Flying fish and diving birds often navigate the interface between water and air in their flight, but few studies have actually looked at the effects of this transition on lift. In this work, researchers measured forces on a small, fixed wing as it egresses from water into air at a constant velocity.
The tests showed that exit velocity had a large effect on lift generation. At low speeds, an exiting wing experienced a strong, positive lift spike as soon as the leading edge broke the surface. But that lift changed to strongly negative as the wing continued out of the water. At higher speeds, the wings had no lift reversal but also reached lower peak lift coefficients. The team studied the effects of angle of attack and starting depth as well, concluding that any vehicles intended to navigate the water-air transition will need robust control systems prepared to deal with fast-changing forces. (Image credit: fish – J. Cobb, wing – W. Weisler et al.; research credit: W. Weisler et al.)