Prediction of jet noise in flight from static tests

The sound intensity of jet noise from aircraft in flight is derived in a co-ordinate system fixed to the jet engine. For this reason a convected form of the Lighthill equation is solved, with special care taken of jet temperature effects. Under certain assumptions and approximations, the in-flight and static sound intensities are related in a simple manner. Thus the directivity of jet noise in flight can be predicted. The theoretical result is checked with measurements. The agreement is remarkably good.