The buzz: Wings flip, air whirls, bugs lift

Science News, June 19, 1999 by P. Weiss

Insect flight has long puzzled scientists. In conventional wind-tunnel studies, insect wings generate so little upward force, or lift, that researchers have wondered how most bugs stay aloft. Now, a novel study of the forces on robotic wings may give the first complete picture of insect wing action--albeit a broad-brush portrait--the experimenters say.

As pitchers spin baseballs to make them curve, so flying bugs rotate their wings to gain lift, report Michael H. Dickinson and Sanjay P. Sane of the University of California, Berkeley and Fritz-Olaf Lehmann of the University of Wurzburg am Hubland in Germany. Insects also obtain upward boost in each thrust, the study shows, by recapturing energy from the wake of the prior stroke.

Moreover, the new measurements, reported in the June 18 SCIENCE, also confirm 1996 experiments showing that insects acquire lift by tilting their wings sharply into the airflow (SN: 12/21&28/96, p. 390).

"With these findings, we basically have the palette of mechanisms [for creating lift] that are available" to insects, Dickinson asserts. "It's like a unified theory of insect flight." Nonetheless, because insects exhibit so much variety in their wing-motion patterns, "there is an enormous amount of work remaining to find out how they use these mechanisms," he cautions.

Most insects flap their wings forward and backward, not up and down. The wings move through the air like flattened hands sweeping to and fro. The thumb-side edge would be tilted upward into the oncoming flow, and between strokes, the wings flip, as if the hands were to change from palm down to palm up or vice versa. The insect changes the precise timing of the flips to control flight because those semicircular flips create surges of force on the wings, the new findings indicate.

"This is important stuff," comments Adrian L.R. Thomas of Oxford University in England, a researcher on the 1996 study, which used robotic wings and intact moths. "We suggested [then] that rotational mechanisms were not important," he says. "The whole system is a lot more complicated than we thought."

In general, an airborne object that is turning--be it a ball or a wing--drags a thin layer of air along with its rotation. The speed of the air layer peaks where the rotation adds to the forward motion and dips on the opposing surface, creating a pressure difference and therefore a force. Such forces can make a baseball arE deviously or push a wing up or down, Dickinson says.

A decade ago, the Berkeley researcher noticed that fruit flies adjust wing-flip timing relative to overall stroke timing, but he didn't understand why. The new experiments suggest that the timing change alters the force and direction of the push on the wings, giving the animals exquisite maneuverability, he claims.

Besides satisfying scientific curiosity, the new findings should aid efforts to build tiny flying robots for military purposes, scientists say.