Aerodynamic Effects of Phase Offset Between Synchronized Propellers in Hover

Developments in aviation technology are creating new opportunities to use distributed electric propulsion (DEP) on novel aircraft concepts in support of urban air mobility (UAM). The propulsion system for DEP vehicles spread many small electric motors and propellers around the aircraft. The inevitable decrease of propeller spacings on DEP vehicles causes complex interactions that can be detrimental, with some researchers reporting thrust fluctuations, decreased performance, and large noise signatures. This study aims to explore the propeller-propeller aerodynamic interactions between two counter-rotating, synchronized propellers in close proximity. This study uniquely phase locks the propellers and quantifies the impact of phase control on overall performance under hover conditions. 2-D and 3-D PIV experiments were performed to characterize the flow generated by the interacting propellers at different relative phases. Variations in tip vortices, momentum fluxes, and streamwise velocities were explored between the phase offset cases. The interactions between the propellers causes significant alteration to the tip vortex trajectories and rate of tip vortex decay when compared to a single propeller control test. Larger momentum fluxes through the propellers and induced streamwise velocities were also observed for the dual propeller cases. However, our current PIV results show very minor aerodynamic differences between dual propeller cases at varying phase offsets. Because other researchers have reported potential noise benefits realized by controlling phase offset, the results of this study suggest that the benefits of phase control on the directivity patterns and overall sound pressure levels (OASPLs) can be realized with negligible sacrifice in aerodynamic performance.