For aircraft of the future, some clues from the lowly dragonfly
If such pioneer investigators of the aerodynamics of flight as Leonardo da Vinci and Otto Lilienthal had used the humble dragonfly, rather than the bird, as their basic model, the history of aviation would have been far different - probably a lot smoother.
Recently, some students of flight have taken the flying capabilities of this prehistoric-looking insect seriously. And what they are learning could well influence the design of future aircraft.
People have long equated flight with birds - the root of the word aviation is aves, Latin for ''bird'' - and ignored insects, largely because birds seemed more dramatic and appealing. What self-respecting poet, for instance, has yearned to become a dragonfly to experience the thrill of flight and the joy of absolute freedom?
Yet anyone who has watched one of these large insects dart around the fringes of a lake on a hot summer day can hardly have missed the fact that they are superb flying machines. In fact, they are capable of aerobatic feats that few birds can match.
For instance, a dragonfly can hover with apparent effortlessness. It can fly backward and to the side, as well as forward. It moves with startling speed. And it is capable, researchers have found, of lifting 15 times its own weight.
For the last three years, a group of scientists at the University of Colorado have been studying the dragonfly because they realize it is a master of ''unsteady aerodynamics,'' one of the new frontiers in the field of flight.
Soaring birds and the aircraft designed from studying them rely on ''steady state'' aerodynamics, or the interplay of forces set in motion by the steady rush of air across a wing. The dragonfly, by contrast, uses its wings to stir up the air. It creates a controlled turbulence, which it then utilizes in its flight, explains Donald A. Kennedy, a professor of aerospace engineering at Colorado. With colleagues Martin Luttges, C. Y. Chow, and Peter Freymuth, Dr. Kennedy has conducted a study of the dragonfly for the US Air Force Office of Scientific Research.
The field of unsteady aerodynamics was defined about a decade ago when helicopter rotors began cracking thousands of hours before traditional calculations predicted was possible. Researchers soon realized that the rotors were creating turbulent forces similar to those the dragonfly uses, and they set out to discover how helicopter blade life could be extended.
''We decided to see if there might be some beneficial aspects to this,'' Kennedy explains. But turbulence is far more complex and difficult to analyze than smooth air flow. In trying to figure out how to attack this problem, Professor Luttges, a trained biologist, got the idea of studying a ''real practitioner of the art.''
The researchers singled out the dragonfly for two reasons, over and above its flying skill:
* First, the insect has been around for 250 million years and so represents a thoroughly tested design.
* Second, the dragonfly has a relatively simple method for controlling its flight. Its wings are paddles, which rotate, move up and down, and move forward and back. By contrast, the hummingbird, which is one of the few vertebrates able to copy the dragonfly's feats, changes its wing shape to fly as it does.
To monitor the dragonfly's abilities, the scientists catch one, give it a light anesthetic, and glue it at the thorax to a gauge at the center of a two-foot-square box of clear plastic. The box is filled with nontoxic theatrical smoke (which the scientists discovered while attending a campus Shakespeare festival). The smoke graphically demonstrates the air movement when the dragonfly starts using its wings. With high-speed cameras and stroboscopic lights, the researchers have been able to capture the pattern of air flow. After the experiment, the insect is set free.
''Basically, the leading wing creates large vortices and the trailing wing extracts most of the energy,'' Kennedy explains.
Readings on the gauge confirm that the dragonfly can generate stronger lifting forces than traditional, steady-state aerodynamics would indicate. Scientists are optimistic about finding future applications for their newfound knowledge.
In parallel work, the Colorado researchers have shown that the lift of a traditional wing can be enhanced by generating turbulence above it. This is accomplished by mounting a strip of metal along the leading edge running perpendicular to its surface. The strip can be moved up and down. Experiments have shown that, when oscillated at certain rates, lift of the wing is enhanced. The experimenters predict that a similar effect might be achieved by redirecting air from a jet engine to the wing and releasing it in pulsating blasts.
Another possibility the work has suggested is a robotic wing - one that would sense changes in air pressure and alter its lift accordingly. This might enable aircraft to fly through violent gusts.
But these are still theories. Practical applications must await a more thorough understanding of unsteady aerodynamics. ''There is still an awful lot we don't know about this,'' Kennedy notes.
So the dragonfly experiments will continue. So far they have dealt only with hovering. The insect's method of free flight and soaring remains to be analyzed.
Besides the Colorado group, about 60 researchers across the United States are studying unsteady aerodynamics, but no one else is using the dragonfly.