Rapidly pitching plates in decelerating motion near the ground.

Birds utilize rapid pitch-up motions for various purposes: perching birds employ this motion to slow down and come to a complete stop while hunting birds, like bald eagles, employ it to catch prey and swiftly fly away. Inspired by these observations, our research investigates how natural flyers achieve diverse flying objectives by rapidly pitching their wings during deceleration. To explore the impact of ground proximity on unsteady dynamics, we conducted experimental and analytical investigations, focusing on rapidly pitching plates during deceleration in close proximity to the ground. Initially, we executed simultaneous deceleration and pitch-up motion close to the ground. Experimental results demonstrate that the instantaneous lift increases as the pitching wing approaches the ground while the initial peak drag force remains relatively unchanged. Our analytical model conforms to this trend, predicting an increase in lift force as the wing approaches the ground, indicating enhanced added mass and circulatory lift force due to the ground effect. Next, we analyzed asynchronous motion cases, where rapid pitching motions were initiated at different stages of deceleration. The results reveal that when the wing pitch is synchronized with the start of deceleration, larger counter-rotating vortices form early in the maneuver. These vortices generate stronger dipole jets that orient backward in the later stages of the maneuver after impinging with the ground surface, facilitating hunting birds to accelerate after catching prey. Conversely, when the wing pitch is delayed, smaller vortices form, but their formation is postponed until late in the maneuver. This delayed vortex formation generates beneficial unsteady forces late in the maneuver that facilitate a smooth landing or perching. Thus, by strategically coordinating rapid pitch-up motion with deceleration, natural flyers achieve wide range of flying objectives