What makes an airplane fly? — SDH, Vicksburg, MS
While there are several ways to understand how air supports a plane’s weight, I will look at it first in terms of the deflection of the air flowing past the plane’s wings. As the plane moves forward, air flows both over and under the plane’s wings. It flows across the wing from its leading edge to its trailing edge. The air that strikes the inclined lower surface of the wing is deflected downward and leaves the wing’s trailing edge with a slight downward component to its motion. The air that flows over the arced and inclined upper surface of the wing travels a more complicated route, curving up, over, and down before leaving the wing’s trailing edge with a slight downward component to its motion. In both cases, the wing has made the air accelerate downward by pushing the air downward and it is the nature of our universe that the air must push upward on the plane in response. It’s a case of action and reaction: if one object pushes on another, the second object must push back on the first object with an equal but oppositely directed force. So the plane’s wing pushes down on the air and the air pushes up on the plane. When the plane is moving fast enough and the wings are properly shaped and/or tilted, the upward force that the air exerts on the wings can support the weight of the plane and suspend it in the air.
Another important view of flight involves air pressure in the streams of air flowing over and under the plane. When the air passing under the wing curves downward, it actually does so because the pressure just under the wing is higher than the pressure far from the wing—the air stream is experiencing an overall downward force due to this pressure imbalance and this downward force is deflecting the air stream downward. When the air passing over the wing arcs up, over, and down, it is also doing so because the pressure just above the wing is different from that far from the wing. In this case, the pressure just over the wing’s leading edge is quite high—enough to deflect the air stream upward initially. But the pressure over the rest of the wing’s upper surface is very low and the air stream curves inward toward the wing; arcing downward so that it leaves the wing’s trailing edge with a small downward component to its motion. Overall, there is a low average pressure above the wing and a high average pressure below it. This pressure imbalance produces an overall upward force on the wing and supports the plane’s weight.
These two views of flight—one involving deflection of the air stream and the other involving pressure imbalances—are intimately related to one another and really only two descriptions of the same process. Incidentally, the low pressure just over most the wing causes the air flowing over that wing to speed up. That’s Bernoulli’s equation in action—when air following a streamline experiences a drop in pressure, it accelerates in the forward direction.