Bats on the wing

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This fruit bat was photographed in daylight in Sri Lanka, while moving from branch to branch.

Anton Croos

With a small body mounting giant wings built on the highly articulated bone structure of the mammalian arm, the bat’s herky-jerky motion is almost inevitable. But how does their flapping affect the airflow around the wings?

In an extremely complex manner, is the gist of a new study that, based on data about the actual flight of a fruit bat, built a detailed picture of the variations in air pressure that keep the bat aloft and moving forward.

The thousand-odd species of bats are the only true flying mammal, and their wings have the structure of a mammal’s arm and hand, which makes their flight — and the movement of the surrounding air — rather complicated.

The data on flight tracked the position of 50 points on a fruit bat wing during a climbing flight, says senior author Danesh Tafti, a professor of mechanical engineering at Virginia Tech. “The experimenters put markers on the wings, and used two or three cameras to capture the motion. We got the spatial coordinates of these points that tell us where they are in space.”

But the fluid dynamics equations Tafti was using were thirsty for detail. “You have to know the spatial location of each point on the wing at each time point in the simulation,” he says. Since each wingbeat requires tens of thousands of simulation steps, “we have to interpolate a lot of points.”

Because it’s nearly impossible to actually measure air flow and pressure near the wing, computer simulation is the only way to explore the fluid dynamics of bat flight. “Once we have the motion down, we can simulate the air flow generated by the wing and look in detail at how the wing moves the air, and how it generates lift and thrust.”

A bat in flight, species unknown

The Why Files, adapted from bat photo via Shutterstock

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The downstroke of a fruit bat’s wing, shown as the bat climbs toward the left. View is from outside the wingtip, toward the body. The low pressure in the two vortices are the biggest sources of lift and forward propulsion. t = time points in the downstroke.

Modified from original graphic courtesy Danesh Tafti, Virginia Tech¹.

With a little help from a tornado

As expected, the computer simulation shows a vortex—a mini-tornado—forming in front of the wing. With low pressure inside the vortex at the leading edge and higher pressure below it, the wing is forced up, creating lift. A similar imbalance, except running front-to-back, creates propulsion.

The leading edge vortex “is present in all flapping flight—for insects, birds and bats—and is the primary mechanism for generating force,” says Tafti. “The leading edge vortex usually forms on the downstroke, and because of how the wing is usually angled, it will produce lift vertically, and thrust in the direction of flight.”

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Gambian epauletted fruit bat (Epomophorus gambianus) flying at night in Ghana. Notice how the bone structure allows precise control of the outer wing — crucial to the precise maneuvering and hovering that bats are known for.

Credit: Photo from Shutterstock

To be effective, the leading edge vortex must stay close to the wing, Tafti says. “Some motions might produce a large leading edge vortex that quickly moves away from the wing, but that does not do much good.” By controlling the vortex’s shape and motion with the many joints that it inherited from its flightless mammalian ancestors, the bat does a good job of holding the leading edge vortex near the wing.

One surprising finding was this: the bat also generates lift on the upstroke, by creating an “airfoil” shape like what you see on an airplane wing. The result is a pressure difference that briefly creates lift.

The wing’s gyrations also enlarge the wing surface on the downstroke and shrink it on the upstroke. “Some big birds reduce the surface area on the upstroke for the simple reason that the upstroke is useless, energy wasting, it usually creates negative lift,” says Tafti. “By tucking in, the bat reduces the wing area and lowers the negative lift.”

Ultimately, the study is part of “bio-inspired design,” that aims to understand and replicate flapping flight, says Tafti, who notes that nature’s designs are usually more efficient than human ones. “We’d like to take the essentials and use them to design a vehicle that would fly like a bat.”