Blown Flaps For Slow Landings

Constantly increasing air travel has brought commercial airlines constantly increasing headaches. And control of the swift traffic aloft is only a part of the problem. With their high speeds and long landing runs, the big jetliners demand long runways on which to set down, but few cities have that kind of space near by. Convenient, close-in air ports, which are usually small, have become the domain of smaller planes and feeder airlines.

Now pilots of the National Aeronautics and Space Administration are learning how to land the big jets closer to town. At Wallops Island, Va., they bring their Boeing 707 slanting out of the sky at a limping 100 m.p.h.--50 m.p.h. slower than a standard jetliner. They float over the end of the runway and touch down at only 90 m.p.h.--as slow as an old-fashioned propeller-driven DC-3.

How can a big sweptwing jet stay in the air and maneuver safely at such low speeds? The answer can be read in the big landing flaps at the rear edge of its wings. On the landing approach the flaps extend at an unusually sharp angle; sometimes they droop as much as 70DEG. Ordinary flaps would not work effectively at this angle; instead of giving more lift, they would merely create drag as the air passing over them burbled into turbulence.

On NASA's 707, though, the steep-angled flaps have help. Just ahead of their leading edges, where they join the wing, streams of high-pressure air from the compressors of the jet engines spurt out of nozzles and bathe the flaps' upper surfaces, smoothing the air flow and creating extra lift. To supply enough air at 100 lbs. per sq. in., the engines must run at high speed, developing too much thrust for a plane on its landing approach. But the research ship picks up no extra speed; its extra thrust is contained by big clamshell deflectors that can be controlled by the pilot. NASA's 707 drops down to the runway so slowly that its horizontal tail surfaces need special, upside-down flaps, which slant upward from the stabilizer's leading edge, to make the air flow over them properly at low speed.

This system of "blown flaps" was first developed in 1956 by NASA's predecessor, the National Advisory Committee for Aeronautics, but it roused little interest. At that time the airplane industry was more concerned with the high cruising speed of jetliners. Landing speed seemed secondary. Now everyone knows better.

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