Alcids-like flapping wing for exploring the role of wing folding in underwater locomotion.

Alcids showcase excellent aerial-aquatic locomotion abilities benefitting from the morphing wing. In the aerial environment, alcids fully extend their wings, while underwater, they partly fold the wings. In this article, we mimicked the wing morphology of the alcids family to explore the hydrodynamic advantage of the folded configuration in the aquatic environment and four three-dimensional-printed wings including any combination of with feather or without feather, and extended or folded were created. The steady-state experiment reveals that the extended wings of alcids double the lift, counteracting gravity, while folding the wings halves the drag and reduces unnecessary lift, with configurations maintaining a similar lift-to-drag ratio. We investigated the alcid wing kinematics and designed an electronic wing mechanism with flapping and pitching motions to mimic the wing motion of the alcid. Quasi-steady estimation confirmed the presence of an active upstroke during diving in alcids. The experimental results validate the estimation, showing good agreement in force trends and average forward force. Additionally, the flapping wing experiment also demonstrates that pitch motion improves thrust generation, reduces lateral vibrations and increases mechanical efficiency. Under identical kinematics, extended models performed best, but alcids may prefer folded wings underwater for their highest thrust at the same Strouhal number and lower power needs.