THE EFFECTS OF WING ROTATION ON UNSTEADY AERODYNAMIC PERFORMANCE AT LOW REYNOLDS-NUMBERS

The downstroke-to-upstroke transition of many insects is characterized by rapid wing rotation. The aerodynamic consequences of these rapid c hanges in angle of attack have been investigated using a mechanical mo del dynamically scaled to the Reynolds number appropriate for the flig ht of small insects such as Drosophila. Several kinematic parameters o f the wing flip were examined, including the speed and axis of rotatio n, as well as the duration and angle of attack during the wing stroke preceding rotation. Alteration of these kinematic parameters altered f orce generation during the subsequent stroke in a variety of ways. 1. When the rotational axis was close to the trailing edge, the model win g could capture vorticity generated during rotation and greatly increa se aerodynamic performance. This vortex capture was most clearly manif ested by the generation of lift at an angle of attack of 0 degrees. Li ft at a 0 degrees angle of attack was also generated following rotatio n about the leading edge, but only if the downstroke angle was large e nough to generate a von Karman street. The lift may be due to an alter ation in the effective angle of attack caused by the intervortex strea m in the downstroke wake. 2. The maximum lift attained (over all angle s of attack) was substantially elevated if the wing translated backwar ds through a wake generated by the previous stroke. Transient lift coe fficient values of nearly 4 were obtained when the wing translated bac k through a von Karman street generated at a 76.5 degrees angle of att ack. This effect might also be explained by the influence of the inter -vortex stream, which contributes a small component to fluid velocity in the direction of translation. 3. The growth of lift with angle of a ttack was significantly elevated following a 7.5 chord stroke with a 7 6.5 degrees angle of attack, although it was relatively constant under all other kinematic conditions. 4. The results also indicate the disc repancies between transient and time-averaged measures of performance that arise when unsteady mechanisms are responsible for force generati on. Although the influence of wing rotation was strong during the firs t few chords of translation, averaging the performance over as little as 6.5 chords of motion greatly attenuated the effects of rotation. 5. Together, these modeling results suggest that the unsteady mechanisms generated by simple wing flips could provide an important source for the production of aerodynamic forces in insect flight. Furthermore, th e extreme sensitivity to small variations in almost all kinematic para meters could provide a foundation for understanding the aerodynamic me chanisms underlying active flight control.