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Boffins claim breakthrough in hovering robothopter experiments

Paper 'bugs' used subwoofer-powered wing flaps

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Brainboxes in New York say they have made progress on one of the knottiest conundrums facing the technology of humankind today: that of constructing mechanical ornithopters able to fly – and specifically to hover – as well as insects can.

This wouldn't be of use at the scale of today's manned aircraft: nobody's that interested in building mighty 'thopter-craft to replace choppers or jets. Where flapping wings are demonstrably more efficient than airscrews or turbines is at the small scale, where the viscous forces in aerodynamics become more significant compared to inertial ones. Little insects, bats and some kinds of birds, despite the fact that they have very limited power output, can perform feats that aircraft of similar size – small unmanned choppers, for instance – struggle to mimic. In particular they can hover for long periods with great precision.

Military boffins have long sought to develop robotic aircraft which could operate in this low-Reynolds-number flight regime: there is in fact one such mini-ornithopter test vehicle flying. Nonetheless this kind of aerodynamics remains poorly understood.

Thus it was that Professor Jun Zhang and his colleagues found themselves experimenting on the dynamics of flapping-wing flight. Rather than building actual ornithopters (an expensive business: the military test minithopter has cost the Pentagon better than $4m so far), they used pyramid-shaped paper "bugs", kept airborne in a stream of blown air. The "bugs" were made to flap their wings rather cunningly, by using an ordinary audio subwoofer to create oscillations in the air column.

This led to several discoveries: one of them, counterintuitively, was that bugs weighted to be top-heavy actually flew more stably than bottom-heavy ones.

“It works somewhat like balancing a broomstick in your hand,” explains Zhang, boffin at New York uni's Courant Institute. “If it begins to fall to one side, you need to apply a force in this same direction to keep it upright.”

With the top-heavy paper flapcopters, this force is generated naturally as the bug tilts.

The new boffinry is set out in a paper published in the journal Physical Review Letters. We learn from the synopsis:

While the “bug” is far from a realistic analogy for an insect, the unsteady flow mechanisms revealed through these experiments can help address current disagreements among models that assess the intrinsic stability of flying insects. The next step could be to replace the pyramids with a mobile robot for a better simulation.

The full paper can be found here. ®

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