Effects of asymmetric inflow on near-field propeller noise

The aerodynamic and subsequent acoustic effects of nonaxial inflow on wide-chord, advanced high-speed propellers are studied by means of a lifting surface theory. Starting from the flow equations for a perturbed, axially subsonic flow expressions are derived for the pressure and velocity field of a propeller. By using a representation of Green's function in separated, cylindrical coordinates the boundary condition at the hub is naturally incorporated, Application of the boundary condition at the blade surfaces yields an integral equation for the unknown pressure jump distribution over the blades that is solved numerically, The method is applied to study the near-field acoustic effects of propellers in uniform upwash and downwash at low and high speeds. Typical examples show that, for moderate speed, substantial reductions of the side line peak level of the order of 5 dB are possible for the proper amount of downwash, At high speed, however (flight Mach number 0.75, helical tip speed supersonic), downwash does not yield any significant sound reduction.