Experimental Investigation of the Cycloidal-Rotor Concept for a Hovering Micro Air Vehicle

The viability of cycloidal rotor (cyclocopter) concept for developing hover-capable micro-air vehicles was analyzed through performance and flow field measurements. Detailed parametric studies were conducted to identify the dependence of the rotor performance on the amplitude of the blade pitch, blade airfoil profile, blade flexibility, and rotational speed. All experiments were conducted using a three-bladed, model scale cyclocopter that was tested up to the design rotational speed of 2,000 RPM. While higher pitch angles were found to increase the power loading (thrust/power) of the cyclocopter, the bending and torsional flexibility of the blades deteriorated the performance. Similarly, fixed camber proved to be detrimental to the rotor performance. Thrust measurements suggested the presence of a sidewise force (along with the vertical lift), similar to those found with lifting cylinders. Digital particle image velocimetry (DPIV) measurements made in the wake of the cyclocopter provided evidence of wake skewness, resulting in sideward force. The thrust produced by the cyclocopter was found to increase with a geometric pitch of 45° without showing any signs of stall. This behavior was explained through DPIV measurements that showed the presence of high induced velocities, resulting from the trailing tip vortices. These velocities are comparable to the rotor blade velocities, reducing substantially the aerodynamic angles of attack experienced by the rotor blades, thereby, preventing the occurrence of stall. DPIV measurements also identified several interesting flow features that include the presence of a leading edge vortex, similar to a dynamic stall vortex. The slipstream boundary obtained by following the path of the tip vortices was found to contract, as expected for a lifting rotor.