Folded for Speed

Wings spread as it wheels through high, slow arcs, wings tucked back as it plummets in a swift hunting dive, the peregrine falcon is a picture of functional perfection. No airplane has yet come close to copying its easy versatility. But aeronautical engineers have never stopped trying, and the Department of Defense is convinced that government scientists have finally turned the trick. Last week leaders of the U.S. aircraft industry were locked in fierce competition for the privilege of building a "variable geometry" fighter that can stretch its wings at low speeds during take-offs and landings, or fold them like the predatory falcon for highspeed flights.

No matter what company wins the contract (which may well be the last really large award in the fading manned-fighter-plane business), the new plane will be based on the brilliant research directed by Aerodynamicist John Stack at the Langley Research Center of the National Aeronautics and Space Administration.* It is a solution to problems that have expanded as planes have improved.

Early airplane designers did not have to worry about changing their wings in flight. The straight, thick wings that got their ships off the ground served equally well in low-speed flight. But as airplanes became faster, their wings had to be thinned down and shortened to cut drag at high speed. And since thinner, shorter wings have less lift, the new fast planes needed longer runs to get them off the ground. When airplane speeds were boosted by jet engines, designers resorted to swept-back wings, which function better up near and above the speed of sound.

But this dodge has its disadvantages too: at the speeds of landing and takeoff, sweptwing ships are hard to handle. Airmen began to yearn for wings that would be long, thick and straight for takeoffs, yet short, thin and swept-back for highspeed flight.

False Start. The obvious solution was an airplane with wings that could be changed in flight. Several planes of this type were built and rejected; they flew, but not well. Principal trouble was that when their wings were slanted backward, they applied their lift far backward too. Control became all but impossible.

In 1958, Stack and his group started all over again. They finally designed an odd-looking airplane with unusually wide, thick fixed wing roots. Only the outer segment of the wing is movable. On take-off the wingtips are extended, and since they, too, are fairly thick, they give plenty of lift, allowing the plane to take off at slow speeds. As speed increases, the wingtips are slanted farther and farther backward.

But the point at which their lift is applied is only slightly shifted. Part of each wingtip swings into its wing root and ceases to produce lift. The part still exposed also loses lift because the airstream, slanting over it diagonally, is less disturbed by its thickness. Only the fixed wing roots do not change. When the airplane reaches top speed, with wings folded far back, the wide wing roots take over much of the lifting job.

Fast Finish? So far, Stack's models have been tested only in wind tunnels, but they already look singularly promising. A plane built in this manner should be able to take off slowly after a short run, then fold its wings to fight at supersonic velocity. In addition, it will be able to loiter for long periods at slow, fuel-saving speed before accelerating into action. It will also be able to fold its wings and fly extremely fast just above the ground, where air resistance is high but where enemy radars cannot find it. If it lives up to its potential, the variable-geometry plane will be that rare bird--a fighter that can serve equally well for all three military services.

The variable-geometry wing may even supply an unexpected bonus. If it enables an airplane to reach high supersonic speeds a few feet above the ground, its shock wave may be sufficiently destructive for use as a military weapon, knocking holes in most structures that the plane flies near. And when a supersonic airliner is finally developed, it may well have variable wings to get it off the ground at reasonable speed and help it climb to high altitude without using too much fuel.

When the giant plane is high enough (about 45,000 ft.) so that its shock wave will not be annoying at ground level, it will be able to furl its wings and cruise at two or three times the speed of sound.

* In 1958 the National Advisory Committee for Aeronautics (NACA) became NASA, and its quiet, effective work on aircraft was obscured by a blaze of space publicity.

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