Aerodynamic characteristics of flexible wings with leading-edge veins in pitch motions

To demonstrate the effects of wing deformations on aerodynamic performances during the wing reversal, aerodynamic force/torque and flow vector-fields were measured. Wing models consisted of wing planes with various thicknesses and two leading-edge veins, which obstructed spanwise deformations (Case 1 as a rigid wing and Cases 2 to 4 as flexible wings). They also underwent three different pitching periods (t/TR,α=0.1,0.2, and 0.4) to determine the distinct changes in vortical structures and corresponding aerodynamic characteristics. Flexible wings generally showed a negative camber in the stroke motion, causing poor aerodynamic performance relative to a rigid wing. This was also related to the positive camber and camber change to the negative during the pitch motions. After the start of the stroke, the positive camber caused separated and weakened tip vortex (TV) structures, hindering leading-edge vortex (LEV) formations around the wingtip. Depending on the LEV shedding, the flexible wings generated less aerodynamic forces than the rigid wing in stroke motion. During the pitch motions, on the other hand, the dynamic cambers influenced rotational mechanisms such as the wing–wake interaction and rotational force, causing higher or less initial lift increment. As the t/TR,α increased, the amount of the lift augmentation decreased due to the weakened wing–wake interaction, instead showing a downwash. At t/TR,α=0.4, however, the higher initial lift peak was occurred due to the absence of distinct trailing-edge vortex (TEV) generation during the wing-reversal. This lack of the U_TEV2 led to the decline of the downwash. Thus, the delayed U_LEV1 dispersal and the U_TEV2 traces generated similar induced flows to the wing–wake interaction, resulting in higher lift augmentation. Furthermore, Case 2 achieved higher lift augmentations than the other cases in all pitching periods. The slight wing deformations not only reduced the distance between the vortices and the wing surface, but also caused the delayed vortex dispersals. These induced the stronger rapid flows toward the wing surface, causing the higher initial lift peaks. Case 2 also had the highest C¯L/C¯P,t at t/TR,α=0.1 than the other cases. These results suggest that the flexible wing with leading-edge veins can have higher aerodynamic efficiency in its specific chordwise flexibility range.