In-Flight Array Measurements of Tail Rotor Harmonic Noise

Acoustic data that utilized a unique flying microphone boom array was processed and 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 experimental data show good agreement and indicate that the average periodic 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 associated with the overall sound pressure level (OASPL) with respect to both forward velocity and descent angle. A significant variance about the tail rotor harmonic noise is also shown, possibly indicating that other sources of aperiodic tail rotor harmonic noise may be important.