Learning to Fly
Flies have a lot to teach us about aviation that can't be learned from studying fixed-wing aircraft. For years, there was little known about the mechanics of insect flight, yet they are the world's oldest group of aviators, sometimes called nature's fighter jets. You may have heard about how bumblebees can't fly according to conventional aerodynamics. That's because the principles behind insect flight are far different from those behind fixed-wing airplane flight.
"Engineers say they can prove that a bumblebee can't fly," said Michael Dickinson, a biologist at the University of California, Berkeley. "And if you apply the theory of fixed wing aircraft to insects, you do calculate that they can't fly. You have to use something different."
Dickinson is part of the Micromechanical Flying Insect (MFI) Project, which is developing small flying robots using the flight principles of insects. The project is in cooperation with DARPA. The MFI Project is proposing a robotic insect that is about 10 to 25 millimeters (0.39 to 0.98 inches) in width, which is much smaller than DARPA's size limit of 6 inches (15 cm), and will use flapping wings to fly. The project's goal is to recreate the flight of a blowfly.
If you read the article How Airplanes Work, you know that airplanes generate lift due to the air travelling faster over the top of the wing than along the bottom of the wing. This is called steady-state aerodynamics. The same principle cannot be applied to flies or bees, because their wings are in constant motion.
"Unlike fixed-wing aircraft with their steady, almost inviscid (without viscosity) flow dynamics, insects fly in a sea of vortices, surrounded by tiny eddies and whirlwinds that are created when they move their wings," said Z. Jane Wang, a physicist at Cornell University's College of Engineering. An eddy is whirlpool of air that is created by the wing, and the air in the eddy is flowing in the opposite direction of the main current of air.
The vortices created by insect wings keep the insects aloft. Dickinson's group outlines these three principles to explain how insects gain lift and stay airborne:
- Delayed stall - The insect sweeps its wing forward at a high angle of attack, cutting through the air at a steeper angle than a typical airplane wing. At such steep angles, a fixed-wing aircraft would stall, lose lift and the amount of drag on the wing would increase. An insect wing creates a leading-edge vortex that sits on the surface of the wing to create lift.
- Rotational circulation - At the end of a stroke, the insect wing rotates backward, creating backspin that lifts the insect up, similar to the way backspin can lift a tennis ball.
- Wake capture - As the wing moves through the air, it leaves whirlpools or vortices of air behind it. When the insect rotates its wing for a return stroke, it cuts into its own wake, capturing enough energy to keep itself aloft. Dickinson says that insects can get lift from the wake even after the wing stops.
"It would be real spiffy if we could exploit these mechanisms, too, by building an insect robot. But you can't build them now based on known principles -- you have to fundamentally rethink the problem," Dickinson said. In the next section, you will learn how researchers are taking these principles and applying them to the creation of robotic flying insects.