Quadcopter Noise Variation Due to Relative Rotor Phasing

This study examines the acoustics in hover for manned-size, multi-rotor, eVTOL aircraft in a quadcopter configuration. The rotors on such larger aircraft could have collective pitch control allowing them to operate at a fixed rotational speed. This paper seeks to explore how the relative phasing between the rotors affects the acoustics. Quadcopters with three different rotors are considered: a baseline solidity σ rotor with number of blades N = 2, a 3σ rotor with number of blades N = 2, and a 3σ rotor with number of blades N = 5. The simulations use the Rensselaer Multicopter Analysis Code (RMAC) for the aerodynamic loads on the blades, coupled to an acoustic propagation code for noise predictions at observers in the plane of the quadcopter and at elevations of 30 deg and 60 deg (below the quadcopter). The starting phase of rotors 2, 3, and 4 are varied relative to rotor 1, resulting in 216 total phasing cases for each rotor. From the simulation results in this study, the range of variation in tonal noise (due to thickness and loading) was between 21-30 dB in overall sound pressure level (OASPL). If there is phase locking between rotors, for 2-bladed rotors orthogonal phasing was generally observed to produce low average noise, while tip-to-tip phasing produced higher averaged noise, but this observation did not hold for 5-bladed rotors. For high-solidity 2-bladed rotors, the unweighted OASPL from tonal noise (thickness and loading) is greater than that from broadband noise, especially in-plane and at low elevation angles. But for high-solidity 5-bladed rotors, OASPL from broadband noise was observed to be higher than from tonal noise.