Swedish researchers set up honey water feeding stations in a massive wind tunnel and used fog, lasers and high speed cameras to track exactly how the bats flew.
They found that when the bats flapped their wings downward they created tiny air cyclones above the wings called a leading vortex which pulls the animal upward and allows them to hover in place without expending nearly as much energy as simply flapping their wings.
Without this trick they would not have the strength to hover in place in order to feed as the vortex provides as much as 40 percent of the lift force which keeps the bats in the air.
The bats used the thumbs and fingers embedded in the skin membrane of their flexible wings much like flaps on an airplane to alter the curve of the wing and create the lift force necessary to hover.
“To be able to generate these vortexes they need this exquisite control of their wing surface and it’s a really delicate thing to control the stability of this vortex,” said lead author Anders Hedenström of Lund University.
“It’s like a separation bubble that develops behind the leading edge (vortex) on top of the wing,” he said in a telephone interview.
“It remains attached to the wing surface throughout the wing stroke and that’s very important because if it had broken off in the down stroke it would have caused turbulence and no extra lift.”
While this trick has been observed in insects, and has even been used on fighter planes, this is the first time that researchers have observed a bat or a bird using this complex wing stroke, Hedenström said.
“We believe this is actively controlled by the bats while insects, which have much thicker wings, can’t control it like the bats do but they can develop these vortexes because they beat their wings very quickly,” he said.
The complex way that the bats control their wings to create these vortexes can be used to improve upon designs of micro airplanes which are used for various types of surveillance.
“It’s an important piece of information to know how to generate the control of the wing shape,” he said.
“This shows we still have lots of engineering design inspiration to recover from nature.”
Hedenström’s team studied the Pallas long-tongued bat, which weighs about 12 grams, has a body about nine centimeters in lengths and a wing span of 24 centimeters and lives in Central and South America.
He will next test the Mexican lesser long-nosed bat which is more than twice the size to see if it also is capable of creating these vortexes.
AFP’s Mira Oberman