Aerodynamic and aeroacoustic investigations of multi-copter rotors with leading edge serrations during forward flight

Inspired by the silent flight of owl wings, this paper adopts leading edge (LE) serrations for noise reduction of isolated multi-copter rotors. By choosing different serration parameters of height and wavelength, aerodynamic loading and noise emissions of these multi-copter rotors are measured and analyzed at the forward flight condition through a force balance and microphones in an anechoic wind tunnel. Rotors with add-on LE serrations generate slightly lower thrust than the Baseline (BL) rotor at the same rotation speed. In order to assess the noise reduction of different serration designs from the application perspective, a comparison of rotor noise emission is conducted at the same thrust. All serrated rotors considered in this study show lower tonal, broadband, and overall noise levels than the BL rotor. The noise reductions of all serrated rotors are observed to generally follow a scaling law with the Strouhal number in the mid-to-high frequency range. The height-to-wavelength ratio is a potential critical parameter that determines the coupled aerodynamic and aeroacoustic performance of the serrated rotors. The directivity of the rotor noise reduction is also shown through the data of a circular microphone array.