Can Dragons Teach Us to Fly
Scientists are learning the secrets of aerodynamics from miniature masters
Aerospace engineer Marvin Luttges witnessed an aerodynamic miracle in Colorado ten years ago, and it changed his life. It also helped open up a new frontier in aircraft design for the U.S. Air Force and Navy. The miracle? Ten years ago Marvin Luttges watched several dragonflies catch mosquitoes in midair.
Dragonflies have been nailing prey on the wing for centuries, but Luttges spotted the potential value of understanding that feat. Dragonflies can hover without visible effort, fly backward with ease, make about-face turns within the length of their bodies, speed as fast as 35 miles an hour and then stop on a dime. One South African species does somersaults. In other words, these insects can easily outmaneuver the finest human-designed aircraft. Perhaps, thought Luttges, they can show us better ways to fly. The researcher has been trying to find those ways ever since. In the process, he has uncovered some amazing facts about the capabilities of the tiny creatures.
"I've been enamored of dragonflies since I was a boy," says Luttges. They existed long before dinosaurs and have survived long since—not a bad record. The ancient dragonflies were the largest insects ever to have lived. With wingspans more than 2 feet across, they darted through jungles of giant ferns some 300 million years ago. Today, though miniaturized, the insects have changed little in terms of their wing structure and aerodynamic prowess.
Luttges threw himself into pursuing the dragonfly's aerodynamic secrets, directing the work of more than 30 students at the University of Colorado and collaborating with three other professors. In 1983, the Air Force began funding some of his studies, and, three years later, the Navy began to support work on the dragonfly's flight control mechanisms. Though Luttges had high hopes for dragonfly research, the early studies clearly astounded him.
His first goal was to determine the lift that a dragonfly could generate. But before he could do that, he needed to capture the agile creatures. He discovered that a resting dragonfly takes off at a 45-degree angle when startled, a strategy that gives the insect its fastest getaway. So catching the creatures turned out to be relatively easy: Position a butterfly net in front of the insect; scare it, and it shoots right into the net.
Mark Kliss, one of Luttges' former graduate students, remembers spending hours in the field trying to snag dragonflies. On one occasion, a fisherman, watching him flap a net again and again along the water's edge, asked, "Son, didn't you ever get interested in bicycles, women or cars?"
When Kliss or another student brought in dragonflies, Luttges refrigerated the insects for a half hour to slow them down enough to handle in the lab. Using a minute instrument that detects small forces, he measured the lift generated by the wings of several local species. (About 450 dragonfly species are native to the United States.) Luttges discovered that one commonly called the widow (Libellula luctuosa), which weighs only one-seventh as much as a dime, generates three times the lift for its weight of our most efficient airplanes. To check his finding, he tied on tiny weights—two to two-and-a-half times the weight of the dragonfly itself. The insect lifted these with ease.
How could dragonflies do this? Luttges photographed them flying in a wind tunnel. Using a fogger to create nontoxic smoke, he and his students tracked the air currents created by the wings.
"The photographs revealed that dragonflies twist their wings on the down-stroke," explains Kliss. This spin creates tiny whirlwinds—engineers call them "unsteady airflows"—on the top surface of the wings. This turbulence moves air fast over the upper surface, lowering air pressure there and giving the dragonfly phenomenal lift. Modern airplanes depend mainly on steady airflows, but dragonflies tap the power of the whirlwind.
Luttges and his team moved on to study how the dragonfly's muscles and nerves tame these unsteady airflows. Kliss discovered that the insect's wing motions follow regular patterns without much variation. For example, Kliss detected what he calls "a kind of program or 'tape' for hovering flight." This program is already wired in, via the dragonfly's genes. To hover, the insect taps into its program and doesn't have to monitor, and adjust to, every breeze and eddy. Could such a system be applied to mechanical flight?
"It's a promising idea," says Kliss. "Planes may not need to be designed with complex feedback and updating systems to take advantage of unsteady airflows."
Luttges sees unsteady aerodynamics—the study of unsteady airflows—as "the new frontier in aviation, one of the most challenging fields for the next several decades." He guesses that "planes of the future may never bend or flex their wings as does a dragonfly, but they may be able to elicit and use unsteady airflows by manipulating flaplike mechanisms on their wings." NASA is already investigating this possibility at its research center in Virginia. If planes had more lift, explains Luttges, they could take off more easily, turn faster and touch down on tiny landing fields. The space program might also benefit. Today's spacecraft are "chubby guys that fly like rocks," laments Luttges. But if they could be redesigned to extract the high lift of unsteady airflows, they might fly more like airplanes, perhaps even landing at commercial airports.
How did dragonflies develop such aerodynamic wizardry? Entomologist Sidney Dunkle explains it this way: Dragonflies existed millions of years before birds. When birds arrived on the scene, dragonflies, and other insects as well, were forced to evolve mechanisms to evade the new winged predators. Some insects hid or developed poisons; others scurried about only at night. But for dragonflies, the answer was to outmaneuver the birds, "to fly better and faster," says Dunkle.
Dunkle manages the International Odonata Research Institute, a branch of the International Dragonfly Society, in Gainesville, Florida. The nonprofit organization focuses its efforts on the Odonata, the insect order that includes both dragonflies and damselflies. Generally smaller than dragonflies, damselflies usually fold their wings above their bodies when at rest. Dragonflies stretch their wings out flat while perching. Many scientists, as well as the International Dragonfly Society, use the word dragonfly when referring to both the dragonfly and the damselfly.
The myriad species (5,000 known worldwide) are most varied and numerous in the tropics. From the equator, they fan out north and south, finally disappearing at the tree line and leaving the air to birds. "Dragonflies are the bird-watcher's 'mini-birds,' " says Dunkle. "You can get closer to dragonflies, see more behaviors like territorial defense and mating and egg-laying, and see them more often."
Seeing dragonflies mate is a memorable experience. A male dragonfly catches a willing female in flight with his legs and curves his abdomen around, hooking it to the back of her head. The pair can continue to fly in this position.
The male then transfers sperm from the tip of his abdomen to a pouch in a more accessible spot. The female swings her long abdomen around to the sperm pouch, until the pair's bodies form a rough wheel. Depending on the species, the two stay in this position for a few seconds to more than an hour.
The pair's eggs hatch into homely underwater creatures "unique at both ends," says Dunkle. A dragonfly larva's lower lip—a big, armlike projection—can dart out in a hundredth of a second to grab food. And at the other end of the larva's body, its rectum holds its gills. When a predator attacks, the gills shoot out a jet of water with such force that the whole body jerks forward several inches.
To transform into a winged dragonfly, the larva climbs out of the water, usually at night. Then it gulps air until its ballooning body splits the outer skin. The soft, moist adult "rises up out of the larval skin like a pale ghost," says Dunkle. In perhaps an hour and a half, its body hardens. The dragonfly takes its first flight before dawn, performing immediately the miracles that today's most sophisticated human aviators can only envy.
Botanist Mariette Nowak directs Milwaukee's Wehr Nature Center.