Performance of a Mach-Scale Coaxial Counter-Rotating Rotor in Hover

A Mach-scale rotor system, 2.03 m in diameter, was built and hover tested in three configurations: two-bladed single rotor, four-bladed single rotor, and two-bladed coaxial counter-rotating rotor. The blades were untwisted with a VR-12 airfoil profile and a constant chord of 76.2 mm with a 3.8 mm trailing-edge tab. The hubs were rigid and had a vertical spacing of 13.8% rotor radius. Individual rotor steady and vibratory hub loads as well as lower-rotor pushrod loads were measured for several blade loadings up to 0.095. Mean loads were used to analyze rotor performance with an analytical momentum theory model as well as to validate an in-house, free-vortex wake model. Statistical analysis of the measured data revealed clear trends with a known confidence level. Because of mutual interference, the upper and lower rotors of the coaxial configuration consumed 18 and 49% more induced power than that of an isolated two-bladed rotor. The coaxial counter-rotating configuration was found to consume 6% less induced power than an isolated four-bladed single rotor of equal solidity. While torque-balanced, the upper rotor was found to produce 54% of the total system thrust regardless of blade loading. Rotor performance was not affected with an unbalanced torque of up to 5%. The free-vortex wake model was used to gain insight into the flow physics responsible for the interference effects by exploring the radial inflow and thrust distributions.