Ducted Rotor Noise Analysis

This paper provides a description of the important noise sources for low-Mach number ducted propellers. It presents an analysis of various noise source strengths using generic acoustic models from the literature. The paper shows that for low-Mach number applications, unsteady noise sources are of major importance. The duct expansion ratio is shown to reduce tonal noise from steady loading because as the expansion ratio increases, the rotor thrust is reduced. Haystacking noise, created by multiple cuts of the same eddy by subsequent rotor blades, is analysed using an analytical approach. The time and length scales within the ducted propeller are shown to significantly affect the intensity of the noise created by this interaction. The relationship between rotor thrust, duct geometry, turbulence distortion and noise is found to be an important research gap that needs to be addressed by future research programs.