Now that you know about push and lift, can you see how these forces might relate to airplanes? If we can make air speed up over a wing, the pressure of the air over the wing will drop. The higher pressure air below the wing then pushes the airplane up. How would you shape a wing so that the air moves more quickly over the top than under the bottom?
The "Squeeze the Stream" activity shows what happens when a fluid is forced to flow from a wide space through a narrower channel. For the water to squeeze through a thinner space, something must either compress the water (think of pulling a sponge through a bottle neck) or speed it up. Freely flowing water does not compress easily. Instead, it speeds up as the channel narrows. Water also speeds up as it moves around an object, such as a rock in a river. Air is a fluid, too, and it behaves like water when it moves through a narrow channel or around an object: It speeds up. As you saw with the other activities, when air moves faster, its pressure drops and it pushes less.
When an airplane flies, it pushes air out of the way. That air must go somewhere so it "squeezes" between the wings and the surrounding air. The wings are shaped and tilted so that the air moving over the top has less room than the air moving below the wings. Because it has less room, the air moving over the top must speed up more than the air below the wing. As it moves faster, the air on top of the wing also loses pressure and push. The slower moving air below the wing maintains more of its pressure, which pushes the wing, and the plane, up.
An airplane wing affects moving air much like a rock in a stream affects moving water. Remember that the space around the wing is already jammed full of air, so there's no empty space for more air to move into. As oncoming air hits the wing and moves either over or under it, it speeds up and "squeezes" between the wing and the surrounding air.
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Many books state that air speeds up over a wing because it has further to travel than air moving under the wing. This explanation implies that air separates at the front of the wing (point A) and rejoins behind the wing
(point B), but this isn't true. Air moving over the top of a wing speeds up so much that it arrives at point B
sooner than air that travels beneath the wing.
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