Experiments on the optimization of MAV-scale cycloidal rotor characteristics towards improving their aerodynamic performance
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Studies of a MAV-scale cycloidal rotor (cyclocopter) were performed using performance and flow field measurements. Detailed parametric studies were conducted to determine the effects of rotational speed, the amplitude of the blade pitch angle, and the number of blades on cycloidal rotor performance. It was found that a 5-bladed cyclocopter system operating with 40 pitch produced the best power loading (thrust per unit power), with a figure of merit (based on the definition given in the paper) of 0.4. Thrust measurements suggested the presence of a sidewise force along with the vertical lift, similar to those found on rotating circular cylinders. This was confirmed using particle image velocimetry (PIV) measurements, which showed evidence of a wake skewness. The magnitudes of the measured vertical and sidewise forces were confirmed from the flow field measurements using a momentum balance technique. The thrust produced by the cyclocopter was found to increase until a geometric pitch of 40 was reached without showing any signs of blade stall. This behavior was explained using the PIV measurements, which showed the presence of high induced velocities in the rotor wake, which reduced angles of attack and delayed the occurrence of stall. Blade lift and drag were estimated from the flow field measurements, and were found to correlate well with the balance measurements. The PIV measurements clearly showed the rotational nature of the flow inside the cycloidal rotor, which is a significant source of energy loss, as well as the fact that the lower half of the rotor operates in the wake of the upper half. The induced power loss manifested as a large induced power factor in the modified momentum theory when compared to measurements. Copyright 2009.
