Inlet him chat to a shopper at a family bike store in Dayton, Ohio, Wilbur Wright thoughtlessly twisted a piece of cardboard that once had an inner tube when he came up with the idea. The “semi-rigid” way in which cardboard can deform but still retain its hardness could, in his opinion, provide an answer to a small problem he was working on with his brother Orville: how to design a wing for a heavier-than-air aircraft .
From ornithological observations the Wright brothers knew that if a wing on one side of a bird’s body encounters a flow of air coming at a greater angle than on the opposite wing, then that wing will rise. The wing of the plane, which could be made to rotate like that, would help the pilot roll and turn. Using mechanical gears and levers to turn the entire fully rigid wing to make the plane too heavy. The semi-rigid wing, however, could be bent at different angles using a lightweight series of wires and pulleys. Thus, on December 17, 1903, the brothers made the first controlled and long flight of a moving aircraft.
The Wright brothers called their system a skew wing. But this did not continue. Within a few years the aviators began to use a more robust form of control that fitted hinged ailerons and flaps to the rear edges of the wings, and in 1915 Orville (Wilbur died three years earlier) gave way and followed suit. But in a slightly different guise the twisting of the wings returned. And not only that. In their efforts to make the wings more fuel-efficient, aerospace engineers are also looking for inspiration in the wings of birds in other ways, borrowing the trick from the most experienced long-range pilot, the albatross.
Airbus, a giant European manufacturer of passenger aircraft, recently completed a series of tests of new forms of wing control using a model in a wind tunnel at Filton, near Bristol, in south-west Britain. As a result, the company is now installing these systems on a specially built wing, which will be used as an experiment to replace one of the wings of the Cessna Citation business jet. This aircraft will then test new designs in real flight conditions.
One of the features of the eXtra Performance Wing wing, as it is called, is that instead of movable surfaces attached to the wing by hinges, mechanical drives will change the shape of the semi-curved surface at the rear edge. These surfaces will be multifunctional, which means that by moving them up or down, they can be used either as ailerons (which allow the pilot to tilt and turn the aircraft) or as flaps (which provide additional lift). Pop-up spoilers, which appear from the upper surface of the wing, on the contrary reduce the lifting force and help slow down the aircraft during landing.
The wing itself will have a high aspect ratio – in other words, a long slim shape that helps (due to maneuverability) reduce aerodynamic drag. However, the problem with the large wingspan of passenger aircraft is that the aircraft may not enter the airport gates. So the idea is to fold the wing tips up as soon as the plane is lying on the ground.
Hinge and bracket
Folding wings is not a new idea. Naval aircraft, which already have a low aspect ratio, have long folded their wings to fit into the tight confines of aircraft carriers. And Boeing, an American firm that is a major competitor to Airbus, is developing a new version of its aircraft 777, 777X, which will also make up the wing tips. (When expanded, they will add 3.4 meters to the length of each wing.)
Airbus, however, is giving this idea a new twist by borrowing a trick from the albatross. During long flights the albatross fixes the elbow joints of its extended wings to make them stiff. Recorded in this way, they operate similarly to a glider (the type of aircraft that has the highest aspect ratio of all). The bird unlocks its wings and flutters them when it needs to maneuver or cope with gusty conditions.
The tips of the Airbus wing will do something similar. If they are unlocked in flight, they will be able to swing freely up and down during gusts of wind or periods of turbulence. Thus, says Oliver Family, who manages the project, the aerodynamic load on the wing will be reduced, which will make the wing lighter, which in turn improves fuel economy. Waving tips will also help ensure a smoother flight. In addition, the project will study the use of sensors that could detect gusty conditions in front of the aircraft and prepare the wing tips for swing.
The redesigned Citation, which is due to fly by 2024, will be piloted remotely by a pilot. This is a safety measure because Airbus intends to use its test flights to push the aircraft to the limits. The company says it does not seek to use these new systems in future aircraft. But if one or the other proves its worth, passengers who thoughtlessly look out the window of future airliners are more likely to see wings on their planes moving in an unusual way. ■