ROTOR NOISE PREDICTION AND VALIDATION

An evaluation of the state-of-the-art linear rotor rotational noise prediction models is presented with the help of the state-of-the-art rotor aerodynamic codes and a high confidence acoustic flight test data of a MD-500E helicopter, The noise prediction models evaluated ranged from a compact line source model, LRN, used in NASA's Phase I ROTONET prediction system to a noncompact full surface model, RTN, available in NASA's Phase II ROTONET system. Noise predictions were made both for the main and tail rotors and combined before comparing with the flight test data. Rotor airloads input needed by these models were generated using aerodynamic codes ranging from the lifting line wake code CAMRAD to a state-of-the-art 3-D, rotor transonic full potential flow code RFS2 developed by MDHC. Comparison of noise predictions with flight test data showed that the prediction models, especially the noncompact source models did a good job in predicting the noise for shallow observer angles when the helicopter is approaching the observer, due mostly in part to the accurate thickness noise prediction for the tail rotor. For the overhead observer angle, and the angle corresponding to the observer position behind the helicopter the noise levels, especially at frequencies corresponding to higher tail rotor noise harmonics (beyond the second or third) were grossly underpredicted both for the centerline and the sideline observer positions. The correlation with experimental data at these frequencies, can be improved if higher harmonic airloads input is available for tail rotors.