Fruit Flies Highlight Aerodynamics of Insect Flight

John Roach
for National Geographic News
April 23, 2003
To swat a fly can be a lesson in futility. The insect darts from each swipe with uncanny precision, altering its course to zip off in nearly the opposite direction.

Precisely how a fly achieves its aerial acrobatics is more than a curiosity of annoyance for Michael Dickinson, a bioengineer at the California Institute of Technology in Pasadena. Dickinson has built an entire research lab, not to mention professional career, seeking an answer to just how a fly's brain controls its muscles in precision flight.

"An interest in the brain led to an interest in flight and aerodynamics," said Dickinson. "I've spent a lot of time with folks in the lab trying to figure out the basic aerodynamics of insect flight."

Together with Steven Fry, a biologist at the University of Zurich in Switzerland, and Rosalyn Sayaman, a research assistant also at the California Institute of Technology, Dickinson determined how common fruit flies use their wings to make 90-degree turns at speeds faster than a blink of the human eye, let alone the swoosh of a swatter.

The researchers discovered that the mechanics of how flies execute such turns were contrary to what they initially believed.

To turn, a flying creature must generate enough twisting force, or torque, to offset two forces working against it—the inertia of its own body (think forward motion on a bicycle, once you stop pedaling) and the viscous friction of the air, which for small insects is thought to be like syrup.

Scientists had long assumed that the viscosity of the air, and not inertia, was the greater force for insects such as flies to overcome. Inertia, they believed, was primarily the bane of larger animals like birds.

"No one challenged the notion because there was no indication that it might be different," said Fry. "The results actually proved the opposite."

The research team found that fruit flies make subtle changes in the tilt of their wings relative to the ground and the size of each wing flap to generate the forces that allow them to turn. Flies then create an opposite twisting force with their wings to stop the inertia of the turn, preventing an out of control spin.

This finding, say the researchers, indicates that inertia, and not friction, is the greater force for the fruit fly to overcome in the turn.

Ron Fearing, an electrical engineer at the University of California at Berkeley who is developing a tiny robotic insect capable of autonomous flight, said that the research team has "shown a quite surprising result, in that very subtle changes in wing motion are responsible for rapid maneuvers."

The Fly Tests

Using three high-speed infrared video cameras, the researchers filmed fruit flies in an enclosed arena flying towards a cylinder laced with a drop of vinegar. As the flies approached the cylinder, it loomed in their field of view, triggering a rapid turn that helped them avoid a collision.

These rapid turns were captured by the three cameras, allowing the researchers to analyze in three-dimensions the wing and body position of the flies as they executed the turns. The team determined that fruit flies perform banked turns similar to those observed in larger flies and hummingbirds, which must overcome inertia.

If friction dominated the turn, flies would have to continuously use their wings to generate torque during the entire turn to overcome the viscosity of the air. Otherwise the momentum of the turn would halt, the scientists said.

Dickinson likens such a scenario to trying to spin a toy top in vat of honey. "As long as you work to move it, it would move, but the moment you let go it would stop," he said.

Instead, to execute a turn, a fruit fly generates torque to accelerate into the turn and then the fly has to actively counteract the inertia of the turn by producing torque in the opposite direction, bringing the rotation of the body to a halt, according to the scientists. Once the flies have achieved their desired turn angle, they buzz off.

"In some ways it flies like a helicopter," said Fry. "It has to adjust its body orientation in space and does so using subtle changes in wing motion."

To make sure their measured patterns of wing motion were sufficient to explain the rapid turning ability of the fly, the researchers played the sequences through a dynamically-scaled robotic model, whimsically named Bride of Robofly. The robot does not fly, but rather is a larger version of a fly's wings that flap in a tank of mineral oil, allowing the scientists to measure the forces on the wing.

The researchers found that their calculations of the flies' movements based on their observation from the three-dimensional video matched well with the calculations they derived from the wing motion of the robot.

"Through use of the scaled robot fly model in the oil tank, they have quantified the resulting change in forces due to these small motion changes," said Fearing.

Flying Robotic Flies

The research provides insight to the lightening-quick control mechanisms that are active in flies, demonstrating that flies are able to control changes in their body orientation within just a few wing beats, said Fry.

"A fly's wing flaps at 200 wings beats per second," he said. "The control has to be similarly fast."

Insights into the control mechanisms of the fly will help researchers build a robot that flies like a fly. Such potential robots would be highly prized for search and rescue, spying, and surveillance operations by the U.S. military, which is funding Fearing's robotic fly work at the University of California at Berkeley.

A summary of the research by Fry, Sayaman, and Dickinson appeared Friday in Science, the journal of the American Association for the Advancement of Science.

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