## What Keeps An Airplane In The Air?

Many science books and articles say that the Bernoulli Principle is what keeps an airplane in the air. But, is it really?

Airplane wings are shaped so that a cross section looks something like this. Air has farther to move over the top of the wing than it does under the wing. This makes the air move faster over the top of the wing. And this makes the air pressure less on the top surface of the wing. This lowering of the air pressure is the Bernoulli Principle, and it makes the air suck (more or less) the wing upward.

But, the Bernoulli Principle is not the only force pushing (or pulling) the airplane upward. It turns out that the wing is normally tilted upward (as in the diagram), so that the moving air is actually pushing the wing up, much as water pushes on a water ski. Those little balsa-wood gliders that little kids throw around have flat wings, and do not use the Bernoulli Principle at all. And they fly just fine. And this actually is normally a greater upward force on an airplane's wings, than the Bernoulli Principle is.

When an airplane is climbing, the propellor provides a significant upward force. But in level flight, it does not provide much upward force, if any.

Glider pilots (and many birds) seek out upward-moving air currents. This is also a significant upward force.

A Bernoulli Box is a kind of hard disk, for computers. The Bernoulli Principle is used to keep the head from scratching the surface of the magnetic disk of this device. The head essentially has little wings, and there is a current of air over the spinning disk. The Bernoulli Principle keeps the head at just the right height over the disk.

Perform this experiment. Fold a small piece of paper or note card, like this picture. Set it on a table. I'll bet you (a zillion dollars) that you can't blow at the paper and flip it over. The harder you blow, the more the paper sucks down toward the table. This is the Bernoulli Principle. You may be able to flip the paper over by blowing at the side instead of the opening. By the way, I think I owe my brother several zillion dollars.