Data from: The wake of hovering flight in bats

Hovering means stationary flight at zero net forward speed, which can be
achieved by animals through muscle powered flapping flight. Small bats
capable of hovering typically do so with a downstroke in an inclined
stroke plane, and with an aerodynamically active outer wing during the
upstroke. The magnitude and time history of aerodynamic forces should be
reflected by vorticity shed into the wake. We thus expect hovering bats to
generate a characteristic wake, but this has until now never been studied.
Here we trained nectar-feeding bats, Leptonycteris yerbabuenae, to hover
at a feeder and using time-resolved stereoscopic particle image
velocimetry in conjunction with high-speed kinematic analysis we show that
hovering nectar-feeding bats produce a series of bilateral stacked vortex
loops. Vortex visualizations suggest that the downstroke produces the
majority of the weight support, but that the upstroke contributes
positively to the lift production. However, the relative contributions
from downstroke and upstroke could not be determined on the basis of the
wake, because wake elements from down- and upstroke mix and interact. We
also use a modified actuator disc model to estimate lift force, power and
flap efficiency. Based on our quantitative wake-induced velocities, the
model accounts for weight support well (108%). Estimates of aerodynamic
efficiency suggest hovering flight is less efficient than forward flapping
flight, while the overall energy conversion efficiency (mechanical power
output/metabolic power) was estimated at 13%.