Efficiency-optimal rigidity configuration of passive torsional flapping wings

In this work, a simple analytical tool for the efficient, optimal design of passive torsional flapping wings, which is usually done with the complex fluid–structure interaction method, is proposed. For thin-skin-wrapped spar–rib wing structures, the propulsion efficiency was modeled based on the momentum theorem, and the wing passive torsion was modeled based on the thin-wall-tube torsional vibration theorem. A forward efficiency-optimal design method for passive torsional flapping wings was thereby developed. The proposed method was validated through numerical simulation and a wind-tunnel experiment for a 0.85-m-span flapping-wing model with multiple wing torsional rigidity configurations. The group with the torsional rigidity optimized by the presented method displayed the best efficiency performance. The proposed method has potential applications for the concept design and performance evaluation of flapping-wing aircraft.