Noise Characteristics of Helicopter Rotors at Tip Speeds up to 900 Feet Per Second

Evidence is presented which suggests that the noise of full‐scale helicopter rotors results mainly from conditions of unsteady flow. Measurements of the sound‐pressure levels and spectra are presented for test conditions where gear train, engine, and other propulsion system noises are minimized. These data cover a range of tip speeds from 100 ft/sec to 900 ft/sec for various rotor disk loadings. Results indicate that both tip speed and disk loading have an important influence on the noise radiated from the rotor. During stall, the sound pressure levels increased at all frequencies, but particularly at the high end of the spectrum. As a matter of special interest, a highly‐peaked wave form due to possible doppler effects was noted to be associated with high‐tip speed operation.