Impact of Phase Offset on the Flowfields Downstream of Synchronized Propellers

This paper reports on an experimental investigation of the interactions of two side-by-side propellers in hover ([Formula: see text]) using stereoscopic particle image velocimetry (SPIV). The propellers employed were DJI 9443 propellers rotating at 4860 RPM with a spacing of 5% of the propeller diameter. The SPIV measurement images were phase-locked with the synchronized propeller rotation. Three experimental scenarios were considered: a) a single propeller; b) dual, counter-rotating propellers rotating exactly in phase; and c) dual, counter-rotating propellers rotating 90 deg out of phase. The propeller tip vortices, full-field velocity measurements, and overall momentum flux measurements were analyzed for the three scenarios. The interactions of adjacent propellers caused a 55% faster decay in the peak vorticity of the tip vortices and significantly altered the tip vortex trajectories when compared to a single-propeller scenario. There was negligible change in the magnitude of the peak vorticity in tip vortex cores, the dissipation of the tip vortices, and axial velocity flowfields downstream of the propellers for the two differing phase-offset dual-propeller configurations. For both dual-propeller scenarios, the average momentum flow rate was 2.2% higher than for the single-propeller case, while the peak value was 5.2% higher.