No, the myth isn’t that airplane fly at all — we know they do, but how do they do it? This is one that really bothered a bunch of us when we were in graduate school in physics. How does an airplane fly? I have a substantial investment in knowing that the physics of these big metal monsters is sound. The reason we were worried is the Bernouilli effect. The Bernouilli effect is what sucks papers out of your car window when you’re speeding down the highway — it says that the faster a fluid (e.g., air) moves, the lower its pressure. That’s why the papers get sucked out of the window — they’re drawn towards the lower pressure outside the window, where the air is moving quickly.

For airplanes, the Bernouilli argument goes that the air moving over the top of the wing (where it’s curved, see below) must travel farthe than that moving under the wind (where it’s flat). So, the lift is caused by the lower pressure on the top of the wing relative to the bottom of the wing.

Fine. But then how do planes fly upside down?

The Bernouilli argument above is flawed. There is no reason why two air molecules which hit the front of the wing at the same time must rejoin each other at the trailing edge, which is what the above argument suggests with its “air must go faster along the top of the wing because it’s traveling further than if it had gone below the wing.”

The key lies instead in the “angle of attack” shown in the above diagram. The wing is slanted upward slightly. As the wing moves forward, it pushes air in front of it, which “piles up” under the wing, becomes compressed, producing high pressure on the underside of the wing. At the same time, the upper surface is being pulled away from the air behind it as the plane moves forward. This leaves a low pressure area along the upper surface of the wing. This produces lift.

Another force lifting the wing is that the lower surface of the wing hits air molecules downward as it moves. Every action produces an equal and opposite reaction, so just as when two balls hit each other and move off in opposite directions, the wing hits air downward and this throws the wing upward slightly. This gives some more lift.

This post was adapted from Kenneth Fuller’s website.

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