Helicopter Rotor Rotational Noise Prediction and Correlation. Volume 1. Rotational Noise Prediction and Correlation Under Nonuniform Inflow Conditions
Final rept. 27 Jun 1966-31 May 1968
UNITED TECHNOLOGIES CORP STRATFORD CT SIKORSKY AIRCRAFT DIV
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The results of a measurement, prediction, and correlation study of rotational noise of the Sikorsky NH-3A compound helicopter are presented. Measured and predicted levels correlated well for the first four harmonics of rotor noise during forward flight. Harmonic levels above the fourth could not be extracted from the measured noise data for correlation. Predicted levels grossly underestimated measured levels for low-altitude hover due to acoustic directionality and near-field effects, and due to aerodynamic ground effects. Harmonics of differential pressure are tabulated for up to 30 harmonics of revolution of the main rotor 30rev at each of 25 locations on a main rotor blade. Spanwide locations of the measured differential pressures are 40, 75, 85, 95, and 98. Flight conditions are 165 knots, 190 knots, and hover IGE.