Engineers designing miniature flying machines for the military should look to bats for inspiration, researchers say.
Bats are unique among flying animals, with wings made of highly jointed skeletons and elastic membranes. Now remarkable footage has revealed the extreme flexibility of those wings, which allow the bats to generate and manipulate lift in unusual ways.
Kenneth Breuer and Sharon Swartz at Brown University in Providence, US, used high-speed video cameras to record the 3D wing and body movements of flying lesser short-nosed fruit bats, Cynopterus brachyotis (see the video of the bat flying in a wind tunnel, 5.4 MB, mpeg format).
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The down stroke looked similar to that of birds, but with more sweeping and billowing, says Breuer, an engineer who studies animal flight mechanics. The upstroke was very different, however. 鈥淭he bat almost completely collapses its wings,鈥� he says.
Aerial footprint
To better understand how these differences affect flight performance, the team studied the patterns of air movement in the animal鈥檚 wake. They filled the flight cage with a mist of particles and tracked the particles鈥� movement and speed with lasers and another set of cameras (watch a video of the bat in the smoky wind tunnel, 1.8 MB, MB, mpeg format).
The resulting wake pattern is 鈥渞eally the footprint of the bat鈥�, says Swartz, a biologist who studies the mechanical performance of vertebrate skeletons. 鈥淚t鈥檚 a physical record of the how the aerodynamic forces are generated.鈥�
The stretchy skin on a bat鈥檚 wings interacts with air differently compared to the firmer wings of birds and insects, deforming in ways that give more lift at higher angles of attack 鈥� the angle at which the wing meets the air on its down stroke. This may allow bats to fly more efficiently and reduce the risk of stalling at low speeds.
Scaling down
The wings鈥� elasticity, combined with dozens of joints, also allows bats to generate unusual shapes and motions, such as folding the wings very close to the body to reduce drag. 鈥淚t鈥檚 pretty clear the bats are taking advantage of these stretchy, flexible wings,鈥� Swartz says.
Next, the researchers aim to study bats鈥� wakes using higher-resolution cameras provided by the US Air Force Office of Scientific Research. That information could help design tiny flying vehicles, such as those being developed for military reconnaissance.
鈥淭he knowledge of the aerodynamics in that size range is very limited,鈥� says Geoffrey Spedding, a mechanical engineer who studies tiny aircraft at the University of Southern California in Los Angeles, US. Standard wing shapes do not work well at small scales, he says. 鈥淗ere鈥檚 a system that works so it鈥檚 a good idea to take a look at it.鈥�
Journal Reference: Bioinspiration & Biomimetics (vol 1, p S10)