Tuesday, October 29, 2013

Flying Machine Mimics Graceful Swimmers

When it comes to the skillful art of flight, insects must be doing something right.
About 350 million years ago, they were the first life on Earth to achieve lift off. Fast-forward to the 21st century and scientists and engineers realize that flying like an insect is harder than it looks.

In particular, the motion of insect wings lends little stability to the object itself. So, many prototype flying machines meant to mimic a flying insect often tumbled through the air like a somersaulting astronaut in zero-g. Attached technological sensory mechanisms can enable stability but also add weight, so the machine must expend more energy to boost itself off the ground.

A pair of mathematicians at New York University have overcome the obstacle of stability in a different way and built the first machine of its kind that is inherently stable. The bizarre part is that the machine moves unlike any insect in nature and instead resembles more of a jellyfish in air.

Leif Ristroph and Stephen Childress will present their work this November at the 66th Annual Meeting of the APS Division of Fluid Dynamics in Pittsburgh, PA.

The machine has a mass of 2.1 grams – the equivalent of two dollar bills – and consists of a small motor, four wings and a spherical cage for structure. Each eight-centimeter-long wing is tear-drop shaped with the pointed tip attached around the top of the cage at four equidistant locations. The motor flaps the wings back and forth against the cage about 20 times per second, eventually achieving lift as shown in the video above.

“It’s the simplest design,” said Ristroph. “You just have a motor and wings and that gives you stability. It’s especially relevant for making things smaller down to the 2-centimeter scale because how are you going to put sensors and circuits that help with stability on something that small? My feeling is you want it as simple as possible with no extra parts.”

In the video, the wires above and below the apparatus are feeding electricity into the motor. If such a design were eventually built for free-flying, the design would have to account for the additional weight of a battery.

“The interesting features of this study are one: the approach of taking inspiration from nature without following her slavishly, and two: the inherent stability of the design,” said a University of Oxford lecturer in mathematical biology Graham Taylor who was not involved with the study. Taylor studies control and stability in animal flight and has analyzed how flying insects use sensory systems to maintain stability.

“The design is obviously capable of slow forward flight and stable hover, but I would not expect this to be an efficient mechanism for fast forward flight. The mechanism is reminiscent of a jellyfish, and they of course do not swim fast either,” Taylor said.

The machine’s low center of mass combined with the wings’ motion is how it manages to remain stable during flight. The low center of mass acts as a stabilizing pendulum while the flexible, flapping wings achieve lift off.

In fact, after studying the model’s aerodynamic properties, the scientists discovered that the flexible nature of the wing’s material is why the machine can generate enough lift to fly. If the wings were rigid, Ristroph suspects that they would have a flightless dud.

“That suggests there’s a lot more work to do,” Ristroph said. “We did not optimize lift. If we understand more about how these flexing wings are generating lift then we might find that there’s even more lift to be had.”

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