience an increased load and the up-traveling
blade on the other side of the propeller disk will experience a decreased
load. Under normal circumstances, therefore, the maximum effects of
inclination will occur along the horizontal diameter.

This analysis should also apply for angles of yaw or combinations
of angle of attack and yaw. In any case, the maxim effects should be
experienced along a propeller diameter perpendicular to the plane of
resultant angle variation.