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When I was learning for my license, one of the first diagrams I remember was about the wing profile. The air going around the wing and on the upper side it has to travel a longer way, thus generating lower pressure and bang, plane is flying. Same explanation already back at school.

See my other question: if the theory was right, why can planes fly inverted?

So here's the follow up: why is this wrong theory so popular and still part of books?

Wouldn't it make sense to teach students how a wing really works? I mean just look at any RC plane meeting - you'll be amazed what weird designs are capable of flying if there's enough engine power.

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  • $\begingroup$ Because it's simple enough for a fifth grade to understand. :-) Oh, and I usually say that if you have enough power that you can make a plate fly. Also, when considering the explanation, think about aerobatic airplanes with symmetrical airfoils. They shouldn't be able to fly either! $\endgroup$
    – Lnafziger
    Jan 19, 2014 at 14:22
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    $\begingroup$ But it's also simple to say that storks deliver babies. That doesn't mean it's correct. $\endgroup$
    – Krumelur
    Jan 19, 2014 at 14:24
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    $\begingroup$ I never said that it was! :) (and yet there are still people who believe in storks too....). In all seriousness, the "real" reason that wings produce lift is a terribly complicated subject with a lot of different factors, and the average pilot simply doesn't need to understand it. On the other hand, when someone asks a pilot what makes an airplane fly, it sounds better than "magic" or "I don't know, it just works". $\endgroup$
    – Lnafziger
    Jan 19, 2014 at 14:30
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    $\begingroup$ This answer (not the accepted one by the way) shows some of what you would have to do to give a "real" explanation of lift: physics.stackexchange.com/a/77735. NASA also has a webpage about the different explanations and what is wrong with them: grc.nasa.gov/WWW/K-12/airplane/lift1.html. Wikipedia also covers it nicely: en.wikipedia.org/wiki/Aerodynamic_lift $\endgroup$
    – Lnafziger
    Jan 19, 2014 at 17:56
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    $\begingroup$ @Lnafziger Symmetrical airfoils don't generate lift without angle of attack, so that doesn't really discredit 6th grade approximation. Since with angle of attack, you move the stagnation point, making one side longer than the other. $\endgroup$
    – OSUZorba
    Apr 26, 2016 at 22:13

2 Answers 2

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Long story short is that both Newton's third law and the Bernoulli's effect are two different ways of explaining the same phenomena. Accurate equations have been written to explain both. Newton's third law, the air being forced down and forcing the airplane up, is actually the effect of lift though not the cause. The cause of lift is the change in pressure.

The falsehood that is taught is usually that the two little air particles actually meet up at the same point downstream at the trailing edge of the wing at the same time. This is completely false. In fact, if you were to actually track two air particles, one over the top, and one across the bottom, the one going over the top of the wing will probably arrive much sooner than the one on the bottom! [Even though the distance IS longer] It picks up that much speed. Also they don't actually really meetup again afterwards at all.

The real pivotal relation (which I learned in my Advanced Aerodynamics class at ERAU using John. D. Anderson's book Intro to Flight) is the Kutta-Joukowsky theorem. Taking the time to explain this to a Private Pilot student would probably be a waste of your time and theirs but I will sum it up here anyway to be fair to the question. I will assume incompressible flow because after about Mach .3< it starts to get more complicated.

The Kutta-Joukowsky theorem takes the integral of velocity times the cosine of the incremental angle of the distance along the closed curve. This is a quantity called circulation. The lift equation is then formed as lift equals density (altitude basically) times velocity times circulation. So if you really want get in-depth here is an article from MIT that explains potential flow theory.

What you will find is that the cause of lift is indeed directly related to Bernoulli's principle in that the change in pressure is what creates the lift. However it has nothing to do with particles meeting up at the trailing edge because they never really do meet up. This doesn't discount Newton's third law either: lift can be explained that way but it is the effect of lift, not the actual cause. And yes, like you mentioned, even a flat-plate for example can create lift when angled right in a wind tunnel.

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    $\begingroup$ The curved wing and a flat plate both create lift in the same manner. It's actually drag that is reduced more by the curved wing. Although to be fair keeping the boundary layer attached longer allows for more lift. The shape of the wing is more for keeping the airflow attached longer. Generally you want more INVISCID flow but it is actually the viscous boundary layer that keeps that inviscid flow attached. So the shape of the wing becomes a balance of creating the smallest boundary layer that can still hold the flow without getting airflow separation (stall). $\endgroup$
    – p1l0t
    Jan 19, 2014 at 18:43
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    $\begingroup$ The lift produced by a wing is exactly equal to the mass of the air deflected downward by the wing times acceleration imparted to the air as the wing moves forward. Newton's second law is why wings produce lift. Bernoulli's equation is derived from F=MA. $\endgroup$
    – Jim In Texas
    Jan 21, 2014 at 6:02
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    $\begingroup$ @p1l0t I don't buy the causality arguments here. If 'the change in pressure is what creates lift', then what causes the change in pressure? You cannot simply say 'Bernoulli' or 'Kutta-Joukowsky' because the equations are agnostic about causation, and anyway you would have to explain why they apply in the situation of a wing moving through an otherwise unconstrained air mass. All chains of causality lead to Newton, and to lift being the reaction to the wing's downwards acceleration of the air it passes through. You need hydrodynamics to explain the details, not the root cause. $\endgroup$
    – sdenham
    Jul 9, 2014 at 15:10
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    $\begingroup$ The best part about science is that it is still true whether you buy it or not. Anyway I think I explained the difference between the cause and effect rather well and included some great sources for further reading. You can try and say the pressure change is an effect of something else which is an effect of something else etc.. etc.. but that could go on forever. My point is still valid. Also I didn't just SIMPLY say 'Kutta-Joukowsky' I explained it as best I could (an entire paragraph) and cited an article from MIT that explains it further. $\endgroup$
    – p1l0t
    Jul 10, 2014 at 15:51
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    $\begingroup$ @sdenham: You need fluid dynamics to explain that the wing deflects the air downwards in the first place. $\endgroup$
    – Jan Hudec
    Jul 17, 2014 at 4:52
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Wouldn't it make sense to teach students how a wing really works?

No, not really.

Ask any (good) CFI and they'll tell you that there are certain topics that a student really needs to understand properly. Examples:

  • What's a stabilized approach and how do you fly one? (A disappointing number of students tell me it means holding the same airspeed all the way down)
  • Why do we clear the prop area? (You'd think this would be too simple to screw up, but there's been more students than I've got fingers on my throttle hand who haven't yelled "clear!" until they're already cranking the engine.)

And so on.

How a wing really works is not one of these topics - the consequences of misunderstanding a stabilized approach are that you screw it up and your landings are terrible (or even dangerous). The consequences of misunderstanding prop safety are that someone loses a hand or dies. The consequences of misunderstanding how a wing generates lift are...well...the wing keeps flying anyway.

Now, if the student is an aerospace engineer, they'll need to know the real story, but for a pilot, they can continue in their ignorance for their entire career and never be negatively affected (unless they meet an aerospace engineer in a bar and get into a fight).

To extend the point, I myself tell students that to properly understand GPS, you need to get into the theory of relativity, but for a functional but incorrect understanding, you can think of it as "DME-in-space". It's very wrong on several levels, but it's enough to satisfy the needs of that category of student - same as the wing pressure-differential explanation.

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    $\begingroup$ I agree with this answer, but it would be nice if there were an "easy" description of how a wing generates lift that wasn't so obviously wrong, and couldn't even possibly explain several common scenarios. I mean what else have they been lying to us about all of this time??? :) $\endgroup$
    – Lnafziger
    Jan 19, 2014 at 17:28
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    $\begingroup$ @Lnafziger - Well, for one, the real reason for TBO time is to swap out the hamster on the wheel in there. $\endgroup$
    – Steve V.
    Jan 19, 2014 at 17:33
  • $\begingroup$ Haha, that must be one strong mutant hamster! $\endgroup$
    – Lnafziger
    Jan 19, 2014 at 17:35
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    $\begingroup$ I also agree with this answer, but I did take a shot at explaining lift that wasn't so obviously wrong in my own answer with some links for further reading. Still, gave him the vote up though. As far as students they need to know that they can pull up for more lift until the critical angle and no more. See what I did there. $\endgroup$
    – p1l0t
    Jan 19, 2014 at 18:29
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    $\begingroup$ Upvoted both of you. $\endgroup$
    – Greg M. Krsak
    Jan 19, 2014 at 20:24

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