6 March 2008

Nature teaching humans: Bat flight could aid robotic aircraft

One of the secrets behind robotic flying machines comes from honeybees and hummingbirds that can hover for minutes at a time while staying aloft in a swirl of vortices.

Now a larger, heavier creature is joining the cast of characters teaching researchers how to hover-a bat.

In a follow-up study of bat aerodynamics, researchers were able to measure the velocity field immediately above the flapping wings of a small, nectar-eating bat as it fed freely from a feeder in a low-turbulence wind tunnel, said Professor Geoffrey Spedding of the Department of Aerospace and Mechanical Engineering at the University of Southern California.

"Thanks to a very reliable behavior pattern where bats learned to feed at a thin, sugar-filled tube in the wind tunnel, using the same flight path to get there every time, and the construction of side flaps on the feeder tube, we could make observations with bright laser flashes right at mid-wing without harming the bats," Spedding said. "Before this, we had no direct evidence of how the air moved over the wing itself in these small vertebrates."

The strong vortices associated with the unsteady aerodynamics of bat flight at slow speeds. The vortices cause the surrounding air to rotate rapidly around them, and this motion around the leading-edge vortex on top of the wing increases the lift force on it. Just like fixed-wing planes, the bat also leaves tip vortices in its wake; but the overall flow is further modified by the start vortices created at the beginning of the downstroke.

The researchers' findings challenge quasi-steady state aerodynamic theory, which suggests slow-flying vertebrates should not be able to generate enough lift to stay above ground, Spedding said.

Using digital particle image velocimetry, researchers found Pallas' long-tongued bat, increased its lift by as much as 40% using a giant and apparently stable, re-circulating zone, known as a leading-edge vortex (LEV), which completely changed the effective airfoil shape.

"The air flow passing over the LEV of a flapping wing left an amazingly smooth and ordered laminar disturbance at the trailing edge of the wing, and the LEV itself accounted for at least a 40% increment in lift," Spedding said. The LEV makes a strong lift force, but it may be equally important that the smooth flow behind it may be associated with low, or at least not increased, drag.

"The sharp leading edge of the bat wing generates the LEV," Spedding said, "while the bat's ability to actively change its wing shape and wing curvatures may contribute to control and stability in the leading-edge vortex."

Spedding and his colleagues believe observations of LEVs in active, unrestricted bat flight have important implications for overall aerodynamic theory and for the design of miniature robotic flight vehicles, which have been undergoing dramatic modifications in recent years.

"There's much to be learned from bat flight about unsteady flows and forces on small bodies," Spedding said. "We have suspected for a while that insects weren't the only creatures affected by highly unsteady viscous air flows, but now we know that larger animals adapted for slow and hovering flight, such as these nectar-feeding bats, can-and perhaps must-use LEVs to enhance flight performance."