Fly Like a Fly

One of the old saws passed down by beginning engineers is that they can apply the laws of flight to prove that it is impossible for a bumblebee to fly. Bees are simply too bulky and heavy for their paltry wings to lift them off the ground, they say. Yet humble bees and other lowly insects do, indeed, go aloft, where they also perform a plentitude of apparently impossible feats of aerobatics. As anyone who has tried to bag a buzzing housefly with a swatter well knows, they can hover, soar, dive and turn in a blink--even land upside down.

Clearly, the engineers missed something--but what? Biologists analyzed insects' intricate muscles that control flight; biomechanics researchers tethered tiny flies and tried to measure the forces they generated; others photographed their beating wings with strobe lights. Yet the exact aerodynamic mechanisms of insect flight eluded investigators.

Now, a group of researchers headed by Michael Dickinson, an assistant professor of integrative biology at the University of California at Berkeley, may have cracked the riddle. "If you apply the theory of fixed-wing aircraft to insects, you do calculate they can't fly," says Dickinson. "You have to use something different."

To discover that different something the team built a scaled-up pair of wings that match those of the tiny fruit fly Drosophila melanogaster. These behemoth wings, dubbed the "robofly," are cut from Plexiglas and measure 25 centimeters long. Instead of flapping in air, which would be far too thin to approximate the atmospheric forces on the fly's tiny one-millimeter wings, robofly beats away in a tank of viscous mineral oil.

Six motors--three on each wing--move the wings back and forth and up and down, and rotate them, reproducing the exact motions of a fruit fly's wings. Sensors at the base measure the forces on the wings. "This device gives us the instantaneous force on the wing--that is, how much the forewings are working throughout the stroke," Dickinson says. "We can make it fly any way we want and measure the resulting forces."

Experiments with robofly identified three distinct and interacting techniques that insects employ to gain the lift needed to counter gravity. The researchers confirmed that a force called delayed stall, which was identified about five years ago, creates lift during the up and down portions of the wing stroke. But they also found evidence for two previously unknown forces that occur at the end of each half-stroke, when the direction of the wing reverses--forces they describe as "rotational circulation" and "wake capture."

Delayed stall appears to be the primary lifting mechanism. It occurs as the insect sweeps its wings forward at a high angle of attack, cutting through the air at a steeper angle than that of a typical airplane wing. A fixed-wing aircraft would stall--or lose lift-- at such a high angle, suffering increased drag and possibly a disastrous ending. But the insect moves its wings in a way that creates air currents known as a "leading edge vortex," which forms on the top surface of the wing and creates lift.

Dickinson's findings also put a new spin on insect flight: the phenomonen called rotational circulation. As the insect wing nears the end of its stroke, it rotates backward, creating backspin that lifts the insect just as it does when applied to a tennis ball or a baseball.

In addition, the maneuver dubbed wake capture allows insects to utilize energy that would normally be lost. As the wing moves through the air, it leaves trailing whirlpools, or vortices, of air behind it. If the insect rotates its wing before starting the return stroke, the wing is buoyed up on its own overtaking wake, just as a boat will rise on the following wave if its engine is shut down. "It's a nifty way of capturing energy that is lost," Dickinson says. "Insects can get lift from the wake even after the wing stops."

All these mechanisms have little in common with the usual way of understanding aerodynamics--that air flows faster over the curved upper surface of a wing, creating a vacuum that allows air pressure from below to force the wing upward. Indeed, wing curvature seems to play almost no role in insect flight; the wings are surprisingly rigid and flat, Dickinson notes.

"Steady-state aerodynamics of airplanes works well for birds, for the most part, but insects have always been a problem," Dickinson says. "If you treat a bird wing like an airplane wing and at any given time calculate the speed and lift, then sum it up over the entire stroke, it works fairly well to explain how the bird can stay aloft. With insect flight it fails miserably."

Yet insects were the first earthly organisms to take to the air. And because of their small size--they average only four to five millimeters long--they evolved ways of flying that are very different from that of birds. "You can't fly like a bird if you're the size of an insect," Dickinson observes.

According to Dickinson, different insects use these mechanisms to varying degrees. For example, the most acrobatic insect around, the hover fly, appears to use delayed stall very little, but makes great use of rotation circulation and wake capture. Butterflies, on the other hand, rely primarily on the up-and-down flapping motion of their wings and classic aerodynamics to glide.

The research team, which reported its findings in the June 18 issue of Science,believes their work provides "a unified theory of insect flight aerodynamics," Dickinson says. The next step? Use the technology in their primitive robofly and the principles they have discovered to create tiny flying machines with beating wings. Already, Dickinson is part of a group of engineers and scientists at Berkeley designing such robotic insects.

So look closely the next time something buzzes past your ear--it could be a robofly.