The Effects of Rotor Shaft Spacing and Rotor Rotation Direction on Aerodynamic Interference of Mars Multicopter Rotors

This study investigates how rotor shaft spacing (d/D) and rotor rotation direction affect the aerodynamic performance of overlapping Mars multicopter rotors using three-dimensional unsteady Navier–Stokes simulations. The rotors operate in a Mars-relevant regime at Re = 10,000 and tip Mach numbers of 0.21 and 0.77. As d/D decreases, the performance of the lower rotor deteriorates due to the wake of the upper rotor, leading to a reduction in the overall figure of merit (FM). For counter-rotating rotors, however, the lower rotor maintains its FM at small d/D when the target thrust coefficient is high (C_T,ave = 0.04) because of swirl recovery from the upper-rotor wake. This swirl recovery effect becomes stronger as the thrust coefficient increases, which enhances the FM difference between counter- and co-rotation. By contrast, at large d/D, co-rotation is slightly more efficient because it can recover swirl between neighboring rotors. Finally, the tip Mach number has little influence on the trends of rotor performance with respect to d/D, although compressibility effects reduce FM by about 10% at C_T,ave = 0.04. These results highlight that rotor shaft spacing, rotation direction, and thrust coefficient must be considered together when designing efficient rotor layouts for Mars multicopters.