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Technology

# The Physics of . . . Airplanes

## An old, lofty theory of how airplanes fly loses some altitude

January 20, 2001 12:00 AM

A Cessna Citation leaves a deep trough in the clouds beneath it, proof that it stays aloft by pushing air down. Photo Courtesy of Cessna Aircraft Company

Flight Instructions The forces acting on an airplane's wings are as subtle as they are elemental. According to a principle known as the Coanda effect, air flowing over the top of the wing sticks slightly to the surface and is pulled downward. This produces a low-pressure zone above and a high-pressure zone below, which pushes the wing up. The greater a wing's "angle of attack," the more powerful its lift. Graphics adapted from Understanding Flight, David F. Anderson and Scott Eberhardt, McGraw-Hill, 2001; Graphics by Matt Zang

To bend the air downward, the wing has to exert a force on it (that's Newton's first law). That action inevitably elicits an equal and opposite reaction (Newton's third law). By means of the low-pressure zone above the wing and the higher pressure below it, the air exerts an upward force on the wing: That's lift. The size of the force is equal to the mass of air the wing has diverted downward multiplied by the acceleration of that air (Newton's second law). A pilot can increase the lift by flying faster (adding power) or by increasing the angle of attack (pulling back on the stick); either way the wing diverts more air down and behind the plane. The wings of a 250-ton airliner, Anderson calculates, pump down about 250 tons of air every second. "That's the problem with Bernoulli," he says. "There's no work done— it's all magic. It doesn't make sense that a wing could cut through the air like a knife, leave a small transient ripple in the air, and hold up a 250-ton airplane. A 250-ton airplane is working. It's holding itself up by brute force." Bernoulli, RIP? Well, not quite. After all, aircraft designers do use the Bernoulli principle, with obvious success, in their complex calculations of airflow. And you too can use it, if you want, to understand lift. But then think of things this way: The leading edge of a wing is an obstruction, like a bottleneck in a pipe. It squeezes the air flowing around it, and it squeezes more where the path is more curved— over the top of the wing. Obeying Bernoulli, the flow there speeds up, the press-ure drops, and voil&225;: lift. Both explanations are correct, really; there's no reason to get hot about the subject. But as far as I'm concerned, Anderson and Eberhardt ace a crucial test: They're a lot easier to explain to my third-grader than Bernoulli ever was. We'll see how they fly with the readers.

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