Quantification of the Aerodynamic Interference for Counter-Rotating Coaxial Rotors In-Ground Effect

The performance of a coaxial rotor hovering in-ground effect (IGE) is compared against the out-of-ground effect (OGE) condition to quantify the rotor-ground interaction and against an isolated rotor IGE at equivalent blade loading to quantify the rotor-rotor interaction. It is observed that the performance of the coaxial rotor improves when it hovers IGE. However, the rotor-rotor and rotor-ground interactions compete, which affects the performance of the coaxial rotor. This paper aims to quantitatively measure the aerodynamic interactions of the CCR in IGE by developing a theoretical framework based on momentum theory. This formulation introduced induced power factors to understand the aerodynamic interactions of a CCR operating IGE. The performance measurements show that the rotor-ground interaction in the CCR system behaved similarly to a single rotor operating in IGE conditions. The interactional effects significantly influence the individual rotors as the rotor-rotor interactions get modified in the presence of ground. At the extreme ground height, the rotor-rotor interaction on the upper rotor decreased, while that for the lower rotor increased when the ground height was reduced. However, with an increase in the inter-rotor spacing, the performance of the lower rotor degrades while that of the upper rotor improves. At the (z/R)l = 0.4, the performance of the upper rotor in IGE improves by ∼ 8%, whereas the lower rotor performance degrades by 8% when compared to their respective OGE conditions.