Experimental and computational studies to understand the role of flow curvature effects on the aerodynamic performance of a MAV-scale cycloidal rotor in forward flight
This paper describes the systematic experimental and computational (2-D CFD) studies performed to obtain a fundamental understanding of the physics behind the lift and thrust production of a cycloidal rotor (cyclorotor) in forward flight for a unique blade pitching kinematics. The flow curvature effect (virtual camber and incidence due to the curvilinear flow) was identified to be the key factor affecting the lift, thrust and power of the cyclorotor in forward flight. The experimental study involved systematic testing of an MAV-scale cyclorotor in an open-jet wind tunnel using a custom built three-component balance by varying rotor chord/radius ratio and blade pitching axis location, since these two parameters have a strong impact on flow curvature effects. Because of the virtual camber/incidence effects and the differences in the aerodynamic velocities around the azimuth, the blades produce a small downward lift when they operate in the upper half of circular trajectory and a large upward lift in the lower half producing a net lift in the upward direction. The magnitude of this lift depends on the chord/radius ratio and the blade pitching axis location and the direction of lift depends on the sense of rotation. The positive thrust on the cyclorotor is produced when the blades operate in the rear half of the rotor, while they produce a small negative thrust as they operate in the frontal half. The lift per unit power of the rotor is increased with chord/radius ratio until a c=R of 0.67. Moving the pitching axis location closer to the leading edge also increased the lift producing efficiency of the cyclorotor. It was observed that the optimum chord/radius ratio for maximum thrust per unit power decreased with forward speed. A key conclusion was that the lift producing efficiency (lift per unit power) of the rotor (for a constant thrust) increased with forward speed while the thrust producing efficiency (for a constant lift) decreased with forward speed. This study also disproves the conventional argument that a cyclorotor needs two completely different pitching schedules for efficient hover and forward flight because it is clearly shown that a simple phase shifting of the hover kinematics could result in an efficient forward flight kinematics provided the cyclorotor has a high chord/radius ratio. 2012 AIAA.