Aerodynamic Interactions of Counter-Rotating Coaxial Rotors Hovering in Ground Effect

With the renewed interest in counter-rotating coaxial rotor (CCR) vehicles, a comprehensive understanding of its performance under out-of-ground effect and in-ground effect (IGE) operating conditions is imperative. The current work reports on an experimental study of a modular three-bladed CCR system during IGE hovering conditions. Thrust and torque produced by the upper and lower rotors were individually measured for two different axial spacings between the rotors with varying rotor heights from the ground. Both fixed-collective (untrimmed) and constant thrust, torque-matched (trimmed) experiments were performed to understand the interdependent effects of rotor-to-rotor interactions and ground effect on the performance of the CCR and its individual rotors. The CCR performance increased monotonically with decreasing rotor height with performance qualitatively similar for the two rotor spacing cases. At the same time, individual rotor performance variation with rotor height showed nonmonotonic behavior due to complementary or opposing effects of the rotor-to-rotor interactions to the ground effect. In general, the lower rotor in CCR experienced the most relative improvement in performance during IGE operations and was also observed to degrade the upper rotor performance.