Flies may not have the nicest of personal habits, but no one can fail to be amazed by their remarkable agility as they twist, turn and hover in the air. And their manoeuvrability has not escaped the notice of aeronautic engineers;they'd love to design a minute flying machine that hovers as well as a fly. But if engineers are to be successful, they must understand what keeps flies aloft. According to aeronautical engineer David Lentink, from Wageningen University in the Netherlands, the leading edge vortex keeps hovering flies in the air. Charlie Ellington found in 1996 that the leading edge vortex, a spinning mini-tornado, develops over the wing when small flies hover, reducing the air pressure above the wing to generate the lift necessary to keep the insect aloft. Amazingly, this vortex sticks to the wing and is not ripped off by the air flow, so when Lentink visited Michael Dickinson's California Institute of Technology lab, Dickinson suggested that Lentink looked into the air flows that keep the leading edge vortex in place to get a better understanding of what keeps hovering insects airborn(p. 2691, p. 2705).
Lentink realised that the only way to find out what goes on in the air flowing over a hovering fly's wings was to investigate an extremely complex suite of equations, known as the Navier–Stokes equations(p. 2691). These equations describe how fluid flows around flying insects and other flying animals and relates the animal's body shape and movements to the aerodynamic forces that keep the animal aloft. After a month of complex mathematical derivation Lentink came up with a series of equations that describe the fluid flows around flapping, translating and spinning wings.
Next Lentink began thinking about the ways that flapping wings move while an insect is hovering: they sweep back and forth horizontally as the wing revolves (swings) around the shoulder, so that the front edge of the wing points forward as it sweeps forward and backwards as it sweeps back. Calculating which motion provided the fluid flows that held the hovering insect's vortex in place, Lentink was surprised when he realised that the swinging motion about the shoulder joint must hold the vortex in place while the insect hovers.
Lentink explains that as the wing revolves about the shoulder joint, air sitting near the joint is forced to move outward under the influence of centrifugal acceleration. This forced airflow along the length of the wing experiences a Coriolis acceleration, which allows the air to keep up with the wing. Lentink realised that this accelerating air flow effectively pins the leading edge vortex in place, allowing the vortex to generate the lift required to keep a hoverer aloft. It was also clear that wings that spin around (like helicopter blades or falling spinning plant seeds) would be the most efficient way for a hovering machine to stay aloft.
However, there is a catch. Lentink explains that the leading edge vortex significantly increases the drag acting on the wing, but adds that this isn't really a problem for flies and other small insects. He explains that the drag forces that they experience while trying to move through viscous air are so huge that the extra drag incurred by the leading edge vortex hardly troubles them at all.
Having found that his calculations predicted that Coriolis acceleration was the key to holding the leading edge vortex in place, Lentink moved on to test how fluids flow over moving insect wings to see if his predictions held up in practice. But instead of measuring the fluid flows over tiny insect wings,Lentink turned to Dickinson's scaled up robotic fly, Robofly, which he could use to test out a wide range of flight conditions while watching to see whether leading edge vortices developed and stayed in place(p. 2705).
Simulating the viscosities that flies experience in air by filling Robofly's tank with liquids ranging from thick mineral oil to water, Lentink programmed a large scale Perspex® model of a fruit fly wing to flap in a variety of styles while he monitored the flow patterns across the wing with a stream of air bubbles. Lentink also monitored the lift and drag forces exerted on the flapping wing to find out which movements generated the most lift and drag.
Testing wing beats ranging from the wing spinning horizontally around its shoulder (like a helicopter blade), all the way through to a swinging hovering flap and an aeroplane-like translation, Lentink could see the vortex forming at the front of the wing in all of the flight modes. However, it only remained in place when the wing revolved like a helicopter or swung like a hovering fly's flapping wing.
Lentink also measured how effectively the wings generated aerodynamic lift and he found that the translating fly wing consumed up to 25% less energy than a hovering fly generating the same amount of lift, while the spinning fly wing did even better, generating the same amount of lift as a flapping hovering fly while consuming only half of the energy. `Flapping wings waste a lot of energy accelerating the air back and forth,' explains Lentink.
So what does all this mean for engineers keen on designing microfliers?`Engineers have been thinking that fly sized flying machines would have to fly like a fly,' says Lentink, but he now realises that this is not the case. Having shown that Coriolis acceleration along a stubby spinning wing holds the lift-generating leading edge vortex in place, Lentink explains that engineers could use the same trick to build a fly sized aeroplane. By copying the fly's stubby wings, which hold the leading edge vortex in place, and coupling them with the energy efficient helicopter-style `spinning' motion, engineers could build a fly sized hovering microflier that only consumes half the energy of a flapping fly.