PASADENA, Calif. –How does a fly fly and why should we care? To the first, says Michael Dickinson, a professor of bioengineering at the California Institute of Technology, the short answer is different from what we have thought, and he and his colleagues used a dynamically-scaled flapping robot (aka Robofly), a free flight arena (aka Fly-O-Rama), and a 3D, infrared visual flight simulator (Fly-O-Vision) to prove it.
And we should care, says Dickinson, because the simple motion of a flying fly links a series of fundamental and complex processes within both the physical and biological sciences. Studying a fly may eventually lead to a model that will provide insight into the behavior and robustness of complex systems in general, and, for roboticists, may help them in the design of flying robots that mimic nature.
In a paper entitled "The Aerodynamics of Free Flight Maneuvers in Drosophila," Steven Fry of the University of Zurich, Rosalyn Sayaman, a Caltech research assistant, and Dickinson show how tiny insects use their wings to generate enough torque to overcome inertia, and not--as conventional wisdom has held--friction. The paper will appear in the April 18 issue of the journal Science.
Flies and other dipterans (insects within the family that includes houseflies, hoverflies, and fruit flies), are capable of making rapid 90-degree turns, called saccades, at "extraordinary" speeds, says Dickinson, less than 50-thousandths of a second. That's faster, he says, "than a human eye can blink." To make the turn, a fly must generate enough torque, or twisting force, to offset two forces working against it--the inertia of its own body and the viscous friction of air.
Until now, it's always been assumed that viscosity, a resistance to flow, is the enemy for small critters, while inertia is the bane of larger animals like birds. But the theory has never been tested.
To study the aerodynamics of active flight maneuvers, the researchers employed infrared, three-dimensional, high-speed video (the Fly-O-Vision) to capture the fruit fly, Drosophila melanogaster, performing saccades in free flight. The animals were released in a large, enclosed arena (the Fly-O-Rama), and lured toward a vertical cylinder laced with a drop of vinegar. As the flies approach the cylinder, it looms within their field of view, triggering a rapid turn that helps the fly avoid a collision.
Many flies performed saccades within the intersecting fields of view of the three cameras, which allowed the researchers to film the turn, measure the wing and body position throughout the maneuver, and calculate the velocity of its path.
The improved resolution of the 3D video showed that, despite its small size and slow speed (relative to other animals), the fly performed a banked turn, similar to those observed in larger fly species, first accelerating, then slowing as it changed heading, then accelerating again at the end of the turn. This suggests that the time and velocity of the small fly are dominated by body inertia and not friction.
To see if the measured patterns of wing motion were sufficient to explain the saccades, the researchers played the sequences through a dynamically scaled robotic model (you guessed it, Robofly) to measure the aerodynamic forces as they vary by time. They found that the time and torque they calculated based on the fly's body morphology and body motion from the video matched "amazingly well," says Dickinson, with the calculations derived from the wing motion of the robot. These results, he notes, further support the notion that even in small insects the torques created by the wings act primarily to overcome inertia and not friction.
Although these experiments were performed on tiny fruit flies, says Dickinson, the results impact nearly all insects, because the importance of inertia over friction increases with the size of the animal. The results also provide a basis for future research on the neural and mechanical basis of insect flight, and, for roboticists, may offer insights for the design of biomimetic flying devices. It may also yield a little respect for the common fly. As Rosalyn Sayaman puts it on her web page, "I now love flies. I used to just shoo and swat. Now, I can't even swat anymore."
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