Basic mechanisms of noise generation by helicopters, V/STOL aircraft and ground effect machines☆

The basic mechanisms of noise generation due to mass introduction, applied force and applied stress are discussed with reference to their implications for helicopters, V/STOL aircraft and ground effect machines. The significance of the dimensional dependence of monopole, dipole and quadrupole fields is shown. The results of a new theory giving the effects of system accelerations on noise are presented. This theory is especially relevant to noise problems for these machines because of the centrifugal accelerations associated with many of the noise generating components. System accelerations give rise to higher order poles in the sound field which become particularly important at high speeds. An expression for the sound field produced by fluctuating lift and drag forces in a rotating and convected system is given. As a further example of the application of the general theory the sound field radiated by a hovering helicopter is analyzed. It is shown how a previously unrecognized source of sound arises from the outward components of force induced by the effects of blade coning angle and lag. The source of sound has its maximum in the plane of the rotor disc. The importance of including the proper momentum terms in calculations of noise radiated by moving mass sources is demonstrated.