Against all odds: how bugs take wing.
Conventional aerodynamics is concerned mainly with steady motion, such as a wing moving at constant velocity. The flapping motion of an insect's wing is highly unsteady - it keeps changing direction and speed, explained R. McNeill Alexander, biologist at the University of Leeds in Leeds, England. Upstrokes alternate with downstrokes, up to hundreds of times per second, and at each shift of stroke the wing rotates about its long axis, tilting to the appropriate angle for its new direction of motion.
Early attempts to explain insect flight assumed that, at each instant during the wing-beat cycle, the lift forces were the same as if the wing were moving steadily at a given speed, Alexander said. In most cases, this "quasi-steady" approach failed to account for the missing lift that lets insects take wing.
Researchers realized that some unconventional mechanism must be responsible for creating sufficient lift for insect flight. All mechanisms that were suggested involved the development of a lift-augmenting vortex - a cylinder of air made to rotate by a puff of air - at the leading edge of each wing. Leading-edge vortices are regions of low pressure and increased air circulation around the wing. According to Alexander, such vortices can be generated in two ways: by rotation of the wing about its long axis as it is tilted at the start of each stroke, and by translation of the wing during the stroke.
One of several suggested means to produce enhanced lift is a translation-based phenomenon called dynamic stall. "If a wing takes high angle of attack into the air, you get a lot of lift for a short period, but then it stalls badly," Ellington said. The proposed delayed stall effect could enable the vortex to be stronger, and thus the lift to be greater, shortly after the motion starts. However, while the stall is not immediate, it seemed that the wings had to travel too far during the flapping cycle for them not to stall out, he said.
The researchers decided to look in detail at the airflow around insect wings using dual-camera stereophotography. They chose Manduca sexta, a gray hawkmoth from Florida with a 10-centimeter wingspan. The researchers photographed streaks of smoke passing over the wings of the tethered moths as they flapped 26 times per second in a stream of moving air.
"As expected, we saw the leading-edge vortex," Ellington said, "but as the smoke flowed around the vortex, we saw it suddenly make a right-angle turn and flow out strongly toward the wing tip." The moths, however, were still too small to see exactly when the vortex was being created.
The team constructed an oversize mechanical model that simulated the moth's wing motion to show the fine details of the airflow. This "flapper" device had 10 times the length and breadth of the moth, and it operated at one-hundredth of the frequency. These adjustments were needed to satisfy the scaling criteria for generating airflow with the same Reynolds number - the ratio of inertial to viscous forces. The wings themselves were constructed of elastic cloth stretched over hollow brass tubes, through which smoke could be directly injected into the vortices.
Before designing the flapper gearbox, Ellington looked at robotic wrist joints. The most elaborate existing wrist design he found, however, had only three degrees of freedom operating through a single point. "We needed four degrees of freedom: the ability to flap up and down within a plane; the ability to extend forward; and the ability to alter the angle of attack at the wing base and at the wing tip."
Stereo photos of the flapper in action confirmed that air in the vortices does not simply revolve in circles but flows rapidly out to the wing tips in ever-widening helices that then extend back to form a circle around the puff of air generated by the downstroke. The air spirals out to the wing tip at a spanwise velocity comparable to the flapping velocity at the tip. This outward flow is thought to help stabilize the high-lift vortex, enabling it to retain its early strength.
The work by Ellington and his colleagues provides the first clear evidence that the delayed stall mechanism is the correct one. They have shown how an insect flies.
This kind of stable three-dimensional flow pattern is similar to the conical leading-edge vortices found on delta-shaped aircraft wings where the angled spanwise flow passively stabilizes the vortices, Ellington said. He would like to capitalize on this new unsteady effect in a similar manner, but he's not sure how. "The effect might be peculiar to that Reynolds-number regime, so it might best be applied to microelectro-mechanical systems. We're also looking at things like propellers that rotate in a nonsteady fashion."
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|Title Annotation:||construction of a mechanical moth to study insect flight|
|Date:||Mar 1, 1997|
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