Easy flyer.
Color-coded vorticity fields from a bat during slow flight in a wind tunnel. Red indicates strong wake vortices; blue indicates weak vortices.

Bats Best Birds at Slow Flight

Birds get all the credit for aerial virtuosity. But bats have some fancy tricks as well, especially at low speeds. Researchers have found a possible reason why: Bats cultivate a unique pattern of turbulence behind their wings. The findings could one day be used to design new flying machines, such as unmanned micro-air vehicles.

At fast speeds, bats and birds fly in much the same way. But at slower speeds, they take different approaches. The reason is anatomical. Birds can separate their feathers on the upstroke to minimize drag and maximize lift. But bats have an elastic membrane for their skin, so they do another trick to keep aloft: They flick their wings backwards and almost upside down.

Turbulence impacts performance at slow speeds, too. In previous studies, biologist Anders Hedenstrom of Lund University in Sweden and colleagues had measured vortices produced by a thrush nightingale and a robin and found their wings create a single vortex, or swirl of turbulent air, as they fly. Next, they decided to assess the wake in bats. The researchers put two nectar-feeding bats into a low-turbulence wind tunnel. As the researchers report in tomorrow's Science, the aerodynamic wakes for bats are much more complicated than previous studies suggested. In contrast to birds, bats form one vortex behind each wing, a "totally unexpected" find, says Hedenstrom.

What accounts for the difference? Hedenstrom guesses it could be because nectar-feeding bats don't have tails, which connect vortices in birds. Isolated wake loops might enhance maneuverability, he says, because the wings are more aerodynamically independent.

The results provide interesting and complex information about the individual vortex structures, a phenomenon never witnessed before, says zoologist Ulla Lindhe Norberg of Göteborg University in Sweden. The research also has broader implications for flight, says biologist Bret Tobalske of the University of Portland in Oregon. It appears to indicate that the bat wing--with lift produced during the upstroke thanks to a flexible membrane--is a more efficient design for slow flight compared with a feathered wing, he says, which could be of interest to developers of miniature autonomous flying vehicles.

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