Investigation of Rotor–Airframe Interaction Noise Associated with Small-Scale Rotary-Wing Unmanned Aircraft Systems

In this study, acoustic measurements of a hover condition are taken on isolated rotor–airframe configurations representative of small-scale, rotary-wing unmanned aircraft systems (UAS). Each rotor–airframe configuration consists of two fixed-pitch blades powered by a brushless motor, with a simplified airframe geometry intended to represent a generic multicopter arm. In addition to acoustic measurements, computational fluid dynamics–based aeroacoustic predictions are implemented on a subset of the experimentally tested rotor–airframe configurations in an effort to better understand the noise content of the rotor–airframe systems. Favorable agreements are obtained between acoustic measurements and predictions, based on both time- and frequency-domain postprocessing techniques. Results indicate that close proximity of airframe surfaces results in the generation of considerable tonal acoustic content in the form of harmonics of the rotor blade passage frequency (BPF). Analysis of the acoustic prediction data shows that the presence of the airframe surfaces can generate noise levels either comparable to or greater than the rotor blade surfaces under certain rotor tip clearance conditions. Analysis of the on-surface Ffowcs Williams and Hawkings source terms provides insight as to the predicted physical noise-generating mechanisms on the rotor and airframe surfaces.