  Birds employ rapid pitch-up motions for different purposes: perching birds
use this motion to decelerate and come to a complete stop while hunting birds,
like bald eagles, employ it to catch prey and swiftly fly away. Motivated by
these observations, our study investigates how natural flyers accomplish
diverse flying objectives by rapidly pitching their wings during deceleration.
We conducted experimental and analytical investigations focusing on rapidly
pitching plates during deceleration in close proximity to the ground to explore
the impact of ground proximity on unsteady dynamics. Initially, we executed
simultaneous deceleration and pitch-up motion close to the ground. Experimental
results demonstrate that as the pitching wing approaches the ground, the
instantaneous lift increases while the initial peak drag force remains
relatively unchanged. Our analytical model confirms 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
examined 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, which hunting birds recover 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
facilitates a smooth landing or perching.
