ROANOKE TIMES
Copyright (c) 1996, Roanoke Times
DATE: Thursday, December 19, 1996 TAG: 9612190036
SECTION: NATIONAL/INTERNATIONAL PAGE: A-3 EDITION: METRO
DATELINE: NEW YORK
SOURCE: Associated Press
SCIENTISTS PUT TINY LEASHES on hawkmoths and placed them in a wind tunnel. The wind contained thin streams of smoke to show how the air flowed over their wings. The researchers took 1,000 pictures a second with a videocamera and finally discovered how they did it.
Researchers who put tiny leashes on moths and flapped oversized mechanical wings in the name of science have apparently solved the mystery of how insects can fly.
The puzzle has been how those tiny flapping wings can get the creatures off the ground.
In today's issue of the journal Nature, Cambridge University zoologist Charles Ellington and his colleagues came up with an answer: Insects create a whirling cylinder of air above their wings that provides lift.
The finding is ``as close as we've come to explaining it,'' said Ellington, who has studied insect flight for 25 years.
To figure it out, Ellington's team studied hawkmoths with a wingspan of about 4 inches. Hawkmoths are big enough and beat their wings slowly enough that the researchers had a good chance of seeing what was going on.
In one set of experiments, hawkmoths were snared in a loop of thread attached to a stiff wire to keep them in the breeze from a wind tunnel. The wind contained thin streams of smoke to show how the air flowed over the wings. The researchers took 1,000 pictures a second with a videocamera.
The telltale evidence appeared above and just behind the front edge of the wing when the moth flapped down.
Rather than just flowing over the top, the layer of air curled up into a cylinder, like a window shade wrapping around its roller. That cylinder maintained low pressure that pulled the top of the wing upward, providing enough lift for flying.
Most scientists had thought that such a ``leading-edge vortex'' was responsible for insect flight, but prior experiments had either failed to uncover one or found it was unexpectedly small, Ellington said.
The next step was to show how insects produce this air cylinder. To do that, Ellington and colleagues built a computer-controlled mechanical moth with a 3-foot wingspan, using elastic cloth stretched over a framework of brass tubes.
It turned out that the answer would be familiar to anybody who has experimented with a paper airplane.
If you tilt the wings so that the back edge is lower enough than the front edge, you get great lift for a moment. Then your plane stalls and drops like a rock. The problem is that the rolled-up cylinder of air quickly grows until it breaks free of the wing.
Ellington and colleagues found that insects use the same strategy, but with a trick that prolongs the lift. The cylinder of air continuously slips toward the wingtip, which apparently keeps it from growing too big.
The results should apply to insects in general, not just hawkmoths, Ellington said.
Ellington's work is ``the best explanation for one particular problem, getting sufficient lift,'' said Steven Vogel, a Duke University zoology professor who studies the aerodynamics of living things. ``There are other mysteries, such as how insects can maneuver and cope with winds.''
Some military jets also get help from a leading-edge vortex. But airplanes generally get their lift from the flow of air over the wing: Air flows faster above the wing than below, creating lower air pressure above the wing. That pressure difference supports the wing.
Propellers work in much the same way to pull an airplane along.
LENGTH: Medium: 70 linesby CNB