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Bats have a clever aerodynamic trick to make flying easier, researchers
have found: the sharp edge at the front of their wings cuts through the
air in such a way as to create a vortex on top of the wing, producing up
to 40% of the lift needed to stay aloft.
Hedenstrm and his team showed that the down-stroke of a bat's wing moves
forward as well as down, and is tilted at a sharp angle just like that of
an insect in flight. This produces a powerful lifting vortex. The swirling
air closely follows the surface of the wing during the down-stroke: that's
a good thing, because if the vortex moved away from the wing, a
slow-flying bat would stall, dropping out of the sky. To keep the vortex
close to the wing takes some delicate wing movement. The bats control
their wing curvature in a very subtle way, says Hedenstrm.