October 2, 2009

Locust flight simulator helps robot insects evolve

Right:Smoke signals helps robots fly better (Image: Simon Walker, Animal Flight Group, Oxford University)

A LOCUST flight simulator could be the key to perfecting the ultimate surveillance machine: an artificial flying insect. The simulator can model the way wings of varying shapes and surface features beat, as well as how they change their shape during flight.

The device was created using extremely high-speed flash photography to track the way smoke particles flow over a locust's wings in a wind tunnel - a technique called particle flow velocimetry. This allowed researchers at the University of Oxford to build a computer model of the insect's wing motion. They then built software that mimicked not only this motion, but also how wing surface features, such as structural veins and corrugations, and the wings' deformation as they flap, change aerodynamic performance.

The work has shown that wings' surface structures are crucial to efficient lift generation, says lead researcher Adrian Thomas (Science, DOI: 10.1126/science.1175928).

The simulator could be a big step forward for the many teams around the world who are designing robotic insects, mainly for military purposes, though Thomas expects them to have a massive role as toys, too. "Imagine sitting in your living room doing aerial combat with radio-controlled dragonflies. Everybody would love that," he says.

Imagine sitting in your living room doing aerial combat with remote-controlled dragonflies

Until now, modelling insect wings involved building physical replicas from rigid materials and estimating how they might move from observations of insect flight. Thomas hopes the simulator will take the guesswork out of the process, especially as every flying insect has uniquely shaped wings and wing beat patterns.

Building miniature aircraft is of great interest to the armed forces. In the UK, for example, the Ministry of Defence wants to create a device that can fly in front of a convoy and detect explosives on the road ahead. In the US, the Pentagon's research arm DARPA is funding development of a "nano air vehicle" (NAV) for surveillance that it states must weigh no more than 10 grams and have only a 7.5-centimetre wingspan.

Last month, DARPA contractor AeroVironment of Monrovia, California, demonstrated the first two-winged robot capable of hovering flight (see video at http://bit.ly/18LR8U). It achieved a stable take-off and hovered for 20 seconds. Other DARPA-funded projects by Micropropulsion and Daedalus Flight Systems are also thought to have achieved hovering robotic flight this year.

"Getting stable hover at the 10-gram size scale with beating wings is an engineering breakthrough, requiring much new understanding and invention," says Ronald Fearing, a micromechanics and flight researcher at the University of California, Berkeley. "The next step will be to get the flight efficiency up so hover can work for several minutes."

But how can such machines be made more efficient? Better batteries and lighter materials will help, but most important will be improving wing structure so the aircraft more accurately imitate - or even improve upon - the way insects fly.

So how do insects fly? For a long time no one really knew. In 1919, German aeronautical engineer Wilhelm Hoff calculated that a pollen-laden bumblebee should not have enough lift to get airborne according to the rules of aerodynamics as understood at the time.

It wasn't until 1981 that Tony Maxworthy of the University of Southern California hit on a possible reason: his working model of a fly's wings, immersed in oil, showed large vortices were spinning off the leading edge of the wing as it beat (Annual Review of Fluid Mechanics, vol 13, p 329). Within the vortices air is moving at high velocity, and is therefore at low pressure, hinting at a lift-creating mechanism unlike that of conventional aircraft, in which an angled wing travelling forward deflects air downwards, creating an opposing upward force.

In 1996 Thomas was a member of Charles Ellington's team at the University of Cambridge, which identified the mechanism by which bugs created high lift forces - using a model of a hawkmoth. "We found a leading-edge vortex that was stable over the whole of the downstroke," says Thomas.

The nature of the leading-edge vortex is dependent on the size of the wings, their number, the pattern described by the beating wing and the wing structure.

This work has laid the foundations for researchers such as Robert Wood and his team at Harvard University, who are investigating ways to make insect wings (Bioinspiration and Biomimetics, DOI: 10.1088/1748-3182/4/3/036002). They have developed a new way to build flexible wings from moulds using microchip manufacturing techniques. Using elastic polymers and elegant, vein-like supporting structures, the researchers can build wings with variable camber, and with different corrugations embossed in them, in an attempt to mimic the in-flight aerodynamics and deformation of real insect wings.

Thomas is also focusing on the way insect wings deform in flight. "If we use a wing model with all the complex curves, twists and corrugations of the real insect it is 50 per cent more efficient than a model with rigid flat-plate wings, for the same lift generation. That would be a huge saving in power for a micro air vehicle," he says.

Although the Oxford team's simulator is geared for locust wings at present, the researchers are adjusting the software to model the hoverfly - with other insect types to follow.

"What we've shown is that modern aerodynamics really can accurately model insect flight," Thomas says. "That old myth about aerodynamics not being able to model bumblebee flight really is dead now."


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