Aerodynamic characteristics of flexible flapping wings depending on aspect ratio and slack angle

Abstract

Experimental investigations are made for the combined effects of aspect ratio (AR), slack (β_S), and pitch angles on the aerodynamic characteristics of flexible flapping wings in hover. β_S is introduced as a way to indirectly alter the flexibility of the wing. An optimum AR range of 3–5 based on the lift coefficient is observed depending on the flexibility. For a constant AR, the intensity of the leading-edge vortex (LEV) with corresponding circulatory-based lift mitigates as β_S increases beyond 2.5°. The variation of β_S affects the magnitude of the shed trailing-edge vortices (TEVs) but the vorticity core is maintained. We found the shed TEVs to be the key vortical feature of twistable flexible wings in comparison with the rigid (untwisted) cases. More intriguingly, the negative wing twist played a significant role in sustaining the circulatory lift at the outboard section for even high AR cases. The primary LEV trace is found to be an indicator for the effective spanwise limit of the LEV. Although an increase in AR reduces the effective spanwise limit, it is found that wing flexibility further decreases the radial distance. Again, the study reveals that lift enhancement in the rigid wing requires a wider effective downwash area induced by the outward movement of the LEV traces to merge with the tip vortex. Contrarily, the flexible wing requires an elongated downwash area induced by the wing twist to enhance the aerodynamic performance.