Wing design - the unending quest for new ways to fly

Aircraft wings have long provided a fertile outlet for the imagination - from Leonardo da Vinci's sketches of craft with birdlike flapping wings to contemporary visions of circular-wing planes.

Most of the earlier ideas didn't - and probably couldn't - fly. They were victims of too little technical know-how or too much inventiveness.

Today, however, a new generation of wing designs is rolling off the drafting tables. These new ideas are driven by the need to boost fuel efficiency in planes. They could lead to dramatic performance improvements in some military and commercial aircraft by 1990.

Many of the designs have been around for decades, but new materials and manufacturing techniques are only now making them feasible.

One concept involves perforating the tops of wings with thousands of tiny holes to reduce drag. With conventional wings, the friction of the airflow over the surface causes a degree of turbulence. By pumping air through the tiny holes , scientists believe they can create a cushion of smooth-flowing air.

Indeed, engineers at McDonnell Douglas Corporation claim this technique, called ''laminar flow control,'' could boost fuel efficiency on aircraft by 20 percent or more. The aerospace firm has been working on a design with engineers from the National Aeronautics and Space Administration. NASA is expected to flight-test a partially perforated wing by year's end, though a commercial version is still several years away.

The idea of perforating wings has been around at least since the 1930s. But it wasn't until the recent development of new composite materials (usually graphite or glass fibers embedded in plastic) and better manufacturing tools (such as electron-beam drilling) that engineers considered the concept practical.

Snags remain, though. Better ways of keeping the wings clean are needed; bug spots can ruin the effect of the system. ''We're still not sure it will work,'' concedes one McDonnell Douglas official.

In a more dramatic design alteration, Grumman Aerospace is developing a jet fighter with wings that sweep forward from the rear of the aircraft. A prototype of the backward-looking plane is expected to fly within a year. The possible advantages: less drag, better handling in some conditions.

Here, too, designers have been well aware of the aerodynamic advantages of the idea for decades. But they couldn't come up with a metal wing that was strong, yet still lightweight enough to withstand the punishment of high-speed flying. The answer apparently has been found in new plastic composites reinforced with graphite and boron fibers. The first wave of forward-swept wings will be used in jet fighters. But there's no reason, experts say, that the design can't eventually be adapted to jumbo jets as well.

A more subtle change under development is the ''mission adaptive'' wing, which to a degree mimics the pliancy of a bird's wing in flight. Hydraulic controls tilt the leading and trailing edges of a wing to suit the flying conditions. In theory the system works like conventional wing flaps. But in practice it is much different. Instead of a separate fin tilting up or down, the edge of the wing - a skin of tough but flexible fiberglass composite - bends and changes shape. Result: no break in the wing surface, better performance.

Wind tunnel tests have convinced engineers such a system will squeeze 25 percent more from a gallon of jet fuel. That theory will be tested next summer, when NASA and the Air Force put up a Boeing-built aircraft with adaptive wings. Initially, the system will be manually controlled. But within two years, engineers expect to have computers and electronic sensors on board that will automatically adjust the wing.

Farfetched though it sounds, this technology could lead to an entire aircraft - fuselage and wings - that changes shape in the air. ''The ultimate would be when you can design the airplane as you go along - in flight,'' says Kenneth Szalai, engineering director at NASA's Dryden Flight Research Center in California.

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