The Frequency Doubling Of Helicopter Rotor Noise Due To Retreating Blade Stall

A detailed examination of the measured blade noise of a helicopter in forward flight is presented. The data was acquired during the AH-1G helicopter operational loads survey. We observe that for certain level flight conditions the fundamental period of the acoustic signal appears to be at half the blade passing period. This effect is demonstrated by showing that the autocorrelation function of this signal has a peak at this interval. This phenomenon is apparently due to retreating blade stall. Acoustical calculations based on the measured blade loads show that this effect occurs only when loading noise is greater than thickness noise. Furthermore, by basing the calculation on the measured blade loads that include retreating blade stall, we were able to analytically reproduce the observed effect.