The superior aerobatic ability of the bat in flight is due to the wing segmentation provided by the skeletal frame, the flexibility of the membrane segments, and the very fine controls provided by the wing musculature. To date, the best analytical theory of animal flight is the vortex theory first introduced by Ellington in 1978 and further developed by Rayner in 1979. According to vortex theory, bats fly by generating volumes of circulating air (called vortices, singular vortex) that create pressure differences on different parts of the bat’s wing. The resulting fluid forces push the animal in the direction it wants to go, at the speed it wants to go. The bats’ flight motions are similar to the motions of a human swimmer doing the butterfly stroke.
During the downstroke, the wing is fully extended. It envelops the maximum possible volume of air and pushes it down, generating a region of high pressure beneath the wing and low pressure above the wing. The pressure differences add up to a resultant force that has two components: a thrust component that opposes the drag exerted on the animal by its motion through the air, and a lift component perpendicular to the drag that opposes the action of gravity on the mass of the animal (the animal’s weight). The numerical value of each component depends on the angle of attack. The steeper the angle, the higher the lift and lower the thrust. During steep-