Experiments on Rigid Wing Undergoing Hover-Capable Flapping Kinematics at Micro-Air-Vehicle-Scale Reynolds Numbers
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2015 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. The paper focuses on understanding the mechanism of force production on a hover-capable flapping-wing system by using a combination of direct force measurements and flowfield studies. The experiments were conducted in air at a Reynolds number of approximately 25,000 (based on mean chord and maximum tip speed), which is the typical operating regime of small flapping-wing micro air vehicles. The forces and moments were measured using a miniature six-component force transducer installed at the wing root. The wing was flapped in air and vacuum at the same frequency and wing kinematics, and the vacuum forces were subtracted from the total forces to obtain the pure aerodynamic forces. Flow visualization and particle image velocimetry were used to characterize the formation, strength, and structure of the leading-edge vortex on the flapping wing. A rapid increase in wing lift coefficient was associated with the growth of the leading-edge vortex and a progressive reduction in lift coefficient with the convection of the leading-edge vortex over the chord. The leading-edge vortex was observed to be stable for the spanwise locations closer to the root (i.e., the 25% span location) and burst for locations away from the root (50 and 75% span locations) just after the midstroke. High instantaneous lift coefficient values (Cl max =1.85) were measured during the translational phase, clearly showing the role of leading-edge vortex in augmenting lift. However, lift-to-drag ratios were always less than 1. Aerodynamic efficiency during the translational phase was quantified in terms of figure of merit, which improved with decreasing translational wing pitch angle. The maximum figure of merit value measured was 0.36 at 40 deg translational pitch angle.
