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How Fruit Flies Execute In-Flight Turns

Micro air vehicles could benefit from high speed film footage revealing how fruit flies control their flight.

The breakneck flying abilities of insects is astounding to watch. But it is also a head scratcher for physicists and engineers who have long known of the scale-dependence of aerodynamics that makes flight a rather different matter for a fruit fly than it is for a pigeon or a jumbo.

While we’ve known for centuries about the principles of lift that keep an aircraft in the air, physicists have discovered only in the last 10 years or so, how insect wings create lift through the generation of, and interaction with, vortices.

Having solved that conundrum, the attention is turning to the way insect fliers control their movement. The question is how do these fliers manipulate their wings to execute in-flight turns, sometimes extremely sharply and at relatively high speed?

The answer comes today from Jane Wang and buddies at Cornell University who have studied the way fruit flies turn using three cameras at right angles recording images at the rate of 8000 frames per second, or about 35 frames per wing beat.

Fruit flies generate lift and propulsion by paddling their wings back and forth in a kind of rowing motion. High speed movie footage shows that, In steady flight, each wing’s angle of attack is about the same during the forward and backward strokes and that the movement of the left wing mirrors the right. So in steady flight, the torque generated by each wing cancels out.

But during a turn, Wang and co say a fruit fly changes the angle of attack of one wing by around 9 degrees, generating extra drag that causes the fruit fly to turn.

So imagine looking down on a fruit fly in flight. A change in the angle of attack of the rigth wing generates drag that causes the fly to turn clockwise.

What’s interesting about the Wang team’s analysis is their conclusions about how a fruit fly controls this change in wing angle. It looks as if much of a fly’s capacity to fly is a function of the strength and elasticity of the wings themselves: they are designed to “row”. That immediately reduces the command and control burden on the fruit fly’s central nervous system.

This is an idea called passive dynamics in which a system’s design includes the capacity to control movement by default, like a shuttlecock’s ability to orient itself in flight.

Wang and co say that turning looks as if it is executed by a very small modification to the spring-like behaviour of the wing hinge. And that this can easily be done using a relatively small muscle.

“Our model predicts, that flight muscles of flies can act over several wing beats to bias the pitch of the wings and yet generate the sub wing-beat changes in wing motion that aerodynamically induce the maneuver,” say the team.

That may sound somewhat esoteric but it should turn out to be of great value for the growing number of engineers attempting to build robotic micro air vehicles with insect-like wings.

Wang and co conclude: “The simple mechanism used by fruit flies may be quite general and should likewise simplify the control of flapping flying machines.”

Anyone hear that buzzing noise?

Ref: arxiv.org/abs/0910.0671: Fruit Flies Modulate Passive Wing Pitching to Generate In-Flight Turns

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