Prediction - and its Validation - of the Acoustics of Multiblade Rotors in Forward Flight Utilising Pressure Data from a 3-D Free Wake Unsteady Panel Method

Abstract

A newly developed high resolution 3-D Panel method to predict the unsteady aerodynamics of multiblade helicopter rotors has been used to generate the input data for an also new Acoustics code based on Farassat's Formulation 1A of the FWH/equation. The combination of this aerodynamics/aeroacoustics codes is used to predict the acoustics of multiblade rotors in hover and forward flight. Of special interest are cases of low speed descent where severe BVI is known to occur. The use of panel method excludes the treatment of shocks in the rotor flow field. No restrictions are placed on the blade profile, planform or other geometry characteristics as well as blade motions. The acoustic pressure-time history, mid-frequency sound pressure levels and other predictions are validated with flight test and wind tunnel data for two and four blade rotors in hover and forward flight modes. Some comparisons with the predictions of the well known WOPWOP code have also been performed.