8 ms·

Incorrect Lift Theory

66 points by ColinWright 5y ago

foreigner 5y ago
Clicking through the next couple pages their answer is ultimately "it's too complicated to explain here". How disappointing.
- bondarchuk 5y ago
  This is the correct one: https://www.grc.nasa.gov/WWW/k-12/airplane/right2.html https://www.grc.nasa.gov/WWW/k-12/airplane/right2.html
- ColinWright 5y ago
  This video does a fair job of explaining what's going on: https://www.youtube.com/watch?v=E3i_XHlVCeU https://www.youtube.com/watch?v=E3i_XHlVCeU It does say "it's complicated" and "there's a lot going on" but it does show the interaction between several factors.
- lolc 5y ago
  I'm wondering whether I'm missing something, because whenever this topic comes up I think "the wing accelerates air downwards and the inertia of the air lifts the wing". What's incomplete or complicated about that? I like searching for "downwash clouds" to see the effect :-)
  - wbeaty 5y ago
    It's not complicated, instead it's HERETICAL. It directly violates Bernoulli/Euler, because with downward accelerated air, the parcels continue moving down, long after the wing has passed by ...meaning that energy has been injected into the atmosphere. Yet Bernoulli equation is entirely based on having zero energy injected. Bernoulli only works if we eliminate all net downward acceleration (so, the parcels MUST be left unmoving after the wing has passed by. As an example of this, look at any 2D airfoil diagram. The air approaches horizontally, and leaves horizontally.) In my opinion, that issue seems to be the source of the entire controversy. --- In real wings, Bernoulli equation cannot be employed, because parcels remain moving downwards, which means that lift is an example of propulsion. Bernoulli does not describe propulsion. Real wings are air-pumps, and they fling air downwards, leaving behind a wake of descending air. The wing is adding net energy to air parcels. Yet in fluid dynamics for beginners, all the low-level explanations reject this notion, and instead eliminate the air-pump (eliminate the energy-injection, eliminate the vortex-creation, eliminate the downward "exhaust-plume" created by all short wings. That way Bernoulli equation can apply.) How do they do this? Just get rid of the wingtips! Make the wingspan be infinite. In other words, we only analyze the flow around airfoils in a 2D world. In a wind-tunnel, a two-dimensional airfoil must have sliding contact with the walls of the tunnel, and the airfoil doesn't pump any net air downwards. It acts as a small slice of an infinitely-wide wing. It creates a pressure distribution which presses against the floor and ceiling of the wind tunnel ...even if the wind tunnel is very very tall. To make Bernoulli equation applicable, we must only analyze airfoils having infinite wingspan. But unfortunately, a 2D airfoil only depicts "ground-effect flight," because an infinitely-wide wing can never fly high enough to escape 100% ground-interaction. (It would have to fly higher than infinite altitude. A wing is in 100% ground-effect whenever the altitude is << wingspan, and if wingspan is infinite, then we're only explaining venturi-effect flight close to the ground surface, where no air is flung downwards on average, and the Newton's-3rd force-pair is between the wing and the ground surface.) In other words, our explanations of lift have the ground surface built into the explanation (which then lets us employ Bernoulli/Euler equations, since zero energy is injected into the air.) The wing pushes indirectly against the ground, and the ground pushes indirectly against the wing. No need for net acceleration of the air. Yet real wings only push upon air, and the ground-surface plays no role. But then we don't bother to mention any of this to our students. And, our 2D flow diagrams completely violate Newton's 3rd, because they don't even depict the ground! Instead, real wings only create propulsion forces, and they fly much like helicopters: flying far from the ground, while leaving behind a momentum-carrying "exhaust plume" of descending air. Indeed, if first we explain a hovering helicopter, then later shove it rapidly sideways, we end up with a complete explanation of lifting force in fixed-wing aircraft. But then we're forbidden from applying Bernoulli, since Bernoulli cannot tolerate propulsion, or air pumps, or descending plumes left behind by airfoils. I'm reminded of the car-keys joke. Someone is searching for their car keys at night, only searching under a bright streetlight, because the light is brighter there. But they dropped their keys far away, in the dark. We've done the same with airfoil explanations: altering them until they no longer can explain flight ...but they do allow us to use much simpler math (employing a Bernoulli-based description.) Unfortunately, if we want an intuitive and satisfying explanation of lifting force, first we must decide to remove Bernoulli entirely. Then sit ourselves down and figure out an alternative approach. This might even be easy! But it remains totally impossible while, right at the start, we insist on using Bernoulli concepts, where helicopters and propulsion and downwash-plumes are utterly forbidden.
    - lolc 5y ago
      I sure appreciate a technical rant on the topic :-) Haven't thought about conservation of energy in this context. I wonder how this concept came to take hold? If it has no explanatory power, why even offer it?
      - megameter 5y ago
        There's a longstanding interest in the acadame in finding "clean" closed-form analytical explanations for all sorts of real-world problems, which mostly speaks to the historical lack of computing power to do a complete simulation leading to acceptance of bad approximations. Economics is also full of these kinds of equations, and many of the ones taught in undergrad are barely beyond a working hypothesis.
      - wbeaty 5y ago
        It was started by a famous paper from Ludwig Prandtl, in ?1920?. The math is easy, if vortex-shedding is removed, and our wing has infinite span. Nobody realizes that an infinite wing is permanently trapped in ground-effect mode, and only works by instant ground-forces, just like a venturi. Just like those snow-speeders from Star Wars. Other Prandtl papers analyze short, non-infinite wings. But they avoid all the insoluable equations by having the wing fly at infinite velocity! This way, no air moves downwards on average. Prandtl forgets that if the tip-vortices don't move downwards, then also the vortex-shedding goes to zero, and the tip-vortices vanish! By flying at infinite velocity, then pretending that tip-vortices are still created, Prandtl is "searching for car-keys under bright streetlights, where the job is easier" when the actual explanation of lifting force is still hidden elsewhere, out in the darkness. Heh, another of Prandtl's papers rigorously described the equal-transit-time theory, giving us diagrams, and including it as the explanation for lifting-force. Yet nobody could contradict the Great Prandtl, since his papers were huge walls of interlocking equations, which were all correct. Only his initial assumptions were wrong, and it took about seven decades before physics teachers started seeing the problem. See: http://amasci.com/wing/Weltner1a.gif http://amasci.com/wing/Weltner1a.gif
- FabHK 5y ago
  An excellent source is See How it Flies, a free online book by John S. Denker. Relevant for this discussion is chapter 3. http://www.av8n.com/how/ http://www.av8n.com/how/
dmkirwan 5y ago
Funnily enough, there's another web page on the NASA domain that outlines the "longer path" or "equal transit" theories as the correct cause of lift on an aircraft wing: https://www.grc.nasa.gov/www/k-12/UEET/StudentSite/dynamicsofflight.html https://www.grc.nasa.gov/www/k-12/UEET/StudentSite/dynamicso... When I was studying for my PPL, they taught us using the equal transit theory, though the instructor couldn't help himself from telling us that it was a simplification. This video from Cambridge University illustrates what happens to the airflow over an aerofoil: https://www.youtube.com/watch?v=UqBmdZ-BNig https://www.youtube.com/watch?v=UqBmdZ-BNig
- nabla9 5y ago
  Euler equations explain the lift, but it can be still hard to grok. Newton’s Laws of Motion gives the clearest correct explanation: If the wing stays in the air, there must be a force that is equal but opposite to the gravity. If you divide the air around the wing into equal mass chunks of air (not equal volume) and visualize the velocity and acceleration of each chunk, you get very informative picture of where and when of the lift. If the wing stays in the air, air around the wing must be accelerating downwards.
  - VBprogrammer 5y ago
    Expanding on that, the downwash theory is the easiest to understand in my opinion. If you think about the flow of water over the back of a spoon then imagine increasing the amount of the water flow and the size of the spoon until you could eventually feel the force pulling your hand opposite the direction the water is deflected.
    - bbojan 5y ago
      You don't have to imagine, simply put a spoon sideways against a flow of water from the faucet and you can instantly feel a pull.
  - wbeaty 5y ago
    In 2D airfoil diagrams, first the parcels approach horizontally. Then they are mysteriously accelerated upwards. Then the airfoil accelerates them downwards. Then behind the airfoil, they're mysteriously accelerated upwards again, so they depart horizontally, for zero net acceleration applied to each parcel. As a whole, air around the wing ISN'T accelerating downwards. As long as we ignore the "mysterious" part, or try to sweep it under the rug, our explanations won't satisfy. A complete explanation must include the ground surface ...and then we discover that we're actually explaining a kind of "venturi effect," where the ground is a part of the system, and those mysterious accelerations before and behind the airfoil are caused by interaction with the ground. This is a "big AHA!" situation: a 2D airfoil can only explain ground-effect flight, where the Newtonian force-pair is located between ground and the airfoil, so no net downward acceleration of air exists. (In other words, if we erase the ground from our diagram, we've just violated Newton's 3rd law.) A 2D airfoil diagram without the ground surface ...that's a sort of "inertial drive," where the upward force on the airfoil has no corresponding down-force (it does not give a net downward acceleration to the air as a whole.) Easy to fix. Just add the ground surface back in. And then confront the fact that this is a venturi, and not an explanation of flight.
bogdan 5y ago
Related to this, I read this article on my commute last week. https://www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/ https://www.scientificamerican.com/article/no-one-can-explai...
- ColinWright 5y ago
  I remember showing that article to a colleague of mine who became Very. Angry. Indeed. They said that all the articles and explanations that are being quoted then picked apart and shown to be inadequate are, were, and always have been simplifications intended to give the non-technical person some insight into what's going on, but have never been intended to "explain flight". My colleague went on to say that starting with each of the simplified and incorrect descriptions, one can gradually refine each, then successively combine into a bigger picture, until one ends up with a description that "feels right" and agrees with all the numerical simulations. But they've also stopped trying to be helpful pretty much exactly because of articles like this, that take half-truths as if they are intended to be complete explanations, then debunks them. So don't read this article from Sci.Am. and believe it uncritically.
  - _dain_ 5y ago
    This feels like a motte/bailey tactic. I remember clearly being taught the equal-transit-time theory as a kid, and it was taught as actual fact, not a simplification. I was uneasy with it but couldn't vocalize why (it didn't occur to me to ask why planes can fly upside down). If it is meant as a simplification, many HS teachers have not gotten the memo.
    - NovemberWhiskey 5y ago
      Substantially everything taught as fact in HS science is a simplification or a poorer approximation of a better answer though, isn't it?
      - hobbes78 5y ago
mansoor_ 5y ago
For those that want a TL;DR explanation: Almost all object that are airborne, stay so by pushing air in the opposite direction that they want to go. This is Newton's third law. Aeroplane wings take the air coming towards them (caused by the existing velocity of the aeroplane) and direct it down and backwards; this creates a resultant force that is forward and upwards. It is really that simple
- closewith 5y ago
  The second page of the article seems to address this and calls it an incorrect lift theory. > It is really that simple. The whole point of the article is that it's not that simple: > The real details of how an object generates lift are very complex and do not lend themselves to simplification.
  - mansoor_ 5y ago
    > The second page of the article seems to address this and calls it an incorrect lift theory. If it does, it's wrong. Edit: Now that I have read it, I would have to disagree with you. The article only references the Bernoulli pressure arguments, it does not mention Newton's third law or airflow circulation. > The real details of how an object generates lift are very complex and do not lend themselves to simplification. There is no scientific contention about flight. Nobody is currently publishing papers on this topic ("journal of fluid mechanics" if you are interested), it was well understood in the previous century. Newton's third law explains at a high level how object fly. Yes you can dig into the details of fluid motion, turbulent flow, boundary layers etc, but this does not detract from the high-level picture.
- hugh-avherald 5y ago
  That's half of it. Lift is produced by airflow over both sides of an aeroplane wing not just the underside.
  - mansoor_ 5y ago
    It's not just the underside that is responsible for the circulation of the airflow. In fact the majority of the circulation is generated by the top surface.
lordnacho 5y ago
This was one of the weirdest parts of the engineering course. There's so many wrong explanations of why planes fly, and we went through them. It's not equal-time-to-the-end-of-the-wing, and it's not pushes-flow-down-so-goes-up. It's a good time to do some critical thinking, though: 1) We observe that planes can fly upside down. Does the theory under evaluation explain this? 2) It's hard to take off behind another plane that's just taken off. Why?
- formerly_proven 5y ago
  And some planes can go straight up without (?) being deflected to the side.
  - lmilcin 5y ago
    Those planes work in a little bit different regime. For a plane flying straight up you have (usually) two things happening: 1. It is trading kinetic energy for potential energy. 2. They have so much power that their engines can literally lift the entire plane. These planes do not fly. They just use the air to keep the right direction but do not get meaningful lift when going straight up in a straight line.
    - meheleventyone 5y ago
      A wing generates lift in relation to the relative airflow over it, so a plane with some velocity "straight upwards" generates lift exactly equivalent to the lift generated when flying "horizontally" at the same velocity. You can have aircraft with engines powerful enough that they can overcome gravity with thrust alone so can fly upward indefinitely but their wings will still be generating lift, even if they are travelling vertically and it's not useful for overcoming gravity.
      - dTal 5y ago
        Through surfaces on the horizontal stabilizer, the pilot controls the angle of attack of the wing, and hence the "lift" force perpendicular to it. If the pilot doesn't want any lift, there's no lift. The "vomit comet" for instance achieves weightlessness by flying a totally ballistic trajectory; the wings produce no lift for up to 30 seconds at a time.
AndrewThrowaway 5y ago
I love these theories that seem to explain things and then fall short just like that. "So how does an airplane fly upside down?" An the theory goes out the window. Relevent XKCD: https://xkcd.com/1145/ https://xkcd.com/1145/ "So why the sky is not violet?" Actually things like that is what got me interested in physics. My physics teacher asked "so does lightning strike from the sky, downwards". Everybody in class agreed. We all "know" this. We all "seen" it in cartoons. We were learning about electricity (electrons) at this time. "But if electricity is electrons moving, ground obviously has more electrons that the sky, lightning must go from ground to the sky". I was so fascinated about actually "learning" something new. Remember running home to tell my father how clever I was now. Lightning is much more complicated that this obviously: https://youtu.be/RLWIBrweSU8?t=77 https://youtu.be/RLWIBrweSU8?t=77
quantum_mcts 5y ago
There is an analogy between this and infeasibility of "Explainable AI". For the Lift Theory we have Navier-Stokes equations that give you the lift if you solve them numerically or, maybe, figure an analytical solution. But most people won't accept it as an "explanation" - they want a simpler description in more coarse-grained concepts and relationships. And, it seems, that there is no such satisfactory reduction for the Lift Theory - there are just too many interrelated things happening at the same time. Same thing goes for some decision-making AI. One might like to have an "explanation" for why AlphaZero decided to make a particular move. And sometimes there could be a general high-level pattern rule, that one could extract from the model. But, it seems, that in most cases no simple rule can be expected.
- H8crilA 5y ago
  There is a simple "explanation" of wing lift, though: the wing pushes air down, it applies downward force on air masses. This is a trivial consequence of Newton's third rule and the facts that a) there's gravity, b) airplanes don't fall from the sky. If you want to know what exactly is happening to the air around the wing you need fluid dynamics of course. I don't know if we'll get any such "conservation of X" principle from the AI world, but that would be pretty cool.
  - quantum_mcts 5y ago
    I'd argue that that is not really an "explanation", because you cannot meaningfully generalize it. Yes, there is a lift because there is upward force on the wing, because there is a downward force on the air... But, how do you use it in another context? That feels more like a sequence of tautologies, than an explanation.
    - H8crilA 5y ago
      Generalize to what? Also, what other context?
  - wbeaty 5y ago
    But if air is pushed down, then it must continue moving down (much like with a hovering rocket, or a helicopter.) But this effect does not appear in typical airfoil flow-diagrams. Instead, the air approaches horizontally, and leaves horizontally. It's a symmetrical pattern, as guaranteed by "circulation theory." In these diagrams, the wing doesn't fling any air downwards. (Heh, the same thing happens with helicopters that fly inside a giant vertical pipe, where the blade-tips slide along the inside surface of the pipe.)
GeorgeOu 5y ago
The best explanations I've heard is that these theores are not mutually exclusive. They can all be true at the same time and merely describe the physics from a different angle.
- neolefty 5y ago
  That's true for some explanations — for example the various proofs of the Pythagorean Theorem — but you still have to watch out for explanations that are flat-out wrong, and can even be harmful because they obscure the real explanation. I think equal-transit theory seems to be one of those, haha. Maybe the damage is limited because very few of us will be designing airfoils. That may also be why it has been so hard to correct! The consequences for getting it wrong are minimal. Something much more dangerous would be an untruth that directly harms people. The debate around those tends to be much more rigorous, although I'm surprised how long some of those can endure when people get attached to them!
  - wbeaty 5y ago
    Many generations of professional pilots have been taught the "equal transit theory." It's often as an answer their licensing exams. So, most pilots have little idea how wings actually work. (But since the rise of the internet, things have been sloooowly changing. At least the pilots have started questioning and arguing!)

66 points by ColinWright 5y ago

Clicking through the next couple pages their answer is ultimately "it's too complicated to explain here". How disappointing.

bondarchuk 5y ago
This is the correct one: https://www.grc.nasa.gov/WWW/k-12/airplane/right2.html https://www.grc.nasa.gov/WWW/k-12/airplane/right2.html
ColinWright 5y ago
This video does a fair job of explaining what's going on: https://www.youtube.com/watch?v=E3i_XHlVCeU https://www.youtube.com/watch?v=E3i_XHlVCeU It does say "it's complicated" and "there's a lot going on" but it does show the interaction between several factors.
lolc 5y ago
I'm wondering whether I'm missing something, because whenever this topic comes up I think "the wing accelerates air downwards and the inertia of the air lifts the wing". What's incomplete or complicated about that? I like searching for "downwash clouds" to see the effect :-)
- wbeaty 5y ago
  It's not complicated, instead it's HERETICAL. It directly violates Bernoulli/Euler, because with downward accelerated air, the parcels continue moving down, long after the wing has passed by ...meaning that energy has been injected into the atmosphere. Yet Bernoulli equation is entirely based on having zero energy injected. Bernoulli only works if we eliminate all net downward acceleration (so, the parcels MUST be left unmoving after the wing has passed by. As an example of this, look at any 2D airfoil diagram. The air approaches horizontally, and leaves horizontally.) In my opinion, that issue seems to be the source of the entire controversy. --- In real wings, Bernoulli equation cannot be employed, because parcels remain moving downwards, which means that lift is an example of propulsion. Bernoulli does not describe propulsion. Real wings are air-pumps, and they fling air downwards, leaving behind a wake of descending air. The wing is adding net energy to air parcels. Yet in fluid dynamics for beginners, all the low-level explanations reject this notion, and instead eliminate the air-pump (eliminate the energy-injection, eliminate the vortex-creation, eliminate the downward "exhaust-plume" created by all short wings. That way Bernoulli equation can apply.) How do they do this? Just get rid of the wingtips! Make the wingspan be infinite. In other words, we only analyze the flow around airfoils in a 2D world. In a wind-tunnel, a two-dimensional airfoil must have sliding contact with the walls of the tunnel, and the airfoil doesn't pump any net air downwards. It acts as a small slice of an infinitely-wide wing. It creates a pressure distribution which presses against the floor and ceiling of the wind tunnel ...even if the wind tunnel is very very tall. To make Bernoulli equation applicable, we must only analyze airfoils having infinite wingspan. But unfortunately, a 2D airfoil only depicts "ground-effect flight," because an infinitely-wide wing can never fly high enough to escape 100% ground-interaction. (It would have to fly higher than infinite altitude. A wing is in 100% ground-effect whenever the altitude is << wingspan, and if wingspan is infinite, then we're only explaining venturi-effect flight close to the ground surface, where no air is flung downwards on average, and the Newton's-3rd force-pair is between the wing and the ground surface.) In other words, our explanations of lift have the ground surface built into the explanation (which then lets us employ Bernoulli/Euler equations, since zero energy is injected into the air.) The wing pushes indirectly against the ground, and the ground pushes indirectly against the wing. No need for net acceleration of the air. Yet real wings only push upon air, and the ground-surface plays no role. But then we don't bother to mention any of this to our students. And, our 2D flow diagrams completely violate Newton's 3rd, because they don't even depict the ground! Instead, real wings only create propulsion forces, and they fly much like helicopters: flying far from the ground, while leaving behind a momentum-carrying "exhaust plume" of descending air. Indeed, if first we explain a hovering helicopter, then later shove it rapidly sideways, we end up with a complete explanation of lifting force in fixed-wing aircraft. But then we're forbidden from applying Bernoulli, since Bernoulli cannot tolerate propulsion, or air pumps, or descending plumes left behind by airfoils. I'm reminded of the car-keys joke. Someone is searching for their car keys at night, only searching under a bright streetlight, because the light is brighter there. But they dropped their keys far away, in the dark. We've done the same with airfoil explanations: altering them until they no longer can explain flight ...but they do allow us to use much simpler math (employing a Bernoulli-based description.) Unfortunately, if we want an intuitive and satisfying explanation of lifting force, first we must decide to remove Bernoulli entirely. Then sit ourselves down and figure out an alternative approach. This might even be easy! But it remains totally impossible while, right at the start, we insist on using Bernoulli concepts, where helicopters and propulsion and downwash-plumes are utterly forbidden.
  - lolc 5y ago
    I sure appreciate a technical rant on the topic :-) Haven't thought about conservation of energy in this context. I wonder how this concept came to take hold? If it has no explanatory power, why even offer it?
    - megameter 5y ago
      There's a longstanding interest in the acadame in finding "clean" closed-form analytical explanations for all sorts of real-world problems, which mostly speaks to the historical lack of computing power to do a complete simulation leading to acceptance of bad approximations. Economics is also full of these kinds of equations, and many of the ones taught in undergrad are barely beyond a working hypothesis.
    - wbeaty 5y ago
      It was started by a famous paper from Ludwig Prandtl, in ?1920?. The math is easy, if vortex-shedding is removed, and our wing has infinite span. Nobody realizes that an infinite wing is permanently trapped in ground-effect mode, and only works by instant ground-forces, just like a venturi. Just like those snow-speeders from Star Wars. Other Prandtl papers analyze short, non-infinite wings. But they avoid all the insoluable equations by having the wing fly at infinite velocity! This way, no air moves downwards on average. Prandtl forgets that if the tip-vortices don't move downwards, then also the vortex-shedding goes to zero, and the tip-vortices vanish! By flying at infinite velocity, then pretending that tip-vortices are still created, Prandtl is "searching for car-keys under bright streetlights, where the job is easier" when the actual explanation of lifting force is still hidden elsewhere, out in the darkness. Heh, another of Prandtl's papers rigorously described the equal-transit-time theory, giving us diagrams, and including it as the explanation for lifting-force. Yet nobody could contradict the Great Prandtl, since his papers were huge walls of interlocking equations, which were all correct. Only his initial assumptions were wrong, and it took about seven decades before physics teachers started seeing the problem. See: http://amasci.com/wing/Weltner1a.gif http://amasci.com/wing/Weltner1a.gif
FabHK 5y ago
An excellent source is See How it Flies, a free online book by John S. Denker. Relevant for this discussion is chapter 3. http://www.av8n.com/how/ http://www.av8n.com/how/

dmkirwan 5y ago

Funnily enough, there's another web page on the NASA domain that outlines the "longer path" or "equal transit" theories as the correct cause of lift on an aircraft wing: https://www.grc.nasa.gov/www/k-12/UEET/StudentSite/dynamicsofflight.html https://www.grc.nasa.gov/www/k-12/UEET/StudentSite/dynamicso... When I was studying for my PPL, they taught us using the equal transit theory, though the instructor couldn't help himself from telling us that it was a simplification. This video from Cambridge University illustrates what happens to the airflow over an aerofoil: https://www.youtube.com/watch?v=UqBmdZ-BNig https://www.youtube.com/watch?v=UqBmdZ-BNig

nabla9 5y ago
Euler equations explain the lift, but it can be still hard to grok. Newton’s Laws of Motion gives the clearest correct explanation: If the wing stays in the air, there must be a force that is equal but opposite to the gravity. If you divide the air around the wing into equal mass chunks of air (not equal volume) and visualize the velocity and acceleration of each chunk, you get very informative picture of where and when of the lift. If the wing stays in the air, air around the wing must be accelerating downwards.
- VBprogrammer 5y ago
  Expanding on that, the downwash theory is the easiest to understand in my opinion. If you think about the flow of water over the back of a spoon then imagine increasing the amount of the water flow and the size of the spoon until you could eventually feel the force pulling your hand opposite the direction the water is deflected.
  - bbojan 5y ago
    You don't have to imagine, simply put a spoon sideways against a flow of water from the faucet and you can instantly feel a pull.
- wbeaty 5y ago
  In 2D airfoil diagrams, first the parcels approach horizontally. Then they are mysteriously accelerated upwards. Then the airfoil accelerates them downwards. Then behind the airfoil, they're mysteriously accelerated upwards again, so they depart horizontally, for zero net acceleration applied to each parcel. As a whole, air around the wing ISN'T accelerating downwards. As long as we ignore the "mysterious" part, or try to sweep it under the rug, our explanations won't satisfy. A complete explanation must include the ground surface ...and then we discover that we're actually explaining a kind of "venturi effect," where the ground is a part of the system, and those mysterious accelerations before and behind the airfoil are caused by interaction with the ground. This is a "big AHA!" situation: a 2D airfoil can only explain ground-effect flight, where the Newtonian force-pair is located between ground and the airfoil, so no net downward acceleration of air exists. (In other words, if we erase the ground from our diagram, we've just violated Newton's 3rd law.) A 2D airfoil diagram without the ground surface ...that's a sort of "inertial drive," where the upward force on the airfoil has no corresponding down-force (it does not give a net downward acceleration to the air as a whole.) Easy to fix. Just add the ground surface back in. And then confront the fact that this is a venturi, and not an explanation of flight.

bogdan 5y ago

Related to this, I read this article on my commute last week. https://www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/ https://www.scientificamerican.com/article/no-one-can-explai...

ColinWright 5y ago
I remember showing that article to a colleague of mine who became Very. Angry. Indeed. They said that all the articles and explanations that are being quoted then picked apart and shown to be inadequate are, were, and always have been simplifications intended to give the non-technical person some insight into what's going on, but have never been intended to "explain flight". My colleague went on to say that starting with each of the simplified and incorrect descriptions, one can gradually refine each, then successively combine into a bigger picture, until one ends up with a description that "feels right" and agrees with all the numerical simulations. But they've also stopped trying to be helpful pretty much exactly because of articles like this, that take half-truths as if they are intended to be complete explanations, then debunks them. So don't read this article from Sci.Am. and believe it uncritically.
- _dain_ 5y ago
  This feels like a motte/bailey tactic. I remember clearly being taught the equal-transit-time theory as a kid, and it was taught as actual fact, not a simplification. I was uneasy with it but couldn't vocalize why (it didn't occur to me to ask why planes can fly upside down). If it is meant as a simplification, many HS teachers have not gotten the memo.
  - NovemberWhiskey 5y ago
    Substantially everything taught as fact in HS science is a simplification or a poorer approximation of a better answer though, isn't it?
    - hobbes78 5y ago

mansoor_ 5y ago

For those that want a TL;DR explanation: Almost all object that are airborne, stay so by pushing air in the opposite direction that they want to go. This is Newton's third law. Aeroplane wings take the air coming towards them (caused by the existing velocity of the aeroplane) and direct it down and backwards; this creates a resultant force that is forward and upwards. It is really that simple

closewith 5y ago
The second page of the article seems to address this and calls it an incorrect lift theory. > It is really that simple. The whole point of the article is that it's not that simple: > The real details of how an object generates lift are very complex and do not lend themselves to simplification.
- mansoor_ 5y ago
  > The second page of the article seems to address this and calls it an incorrect lift theory. If it does, it's wrong. Edit: Now that I have read it, I would have to disagree with you. The article only references the Bernoulli pressure arguments, it does not mention Newton's third law or airflow circulation. > The real details of how an object generates lift are very complex and do not lend themselves to simplification. There is no scientific contention about flight. Nobody is currently publishing papers on this topic ("journal of fluid mechanics" if you are interested), it was well understood in the previous century. Newton's third law explains at a high level how object fly. Yes you can dig into the details of fluid motion, turbulent flow, boundary layers etc, but this does not detract from the high-level picture.
hugh-avherald 5y ago
That's half of it. Lift is produced by airflow over both sides of an aeroplane wing not just the underside.
- mansoor_ 5y ago
  It's not just the underside that is responsible for the circulation of the airflow. In fact the majority of the circulation is generated by the top surface.

lordnacho 5y ago

This was one of the weirdest parts of the engineering course. There's so many wrong explanations of why planes fly, and we went through them. It's not equal-time-to-the-end-of-the-wing, and it's not pushes-flow-down-so-goes-up. It's a good time to do some critical thinking, though: 1) We observe that planes can fly upside down. Does the theory under evaluation explain this? 2) It's hard to take off behind another plane that's just taken off. Why?

formerly_proven 5y ago
And some planes can go straight up without (?) being deflected to the side.
- lmilcin 5y ago
  Those planes work in a little bit different regime. For a plane flying straight up you have (usually) two things happening: 1. It is trading kinetic energy for potential energy. 2. They have so much power that their engines can literally lift the entire plane. These planes do not fly. They just use the air to keep the right direction but do not get meaningful lift when going straight up in a straight line.
  - meheleventyone 5y ago
    A wing generates lift in relation to the relative airflow over it, so a plane with some velocity "straight upwards" generates lift exactly equivalent to the lift generated when flying "horizontally" at the same velocity. You can have aircraft with engines powerful enough that they can overcome gravity with thrust alone so can fly upward indefinitely but their wings will still be generating lift, even if they are travelling vertically and it's not useful for overcoming gravity.
    - dTal 5y ago
      Through surfaces on the horizontal stabilizer, the pilot controls the angle of attack of the wing, and hence the "lift" force perpendicular to it. If the pilot doesn't want any lift, there's no lift. The "vomit comet" for instance achieves weightlessness by flying a totally ballistic trajectory; the wings produce no lift for up to 30 seconds at a time.

AndrewThrowaway 5y ago

I love these theories that seem to explain things and then fall short just like that. "So how does an airplane fly upside down?" An the theory goes out the window. Relevent XKCD: https://xkcd.com/1145/ https://xkcd.com/1145/ "So why the sky is not violet?" Actually things like that is what got me interested in physics. My physics teacher asked "so does lightning strike from the sky, downwards". Everybody in class agreed. We all "know" this. We all "seen" it in cartoons. We were learning about electricity (electrons) at this time. "But if electricity is electrons moving, ground obviously has more electrons that the sky, lightning must go from ground to the sky". I was so fascinated about actually "learning" something new. Remember running home to tell my father how clever I was now. Lightning is much more complicated that this obviously: https://youtu.be/RLWIBrweSU8?t=77 https://youtu.be/RLWIBrweSU8?t=77

quantum_mcts 5y ago

There is an analogy between this and infeasibility of "Explainable AI". For the Lift Theory we have Navier-Stokes equations that give you the lift if you solve them numerically or, maybe, figure an analytical solution. But most people won't accept it as an "explanation" - they want a simpler description in more coarse-grained concepts and relationships. And, it seems, that there is no such satisfactory reduction for the Lift Theory - there are just too many interrelated things happening at the same time. Same thing goes for some decision-making AI. One might like to have an "explanation" for why AlphaZero decided to make a particular move. And sometimes there could be a general high-level pattern rule, that one could extract from the model. But, it seems, that in most cases no simple rule can be expected.

H8crilA 5y ago
There is a simple "explanation" of wing lift, though: the wing pushes air down, it applies downward force on air masses. This is a trivial consequence of Newton's third rule and the facts that a) there's gravity, b) airplanes don't fall from the sky. If you want to know what exactly is happening to the air around the wing you need fluid dynamics of course. I don't know if we'll get any such "conservation of X" principle from the AI world, but that would be pretty cool.
- quantum_mcts 5y ago
  I'd argue that that is not really an "explanation", because you cannot meaningfully generalize it. Yes, there is a lift because there is upward force on the wing, because there is a downward force on the air... But, how do you use it in another context? That feels more like a sequence of tautologies, than an explanation.
  - H8crilA 5y ago
    Generalize to what? Also, what other context?
- wbeaty 5y ago
  But if air is pushed down, then it must continue moving down (much like with a hovering rocket, or a helicopter.) But this effect does not appear in typical airfoil flow-diagrams. Instead, the air approaches horizontally, and leaves horizontally. It's a symmetrical pattern, as guaranteed by "circulation theory." In these diagrams, the wing doesn't fling any air downwards. (Heh, the same thing happens with helicopters that fly inside a giant vertical pipe, where the blade-tips slide along the inside surface of the pipe.)

GeorgeOu 5y ago

The best explanations I've heard is that these theores are not mutually exclusive. They can all be true at the same time and merely describe the physics from a different angle.

neolefty 5y ago
That's true for some explanations — for example the various proofs of the Pythagorean Theorem — but you still have to watch out for explanations that are flat-out wrong, and can even be harmful because they obscure the real explanation. I think equal-transit theory seems to be one of those, haha. Maybe the damage is limited because very few of us will be designing airfoils. That may also be why it has been so hard to correct! The consequences for getting it wrong are minimal. Something much more dangerous would be an untruth that directly harms people. The debate around those tends to be much more rigorous, although I'm surprised how long some of those can endure when people get attached to them!
- wbeaty 5y ago
  Many generations of professional pilots have been taught the "equal transit theory." It's often as an answer their licensing exams. So, most pilots have little idea how wings actually work. (But since the rise of the internet, things have been sloooowly changing. At least the pilots have started questioning and arguing!)