Aerodynamics: Slotted for Smoothness

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To the casual eye, Northrop Corp.'s brand-new X-21A airplane has the look of an already obsolescent bomber. It is a familiar twin-jet Douglas B66 fitted out with oversize, swept-back wings. But a close look shows a more significant change. There are hundreds of paper-thin slots slicing through the wings' metal skin. And those slots, if the calculations of Northrop's Norair Division scientists prove correct, may well revolutionize the aircraft industry.

Designed by Swiss-born Aerodynamicist Werner Pfenninger, the intricate tracery promises to be the first practical answer to a problem that is as old as airplanes: how to smooth out the turbulent air that burbles along the surface of a moving wing. Every airplane wastes some of its power overcoming the drag of that churning air, but not until modern planes moved up toward jet speeds did the drag demand a remedy. Slow planes can live with their own slight turbulence; a fast ship becomes a fuel-gulping monster as it fights the furious air waves that swirl and eddy over its wings.

Perfect Maze. The solution, surprisingly, has long been obvious. But while engineers knew that the laminar (smooth) airflow they wanted could be had by sucking any turbulent air into a wing's inner cavity, putting theory into practice proved a stubborn puzzle. Dr. Pfenninger worked on his LFC (laminar flow control) wing for 23 years before perfecting its closely packed slits that are only a few thousandths of an inch wide. Under each slit, a small chamber gathers the incoming air and channels it through pin-size holes into ducts that lead to streamlined nacelles hanging under each wing. Inside each of those nacelles, a pair of light, powerful gas turbines—one for the forward part of the wing, one for the more turbulent air in the rear—generate the suction that keeps the system operating.

Northrop engineers, who have run thousands of hours of wind-tunnel tests, say that once the suction is started, there is smooth, laminar flow over both top and bottom of their new wing. Up to 80% of the friction drag is eliminated—and this figure includes compensation for the drag caused by the nacelles and for the power needed to run the turbines. With drag so drastically reduced, an airplane uses much less fuel, thus can fly farther or carry more payload. The null will not have its first flight tests until next month, but Northrop is already making a joint study with Lockheed to apply LFC to Lockheed's EUR-141 jet cargo plane. Project Manager Don Warner is sure that the sucking slots can increase a C-141's payload by 74% or its nonstop range by 50%.