THE aeroplane looks as fragile as a matchstick model. Its aluminium frame shows through its thin, fabric skin, and the engine compartment is protected only by a cloth square stuck on with a few bits of Velcro. But don鈥檛 be fooled: this craft is a high-tech marvel. It is designed to let humans fly the way nature intended-on a pair of flapping wings.
This summer, the 鈥渙rnithopter鈥� and its pilot will finally take to the skies, says its designer, James DeLaurier of the University of Toronto鈥檚 Institute for Aerospace Studies. 鈥淭his is the realisation of humanity鈥檚 most ancient dream of flight.鈥�
It鈥檚 been a long time coming. Perhaps the first attempt to do more than dream was in the 1480s, when Leonardo da Vinci sketched a manned flying machine with flapping wings. In 1871, Frenchman Alphonse Penaud made a working model ornithopter, powered by a twisted rubber band.
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But when inventors tried to build ornithopters big enough to carry people, their contraptions invariably either smashed themselves to pieces or simply sat there, bouncing up and down. After the Wright Brothers had successfully flown their fixed-wing aircraft, flapping flight was discarded as a technological dead end.
A few people did remain interested in ornithopters. Recently, the US Department of Defense has spent a lot of money trying to develop small, remote-controlled air vehicles, including ornithopters, that they hope could someday be as agile as hummingbirds or dragonflies. But no one has yet managed to get a pilot into the air.
Even DeLaurier hasn鈥檛 always been an enthusiast. As a child, his first models weren鈥檛 hugely successful. 鈥淚 built little ornithopters out of sticks and tissue,鈥� he says. 鈥淎nd I thought, 鈥楤oy, this is awful. I鈥檇 rather have a propeller.'鈥�
Then, in 1973, while working at the Battelle Memorial Institute in Columbus, Ohio, DeLaurier met Jeremy Harris. Harris had done graduate work on ornithopters, and the two men began collaborating as a hobby. When DeLaurier moved to the University of Toronto the following year, their collaboration continued, with the help of computer simulations and wind-tunnel tests.
By 1991 they had a remote-controlled flying model with a 3-metre wingspan. In videos of flight tests, the model鈥檚 motions look remarkably bird-like. They show it being thrown by hand from a ridge, then swooping and turning in the sky over a field. The next step was clear-an ornithopter big enough to carry a human being.
Making a model is one thing. Building a full-scale version is quite another. It has to be much stronger and much more powerful, and whereas it is relatively easy to scale up a fixed-wing design, flapping flight presents special problems.
The beauty of the fixed-wing design is that a propeller or jet engine gives you the thrust, allowing a simple, strong wing structure which merely has to provide the lift. Flapping wings, in comparison, are far more complicated because they have to do both. When a bird glides, its wings provide lift the same way an aeroplane鈥檚 do. But when it flaps, it angles its wings so that as well as providing vertical lift, they also provide thrust to propel the bird forward.
Fortunately, says DeLaurier, a lot of the complexity of real birds鈥� wings can be ignored. His goal is more modest than the swooping, delicately controlled flight of birds-he just wants to get off the ground. But that still means a relatively complicated mechanical arrangement, making the wings more flimsy. What鈥檚 more, the mechanical stresses are much greater on a flapping wing than on a fixed wing.
This combination is what broke the early ornithopters. Fortunately, DeLaurier has modern materials to hand. His wings are made from a combination of Kevlar, carbon fibre and epoxy resin. 鈥淲e couldn鈥檛 have done it without composite materials,鈥� he says. 鈥淧eople trying to do this with aluminium tubes and wood were barking up the wrong tree.鈥�
DeLaurier鈥檚 ornithopter has a wing in three sections. The centre section is connected to the engine and pumps up and down (see Diagram). On each side of this are the flapping wing sections, attached with hinges. The wings pivot on the ends of two vertical struts that are fixed about a metre or so out from the side of the fuselage.
The result is that when the centre panel is pushed up, the two wing sections flap down, and vice versa. This three-section design helps cut down on the judder caused by flapping, as the centre of the wing is moving up while the outer portions are moving down.
Cool to flap
The wings are driven by a 24-horsepower petrol engine of the kind often used to power microlight aircraft. It鈥檚 been modified for the ornithopter because far more power is needed in the downstroke than the upstroke. So energy is temporarily stored between downstrokes using a flywheel. The engine also needs a fan to cool it-a job normally done by the propeller-so the engineers have fitted the flywheel with blades and fixed it so that it blows cool air across the engine as it spins.
The ornithopter gets its lift just like an ordinary aeroplane or a gliding bird, but to do that it has to move itself forwards. It generates most of its thrust in a different way from bird wings, using a counterintuitive process called leading edge suction. As the wing flaps down, it forces some of the oncoming airstream that was heading under the wing to make a U-turn around the leading edge and go over the top of the wing instead. When air speeds up rapidly its pressure goes down, so this action creates a low-pressure region in front of the wing, and that sucks the wing forward.
The wings are also designed to twist. That鈥檚 necessary to prevent a phenomenon called flow separation. With a fixed wing, air hugs the curved surfaces like water following the curve of the outside of a glass. But if a pilot tries to climb too steeply, the air breaks away from the surface and moves off at an angle. This can create drag and cause the plane to stall. Likewise, if flapping wings don鈥檛 twist, then flow separation will occur as they move, and the ornithopter would be doomed.
So DeLaurier has built his wing with a split trailing edge, an innovation he calls 鈥渟hearflexing鈥�. This allows the fabric on the top and bottom of the wing to slide freely over waxed internal supports, so the whole wing can twist as it flaps. Because it naturally twists to stay roughly parallel to the airflow, this prevents flow separation.
If all this works, and they actually get the ornithopter in the air, they鈥檒l have to control it. Fixed-wing aeroplanes use flaps on the wings called ailerons: the pilot makes the plane bank by twisting one aileron up and the other down, forcing one wing higher than the other. Then, because the lift is no longer purely vertical, the plane turns.
Roll with it
But the ornithopter doesn鈥檛 have ailerons. That means the pilot will have to rely on the rudder. The rudder controls the yaw of the ornithopter-its turn on a horizontal plane. When the ornithopter yaws to the left, for example, the right wing will suddenly be pointing into the wind, and the left side will be to the leeward. The right wing will get more lift, and that will induce a roll to the left. Then, like an ordinary plane, the ornithopter will turn. But because the process is so involved, it won鈥檛 be turning tightly.
So much for theory. In reality, Project Ornithopter has had its ups and downs. Since runway tests began in 1996, wings have torn, rivets have popped loose and engines have thrown their belts-all of which has kept the aircraft from actually getting into the air.
The main cause of these niggles has been the plane鈥檚 bouncing motion, which isn鈥檛 entirely cancelled out by the three-section wing design. This oscillation starts as the wings begin to flap, at about one cycle per second. By the time the ornithopter approaches take-off speed, it is leaping into the air with each downstroke and then crashing back to the ground with each upstroke. Not surprisingly, that has broken many parts of the plane.
DeLaurier says that if he had simply launched the ornithopter using a ramp or by towing it up to take-off speed, it would have flown already. But he feels that to truly achieve the goal of sustained piloted flapping flight, he needs an aircraft that can take off, fly and land all on its own.
To try to solve the take-off problem, DeLaurier鈥檚 team rebuilt the undercarriage to make the ornithopter鈥檚 nose point down as it taxis for lift-off. The idea is that lift will be suppressed and the aircraft鈥檚 wheels will stay in touch with the runway until it is going fast enough for a clean lift-off, without the destructive bouncing.
But that created another problem during the most recent tests. The vertical spars supporting the wings hadn鈥檛 been designed for the stresses of the new position, and as the ornithopter approached its take-off speed of 92 kilometres per hour a strut buckled and part of the right wing unravelled. It鈥檚 taken the best part of two years to make repairs and get back to the runway.
Flying the ornithopter this summer will be Patricia Jones-Bowman, a flight instructor and pilot. Aside from experience and enthusiasm, Bowman has one other essential quality-she weighs only about 45 kilograms, which will keep the total lift-off weight down.
If she gets airborne, she鈥檒l have to contend with an unusual ride quality, as the ornithopter bucks up and down with accelerations of about 0.5 g. But Jones-Bowman has experienced the sensation in a simulator and is confident she can control the aircraft. 鈥淚t鈥檚 not much worse than riding a horse,鈥� she says.
Her biggest problem will be the view. Pilots rely on their view of the horizon to gauge the angle of the plane. But from the pilot鈥檚 perspective, the horizon will be moving all the time, making it hard to tell how the plane is oriented. So the engineers have designed an instrument that shows the pilot the position of the control surfaces on the tail.
But Jones-Bowman will still be taking a flap into the unknown. 鈥淚t鈥檚 challenging, is what it is,鈥� she says. But the danger doesn鈥檛 daunt her. 鈥淚鈥檓 a well-trained and experienced pilot. I鈥檓 not going to do anything that will result in my immediate demise.鈥�
If the dream is fulfilled and Jones-Bowman is the first human to fly by flapping-what then? What good is a piloted ornithopter? What can it do that fixed-wing aircraft can鈥檛 do more cheaply and reliably, and above all more comfortably?
Probably not much. 鈥淚鈥檇 like to be able to say that ornithopters have a future role in human transportation. Realistically, they probably don鈥檛,鈥� says Nathan Chronister. He isn鈥檛 some nay-saying fixed-wing proponent, but the coordinator for the Ornithopter Society, and works on ornithopters himself. But he thinks practical applications, if they ever come, are a long way off. 鈥淪ome believe that ornithopters will provide a more efficient alternative to the helicopter or a more agile type of aircraft. The technology to realise those benefits is probably a hundred years off if it ever happens at all.鈥�
DeLaurier agrees. 鈥淵ou know that you are talking to people who don鈥檛 understand the project when one of the first questions they ask is, 鈥榃hat are the practical applications?'鈥� But, he adds, it would probably be a hit on the air show circuit.
The funding has been hard to come by. Although DeLaurier has little problem getting money from the US Defense Department for his miniature ornithopter project, he鈥檚 had to scrape around for the 拢140,000 the full-size aircraft has cost.
With luck, that won鈥檛 matter. If all goes well in the next few months, the ornithopter will take off and fly for 10 or 15 seconds-a 鈥淲right Brothers type hop鈥� that will prove it can take off, fly, and land.
And despite all the difficulties he and his team have faced, DeLaurier is confident. 鈥淭here鈥檚 no reason we can鈥檛 make the flight,鈥� he says. 鈥淯nless they change the laws of physics between now and then.鈥�