Myth 5: How do airplanes fly?

by Stephanie Chasteen on April 18, 2008

No, the myth isn’t that airplane fly at all — we know they do, but how do they do it? This is one that really bothered a bunch of us when we were in graduate school in physics. How does an airplane fly? I have a substantial investment in knowing that the physics of these big metal monsters is sound. The reason we were worried is the Bernouilli effect. The Bernouilli effect is what sucks papers out of your car window when you’re speeding down the highway — it says that the faster a fluid (e.g., air) moves, the lower its pressure. That’s why the papers get sucked out of the window — they’re drawn towards the lower pressure outside the window, where the air is moving quickly.

For airplanes, the Bernouilli argument goes that the air moving over the top of the wing (where it’s curved, see below) must travel farthe than that moving under the wind (where it’s flat). So, the lift is caused by the lower pressure on the top of the wing relative to the bottom of the wing.

Fine. But then how do planes fly upside down?

The Bernouilli argument above is flawed. There is no reason why two air molecules which hit the front of the wing at the same time must rejoin each other at the trailing edge, which is what the above argument suggests with its “air must go faster along the top of the wing because it’s traveling further than if it had gone below the wing.”

The key lies instead in the “angle of attack” shown in the above diagram. The wing is slanted upward slightly. As the wing moves forward, it pushes air in front of it, which “piles up” under the wing, becomes compressed, producing high pressure on the underside of the wing. At the same time, the upper surface is being pulled away from the air behind it as the plane moves forward. This leaves a low pressure area along the upper surface of the wing. This produces lift.

Another force lifting the wing is that the lower surface of the wing hits air molecules downward as it moves. Every action produces an equal and opposite reaction, so just as when two balls hit each other and move off in opposite directions, the wing hits air downward and this throws the wing upward slightly. This gives some more lift.

This post was adapted from Kenneth Fuller’s website.

{ 10 comments }

Steve Morgan June 29, 2009 at 11:03 pm

Thanks for the detailed explanation that I could understand. It’s amazing what we, as teachers, teach over the years information that is ‘wrong’. For years I have used the Bernoulli Principle to explain lift and flight. I’m now 66 and have a lesson on flight to teach to 7-8year olds next week. I only do this for fun but the kids take it seriously!!!
I will try my hardest to explain the Angle of Attack approach that you outline and then we’ll get on to designing and testing
paper darts!!!!
Only last week I found out that I been depicting the structure of atoms wrongly for years!
Keep up the good work
Steve

sciencegeekgirl June 30, 2009 at 4:57 am

Glad you found it interesting and helpful, Steve! I’d understood it wrong for years too. Just to be clear, Bernouilli’s principle does play a role, but the standard explanation, of air having to travel further on the top part of the wing to “meet up” with air on the bottom part is just wrong. They had to explain it to me three times because I had the wrong idea stuck in my head so strongly!

Enjoy.

scienceguy January 21, 2010 at 4:30 am

The NASA website disagrees with your theory. Here is a link if you would like to read it

http://www.grc.nasa.gov/WWW/K-12/airplane/wrong2.html

scienceguy January 21, 2010 at 4:32 am

Sorry its not wrong it just not 100% correct.

sciencegeekgirl January 22, 2010 at 2:53 am

Hi scienceguy,

Thanks for the link, which has a very nice visual simulator to show airfoil lift.

However, I think you misunderstood the post. The NASA website indicate that it is incorrect to say that lift is caused by the action/reaction force of molecules hitting the underside of the wing. You can see that’s wrong using their simulation, where there is no lift when the airfoil is not tilted.

My suggestion above (which isn’t really mine, but taken from a bunch of people smarter than me) is that the tilt of the airfoil causes an increase in pressure on the underside of the wing and a low pressure on the upside of the wing. That’s not the same as the action/reaction “wrong” theory that the NASA site debunks (quite nicely, I might add!)

Stephanie

jim-bob July 2, 2010 at 4:18 pm

actually, airplane do fly using the bernoulli principle (otherwise wings would not have to be airfoils, they could just use any old more or less flat shape) and often angle of attack depending on which type of aircraft it is and what it’s doing (e.g. a glider flying level uses bernoulli principle more then a fighter jet doing a steep climb.)

however when flying upside down the angle of attack is increased to the point where it is producing more lift then the bernoulli principle (and gravity) is trying to pull the aircraft down (if you put an aircraft wing upside down it creates downforce as opposed to lift, formula one cars take advantage of this fact.)

so yes in a sense it is a myth, but it does have a rather large nugget of truth attached.

sciencegeekgirl July 2, 2010 at 6:03 pm

Nice distinction, jim-bob. The bernouilli principle does indeed play a role. However, the argument that is given in textbooks — that the pressure difference is caused by the fact that the two air molecules must meet at the other end of the wing — is flawed. And the argument that the bernouilli principle is the only factor is also flawed. So, thanks for pointing out the more honest description!

ZT205 August 14, 2012 at 2:20 am

You still don’t explain how airplanes fly upside down, since doing so reverses the angle of attack. How does this work?

Stephanie Chasteen August 15, 2012 at 3:53 pm

ZT205, it’s true, I didn’t address that part of the real science.

Looking around, it seems that the answer is that the pilot must tilt the nose of the plane to compensate for the upside-downness, so that the angle of attack still provides lift (rather than pushing the plane down with gravity).

Here’s a link that I found that gave a nice explanation, I thought.

http://www.physlink.com/education/askexperts/ae627.cfm

jack cox April 10, 2014 at 5:54 pm

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