Tuesday, 29 July 2014
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Aircraft in a flap

Jon Excell
Ornithopters are largely seen as an amusing example of mankind taking a technological wrong turn. But numerous engineers are now re-examining the concept.

From Icarus' doomed flight to da Vinci's abandoned sketches, aircraft with flapping wings, or ornithopters, are largely seen as an amusing example of mankind taking a technological wrong turn.

However, while early attempts to emulate the flight of birds where frustrated by the power and complexity of flapping wings, numerous engineers are now re-examining the concept in the light of today's more sophisticated technology.

Professor Jim DeLaurier of the University of Toronto - a leading expert on ornithopters - has his eye on one of the last great aeronautical records. Despite the amazing aerospace achievements since the Wright brothers, such as hypersonic flight, voyages to the moon, and travelling around the world without refueling, mechanical flapping flight remains to be accomplished.

Using computer analysis and wind-tunnel testing to improve on an older design, DeLaurier first designed and flew a remotely piloted flapping wing model plane. This is recognized as the first successful engine-powered remotely-piloted ornithopter.

This established the foundation for developing a full-scale aircraft, which began shortly after the model's flights. Construction took approximately a year, beginning in 1995, and in October 1996 the first taxi trials were conducted. These showed that the aircraft was capable of accelerating under its own power. Further taxi tests have been conducted in subsequent years, and the aircraft has self-accelerated on level ground to speeds allowing brief lift-offs.

The full-scale ornithopter is an engine powered aircraft that carries one pilot. All of the thrust and nearly all of the lift is created by the mechanical flapping of the ornithopter's wings.

The two wings of the craft are joined by a centre section which is moved up and down by pylons connected to the drivetrain. The wings' thrust is due primarily to a low-pressure region around the leading edge, which integrates to provide a force known as 'leading-edge suction'. The wings also passively twist in response to the flapping. This is due to a structure that is torsionally compliant in just the right amount to allow efficient thrusting ('aeroelastic tailoring'). It should be noted, though, that twisting is required only to prevent flow separation on sections along the wing. It does not produce thrust in the same way as required by sharp-edged wings with little leading-edge suction.

'Our main motivation is simply to achieve an aeronautical first,' says Delaurier, 'however, the project has also been a mother lode of thesis topics for my students (unsteady aerodynamics, flight dynamics, aeroelastic tailoring, etc.) Also, our knowledge of flapping-wing flight has resulted in a DARPA contract for a microair vehicle concept.'

Flapping-wing vehicles are also under consideration by NASA for the benefits that flapping-wing flight would have on planet's with dense, gaseous atmospheres.

'We are actually looking at application on Mars,' says Delaurier. 'There may be some unique advantages for an ornithopter in that situation.'


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