This diagram showing the cross section of a wing does not fully illustrate how a wing produces lift.
The Colditz Glider
Airfoil Aerodynamics

What causes lift, that upward force that keeps flying machines aloft?

Perhaps in a textbook or in a book written for a general audience you've seen an illustration similar to the one shown here—the kind that shows how air flows around a wing.

In this illustration, the air divides at the leading edge of the wing, with some of it flowing up and around the top of the wing, and some of it flowing straight across the wing's flat bottom. The explanation that accompanies such an illustration usually states that the air moving over the top of the wing flows faster than the air underneath, and that this creates a situation in which lower air pressure exists above the wing than below it. The wing, therefore, "lifts" in the direction of lower air pressure.

The above illustration, as it turns out, is not 100 percent correct. And the explanation, although not technically incorrect, is incomplete.

If the shape of the wing alone were responsible for creating lift, how could an airplane flying upside down stay aloft? Wouldn't the lift from its upside-down wings "lift" the plane right toward the ground?

What the illustration fails to show, and what the explanation leaves out, is that for a wing to produce lift, it needs to push air down.

Newton's Third Law states that for every action, there is an equal and opposite reaction. In the case of an airplane or glider, the action of the wings pushing air down causes the reaction of air pushing the wings up. If you stick your flattened hand out of the window of a fast-moving car, you can experience this principle firsthand. If you angle your hand so that the air meets its underside, your hand shoots upward.

 An airplane's wing produces lift when it pushes the air flowing around it down.
This does not mean that the wing's shape is unimportant. The underside of the wing can certainly deflect the air flowing underneath down, but so can the air flowing over the top of the wing. This is because the air tries to hug the top of the wing as it moves over it. And after it flows beyond the wing, the downward-moving air meets the air flowing underneath the wing, forcing it down too.

Keeping the plane aloft requires the right amount of air to be pushed down. If the plane's flight path is level, the weight of the air pushed down is equal to the weight of the plane. And if it's climbing, the weight of the air pushed down is more than the weight of the plane.

 A wing that is positioned at an angle to the oncoming wind will produce more lift.
How much air the wings push down is also determined by the plane's speed. Fast-moving wings move a lot more air than slow-moving wings and so produce more lift. But this doesn't mean that slow-moving wings can't be made to produce more lift. How? The plane can be angled, relative to the air it's flying into, to allow more of the air to hit its wings.

Now we come back to our upside-down flying plane. As long as the plane's wings push air down, we have lift. The wings won't be working as efficiently as possible, but if they're angled enough, the plane can continue its inverted flight for as long as the pilot wants.

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