Aeroacoustics - Historical Prospective And Important Issues

Developments over the last 25 years are reviewed with particular reference to the ability to predict and understand the total, noise generated by a helicopter. Main rotor rotational and broadband noise, tail rotor noise, and interaction sources are discussed. Considerable advancements have been made in the acoustic theories to take account of fluctuating forces, both discrete and broadband, are adequate for most calculations. Limitations are imposed on the use of such methods by the lack of the aerodynamic input data. Such data is unlikely to be available in the foreseeable future. In the case of rotational noise, generalized 'blade loading' data can be used with limited success. There is still debate on the controlling mechanism for broadband noise and, although atmospheric inflow turbulence is important, self-induced noise can not be ignored. High speed blade slap/thickness noise theories have shown improvement with the inclusion of additional terms. The overall prediction of helicopter noise is, however, still relatively poor.