Directional Noise Reduction Via Rotor Phase-Shifting For Multirotor Aircraft

Abstract

This paper presents a numerical study of rotor phase synchronization as a method for directional noise reduction in multirotor VTOL aircraft with collective pitch control. The implementation of such technology would allow to steer sound emission away from sensitive areas without affecting performance and avoiding the need to modify the flight path. A simulation toolchain was developed by integrating an unsteady aerodynamic solver with an aeroacoustic prediction model. A trimming procedure was implemented to replicate realistic flight conditions by determining the necessary control inputs and vehicle attitude. The entire simulation framework relies on UPM and APSIM solvers for aerodynamic and aeroacoustic modeling, both developed by the German Aerospace Center (DLR) Institute of Aerodynamics and Flow Technology. To find the optimal rotor phases to redirect noise around manned size quadrotor and hexarotor configurations in trimmed flight conditions, two different optimization procedures were employed. In forward flight, the optimization was applied directly to the complete simulation toolchain. In hover, instead, an analytical aeroacoustic model based on compact sources was developed as a faster means to simulate rotor noise, reducing the optimizer's computational effort. Acoustic pressure calculations at a ground plane below the vehicle demonstrated that optimized phase angles effectively redirected noise away from targeted areas, achieving up to 10 dB reductions in average sound pressure level.