Monday, August 3, 2009




In 1804 the English aviation pioneer George Cayley installed a bizarre machine at the top of his staircase. He attached wings of various shapes to a whirling arm atop the device, and as it spun the wings would either climb or descend depending on their ability to generate lift. This helped Cayley to develop the aerodynamic theories that led to his successful manned glider flights, and ultimately to the Wright brothers' powered aircraft.

More than two centuries later, a whirling arm is once again being used to prepare the next revolution in flight technology: micro-aircraft that harness the complex aerodynamics and navigation techniques of insects. In his lab at the University of California, Berkeley, microsystems engineer Ronald Fearing fixes each new version of the mechanical insect he is developing to the tip of a 30-centimetre free-spinning arm he calls a "flight mill". Like Cayley's machine, this allows him to measure how much lift his creation can generate, and to test different ways of controlling it.


Mechanical insects could prove far more manoeuvrable than micro-sized versions of conventional aircraft or helicopters. The insect-like craft could fly unobtrusively around buildings, zipping into open windows, for example. When equipped with different kinds of sensors, they could be used as miniature spy drones, security guards and pollution monitors.

The military in particular are interested. The Pentagon's Defense Advanced Research Projects Agency is developing four flying "robobugs", weighing up to 10 grams each and with wingspans of up to 7.5 centimetres. One of the two companies developing the craft for DARPA - Aerovironment, based in Monrovia, California - aims to have a "rough demonstrator" flying by the middle of 2008.

It is challenging work. If micro-aircraft like Fearing's are ever to fly, they will not only need to generate lift in a similar way to insects, but also mimic the way bugs sense their environment to allow them to maintain stability and land safely. Recent developments in wing mechanics and control systems mean that researchers are now getting close.


The first hurdle for engineers like Fearing is to develop mechanisms that will generate enough lift. Insects do this by rapidly beating their wings down and forward, and then rotating them back and upward (see "Moth in a wind tunnel"). At last week's Society for Experimental Biology meeting in Glasgow, UK, a host of new robotic insect-wing designs and flapping mechanisms were on display. Andrew Conn at the University of Bristol in the UK unveiled a hummingbird-sized wing mechanism driven by a pair of motorised aluminium cranks that reproduce a typical insect wingbeat: one beats the 7.5-centimetre wing up and down, while the other rotates it (see Photo, above). Unlike previous mechanisms, says Conn, the current design's wing motion is adjustable and should allow more manoeuvrability in the air.

However, the team, which is being funded by the UK government's Defence Science and Technology Laboratory, has found that friction in the mechanism is slowing the wing's beating. The device is also currently too heavy to take off, so the researchers plan to replace as much metal as possible with carbon fibre. "We'll probably need to halve our weight and at least triple our lift," says Conn.

These problems come as no surprise to the entomologists at Michael Dickinson's lab at the California Institute of Technology in Pasadena, where they study fly and honeybee wing dynamics. Anyone attempting to mimic insect wing motion using such complex machined gearing may be wasting their time, they say. As the Bristol team is finding, friction dominates at such small scales, so micro-sized versions of conventional gears and pulleys can sometimes seize up. Dickinson's team reckons success is much more likely to come by emulating the way an insect uses muscles to flex its whole thorax, which in turn moves the wings.

This is the approach being followed by Fearing, who has worked closely with Dickinson. For his 0.1-gram Micromechanical Flying Insect (MFI), he has gone for a more insect-like approach. The prototype comprises a 2-centimetre-wide carbon fibre "thorax" with 4-millimetre polyester and carbon fibre wings on either side. To move the wings, two piezoelectric actuators move a concertina-like carbon fibre structure incorporating 15 polyester joints. As the piezoelectric crystals expand and contract they flex the joints back and forth. The flexing thorax is attached to each wing by a hinge to drive the down-and-forward, up-and-rotate-back wingbeat characteristic of insects (4MB .avi video).