Science Puzzle
How Does a Plane Stay Up?
You were probably taught that a wing's curved top makes air travel further and therefore faster, and that this faster air has lower pressure, which sucks the wing upward.
That story is wrong. Aeroplanes fly upside down. Paper darts have flat wings. So what really holds a plane up?
The Answer
A wing pushes air downward, and the air pushes the wing upward in return. That is the whole of it, and it is nothing more than Newton's third law: every push has an equal and opposite push back.
Watch the air leave the back of a wing and you find it is no longer travelling level. It has been turned downward, tonnes of it every second. Throwing that much air down demands a downward force from the wing, so the air must be exerting an equal upward force on the wing. Call it lift.
A wing achieves this by meeting the oncoming air at a slight upward tilt, called the angle of attack, and by having a shape the airflow can follow around without tearing away. The curved upper surface helps the air stay attached and turns it more gently and more efficiently, and yes, the air over the top does move faster and its pressure does fall. All of that is real. The wrong part of the schoolbook story is the claim that air must rejoin its neighbour at the trailing edge, which is simply not true and gives the wrong number when you calculate it.
Tilt a flat plate into the wind and it flies. That is a paper dart. Roll an airliner inverted, tilt the nose up, and it flies too. Turn air down, go up.
The principle: Lift by momentum change. A wing deflects a large mass of air downward, and by Newton's third law the air pushes the wing upward with an equal and opposite force.