How do flies fly?

Not like birds and airplanes, says Dickinson.

Birds and airplanes stay airborne on wings whose shape and angle create lower pressure above the wing, which helps lift them.

Their flight is explained by a theory called "steady state aerodynamics."

But flies' wings are constantly flapping -- nearly 200 times a second -- and the wings move mostly side to side, not up and down.

To understand the aerodynamic forces generated by flies, Dickinson built a huge model of the wings of a fruit fly, Drosophila melanogaster.

Dubbed "Robofly," the contraption mimics the atmospheric effects of a fruit fly's one-millimeter-long wings flapping in air.

They built a 25-centimeter (15 inch) robotic wing, which flaps and rotates at one-hundredth a fly's speed in a two-ton tank of mineral oil. Three motors move the robotic wing back and forth in precise motions determined by a computer. Bubbles pumped into the tank show the aerodynamic patterns. Sensors measure the forces on the wings during each phase of the stroke.

They first confirmed a previous theory, devised by a number of labs over the last 20 years, that a phenomenon called "delayed stall" occurs in the middle of the stroke.

When the insect sweeps its wing forward, a whirlpool or vortex of air is created on top of the wings. This vortex seems to create a low-pressure zone that produces lift.

The team also discovered that two previously unknown forces occur at the end of each half-stroke. When the wing rotates backward to change direction, air is pulled over the top faster than the bottom, a force called "backspin." Like a tennis ball with backspin, the wing is pulled upward by lower pressure.

In perhaps the biggest surprise, another type of lift -- "wake capture" -- is also created when a wing starts to change direction. The wing actually passes through a spinning vortex wake from the previous stroke. Dickinson says the wing can extract enough energy from this previous stroke to create significant upward lift.

Dickinson published these findings in the June 18, 1999 issue of Science. But these discoveries are just the tip of the iceberg, Dickinson says. Many questions are still unanswered. How do flies turn? How do they navigate?

diagram: Wake capture

Quicktime video: a real fly's wing motions

source: Michael Dickinson