January 20, 2002 | David F. Coppedge

Bats Exhibit Aerodynamic Superiority

They may look clumsy fluttering around in the twilight air, but “Flexible, highly articulated wings give bats more options for flight than birds: more lift, less drag, greater maneuverability.”  Thus reads the caption to a picture of a bat in flight on a Brown University press release.  Researchers at Brown U are studying the differences between bat wings and those of insects and birds.  They are finding that “bats have unique capabilities,” and that “a novel lift-generating mechanism may be at work in bats and point to the highly maneuverable mammals as a model for tiny flying machines.”  Those unique capabilities include highly articulated bones, over two dozen independent joints and flexible membranes.  By watching videotapes of bats flying in an aerosol mist, the scientists discerned a number of novel flight mechanisms:

Birds and insects can fold and rotate their wings during flight, but bats have many more options.  Their flexible skin can catch the air and generate lift or reduce drag in many different ways.  During straightforward flight, the wing is mostly extended for the down stroke, but the wing surface curves much more than a bird’s does – giving bats greater lift for less energy.  During the up stroke, the bats fold the wings much closer to their bodies than other flying animals, potentially reducing the drag they experience.  The wing’s extraordinary flexibility also allows the animals to make 180-degree turns in a distance of less than half a wingspan.

The researchers also considered how bat flight might have evolved.  Could bat wings have developed from gliding mammals, like flying squirrels?

[Sharon] Swartz, an associate professor in ecology and evolutionary biology at Brown University, and longtime collaborator with [Kenneth] Breuer, is particularly interested in how bats evolved their capabilities.  “The assumption has always been that bats evolved from some sort of flying squirrel-type animals,” says Swartz.  “Gliding has evolved in mammals seven times.  That tells us that it’s really easy for an animal with skin to evolve into a glider, but going from a square gliding wing to a long, skinny flapping wing has not happened seven times.  It might have happened once.  And now it doesn’t look like bats have any relationship to these gliding things.

The Air Force funded work was published in Bioinspiration and Biomimetics.

The design work was productive; the evolutionary speculation was useless.  Actually, it was useful for one thing: to show that bats did not evolve from gliders, contrary to “the assumption” that has “always been”.  How about fewer assumptions and more videotape?

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