The Uses And Abuses Of The Acoustic Analogy In Helicopter Rotor Noise Prediction

This paper is theoretical in nature and addresses applications of the acoustic analogy in helicopter rotor noise prediction. It is argued that in many instances the acoustic analogy has not been used with care in rotor noise studies. By this it is meant that approximate or inappropriate formulations have been used. By considering various mechanisms of noise generation, such abuses are identified and the remedy is suggested. The mechanisms discussed are thickness, loading, quadrupole and blade-vortex interaction noise. The quadrupole term of the Ffowcs Williams- Hawkings equation is written in a new form which separates the contributions of regions of high gradients such as shock surfaces.