Flow-Field and Force Measurements on a Cycloidal Rotor Blade in Forward Flight

The paper investigates the unsteady forces and flowfield of a cycloidal rotor blade undergoing forward flight motion through water tunnel experiments. A particle image velocimetry (PIV) system is used in conjunction with an instrumented blade to measure both the two-dimensional flow velocity around the blade and the fluid dynamic forces. The flow-field studies reveal the formation and shedding of strong leading-edge vortices in both the frontal and rear halves of the circular blade trajectory, which plays a key role in generating lift as observed from the blade force measurements. Increasing forward speed diminishes the size and strength of these leading-edge vortices due to the reduction in angle of attack, which reflects in the blade forces. With pitch kinematics symmetric between frontal and rear halves of the cycle the blade produced significantly higher forces in the rear half compared to the frontal half, which was attributed to the dynamic virtual camber and the differences in relative velocity. The thrust vector was observed to be highly sensitive to both pitch phase offset and spin direction at high advance ratios and required a phase angle around 40-degree for positive propulsive force and lift. At very high advance ratios the blade extracts power from the flow over a large region in the frontal half.