Experimental Characterization of Phase-Explicit Noise of Tandem-Rotor in Edgewise Flight

This study experimentally investigates the far-field noise on a tandem-rotor assembly in edgewise flight, of which limited knowledge is available in existing literature. A system to control the relative phase between the propellers is presented and evaluated. The relative phase between propellers is varied to reveal highly phase-dependent noise profiles. A unique microphone setup, comprising an array in, and three arrays below the propeller plane of rotation was utilized to characterize the noise emissions from the assembly. The tonal noise at the first three harmonics of the blade passing frequency was found to vary by up to 25 dB depending on the relative phase between propellers. Furthermore, the noise is highly directive possibly due to the mutual interaction between the front and the rear propeller. The acoustic and aeroacoustic interaction is intensified when the axial separation between propellers is reduced. The present work will shed further insights on the nature of multi-rotor noise and the potential of noise mitigation using relative phase control.