Azimuthal Noise Characteristics of Contra-Rotating Propellers: Results of a Wind Tunnel Model Test.

Abstract

Compared to conventional turbofan engines, advanced high speed coaxial/contra-rotating ("CR") propellers promise significant fuel savings. Their noise characteristics, however, differ from those of singe-rotating ("SR") propellers as a consequence of the unavoidable acoustic interaction effects; additional noise is generated by aerodynamic interactions. In the context of an acoustic research program to support the development of future CR-propfans, pilot-type aerodynamic wind tunnel tests on contrarotating model-scale propellers (diameter 0,27 m)were conducted. It was found that, compared to SR-propellers, the noise radiated from CR-propellers exhibits the following specific characteristics: o increased sound pressure levels at higher frequencies, o pronounced azimuthal sound pressure level directivity, o two maxima in the polar directivities of both the high-frequency ha rmonic and the A-weighted levels, o a lower gradient of A-weighted sound pressure levels versus Mach number.