# What really allows airplanes to fly?

What aerodynamic effects actually contribute to producing the lift on an airplane?

I know there's a common belief that lift comes from the Bernoulli effect, where air moving over the wings is at reduced pressure because it's forced to travel further than air flowing under the wings. But I also know that this is wrong, or at best a minor contribution to the actual lift. The thing is, none of the many sources I've seen that discredit the Bernoulli effect explain what's actually going on, so I'm left wondering. Why do airplanes actually fly? Is this something that can be explained or summarized at a level appropriate for someone who isn't trained in fluid dynamics?

(Links to further reading for more detail would also be much appreciated)

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There is an art, or rather a knack to flying. The knack lies in learning how to throw yourself at the ground and miss. ~Douglas Adams, Hitchhikers Guide to the Galaxy :-) – Pratik Deoghare Nov 6 '10 at 11:19
That would be the art of orbiting ;-) – Sklivvz Nov 6 '10 at 11:26
Great question. People always state that it's a misconception to attribute flight to the Bernoulli effect, but it's rarely explained what the real effect is! – Noldorin Nov 6 '10 at 17:01
xkcd did a comic on this recently. – user172 Nov 8 '10 at 4:12
@Noldor: As I understand it, the Bernoulli effect is not entirely a misconception in itself, but a different way of looking at Newton's Third Law. The misconception is the "equal time fallacy" -- the notion that a parcel of air flowing over the top of the wing must reach the trailing edge at the same time as a companion parcel flowing under the wing. See en.wikipedia.org/wiki/… – Fred Larson Nov 10 '10 at 21:56

A short summary of the paper mentioned in another answer and another good site.

Basically planes fly because they push enough air downwards and receive an upwards lift thanks to Newton's third law.

They do so in a variety of manners, but the most significant contributions are:

• The angle of attack of the wings, which uses drag to push the air down. This is typical during take off (think of airplanes going upwards with the nose up) and landing (flaps). This is also how planes fly upside down.
• The asymmetrical shape of the wings that directs the air passing over them downwards instead of straight behind. This allows planes to fly level to the ground without having a permanent angle on the wings.
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I think a clearer way to state this is to say that the wings push air downwards thus producing lift, and the airfoil shape is simply more efficient that a simpler shape, such as a wing with a rectangular cross-section. There is nothing magical about an airfoil except that it produces the least possible drag for a given amount of lift. – Colin K Dec 27 '10 at 17:22
How do you not talk about Bernoulli's principle here? Angle of attack has nothing to do with it. They do not push air downwards. This is a massively upvoted and accepted answer on this site? My word. – Robusto Aug 23 '11 at 1:29
@Colin K: Really? Then why are airfoils shaped the way they are? Why are they curved on top? – Robusto Aug 23 '11 at 1:32
@Robusto: I would make a slight correction to Sklivvz's answer. Wings don't just push air downward, they pull it downward. The top surface of the wing is more important than the bottom. If the flow separates from the top surface, the wing stalls. That's what happens at a high-enough angle of attack, and it is exacerbated by anything that makes the surface rough. – Mike Dunlavey Dec 28 '11 at 13:52
@Sklivvz: Sorry, being a student pilot, and loving physics, this is a favorite topic. Reviewing your answer, I'd point out that aerobatic aircraft have symmetrical airfoils, just so they can fly inverted as well as upright. Positive AofA gives positive lift, and negative negative. Since normal aircraft don't fly inverted, the airfoil is asymmetrical, but that's just an optimization. Aircraft may look like they are level, but the wings are not, they are at an AofA. The slower they go, the higher they must nose-up to get the necessary lift. – Mike Dunlavey Jan 2 '12 at 21:26

From Stick and Rudder by Wolfgang Langewiesche, page 9, published 1944:

The main fact of all heavier-than-air flight is this: the wing keeps the airplane up by pushing the air down.

It shoves the air down with its bottom surface, and it pulls the air down with its top surface; the latter action is the more important. But the really important thing to understand is that the wing, in whatever fashion, makes the air go down. In exerting a downward force upon the air, the wing receives an upward counterforce--by the same principle, known as Newton's law of action and reaction, which makes a gun recoil as it shoves the bullet out forward; and which makes the nozzle of a fire hose press backward heavily against the fireman as it shoots out a stream of water forward. Air is heavy; sea-level air weights about 2 pounds per cubic yard; thus, as your wings give a downward push to a cubic yard after cubic yard of that heavy stuff, they get upward reactions that are equally hefty.

That's what keeps an airplane up. Newton's law says that, if the wing pushes the air down, the air must push the wing up. It also puts the same thing the other way 'round: if the wing is to hold the airplane up in the fluid, ever-yielding air, it can do so only by pushing the air down. All the fancy physics of Bernoulli's Theorem, all the highbrow math of the circulation theory, all the diagrams showing the airflow on a wing--all that is only an elaboration and more detailed description of just how Newton's law fulfills itself--for instance, the rather interesting but (for the pilot) really quite useless observation that the wing does most of its downwashing work by suction, with its top surface. ...

Thus, if you will forget some of this excessive erudition, a wing becomes much easier to understand; it is in the last analysis nothing but an air deflector. It is an inclined plane, cleverly curved, to be sure, and elaborately streamlined, but still essentially an inclined plane. That's, after all, why that whole fascinating contraption of ours is called an air-plane.

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Excellent explanation! – Ingo Feb 19 '11 at 10:50

Since you asked for an explanation appropriate to an non-specialized audience, maybe this will do: "A Physical Description of Flight; Revisited" by David Anderson & Scott Eberhardt. It is a revision of the earlier "A Physical Description of Flight" (HTML version).

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 Really great paper. – Justin L. Nov 6 '10 at 7:34 +1 This looks like a pretty handy explanation. – Noldorin Nov 7 '10 at 23:07