  An analysis is presented for the evaluation of the acoustic field of a
rotating source in a non-uniform potential flow. Other than the restriction to
low flow Mach numbers, the method is exact and general. The variation in
radiation properties with source position is handled by representation as a
Fourier series in source angle, giving rise to an asymmetrically varying
acoustic field evaluated by summation of the series. The method is used to
develop an exact solution for the model problem of a rotor operating near a
cylinder in cross-flow and sample calculations demonstrate the accuracy of the
technique when compared to full numerical evaluation. The calculations show
changes of greater than one decibel in the acoustic field due to flow
non-uniformity at a flow Mach number of 0.15, a typical speed for propeller
aircraft at take-off.
