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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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Krumelur
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  • 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! – Lnafziger Jan 19 '14 at 14:22
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    But it's also simple to say that storks deliver babies. That doesn't mean it's correct. – Krumelur Jan 19 '14 at 14:24
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    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". – Lnafziger Jan 19 '14 at 14:30
  • @Lnafziger If I sit in a piece of plywood and Oracover, 10000ft high, "magic" is what describes it best. :-) – Krumelur Jan 19 '14 at 15:19
  • A link to a source that explains why this explanation is wrong would be great, especially for people not familiar with the subject. I'm not saying you're wrong, but it would help to understand the answers better. – Pondlife Jan 19 '14 at 16:15
  • See also http://aviation.stackexchange.com/questions/466/how-do-i-explain-what-makes-an-airplane-fly-to-grandma – Philippe Leybaert Jan 19 '14 at 16:51
  • I think this might need rewriting - as it stands, it's basically asking for opinions, which means there can't be a right answer. I love the topic, but I think it's a matter of splitting this up into various sub-questions. Perhaps this is a good Topic of the Week idea? – egid Jan 19 '14 at 17:11
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    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: http://physics.stackexchange.com/a/77735. NASA also has a webpage about the different explanations and what is wrong with them: http://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html. Wikipedia also covers it nicely: http://en.wikipedia.org/wiki/Aerodynamic_lift – Lnafziger Jan 19 '14 at 17:56
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    @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. – OSUZorba Apr 26 '16 at 22:13
  • Along the lines of weird designs, there's the old re-entry prototype NASA M2-F2 (unpowered) and M2-F3 (rocket engines), where the "longer" side (bulge) is on the bottom. Also the tail is blunt (that's where the rocket engines go), no sharp trailing edge (Kutta condition ). The lift to drag ratio is low, but this is a re-entry prototype, so a high lift to drag ratio is not a goal, and the space vehicle / aircraft has to deal with hypersonic speeds. Link to youtube video https://www.youtube.com/watch?v=qxe0Do5Jleo . – rcgldr May 01 '18 at 13:50
  • This https://www.youtube.com/watch?v=zp1KzGQdouI shows that motion/lift is possible without Bernoulli. – cibercitizen1 Sep 11 '20 at 08:06

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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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p1l0t
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  • Would it be reasonable to refer to flat plate lift as direct lift and that produced by wing differential pressure (Bernoulli) as induced lift? Also, thanks for the two references. – Terry Jan 19 '14 at 18:30
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    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). – p1l0t Jan 19 '14 at 18:43
  • I should add that much depends on the aircraft and it's mission (aerobatics, training, fighter, airliner, crop duster, etc..) – p1l0t Jan 19 '14 at 18:45
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    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.

     – Jim In Texas Jan 21 '14 at 06:02
  • That is correct. – p1l0t Jan 21 '14 at 16:09
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    @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. – sdenham Jul 09 '14 at 15:10
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    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. – p1l0t Jul 10 '14 at 15:51
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    @sdenham: You need fluid dynamics to explain that the wing deflects the air downwards in the first place. – Jan Hudec Jul 17 '14 at 04:52
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    This is the same argument I always make. I get annoyed all the time with people trying to use this as a gotcha question. I used to design blades in jet engines and they most definitely had a "suction" side and "pressure" side that created the lift. Of course they also changed the direction of the flow, but that was always seen as an "effect." – OSUZorba Apr 26 '16 at 22:21
  • This https://www.youtube.com/watch?v=zp1KzGQdouI shows that motion/lift is possible without Bernoulli. – cibercitizen1 Sep 11 '20 at 08:06
  • @cibercitizen1 Wrong. (nice video though) A flat board still causes changes in pressure. – p1l0t Sep 11 '20 at 13:13
  • @p1l0t Thanks. Well, I think that pressure is higher on windward just because wind hits this side; and there is no wind movement on leeward. – cibercitizen1 Sep 11 '20 at 15:51
  • @cibercitizen1 I'm not sure what you mean by no movement. The concept of a flat plate and a curved wing are the same. The flat plate is just WAY less effcient. If you are making lift it's because you're creating a pressure difference. The only difference between Newton and Bernoulli is one explains the cause and the other the effect. Both are accurate ways of describing (the same) lift. If you had no movement of air on the top (or a mostly static turbulence or something) the wing would be in a stalled condition and not generating lift (or pressure difference). – p1l0t Sep 11 '20 at 16:06
  • @p1l0t Hi. By "no movement"( in the video), I'm trying to mean that on the leeward side, the telltales (the black threads) are totally down, thus showing that no air is flowing on that side of "the sail". My doubt is: no wind motion on leeward, then difference of pressure no due to wind motion. Then motion is due to totally to wind DEFLECTION on windward. Well, don't lose your time too much on this. – cibercitizen1 Sep 11 '20 at 19:37
  • @cibercitizen1 Just because the tell tales aren't moving doesn't mean that there is not a pressure difference. – p1l0t Sep 14 '20 at 18:23
  • @p1l0t Agreed. But it shows (I think) that the pressure difference is not caused by a phenomenon on leeward, and much less by fast air creating a low pressure zone. – cibercitizen1 Sep 14 '20 at 18:26
  • @cibercitizen1 Not necessarily. There is something called a turbulent boundary layer. The air doesn't have to be 'fast.' The pressure simply has to be different. If you create a difference on one side and not the other the air will naturally want to stablize. Also using a hairdryer is kind of a bad experiment because you are already pushing (VERY HOT) air towards the 'wing.' People standing at the beach at St. Maarten still go flying from jet blast and they aren't wings... Has nothing to do with how a wing works. Also the vehicle is moving before the video starts so could be an illusion. – p1l0t Sep 14 '20 at 18:51
  • @p1l0t The video is not fake. You can replicate the experiment. – cibercitizen1 Sep 14 '20 at 18:57
  • @cibercitizen1 Yes well I can blow anything off a table with a jet engine. What does that have to do with how a wing generates lift? – p1l0t Sep 14 '20 at 18:58
  • @p1l0t The dryer is simulating a sailing course (close-hauled) where the angle wind boat-axis is the lowest. This is: "the pushing on sail" component is great at any other course. Anyway, IMHO the video shows motion with no airflow attached at leeward. – cibercitizen1 Sep 14 '20 at 19:01
  • @cibercitizen1 What does that have to do with how a wing works? – p1l0t Sep 14 '20 at 19:03
  • @p1l0t The relationship would be that the main cause of lift is air deflection per-se. And differences in pressure is a consequence of air deflection; and not the other way around. – cibercitizen1 Sep 14 '20 at 19:04
  • @p1l0t A wing and a sail are the same. – cibercitizen1 Sep 14 '20 at 19:04
  • @cibercitizen1 Except that a wing only gets a free push from a strong updraft, it's designed to make its own lift, not really to take advangtage of an external push like a sailboat going downwind which is taking more advantage of drag that is going in the direction it wants to go. An airplane can't reasonably do that. – p1l0t Sep 14 '20 at 19:10
  • @p1l0t Of course, that's clear. That is the reason for the chosen courses in the video and thus the dryer position related to the wing. – cibercitizen1 Sep 14 '20 at 19:12
  • @cibercitizen1 Right and that's fine for a sail but that is not how a wing works. Drag is generally bad in an airplane, we try to avoid it. It's like having a hull full of barnicles. We are trying to move relative to the AIR not the water. A sail can use drag going downwind if it helps them going the way they want relative to the WATER. I fly and sail. They are very similar but also different. – p1l0t Sep 14 '20 at 19:16
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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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Steve V.
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    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??? :) – Lnafziger Jan 19 '14 at 17:28
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    @Lnafziger - Well, for one, the real reason for TBO time is to swap out the hamster on the wheel in there. – Steve V. Jan 19 '14 at 17:33
  • Haha, that must be one strong mutant hamster! – Lnafziger Jan 19 '14 at 17:35
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    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. – p1l0t Jan 19 '14 at 18:29
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    Upvoted both of you. – Greg M. Krsak Jan 19 '14 at 20:24
  • To properly describe something to that detail should not be necessary. If you truly understand something, even GPS, you can describe without math. GPS works because of highly accurate clocks measuring the time it it takes for something to happen, a radio signal in this case, and done multiple times from multiple places really fast. The clocks are really far away to make both the position more accurate and the math easier. They are also far away so they can use fewer of them. – Rowan Hawkins Nov 07 '17 at 17:01
  • @RowanHawkins - You are right. Everything can be described without math. Not everything can be understood without math. – Steve V. Nov 08 '17 at 02:21
  • "more students than I've got fingers on my throttle hand who haven't yelled "clear!" until they're already cranking the engine." - Not only students. I've seen more than one licensed pilot yell "clear" and then, without allowing the necessary time for anyone to hear and move out of the way, cranking the starter. – Wayne Conrad Jul 11 '19 at 04:44