PREDICTION OF BLADE-VORTEX INTERACTION NOISE USING AIRLOADS GENERATED BY A FINITE-DIFFERENCE TECHNIQUE

A numerical finite-difference procedure has been developed for the prediction of helicopter blade loads during realistic self-generated three-dimensional blade-vortex interactions (BVI). Here, the velocity field is obtained through a nonlinear superposition of the rotor flow field computed using the unsteady three-dimensional full potential rotor now solver RFS2.BVI and the rotational vortex flow field computed using the Law of Biot-Savart. Potential blade-vortex encounters are identified and tracked in time at equal increments of rotor azimuth using the lifting-line helicopter/rotor trim code CAMRAD. Utilizing the predicted airloads from the finite-difference full potential computations, a BVI noise prediction based on Farassat's formulation IA was used to compute the acoustic signature of the rotor. The acoustic pressure time histories and spectral analyses for the OLS model rotor were predicted with reasonable accuracy for a range of microphone locations under different rotor operating conditions. Despite the accurate prediction of the acoustic waveforms, the peak amplitude was consistently underpredicted. However, the inclusion of BVI noise source in the acoustic analysis improved the PNLT prediction significantly. For the model OLS rotor, reasonable correlation with the experimental wind tunnel surface pressure and acoustic data was obtained.