Hovering impulsive noise - Some measured and calculated results

In-plane impulsive noise, radiating from a hovering model rotor has been measured in an anechoic environment. The hover acoustic signature was compared with existing theoretical prediction models and with previous forward flight experiments using the same model rotor. These hover tests showed good experimental consistency with forward flight measurements, both in pressure level and waveform character over the range of Mach numbers tested (0.8-1.0). Generally poor correlation, however, was confirmed with current linear theory prediction efforts. Failure to predict both the peak pressure levels and shape was reported, especially with increasing tip Mach number.