Lift Production by a Passively Flexible Rotating Wing

Experiments were performed on flexible and rigid rectangular aspect ratio 2 wings undergoing pure rotational motion. Chordwise wing flexibility was modeled by a 1 degree-of-freedom linkage of rigid panels, free to pivot about the midchord. For each wing, unsteady force measurements were acquired for three-quarter-span Reynolds numbers of 10,000, 15,000, and 25,000 and leading-edge angles of attack from 0 to 70deg. Dye flow visualization was used to establish flow structure similarity over the Reynolds number range. Thin-airfoil theory for finite, low-aspect-ratio wings was compared to the experimental lift measurements. Results show that the lift generated by the flexible wing rotating in a quiescent flow increased with instantaneous geometric angle of attack and wing camber, even at angles of incidence above static stall. Passive deformation of the posterior wing section played an important role in lift generation during wake encounters by increasing the camber of the wing and geometric angle of attack, thus mitigating the loss in lift in the presence of downwash.