Abstract
Farfield noise radiated from low-tip-speed propellers on the YO-3A quiet observation aircraft and measured in flyover tests exhibited unexpected levels and trends that have been called the “quiet airplane paradox.” Levels of rotational propeller noise were found to be higher than predicted by conventional axisymmetric theory, and as propeller tip speed was decreased below a given value, at constant thrust, both harmonic and broad-band noise levels actually increased. Thus a “bucket” was formed in the SPL vs tip speed curve. In addition, the harmonic character of the flyover propeller noise was markedly different than that observed in static propeller tests. These data were declassified in 1973, and a program was conducted to investigate the paradoxical experimeutal results and to develop a theoretical explanation. The theoretical study considered propeller blade aerodynamics, nonuniform inflow through the propeller, chordwise blade loading, and, finally, propeller blade wake/wing interaction. It was concluded that the principal explanation of the paradox lies with the non-uniformity of inflow through the propeller disk causing circumferential variations of blade loads. The blade wake/wing interaction rated second in important. [Original experiments with the YO-3A aircraft were supported by the Army and this research program was supported by the Air Force.]
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