Data from: Flight in ground effect dramatically reduces aerodynamic costs in bats

Most flying animals, from insects to seabirds, perform flights close to
ground or water when taking off or landing, drinking, and feeding or when
traveling near water surfaces. When flying close to a surface within
approximately one wingspan, the surface acts as an aerodynamic mirror,
interrupting the downwash, resulting in increased pressure underneath the
wing and suppression of wingtip vortex development. This aerodynamic
interaction lowers the energy added to the air by the animal, reducing the
cost of flying. Modeling suggests that flapping wings in ground effect can
affect the expected power savings compared to gliding flight, either
positively or negatively, depending on the wing motion. Although
aerodynamic theory predicts substantial power reductions when animals fly
in ground effect, quantitative measurements of savings are lacking. Here,
we show, through wake-based power measurements, that Daubenton’s bats
utilize 29% less aerodynamic power when flying in compared to out of
ground effect, which is twice the predicted savings. Contrary to
theoretical predictions we find no variation in savings with distance
above ground when in ground effect. Given alterations in kinematics with
ground proximity, we hypothesize that modulation of wing kinematics raises
the achievable benefit from ground effect relative to current model
predictions. The savings from ground effect are comparable to formation
flight but are not limited to large bird species. Instead, ground effect
is experienced by most flying animals and may have facilitated the
evolution of powered animal flight.