Prediction of counter-rotation propeller noise

Using asymptotic techniques the complex expressions for the sound radiated from a counter-rotation propeller are reduced to provide simple results which do not involve numerical integration or the evaluation of Bessel functions but retain the main parametric dependences. The unsteady loadings on the blades, needed as input to the radiation formulas, are also evaluated asymptotically. Here consideration is given to the effects of both viscous wakes and bound potential fields, the latter resulting in unsteady loads on both blade rows. Comparisons are made with measurements taken from fly-over tests on the Fairey Gannet aircraft. Excellent agreement is obtained between the asymptotic theory and the data in terms of both absolute level and directivity. In addition, for some interaction tones, it is shown that the potential field interactions are much stronger than the viscous wake interactions.