High-Speed Helicopter Impulsive Noise

Forward flight impulsive noise data from a 1/7-scale UH-1H model rotor have been gathered in an acoustically treated wind tunnel and are compared with full-scale acoustic flight-test data for the same helicopter. Good agreement between model and full-scale waveforms and peak pressure amplitudes is noted when key performance parameters are matched and the data are acoustically scaled. In-plane acoustic radiation characteristics of the model data are presented for variations in thrust, advance ratio, tip-path-plane angle, and advancing tip Mach number. The acoustic waveform exhibits changes in character as advancing tip Mach number (MAT) is increased, becoming almost discontinuous at high MAT. This step increase in acoustic pressure correlates with schlieren photographs of a periodic pressure wave which radiates from the advancing rotor blade to the acoustic far field.