Vtol directional noise reduction via rotor phase synchronization: a numerical trim procedure and flight mechanics investigation

This thesis presents a study on noise reduction strategies for multirotor vertical takeoff and landing (VTOL) vehicles, conducted at the German Aerospace Center (DLR) in Braunschweig, under the Helicopter Department of the Institute of Aerodynamics and Flow Technology. The research specifically focuses on the implementation and evaluation of a numerical framework for analyzing rotor phase synchronization and its impact on noise directivity. This study required the development and validation of a simulation toolchain for variable-pitch propellers and semi-rigid rotors with swashplate control, enabling detailed flight mechanics, aerodynamic, and aeroacoustic analyses. Using this toolchain, the study investigates rotor synchronization strategies and their effectiveness in reducing noise in different realistic operational scenarios. Significant achievements have been made, including the numerical demonstration of Sound Pressure Level (SPL) reductions of up to 10 dB in a ground target area 25 m below the aircraft. This represents an optimal result compared to the 70 dB baseline of an unsynchronized manned-sized hexa-rotor configuration. This thesis specifically focuses on innovative tools for trimming multirotor configurations with synchronized rotors, relying on the coupling between a flight mechanic script and the Unsteady free-wake Panel Method (UPM) from DLR. This has proven to be a robust and reliable methodology for variable-pitch and semi-articulated rotors trim, unlocking new possibilities for multirotor VTOL flight mechanics and performance analysis. In this research, a series of parametric studies are presented. The results highlight the potential of phase synchronization to improve noise directivity control in urban and military applications, achieving significant acoustic benefits with minimal hardware modifications and aerodynamic losses. This work represents a significant step towards the development of quieter VTOL systems.