Separation of main and tail rotor noise from ground-based acoustic measurements

A new method of characterizing the external noise radiation of rotorcraft is presented, making use of ground-based acoustic measurements. The method employs time-domain de-Dopplerization to transform the acoustic pressure time-history data collected from a fixed array of ground-based microphones to the equivalent time-history signals observed by an array of virtual in-flight microphones traveling with the helicopter. The now-stationary signals observed by the virtual microphones are then periodically averaged with the main and tail rotor blade passages, which are inferred from the acoustic signals using wavelet analysis. The averaging process suppresses noise that is not periodic with the respective rotor, allowing for the separation of main and tail rotor pressure time histories. The averaged measurements are then interpolated across the range of directivity angles captured by the microphone array in order to generate separate acoustic hemispheres for the main and tail rotor noise sources. This method facilitates a more direct comparison of ground-based noise measurements of rotorcraft with theoretical predictions and wind tunnel measurements of isolated rotors. The new method is successfully applied to ground-based microphone measurements of a Bell 206B3 helicopter and demonstrates the strong directivity characteristics of harmonic noise radiation from both the main and tail rotors of that helicopter.