Effects of flapping deviation on the hovering performance of tandem pitching-plunging foils

Dragonfly-like flapping wings and foils take an in-line arrangement and their performance is strongly affected by the wing–wing interaction. In this study of tandem pitching-plunging foils, the effects of flapping deviation on the hovering performance are studied numerically. A two-dimensional model is established, and the flapping deviation is represented by the non-dimensional deviation distance ΔL/c, the effects of which are investigated, together with those of the phase angle φ in plunging motion and the non-dimensional inter-foil spacing G/c. In the hovering status, our results support that the wing–wing interaction at ΔL/c = 0 is detrimental to the lifting capacity of tandem pitching-plunging foils. More importantly, we prove that a modification of ΔL/c in which the fore foil is below the hind foil during hovering can compensate for the loss of lift and may even provide a lift greater than that generated by a single foil. The mechanism of this compensation is dependent on the interaction between the wake vortices of the fore foil and the leading-edge vortex of the hind foil. Our results also indicate that the effects of ΔL/c and φ are coupled. The lift enhancement by a negative ΔL/c is maintained within the ranges φ = 45°–90° and G/c<2, beyond which an abrupt drop may occur. These findings suggest that the control of flapping deviation may enhance the performance of dragonfly-like micro air vehicles.