Vortices behavior depending on the aspect ratio of an insect-like flapping wing in hover

Force measurements and digital particle image velocimetry (DPIV) were carried out to reveal the effects of the aspect ratio (AR) of an insect-like flapping wing. A total of seven aspect ratios around that of an insect wing including 1.5, 2, 3, 4, 5, 6, and 8 were taken into account for the same hovering configurations. Time-course forces showed that both lift and drag in the translational phase were maximized in the case of AR = 3, which is the closest ratio to that of a living insect. The chordwise crosssectional DPIV conclusively showed that the leading-edge vortex (LEV) on the wing of AR = 1.5 remained nearly unchanged in all cross sections. In other AR cases, however, the trailing-edge vortices (TEV) were clearly found with LEVs that lifted off the wing surfaces at the outboard cross sections. In each of these cases, the TEV interrupted the downwash, and the overall flows behind the wing became wakes similar to those found over a blunt body. The near-wake flow structures revealed that the tip vortex gradually entered the inner area from the wing tip as the AR increased. Circulations and downwash distributions showed a stretched LEV and asymmetrically developed tip and root vortices as the AR moved away from AR = 3. These results do not only indicate that the AR effects of a flapping wing are characteristics that are definitely distinctive from those of a typical aircraft, but also briefly imply that maintaining an LEV attachment by employing strong rotational accelerations is not the highest priority when attempting to achieve lift enhancements. Among the tested cases, the wing of AR = 3 had a balanced downwash flux as well as the best aerodynamic performance characteristics, including the maximum lift, reasonable efficiency, and a moderate pitching moment. This indirectly explains why the wings of living flyers adept at hovering have this AR, and it also suggests the appropriate AR for a flapping-type micro-air vehicle.