Lichten Runner-Up Paper: Experimental and Computational Investigation of a UAV-Scale Cycloidal Rotor in Forward Flight

Abstract

This paper investigates the unsteady force production on a UAV scale cycloidal rotor blade undergoing forward flight motion via experiments in a water tunnel as well as an unsteady aerodynamic model and a computational fluid dynamics (CFD) model. While time-averaged forces can shed light on cycloidal rotor performance, it is important to be able to accurately measure and predict the time history of forces on the blade in order to understand the highly nonlinear and unsteady fluid dynamics on a cycloidal rotor blade resulting from high-amplitude blade pitch kinematics, unsteady flow curvature effects and complex inflow distribution. Towards this, the instantaneous radial and tangential forces on a cycloidal rotor blade were measured in a water tunnel using a custom-built test-rig over a range of Reynolds numbers (Re = 30,000 – 100,000) while varying the rotor rotational speed, flow speed, blade cyclic pitch amplitude and pitch offset. The advance ratio was varied from 0 to 0.44. An unsteady aerodynamic model of the cycloidal rotor was developed and systematically validated with instantaneous force data from the water tunnel experiments. Once validated, the analytical model is utilized to explain various trends observed during the experimental study. To further investigate the forward speed aerodynamics, a 2-D CFD solver tailored towards the unique kinematics of cycloidal rotor is utilized. It is observed that the lift generated by cycloidal rotor increases continuously up to ±45° pitch amplitude for a low Reynolds number (Re = 29,000). However, this monotonic increase in lift plateaus at a pitch amplitude of ±30° for high Reynolds number (Re = 87,000). Moreover, a significant asymmetry was observed in the forces generated between the frontal half and rear half of the cycloidal rotor because of the dynamic virtual camber effect. A cycloidal rotor needs a phase offset angle in the vicinity of 90° to produce both a positive lift and propulsive force in forward flight. Additionally, this propulsive force was observed to decrease with increasing advance ratio due to a reduced effective angle of attack at all points along the azimuth. Moreover, at higher advance ratios the rotor begins extracting power from the flow over a large region at the frontal half of the cycle.