FOR many years, there has been a need for accurate prediction of the noise of propellers and helicopter rotors.

The increase in the number of general aviation aircraft and helicopters and the awareness of the public to environmental noise pollution have been driving forces for research on prediction techniques. A good review of the works in this field up to the early seventies is by Morfey. J Many other papers have been published since Morfey's review article, with new formulations for prediction of the noise of rotating blades. Most of these results are for noncompact sources. In general, one requires the use of a computer for obtaining the acoustic pressure signature or spectrum of the noise. In this paper, a unified approach is used to derive many of the compact and noncompact formulas for the calculation of the discrete frequency noise of rotating blades. The approach is based on the Ffowcs Williams-Hawkings (FW-H) equation. 2 It is shown that many known results can easily be obtained by manipulating various forms of the solution of the FW-H equation. The methods presented here are not those used in the original derivations. For example, Hanson's results, discussed below, were originally based on the solution of an inhomogeneous wave equation as described by Morse and Ingard. 3 However, it was felt that the approach used in the present paper is more appropriate since it takes advantage of the operational properties of generalized functions and thus reduces the algebra.