Borrowing tricks from birds may result in smoother flights
While chatting to a customer in the family bicycle shop in Dayton, Ohio, Wilbur Wright was idly twisting a piece of cardboard that had once contained an inner tube, when he came up with an idea. The “semi-rigid” way in which the cardboard could be deformed yet still retain its stiffness might, he considered, provide an answer to a little problem he was working on with his brother Orville: how to design a wing for a heavier-than-air flying machine. The Wright brothers knew, from ornithological observations, that if a wing on one side of a bird’s body meets the oncoming flow of air at a greater angle than the opposite wing does, then that wing will rise. An aircraft wing which could be made to twist like this would help a pilot bank and turn. Using mechanical gears and levers to rotate an entire fully-rigid wing to do that would make the plane too heavy. A semi-rigid wing, however, could be warped into different angles using a lightweight series of wires and pulleys. Which is how, on December 17th 1903, the brothers achieved the first controlled and sustained flight of a powered aircraft.The Wright brothers called their system wing-warping. But it did not last. Within a few years, aviators began adopting a more reliable form of control that fitted hinged ailerons and flaps to the trailing edges of wings, and in 1915 Orville (Wilbur having died three years earlier) conceded, and followed suit. But, in a slightly different guise, wing-warping is now back. And not only that. In their efforts to make wings more efficient, which saves fuel, aerospace engineers are also looking for inspiration from birds’ wings in another way, by borrowing a trick from the most accomplished long-range flyer of them all, the albatross. Airbus, a giant European producer of passenger jets, recently completed a series of tests on new forms of wing control using a model in a wind tunnel at Filton, near Bristol, in south-western Britain. As a result, the company is now fitting these systems to a specially built wing that will, as an experiment, be used to replace one of those on a Cessna Citation business jet. This aircraft will then test the new designs in real flying conditions.One feature of the eXtra Performance Wing, as it is called, is that instead of having moving surfaces attached to the wing with hinges, mechanical actuators will change the shape of a semi-flexible surface on the trailing edge. These “morphing” surfaces will be multifunctional, meaning that by moving them up or down they can be used either as ailerons (which allow the pilot to bank and turn a plane) or as flaps (which provide extra lift). Pop-up spoilers that emerge from the top surface of the wing will conversely reduce lift, and help slow the aircraft during landing.The wing itself will have a high aspect ratio—in other words, a long, thin shape that helps (at the expense of manoeuvrability) to reduce aerodynamic drag. The problem with long wingspans on passenger aircraft, however, is that a plane might not fit into the gates at airports. Hence the idea is to fold the wing tips up once the plane is on the ground.Hinge and bracketFolding wings are not a new idea. Naval aircraft, which already have a low aspect ratio, have long folded their wings in order to fit into the cramped confines of aircraft-carriers. And Boeing, an American firm that is Airbus’s chief rival, is developing a new version of its 777 aircraft, the 777X, which will also fold its wing tips. (When unfolded, these will add 3.4 metres to the length of each wing.)Airbus, though, is giving this idea a new twist, by borrowing a trick from the albatross. During long flights, an albatross locks the elbow joints of its extended wings to make them rigid. Thus fixed, they work much like those of a glider (a type of aircraft that has among the highest aspect ratios of all). The bird unlocks its wings and flaps them when it needs to manoeuvre or cope with gusty conditions.The Airbus wing tips will do something similar. When unlocked in flight, they will be capable of flapping freely up and down during gusts of wind or periods of turbulence. In this way, says Oliver Family, who leads the project, aerodynamic loads on the wing will be reduced, allowing the wing to be made lighter—which, in turn, improves fuel economy. The flapping tips will also help provide a smoother flight. In addition, the project will explore the use of sensors that could spot gusty conditions ahead of the plane and prepare the wing tips for flapping.The converted Citation, which is due to fly by 2024, will be operated remotely by a pilot on the ground. This is a safety measure, because Airbus intends to use its test flights to push the aircraft to its limits. The company says it is not committed to using these new systems in future aircraft. But if one or other of them proves its worth, passengers gazing idly out the window of future airliners are likely to see the wings on their aircraft moving in unusual ways. ■