Parameterization of Helicopter Rotor Broadband Noise From Experimental Data

A semi-empirical relationship that represents helicopter rotor broadband noise in terms of blade section parameters has been derived from measured whirl tower acoustic data. The noise generating mechanism is assumed to be discrete vortex shedding from the rotor blade. A mathematical model of the shedding mechanism was developed so that the acoustic radiation of the oscillatory blade forces induced by vortex shedding could be related to the measured pressure time history. The model was used to solve for the oscillatory forces on the blade that would generate the broadband noise characteristics of the measured pressure time history. Acoustic data generated at the Boeing whirl tower facility was used to fit for the oscillatory forces. Since a variety of test conditions were recorded in the Boeing test program, it was possible to obtain a parametric relationship for the oscillatory forces as functions of the section Mach number and angle of attack. The magnitude of the oscillatory forces increase with section angle of attack but decrease with section Mach number. The parametric relationship representing broadband noise was incorporated into a helicopter noise prediction program that predicts rotational as well as vortex noise. The prediction program generates the digital pressure time history produced. by the helicopter rotors at any observer location for hover or steady state flight. This pressure time history corresponds to that which would be recorded by a microphone placed at the observer location. The program output can be Fourier analyzed so that the noise spectrum can be generated. The digital pressure time history can also be converted to an analog signal for subjective evaluation.