STUDIES OF HELICOPTER ROTOR NOISE

Abstract : A comprehensive study of the problem of helicopter noise radiation is presented. After a review of the basic features of the noise, the limited experimental data are reviewed in some detail, and empirical laws are proposed. An exact theoretical expression for the noise is derived. This expression has been used as the basis for the development of a comprehensive computer program to calculate helicopter noise at any field point, including all effects of fluctuating airloads and all possible rigid and flexible blade motions. Under very reasonable approximations, an analytic expression was found for the sound field far from the helicopter, and computations based on this expression were made. The results show all the higher harmonics of the loading, at least up to the sixtieth, to be significant for noise generation. A study of the harmonic airloads is presented. Comprehensive acoustic results from the theory include the effect of various loading inputs, thrust, tip velocity, number of blades, blade motion, and forward speed. Fair agreement with experiment is found for overall levels, and good agreement is found with experimental trends. Design charts are presented which enable routine calculation to be made of noise radiated from any helicopter in hover or forward flight.