This investigation examined some of the effects of atmospheric turbulence ingestion on hovering tail rotor acoustics. The presentation of dBA levels and power spectra provide overall and detailed information concerning changes in radiated sound due to various atmospheric parameters. The data clearly indicate a strong dependence of measured sound levels on the rotor inflow velocity and thrust level. This trend is somewhat unexpected and warrants further investigation. The following conclusions, based on two fixed-location hot-film probes and two fixed-location microphones, are drawn from the presented data. Sound levels measured in dBA increase with increasing rotor inflow velocity and rotor thrust. Current linear theories do not account for the influence of rotor thrust on turbulence ingestion noise. Normalized near-field rms turbulence velocity is independent of atmospheric wind speed. There is no discernible effect of rotor operating condition on the rms turbulence velocity. The widening of peaks with increasing frequency in the measured spectrum is attributed to turbulence ingestion noise. Noise measurements made along the rotor axis indicate that with higher near-field rms turbulence velocity, the amplitude of the lower frequency rotor blade passage harmonics is greater than for the lower u'n case; the amplitudes of the higher frequency harmonics are about the same. Also, with higher near-field rms turbulence velocity, the broadband noise floor increases, about 2 dB at the lower frequencies and about 5 dB at the higher frequencies. No distinct effect of eddy length on measured noise was observed. No distinct effect of atmospheric wind speed on measured noise was observed. The addition of small scale turbulence caused by installing a grid upstream of the rotor increased the sound levels, measured in dBA, for the location on the rotor axis. Also, the amplitude of the broadband floor in the measured spectrum is higher with the grid.
Effects on ingested atmospheric turbulence on measured tail rotor acoustics
Einflüsse der atmosphärischen Turbulenz auf die akustischen Eigenschaften von Heckrotoren
Journal of the American Helicopter Society ; 41 , 1 ; 77-90
1996
14 Seiten, 21 Bilder, 2 Tabellen, 18 Quellen
Aufsatz (Zeitschrift)
Englisch
Effects of ingested atmospheric turbulence on measured tail rotor acoustics
Online Contents | 1996
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