How Does an Airplane Fly?

The moment you doubt whether you can fly, you cease forever to be able to do it.

— J. M. Barrie, Peter Pan


As I sit on an airplane, I ponder the question, how, exactly, is this thing staying up in the air. The particular aircraft on which I am seated, the McDonnell Douglas MD-80 weighs almost 80,000 pounds empty. With crew, cargo, and passengers, we are easily talking about 100,000 pounds plus. Planes, however, rarely crash, and they almost never fall from the sky. What keeps them airborne?

The answer was first described by 18th century Swiss mathematician and physicist Daniel Bernoulli*, who stated that, under the principle of conservation of mass, the same volume of fluid lowers in density and the volume in which it is contained increases. Put simply, the same amount of air is thinner in a big container than in a small container.

*The Bernoulli’s were a very talented clan. Besides Daniel, there was Jacob, who pioneered a fair amount of number theory, Johann, who invented a lot of calculus, as well as Nicholas, Nicholas II, Jacob II, Johann II, and Johann III, all of whom made significant contributions to mathematics, physics, astronomy, and biology. It seems as if they were all very blinded with science.

How does that relate to keeping me from going the way of Icarus? The answer is that marvel of modern engineering, the airplane wing, is curved on top and flat on the bottom. This means, that as air passes over the top of the wing it has to travel farther than the same amount of air traveling under the bottom of the wing. Since that air is going over the top moves a longer distance that the air passing under the bottom, it is less dense. In other words, a moving airline wing has more pressure below the wing than on top of the wing. This provides lift and keeps the plan flying.

As anyone who has been on an aircraft realizes, this principle holds only when the wing is moving. If the wing is not in motion, or if the wings motion is not at a high enough velocity, air will rush in and fill the lower pressure, and lift will not occur. This is one of the reasons a plan must move — and move fast — in order to stay in the air.

Well, time to turn off my electronics (whether or not that can crash a plane is a topic for another Fun Fact), so off we go, into the wild blue yonder.

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