A design principle for selecting the stiffness of a wing rotational flexure hinge on bio-inspired flapping wing robots is presented. In this work, a systematic approach of selecting rotational stiffness values such that the primary mode of resonance occurs at harmonics of the wing stroke frequency is proposed. Using the quasi-steady aerodynamic model as a basis, simulations were performed to assess the effects of wing hinge stiffness on the rotational dynamics, while also evaluating effects on the mean lift coefficient. Based on the results from simulation, an operating mode for the optimal wing hinge stiffness is proposed. As verification, test wings incorporating a flexible rotational hinge were fabricated from laser machined composites and tested at a range of flapping frequencies. Wing kinematics were then extracted from high speed video and the resulting lift generation was evaluated. Experimental data from wing-hinge stiffnesses designed such that the primary mode of rotational resonance occurs near twice the flapping frequency showed near optimal cycle-averaged lift at targeted frequencies and low sensitivity to wing parameter variation. As a result, for a given set of vehicle parameters, the proposed principle serves as design guideline from which an initial rotational hinge stiffness can be selected and iterated to achieve optimal lift.
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