In-Flight Array Measurements of Tail Rotor Harmonic Noise

Abstract

Acoustic data that utilized a unique flying microphone boom array was analyzed using a time domain averaging technique, based on a theoretical tail rotor one-per-rev signal, to obtain tail rotor acoustic time histories for forward and descending flight. This flying microphone array approach provides useful insight into the directivity and noise mechanisms associated with the periodic harmonic noise generated by the Bell 206B tail rotor. Comparisons between linear harmonic noise theory and the time averaged data show good agreement, indicating that the average harmonic noise of the 206B is dominated by thickness noise near the tip-path-plane of the tail rotor and by loading noise at the out-of-plane microphone positions. The measured data also provides trends in the overall sound pressure level (OASPL) with respect to both forward velocity and descent angle. A significant deviation from the average SPL of the harmonic noise levels at high frequencies is also shown.