Predicted changes in advanced turboprop noise with shaft angle of attack

Current interest in advanced turboprop blade designs and new propeller installation schemes motivated an effort to include unsteady loading effects in an existing propeller noise prediction computer program. The present work validates the prediction capability, while studying the effects of shaft inclination on the radiated sound field. Classical methods of propeller performance analysis supply the time-dependent blade loading needed to calculate noise. Polar plots of the sound pressure level (SPL) of the first four harmonics and the overall SPL are indicative of the change in the directivity pattern as a function of the propeller angle of attack. Noise predictions are compared with newly available wind tunnel data and the accuracy and applicability of the prediction method are discussed. It is concluded that unsteady blade loading caused by inclining the propeller with respect to the flow changes the directionality and intensity of the radiated noise. These changes are well modeled by the present quasisteady prediction method.