Fly-O-Rama!

For decades, scientists had been attempting to do just that by testing models of insect wings in wind tunnels. But in 1984, biologist Charles Ellington of the University of Cambridge examined the measurements that had been amassed and found that the numbers didn't add up. No one had managed to explain even half of the lift that insects actually create. Since then, researchers have proposed a host of ways in which insects could be invisibly adding lift to their flight. Evaluating these theories has been impossible, though, because there are simply too many variables to consider: "No computer on the planet could tell us what the forces are," says Dickinson. He decided to take a different approach: "Let's just measure it."




Dickinson knew he couldn't measure the forces at work on a living insect in flight: Even the smallest man-made sensors couldn't fit on a fly's wing. So he decided to use a mechanical fly instead. The tricky part would be to make it experience the same forces a fly does. Air behaves very differently around an insect than around a large animal. To us, it's delicate and slippery. But on the scale of fruit flies, it's thick and sticky.




Working with Karl Gtz at the Max Planck Institute for Biological Cybernetics in Germany, Dickinson created his first robotic wing. He found that a 2-inch-wide wing flapping in sugar syrup would experience the same forces that a much smaller fruit fly wing does in the air. Dickinson and Gtz built the wing and designed a simple computer-controlled motor to make it flap back and forth. As the sensor-laden wing waved through the syrup, they flooded the tank with aluminum shavings. They then took video footage of the swirling shavings and compared it with the forces recorded by the sensors.




A fly, Dickinson discovered, uses a number of tricks to generate lift. One trick involves keeping its wings at steep angles. Instead of gliding smoothly over the wing, the upper stream of air forms a swirling vortex along the wing's leading edge. This vortex lowers the air pressure above the wing, providing an extra upward boost.




Any pilot can tell you that tilting your wings at a steep angle is a risky strategy. The more steeply an airplane climbs, the harder it is for the stream of air traveling over the top of a wing to stay attached to the wing's edge. When the stream pulls away altogether, the plane loses its lift and stalls. But flies have an advantage over planes: They don't have to hold their wings in a fixed position. A fly flaps its wings back and forth so quickly that the vortex at the wing's leading edge doesn't have time to detach before the wings finish a stroke. At the end of each stroke, the fly rotates its wings so that it can flap them in the reverse direction. This creates a new vortex, while the old one slips off harmlessly-without causing a stall.