Directional noise reduction via rotor phase-shifting for multirotor aircraft

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.