6 ms·

Cue the conversation on how wing works.

SonicScrub 5y ago
One of my favourite pointless internet arguments! I usually just point to this link and walk away: https://www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/ https://www.scientificamerican.com/article/no-one-can-explai... Answer: because a surface of a certain geometry produces a force perpendicular to fluid flow as per the Navier-Stokes equations. But that's not a good explanation to give a room full 2nd graders, therefore your pet-explanation will have to make a compromise somewhere. You are all equally right and wrong at the same time. Unless you say "equal transit theory". Then you are just wrong.
- nas 5y ago
  That sounds like the problem of explaining how magnetism works. Engineers and scientists understand very well how airfoils generate lift. It is not some kind of mystery like the article implies. It is true that most of the popular simplified explanations are incorrect. Flat plate airfoils generate lift if they have positive angles of attack. Airplanes can fly upside down. At fractional mach numbers, pressure above and below the wings is essentially equal. If you are not flying near the speed of light, Newton's laws apply. So, if you want simple explanation, the wing deflects air downwards and that pushes the airplane up. If you put your hand outside the car window at an angle, you will feel a force. Should be simple enough for 2nd graders.
  - deleted 5y ago
    [deleted]
  - SonicScrub 5y ago
    The article doesn't imply it's a mystery, only that simple one-liner explanations are insufficient. The headline is reasonably clickbait-y. The momentum theory of lift is simple, intuitive, but unfortunately incomplete (just as the differential pressure explanation). It's covered in the article. > But taken by itself, the principle of action and reaction also fails to explain the lower pressure atop the wing, which exists in that region irrespective of whether the airfoil is cambered. It is only when an airplane lands and comes to a halt that the region of lower pressure atop the wing disappears, returns to ambient pressure, and becomes the same at both top and bottom. But as long as a plane is flying, that region of lower pressure is an inescapable element of aerodynamic lift, and it must be explained. Also I want to address this: > At fractional mach numbers, pressure above and below the wings is essentially equal. Surely you mean density? Air pressure is certainly not the same above and below, as differential pressure integrated over the surface is equal to the lift force generated by the wing. So no, while the Newton's explanation is a great explanation for a second grade classroom, it is not complete.
    - twic 5y ago
      > But taken by itself, the principle of action and reaction also fails to explain the lower pressure atop the wing What? The pressure is lower on top of the wing and higher below because the air is being pushed downwards by the wing. I will happily explain this to any second-grade classrooms you find yourself having trouble with.
      - SonicScrub 5y ago
        And how is that air moving from the upper surface to the lower surface of the wing? is it magically permeating the wing surface? Keep in mind that the vast majority of the pressure differential comes from upper surface suction rather than a pressure increase on the lower surface. At shallow angles of attack there is often little or no increase in pressure on the lower surface; nevertheless lift is produced. Your simplification does not adequately explain this, as addressed in the section of the article subtitled: "Turning on the Reciprocity of Lift" > Nevertheless, there are at this point only a few outstanding matters that require explanation. Lift, as you will recall, is the result of the pressure differences between the top and bottom parts of an airfoil. We already have an acceptable explanation for what happens at the bottom part of an airfoil: the oncoming air pushes on the wing both vertically (producing lift) and horizontally (producing drag). The upward push exists in the form of higher pressure below the wing, and this higher pressure is a result of simple Newtonian action and reaction. > Things are quite different at the top of the wing, however. A region of lower pressure exists there that is also part of the aerodynamic lifting force. But if neither Bernoulli’s principle nor Newton’s third law explains it, what does? We know from streamlines that the air above the wing adheres closely to the downward curvature of the airfoil. But why must the parcels of air moving across the wing’s top surface follow its downward curvature? Why can’t they separate from it and fly straight back?
        strogonoff 5y ago
        > But why must the parcels of air moving across the wing’s top surface follow its downward curvature? Why can’t they separate from it and fly straight back? I’m sure smarter people than I wrote the article, but the way I explain it to myself is that it’s a manifestation of the same basic force or effect where the lower pressure area needs to be filled somehow. Like how the wind blows, cyclones form, etc., except in this case giving the wing lift somehow ends up being part of the most efficient “fill the void” solution. Like how just behind a driving truck there’s an abrupt region of lower pressure; however, the air obviously doesn’t just keep on going straight forever but rushes in (incidentally, giving a boost to whomever happens to be tailgating). The gradual shape of the wing changes the scale of the effect, so that it happens constantly with tiny air ‘parcels’, each filling in the minuscule lower pressure region. (And, probably not unrelated to the fact that it’s intuitively unnatural for air to flow that way, lifting the wing a little apparently turns out to help even out that void most efficiently.) It’s interesting to ponder for sure.
  - WJW 5y ago
    The article does specify that "nobody can explain how wings work", not that "nobody knows how wings work". It also tries to go into "but WHY do the Navier-Stokes equations work like this", which is just not how physics works. But yeah, there is just not an explanation that is both simple and complete and journalists have a pretty rough time dealing with that.
  - nautilius 5y ago
    The pressures above and below the wing are absolutely not equal, even at idealized incompressible flow (zero Mach number). Pressure distribution: https://www.researchgate.net/publication/262976779_Aerodynamic_Optimal_Shape_Design_Based_on_Body-Fitted_Grid_Generation/figures https://www.researchgate.net/publication/262976779_Aerodynam... Pressure contour plot: https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.ripublication.com%2Fijaerspl2019%2Fijaerv14n14spl_19.pdf&psig=AOvVaw2UO2mQLCPRgoYr3uTvxnig&ust=1628125751995000&source=images&cd=vfe&ved=0CAcQjRxqFwoTCOi5s5qXlvICFQAAAAAdAAAAABAF https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.ripubl...
  - amelius 5y ago
    > That sounds like the problem of explaining how magnetism works. https://www.youtube.com/watch?v=36GT2zI8lVA https://www.youtube.com/watch?v=36GT2zI8lVA
- svachalek 5y ago
  It seems similar to explaining monads. If you stick to the math and logic, it's quite clear and hard to confuse. But if you succumb to the common temptation to explain it in a more "intuitive" way, it seems you are doomed to tumble down the stairs in the dark.
- ummonk 5y ago
  That article seems to oddly assume that in the Newtonian explanation, deflection of air is implied to only happen below the airfoil rather than above. At least when I explain lift that way, I gesture air being sent downwards both along the top and bottom of the airfoil. As to explaining why air above the airfoil has to go downwards it seems pretty obvious that the air gets sucked into the gap left by the airfoil, but more importantly the article left out the Kutta condition that air doesn't wrap around a sharp trailing edge. This is both an intuitive phenomenon and crucial to the estimation of lift.
tzs 5y ago
One of the reasons there is so much confusion on this is that there are at least two different valid ways to look at it, but any given person usually only knows one of them (or rather, a simplified version of a serious misunderstanding of one of them). Different people know a different one of them, and thus argument breaks out whenever the topic comes up. NASA has a bunch of aerodynamics educational material on their old Glenn Research Center website. Here's a page on these two different ways to look at lift [1]. Very briefly, the gas flow has to simultaneously conserve mass, momentum, and energy. One approach is to consider conservation of energy. When you work out the implications of that you get different flow velocity over different parts of the wing and different pressure due to those differences. When you integrate the pressure over the whole wing you find that you get lift. The other approach is to consider conservation of momentum. Working out the implications of that, you get velocity differences in the flow. If you integrate those around the whole wing, you find that there is a net turning of the flow downward. Conservation of momentum requires that the wing gets momentum opposite of that and we have lift. [1] https://www.grc.nasa.gov/www/k-12/airplane/bernnew.html https://www.grc.nasa.gov/www/k-12/airplane/bernnew.html
- perl4ever 5y ago
  Does this all work for symmetric wing cross sections?
- dylan604 5y ago
  >or rather, a simplified version of a serious misunderstanding of one of them or both?
- jerf 5y ago
  I think it's a classic case of not understanding equality, hiding behind enough complexity that it's a bit more complicated than usual. Many people have a hard time not reading equalities as an implication... if there's a mass and an acceleration, then there's a force. But the equality says, it's the same. It isn't an if-then, it is that whereever there is a mass that is accelerating, that is force. The force doesn't "cause" the mass to accelerate and the mass accelerating doesn't "cause" the force; they're equal, they're the same thing, trying to wedge in between the two of them is a fatal error, and a very common one in physics education. Both of those approaches are equal. Or, to put it another way, it isn't that the fast moving air and the slow moving air "causing" a pressure differential which then "causes" lift... it's one integrated process. They aren't separable. You can't have different levels of pressure without some effect on air velocity, you can't have forces in the air without some effect on pressure, etc. The whole argument is all "But what causes what?" and the only correct answer is that it's an equality, not an implication. It can only be properly understood as a single whole that can be looked at in several ways.
filoeleven 5y ago
I usually just say “Bernoulli’s principle” with a hand-wave. It’s an ignorant dismissal accompanied by a practical demonstration. Works great at parties inside wind tunnels.

One of my favourite pointless internet arguments! I usually just point to this link and walk away: https://www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/ https://www.scientificamerican.com/article/no-one-can-explai... Answer: because a surface of a certain geometry produces a force perpendicular to fluid flow as per the Navier-Stokes equations. But that's not a good explanation to give a room full 2nd graders, therefore your pet-explanation will have to make a compromise somewhere. You are all equally right and wrong at the same time. Unless you say "equal transit theory". Then you are just wrong.

nas 5y ago
That sounds like the problem of explaining how magnetism works. Engineers and scientists understand very well how airfoils generate lift. It is not some kind of mystery like the article implies. It is true that most of the popular simplified explanations are incorrect. Flat plate airfoils generate lift if they have positive angles of attack. Airplanes can fly upside down. At fractional mach numbers, pressure above and below the wings is essentially equal. If you are not flying near the speed of light, Newton's laws apply. So, if you want simple explanation, the wing deflects air downwards and that pushes the airplane up. If you put your hand outside the car window at an angle, you will feel a force. Should be simple enough for 2nd graders.
- deleted 5y ago
  [deleted]
- SonicScrub 5y ago
  The article doesn't imply it's a mystery, only that simple one-liner explanations are insufficient. The headline is reasonably clickbait-y. The momentum theory of lift is simple, intuitive, but unfortunately incomplete (just as the differential pressure explanation). It's covered in the article. > But taken by itself, the principle of action and reaction also fails to explain the lower pressure atop the wing, which exists in that region irrespective of whether the airfoil is cambered. It is only when an airplane lands and comes to a halt that the region of lower pressure atop the wing disappears, returns to ambient pressure, and becomes the same at both top and bottom. But as long as a plane is flying, that region of lower pressure is an inescapable element of aerodynamic lift, and it must be explained. Also I want to address this: > At fractional mach numbers, pressure above and below the wings is essentially equal. Surely you mean density? Air pressure is certainly not the same above and below, as differential pressure integrated over the surface is equal to the lift force generated by the wing. So no, while the Newton's explanation is a great explanation for a second grade classroom, it is not complete.
  - twic 5y ago
    > But taken by itself, the principle of action and reaction also fails to explain the lower pressure atop the wing What? The pressure is lower on top of the wing and higher below because the air is being pushed downwards by the wing. I will happily explain this to any second-grade classrooms you find yourself having trouble with.
    - SonicScrub 5y ago
      And how is that air moving from the upper surface to the lower surface of the wing? is it magically permeating the wing surface? Keep in mind that the vast majority of the pressure differential comes from upper surface suction rather than a pressure increase on the lower surface. At shallow angles of attack there is often little or no increase in pressure on the lower surface; nevertheless lift is produced. Your simplification does not adequately explain this, as addressed in the section of the article subtitled: "Turning on the Reciprocity of Lift" > Nevertheless, there are at this point only a few outstanding matters that require explanation. Lift, as you will recall, is the result of the pressure differences between the top and bottom parts of an airfoil. We already have an acceptable explanation for what happens at the bottom part of an airfoil: the oncoming air pushes on the wing both vertically (producing lift) and horizontally (producing drag). The upward push exists in the form of higher pressure below the wing, and this higher pressure is a result of simple Newtonian action and reaction. > Things are quite different at the top of the wing, however. A region of lower pressure exists there that is also part of the aerodynamic lifting force. But if neither Bernoulli’s principle nor Newton’s third law explains it, what does? We know from streamlines that the air above the wing adheres closely to the downward curvature of the airfoil. But why must the parcels of air moving across the wing’s top surface follow its downward curvature? Why can’t they separate from it and fly straight back?
      - strogonoff 5y ago
        > But why must the parcels of air moving across the wing’s top surface follow its downward curvature? Why can’t they separate from it and fly straight back? I’m sure smarter people than I wrote the article, but the way I explain it to myself is that it’s a manifestation of the same basic force or effect where the lower pressure area needs to be filled somehow. Like how the wind blows, cyclones form, etc., except in this case giving the wing lift somehow ends up being part of the most efficient “fill the void” solution. Like how just behind a driving truck there’s an abrupt region of lower pressure; however, the air obviously doesn’t just keep on going straight forever but rushes in (incidentally, giving a boost to whomever happens to be tailgating). The gradual shape of the wing changes the scale of the effect, so that it happens constantly with tiny air ‘parcels’, each filling in the minuscule lower pressure region. (And, probably not unrelated to the fact that it’s intuitively unnatural for air to flow that way, lifting the wing a little apparently turns out to help even out that void most efficiently.) It’s interesting to ponder for sure.
- WJW 5y ago
  The article does specify that "nobody can explain how wings work", not that "nobody knows how wings work". It also tries to go into "but WHY do the Navier-Stokes equations work like this", which is just not how physics works. But yeah, there is just not an explanation that is both simple and complete and journalists have a pretty rough time dealing with that.
- nautilius 5y ago
  The pressures above and below the wing are absolutely not equal, even at idealized incompressible flow (zero Mach number). Pressure distribution: https://www.researchgate.net/publication/262976779_Aerodynamic_Optimal_Shape_Design_Based_on_Body-Fitted_Grid_Generation/figures https://www.researchgate.net/publication/262976779_Aerodynam... Pressure contour plot: https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.ripublication.com%2Fijaerspl2019%2Fijaerv14n14spl_19.pdf&psig=AOvVaw2UO2mQLCPRgoYr3uTvxnig&ust=1628125751995000&source=images&cd=vfe&ved=0CAcQjRxqFwoTCOi5s5qXlvICFQAAAAAdAAAAABAF https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.ripubl...
- amelius 5y ago
  > That sounds like the problem of explaining how magnetism works. https://www.youtube.com/watch?v=36GT2zI8lVA https://www.youtube.com/watch?v=36GT2zI8lVA
svachalek 5y ago
It seems similar to explaining monads. If you stick to the math and logic, it's quite clear and hard to confuse. But if you succumb to the common temptation to explain it in a more "intuitive" way, it seems you are doomed to tumble down the stairs in the dark.
ummonk 5y ago
That article seems to oddly assume that in the Newtonian explanation, deflection of air is implied to only happen below the airfoil rather than above. At least when I explain lift that way, I gesture air being sent downwards both along the top and bottom of the airfoil. As to explaining why air above the airfoil has to go downwards it seems pretty obvious that the air gets sucked into the gap left by the airfoil, but more importantly the article left out the Kutta condition that air doesn't wrap around a sharp trailing edge. This is both an intuitive phenomenon and crucial to the estimation of lift.

tzs 5y ago

One of the reasons there is so much confusion on this is that there are at least two different valid ways to look at it, but any given person usually only knows one of them (or rather, a simplified version of a serious misunderstanding of one of them). Different people know a different one of them, and thus argument breaks out whenever the topic comes up. NASA has a bunch of aerodynamics educational material on their old Glenn Research Center website. Here's a page on these two different ways to look at lift [1]. Very briefly, the gas flow has to simultaneously conserve mass, momentum, and energy. One approach is to consider conservation of energy. When you work out the implications of that you get different flow velocity over different parts of the wing and different pressure due to those differences. When you integrate the pressure over the whole wing you find that you get lift. The other approach is to consider conservation of momentum. Working out the implications of that, you get velocity differences in the flow. If you integrate those around the whole wing, you find that there is a net turning of the flow downward. Conservation of momentum requires that the wing gets momentum opposite of that and we have lift. [1] https://www.grc.nasa.gov/www/k-12/airplane/bernnew.html https://www.grc.nasa.gov/www/k-12/airplane/bernnew.html

perl4ever 5y ago
Does this all work for symmetric wing cross sections?
dylan604 5y ago
>or rather, a simplified version of a serious misunderstanding of one of them or both?
jerf 5y ago
I think it's a classic case of not understanding equality, hiding behind enough complexity that it's a bit more complicated than usual. Many people have a hard time not reading equalities as an implication... if there's a mass and an acceleration, then there's a force. But the equality says, it's the same. It isn't an if-then, it is that whereever there is a mass that is accelerating, that is force. The force doesn't "cause" the mass to accelerate and the mass accelerating doesn't "cause" the force; they're equal, they're the same thing, trying to wedge in between the two of them is a fatal error, and a very common one in physics education. Both of those approaches are equal. Or, to put it another way, it isn't that the fast moving air and the slow moving air "causing" a pressure differential which then "causes" lift... it's one integrated process. They aren't separable. You can't have different levels of pressure without some effect on air velocity, you can't have forces in the air without some effect on pressure, etc. The whole argument is all "But what causes what?" and the only correct answer is that it's an equality, not an implication. It can only be properly understood as a single whole that can be looked at in several ways.

filoeleven 5y ago

I usually just say “Bernoulli’s principle” with a hand-wave. It’s an ignorant dismissal accompanied by a practical demonstration. Works great at parties inside wind tunnels.