Effect of Axial Offset, Index Angle, and RPM Differential on the Aerodynamics of Co-Axial Co-Rotating Propellers

Coaxial co-rotating (CCR) propeller systems provide structural simplicity, compactness, and high disk loading, making them attractive for an Electric Distributed Propulsion System (EDPS). However, aerodynamic interactions between the upper and lower propellers can lead to efficiency losses, and the effects of key design parameters on overall performance remain insufficiently understood. This study employs Reynolds Averaged Navier–Stokes (RANS)-based Computational Fluid Dynamics (CFD) simulations to examine the effects of axial offset distance, index angle, and differential rotational speeds on the aerodynamic performance of an 18-inch two-blade coaxial co-rotating propeller. Maximum thrust is typically obtained at an index angle of around 60°, while the maximum Figure of Merit (FoM) is achieved at 90°. Increasing the offset distance from 0.05R to 0.20R improves the FoM by approximately 17.3% and reduces its sensitivity to index angle. When different rotating speeds are applied, assigning the higher rpm to the lower propeller increases thrust by 9.4% and the FoM by roughly 9.2%. These results offer practical guidelines for enhancing aerodynamic performance of a CCR propeller in unmanned aerial vehicle and urban air mobility platforms.