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The major contribution made by the Wright brothers was to give up trying to imitate the flapping of a bird’s wings and to copy, instead, the soaring flight of eagles. Fixed-wing aircraft, plus the air screw or propeller, started man on his flight to the stars.

Aircraft have developed so rapidly in the 40 years between Kitty Hawk and World War II jets that millions of men alive today can remember when flight was a crazy idea, when 60-mph bobsled races made men wonder whether they could live at such high speeds. We know now they can—that there is no limit to the speed a man can withstand though there is a limit in acceleration. But more about that later. For now let’s try to answer the question: What holds an airplane up? The answer is air—more specifically air pressure.

The ordinary airplane is held up by air pressure on its wings and this is explained by a very strange theorem first propounded by the Swiss mathematician Bernoulli. Bernoulli’s theorem states that when a fluid flows past a fixed object the pressure exerted sidewise on the object by the fluid decreases as the velocity of flow increases.

For example, start a water hose spurting a jet of water straight up in the air, then place a Ping-pong ball on top of the jet. The ball bounces and twists but manages to stay on top of the stream of water—defying all apparent logic. What actually happens as the water flows around the ball? First it tends to slip to one side, say to the left. The water divides around the ball, much of it shooting straight up on the right side of the ball; some of it forced to take the long way around to the left. The detour makes the water slow down and as it slows its sidewise pressure builds up. Simultaneously the pressure of the high-speed water shooting up the right side of the ball has lowered, and so the ball moves toward the low-pressure side, away from the high-pressure side and back into the center of the jet.

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