Bat Flapping

["Richard W. Travsky" <rtravsky@uwyo.edu>]

http://www.nature.com/news/2008/080228/full/news.2008.633.html

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.
...