On the wings of a dragonfly; the way a dragonfly flies may inspire new aircraft designs - unsteady aerodynamics

Science News, August 10, 1985 by Ivars Peterson

The dragonfly is an elegant creature of light and air. Seen as a hovering thread, a living flash of light or a luminous windmill in the dusk, this ancient flier flits with an enviable ease and grace.

Remarkably nimble, this insect can hover at will, then instantly dart sideways or backward. It can fly as fast as 30 miles per hour and lift up to 15 times its own weight. It's no wonder that aeronautics engineers are starting to probe the secrets of a dragonfly's aerodynamic agility.

Recent studies show that dragonflies use "unsteady aerodynamics," a mode of flying radically different from the smooth flight of airplanes and soaring or gliding birds. Dragonfly wings churn up the air to create a whirling airflow that a dragonfly controls and uses to provide lift. In contrast, airplanes rely on the smooth flow of air over the upper and lower surfaces of their wings. For these flying machines, turbulence can be deadly.

"Unsteady aerodynamics never got much attention in the past," says Mohamed Gad-el-Hak of Flow Research Co. in Kent, Wash. "It was just too complicated."

Nevertheless, with the help of special wind tunnels and water tanks, computers, scale models, stroboscopic photography and unwitting insect subjects, several groups of investigators, largely funded by the U.S. Air Force, are now looking into unsteady flows. This research may eventually lead to a new generation of "supermaneuverable" figher aircraft or perhaps a more efficient design for turbines used in power plants and elsewhere.

One approach is to see how insects do it. Dragonflies are good subjects because they are relatively simple flying creatures. They each have two pairs of fairly rigid, transparent wings that are always extended. The front and rear pairs are not latched together, but operate independently. In addition, the insects don't change the shape of their bodies or wings in order to take off, glide or fly.

"There are other organisms that we suspect use the same kind of mechanisms," says Marvin Luttges of the University of Colorado in Boulder, "but they're not really so simple as the dragonfly." A hummingbird's wings, for instance, change shape continually during a stroke, while feathers in different locations pop up or stay down at various times.

"The nice thing about dragonflies is that you don't hae to worry about feathers, about changes in wing shape during a stroke," says Luttges. Moreover, the fact that dragonflies have been around for more than 200 million years points to a safe and successful aerodynamic design.

But dragonflies still have to be captured in the wild, something that in certain areas of the world can be done only during the summer, while they are in their flying stage. At the University of Colorado, that task fell to graduate student Chris Somps, who netted dozens of dragonflies in marshes around Boulder. Somps also developed a technique for tethering a temporarily anesthetized insect by gluing its hard body to a small wire. The wire was part of a force balance that allowed the researchers to measure vertical lifting forces generated by the dragonfly as it beat its wings from 25 to 30 times a second.

Using carefully synchronized stroboscopic flash photography and streams of nontoxic smoke, Luttges and Somps observed dragonfly flight. Reporting in the June 14 SCIENCE, they confirmed that rapid changes in a dragonfly wing's speed and angle of attack do indeed generate vortices and unsteady flows. The vortex patterns created by the wings were surprisingly repeatable.

The researchers found that when a dragonfly's front pair of wings generates a small vortex of rapidly whirling air, the back pair, which may be down while the front pair is up or vice versa, captures the extra energy in that vortex's spin. This gives the air flowing over the top of a dragonfly's rear wing a much higher speed than the airflow along the wing's lower surface, and the wing generates more lift.

Applying this principle to aerodynamic design, however, is tricky. "The effect has to be simple and predictable," says Luttges. "No pilot wants an erratic response from an aircraft.

"Until no more than 10 years ago, people saw these kinds of flows as terrible," he says. Turbulence was and often still is viewed as a villain, causing aircraft crashes or robbing turbines of their efficiency. In the cases of helicopter rotors, blades sometimes fail because each blade continually runs into the turbulent wake of the preceding blade, causing vibrations that eventually weaken the metal.

"If we can get to a point where we can control [this unsteady flow]," he notes, "helicopters could be designed to take advantage of it." The same ideas could apply to aircraft designed to land within short distances and the whirling blades of a turbine. One company designing a new racing car is already looking at ways of generating unsteady flows on demand as a way of directing lift downward so that their car tracks better at high speeds.

To get the information needed for new aerodynamic designs, research groups at the University of Colorado, Illinois Institute of Technology in Chicago, Boeing Co. in Seattle and elsewhere are doing more than just studying insects. Computer simulations and wind tunnel tests also play an important role.