To improve lift and thrust and hence flight performance of flapping-wing air vehicles (ornithopters), this study examines the mechanism of both the lift and thrust changing with the stiffness of flapping wings by applying the bionic principles of biological wings. Based on the principle that the stiffness of both variable-stiffness joint mechanisms and Redundantly Planar Rotational Parallel Mechanisms (RPRPM) can be modulated by adjusting the pretensions, a stiffness variation on the flapping direction is achieved by variable-stiffness joints connected to each other. The rotating stiffness modulation of the wings can be replicated by applying RPRPM. The stiffness variation models of the flapping joints and the rotating mechanism with pretensions are established, and then validated by experiments and simulation. The relationships between both the lift and the thrust of the wing and the stiffness of the wing are studied, and it is validated that the lift and the thrust of the wing can be improved by modulating the stiffness of the wing to match the flapping frequency.
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