SOUND FROM A PROPELLER AT ANGLE-OF-ATTACK - A NEW THEORETICAL VIEWPOINT

Accurate prediction of propeller noise is required to satisfy airplane noise certification regulations and environmental concerns. Propeller s typically operate with their shafts at some angle to the flight dire ction but this factor has been ignored in noise calculations until rec ently. In this paper, far field noise formulas are developed by applyi ng the free-space Green's function for the convected wave equation to loading sources on a propeller whose shaft may be at an angle to the f light direction. The resulting inflow angularity influences noise in t wo distinct ways. First, loading is modulated causing generation of mo re efficient radiation modes. Second, the radiation modes themselves a re modified, causing a further efficiency increase. The first effect i s well known; the second effect has only recently been recognized and is the focus of this paper. The noise formulas exhibit the same spinni ng mode behaviour seen in previous analyses but with higher levels rad iated into the crossflow. Since the modes are no longer purely spinnin g, the term 'wobbling mode' has been coined to describe their behaviou r. This paper summarizes the earlier literature on this subject, explo res the radiation formulas in detail, and presents some calculations t hat illustrate the magnitude of the angular inflow effect. It is shown that the inflow angle influences modal efficiency only through changi ng the Mach number of the source relative to the observer.