Wings with stiffness and stroke variations enabling efficient flight and swimming for flapping-wing robots.

Flapping-wing propulsion demonstrates high efficiency and maneuverability, commonly observed in natural species for either aerial flight or aquatic swimming. To chase prey and evade predators, biological flapping wings are specialized for a single medium, posing significant challenges in achieving effective and efficient amphibious flapping. Existing aerial-aquatic robots are primarily driven by artificial propulsion and lack bionic flapping-wing mechanisms. Meanwhile, current flapping-wing robots are operated only in a single medium, without amphibious propulsion capabilities. Inspired by the flapping principles of two different species, birds and manta rays, we design a bimodal flapping wing for both aerial and aquatic propulsion. The wing can switch between aerial and aquatic modes to adapt to the distinct requirements of air and water environments. In the aerial mode, metal tubular spars replicate the high stiffness of bird's hollow bones, while limited rotation strokes of joints emulate asymmetric feather deformation. In the aquatic mode, elastic plates mimic the flexible cartilages of manta rays, and free rotation strokes of joints reproduce the symmetric fin deformation. Simplified analysis, dynamics tests, and robot motion tests have demonstrated that the wing's aerial mode enhances lift generation during aerial flapping, while its aquatic mode decreases power consumption and improves thrust efficiency in aquatic flapping. Furthermore, by carrying auxiliary structures, the flapping-wing robot equipped with bimodal wings shows the potential to perform aerial-aquatic amphibious applications. Our work enhances the amphibious propulsion adaptability of large-scale flapping-wing robots and proposes a feasible engineering design.