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A new actuation mechanism using the twisted string concept to trigger the snap-through of a bistable buckled beam to produce an effective on/off bistable actuator is proposed. The twisted string concept combined with a pin utilizes actuation moment to actuate a bistable beam. The required actuation loads are analytically formulated using the Euler–Bernoulli beam model and solved with the proposed solution algorithm. The actuation mechanism is fabricated to meet the 24.3 N mm actuation requirements. A prototype of the actuator was built, and its performance was evaluated. In a unidirectional actuation, an actuator response time of 104 ms was achieved. The overall response time of the actuator is affected by the length of the string. The twisted string mechanism was also placed in an antagonistic configuration to enable bidirectional actuation. The shape of the input voltage signal also affected the bidirectional performance of the actuator. The actuator produced an actuation bandwidth of 2 and 5 Hz with sine and square input voltages, respectively, while generating 10 mm output displacement.

Experimental investigations are made for the combined effects of aspect ratio (AR), slack (βS) and pitch angles on the aerodynamic characteristics of flexible flapping wings in hover. βS is introduced as a way to indirectly alter the flexibility of the wing. An optimum AR range of 3 to 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 βS increases beyond 2.5{degree sign}. The variation of βS 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 to 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.

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