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The hover performance data of full-scale and model-scale coaxial rotors have been compared with CAMRAD II predictions having a free vortex wake analysis. Performance correlations of a coaxial rotor were made with a variation of key parameters including the rotor spacing and height. To understand aerodynamic behavior of the U.S. Army Aeroflightdynamics Directorate (AFDD) coaxial rotor operating over a range of Reynolds numbers from 36,000 to 180,000, the Reynolds number scaling effect was explored using an airfoil design code, MSES. It was found that the coaxial rotor spacing effect on hover performance was minimal for the rotor spacing larger than 20% of the rotor diameter. The measured performance data showed that more thrust was lost from the lower rotor of a coaxial than the upper rotor due to a larger rotor-to-rotor wake interference effect, and the lower rotor kept only an 81% of the single rotor OGE (out-of-ground effect) thrust whereas the upper rotor maintained a 90%. The lower rotor IGE (in-ground effect) thrust increased quickly by 26% as the rotor approached to the ground from the position of an 80% of the rotor diameter to 10%, and the corresponding IGE power increased by 17%. These thrust and power characteristics were well predicted. Overall, the performance prediction for the coaxial rotor was satisfactory when compared with the measured data. Nomenclature C P rotor power coefficient C T rotor thrust coefficient c chord c d drag coefficient c l lift coefficient D rotor diameter G rotor height above the ground K D scaling factor for the drag K L scaling factor for the lift M Mach number Re Reynolds number r radial distance from the hub center r c vortex core radius r c0 initial vortex core radius S rotor spacing between two rotors t w wake age in time v kinematic viscosity α angle of attack δ vturbulence viscosity parameter

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