Experimental Study of a Hovering Coaxial Rotor with Highly Twisted Blades

The hover performance of a small scale coaxial rotor was measured in both single and coaxial configurations, in and out of ground effect and with various separation distances between the rotors. The performance of the single rotor, when in ground effect, strongly depends on the degree to which it is loaded. When configured as a coaxial rotor, with matching RPM and torque settings, the separation distance between the rotors appears to have little effect. However, proximity to the ground greatly affects the performance of each individual rotor. The flow fields around the single rotor and the coaxial rotor were measured with the rotors out of ground effect. When compared to the wake from a single rotor, the wake from the upper-coaxial rotor contracts to a much smaller radius. The tip vortex trailing from each coaxial rotor blade initially follows a statistically identifiable path, but as the wake ages the position of each vortex becomes more erratic. Detailed time-averaged velocity profiles at different axial positions are compared between the single and coaxial rotors.