Hover Performance Correlation for Full-Scale and Model-Scale Coaxial Rotors

The hover performance data of full-scale and model-scale coaxial rotors have been explored and compared with the prediction by the comprehensive rotorcraft code, CAMRAD II using a free vortex wake analysis. Performance correlation for a coaxial rotor was made with variation of key parameters including the rotor spacing and the rotor height above the ground plane. To understand the aerodynamics behavior of a coaxial rotor operating over a range of Reynolds number from 36K to 180K, the Reynolds number scaling effect was explored using the airfoil design code, MSES. The rotor spacing effect on hover performance was minimal in a coaxial rotor for rotor spacing larger than 20% of the rotor diameter. The lower rotor of the coaxial rotor maintained only 81% of the single rotor OGE (out-of-ground effect) thrust while the upper rotor achieved 90% due to the coaxial rotor-to-rotor wake interference. The lower rotor IGE (in-ground effect) thrust increased quickly by 26% as the rotor ground height decreased from the position of 80% rotor diameter to 10%, while the corresponding IGE power increased by 17%. These thrust and power trends were well predicted. Overall, the performance prediction capability of the coaxial rotor was satisfactory compared with the measured data.