Directional noise reduction via rotor phase-shifting for multirotor aircraft

Abstract

This study investigates rotor phase synchronization as a strategy for directional noise reduction in multirotor vertical take-off and landing (VTOL) aircraft employing collective pitch control. The application of this approach enables the redirection of acoustic emissions away from noise-sensitive regions without degrading aerodynamic performance or requiring alterations to the flight trajectory. A dedicated simulation framework integrates the mid-fidelity aerodynamic solver UPM with the aeroacoustic prediction model APSIM, developed at the German Aerospace Center (DLR), Institute of Aerodynamics and Flow Technology. To reproduce representative flight conditions, a trimming procedure determines the required control inputs and vehicle attitude. Such simulation methodology was validated by reproducing available experimental data for hovering fixed-pitch propellers. To identify optimal rotor phasing for noise redirection in full-scale quadrotor and hexarotor configurations under trimmed flight conditions, two distinct optimization strategies are employed. In forward flight, optimization is conducted using the complete high-fidelity simulation environment. For hover conditions, an analytical aeroacoustic model based on compact source theory provides a computationally efficient alternative that reduces the demands placed on the optimizer. Acoustic pressure evaluations on a ground plane beneath the vehicle confirm that optimized rotor phasing successfully redirects noise away from designated regions.