Robo-moth solves the mystery of `impossible' flight

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Why can insects such as the bumblebee fly? The answer has been solved by a team at Cambridge University, helped by a robot hawkmoth, and it may surprise aeronautical engineers - while leading to the design of a new generation of helicopters and propellers.

According to standard laws of aerodynamics, insects' wings are too small to lift their comparatively huge bodies. But clearly, insects do fly - which led Charles Ellington, of the university's department of zoology, to decide to pin down the answer once and for all.

First, he studied the wing motions of a hawkmoth in an air tunnel, by blowing smoke over it while it flapped its wings.

Then, he built a robotic version, designed to have exactly the same wing motion, but five times larger.

The robot used four servo motors and an elaborate gearbox to drive the movements of the wings, which were made of a framework of rigid and flexible brass tubes, covered on both sides with black elastic cloth. Smoke was used to illustrate the air flow forces on photographic images.

Scientists had previously thought that the missing link in the aerodynamic equation - the extra lift required to keep an insect aloft - might be generated by "rotational lift", through the twirling of the wings as they flap.

But Professor Ellington reports today in the science journal Nature that the extra lift needed to keep the moth aloft is generated during the wings' downstroke, when a spiral vortex of air travels across the leading edge of the wing, from base to tip. The vortex, a region of swirling air, creates a region of low pressure which sucks the wing upwards - creating lift.

The vortices form a complex pattern of loops and spirals which spin away from the wings. Just as one vortex dies out - which would lead to stalling, and cause an earthward plunge - another begins at the body, reinforcing the lift.

Professor Ellington called the process of flying by this method "dynamic stalling": "This is so unlike what we had expected all along that this is a shock, really."

The findings could probably be applied to helicopter and propeller design, as those also use vortices to create lift.

"It's something we are going to start looking at," Professor Ellington said. "It's a way to get something like two or three times more lift."