By all rights, flying insects from midges to monarch butterflies should never leave the ground.
From an engineering standpoint, the best their wings have to offer is a nice paint job; they lack the shape that gives lift to a gull's wing or to an Airbus.
Until now, scientists have turned to conventional principles of aerodynamics to try to explain bug flight, without much success. But a team of researchers using a wind tunnel, hawk moths, and a mechanical "flapper" appear to have solved the mystery.
The key lies in rolling cylinders of air, or vortexes, that are generated along the wings' front edges as they flap, according to a team led by Charles Ellington, a zoologist at the University of Cambridge in Britain. These tiny "leading-edge" vortexes spiral outward from wing root to tip and provide enough lift to keep the bug airborne.
The team, whose work appears in the current edition of the journal Nature, calculates that the lift is sufficient for the moth to carry a payload of half its own weight, if it were so inclined.
"In effect, they have discovered how a typical insect flies," according to Niell Alexander, a biology professor at Britain's University of Leeds. "Birds also generate more lift than conventional aerodynamics can explain," he says, noting that Dr. Ellington's work could point to potential solutions to that puzzle.
Ordinarily, a bird or plane gets its lift from the difference in air pressure above and below its wings as air moves past. A wing's rounded front edge and curved top slows air moving across it, while the wing's flat bottom allows air to pass freely.
Because the slower air atop the wing presents less pressure than the faster air underneath, the wing generates lift. This lift can be strengthened by tilting the wing upward.
But bug wings are thin and flat, and are anything but fixed. They beat at ferocious speeds. Dr. Alexander notes that insect wings' upstrokes and downstrokes alternate at up to hundreds of times a second. Meanwhile, the insects change their wings' angle of attack to flit from one spot to another.
So how do they fly? Researchers used bits of thread to tether hawk moths inside a wind tunnel. The moths' wingspan reaches nearly 4 inches, and the wings beat 26 times a second, giving the team a reasonable chance of recording air-flow patterns on film. Past studies of insect flight have tried to reconstruct three-dimensional flow patterns from a series of two-dimensional measurements, so Ellington and his colleagues set up a strobe light and a pair of 35-mm cameras for 3D stereophotography.
As the moths "flew" through the smoke blowing past them, the cameras caught the vortexes in action. But the detail wasn't fine enough to allow the researchers to determine the aerodynamic mechanism that kept the tiny whorls of air, which had appeared in other studies, intact.
So they built a mechanical model of the hawk moth. It boasted a wingspan of just over 3 feet. The beat rate was 1/100th that of the real moth.
When they mounted robo-moth in the wind tunnel and ran their experiments, they saw more clearly that as the wing moved downward, a vortex appeared along the leading edge that grew in size as it moved toward the wing tip and as the downstroke deepened. As the vortexes reached the wing tip, they then lifted away from the wing's surface and flowed back.
A phenomenon known as "delayed stall" allowed the vortexes to remain stable during the wings' downbeat to generate the lift. In effect, the vortexes were providing the lower air pressure that otherwise is established by curving the upper portion of a wing.