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Intelligent Machines

Sounding an Alarm in the Sky

A “ridiculously simple” device keeps airplanes aloft.

Airplanes are largely the toys or transporters of the middle and upper classes. But one man’s childhood of poverty paved the way for a clever gadget that helps pilots keep their planes in the air.

Born in New York in 1918, Leonard M. Greene was the youngest in a family of five caught in the post-World War I depression. Living on $20 a month, the family had no money for toys, so a 5-year-old Greene began to invent his own playthings. He rejuvenated spent batteries in salt water, built a lighted wagon, salvaged a sewing-machine motor-all with a “trash-can set” of a children’s encyclopedia for reference and inspiration.

Greene called on the same ingenuity at the age of 19, after he and a friend witnessed the nose-first crash of a small plane at a New York airfield. The pilot, Greene’s friend explained, was trying to climb too steeply and caused his plane to lose lift, or “stall.” At that time, stall accidents caused more than half of all aviation deaths.

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As a plane tilts up, the air splits (to flow above and below the wing) at a point progressively farther down along the curve of the wing’s leading edge; if that point is too far down, the air no longer flows properly and lift is lost. So, Greene reasoned, why not put a small vane protruding forward at a spot slightly above the critical point? When the air began to hit the wing at a dangerous angle it would flip the vane up-a contact on the vane could complete an electrical circuit that honked a horn and flashed a warning light in the cockpit.

Greene built his first stall detector from pieces of an alarm clock, and earned a pilot’s license so he could make test flights himself. In 1946, he founded the Safe Flight Instrument Corp. to build the warning system. The $15-to-$25 device, described by one writer as “ridiculously simple in conception and construction,” was hailed in a 1947 Saturday Evening Post story as possibly “the greatest lifesaver since the invention of the parachute.”

Today, you can still see Greene’s simple vane on some commuter flights, but more often you see its complex descendant on the fuselage. Safe Flight continues to supply air-safety technology to major air carriers, the U.S. military and aircraft manufacturers worldwide.

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