Study of Impulsive Sound Generated by Rotor Tips Accelerating Near the Sonic Speed.
Final rept. 25 Jun 69-24 Jun 71,
BOLT BERANEK AND NEWMAN INC CAMBRIDGE MASS
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The principal results obtained are summarized as follows Compressibility effects at the rotor tip during high speed forward flight are an important noise source. An acoustical model appears to give resonable predictions of observed sound levels and directionality. Thickness radiation generally dominates lift radiation. By modifying the tip planform, section, and chord, sound level reductions can be achieved of the order of 10 dB or more. The thickness distributions as derived appear sufficiently smooth so that they should be aerodynamically well behaved. Author