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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 o

by 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.
- nas 5y ago
  > Surely you mean density? The ideal gas law applies, at least nearly enough. So PV = nRT. By saying the density is equal between the top and bottom, you are also saying the pressure is equal. The air around the wing is having it's momentum changed, not it's pressure. At least, at sub mach speeds.
  - SonicScrub 5y ago
    You're confusing static, dynamic and total pressures. Static pressure is the pressure of a fluid on a body when the body is at rest relative to the fluid. Dynamic pressure is the velocity created pressure. Total pressure is the sum of the two, and is what is used in the ideal gas law. To compute lift force static pressure is what is integrated around the wing surface. Total pressure remains constant in the fluid flow for low Mach numbers. Static pressure can and absolutely does change significantly as it accelerates through a streamline such as in low-speed aerodynamics. I understand the semantics on the different kinds of pressure can be confusing. But you should know that when aerodynamics refers to "pressure" as it applies to lift generation, they are referring to static pressure. I should also mention that this pressure absolutely does change a lot over the flow field, and is commonly used to experimentally and mathematically quantify lift. The following is an image of the pressure distribution of a NACA 2412 airfoil at low speeds. https://www.chegg.com/homework-help/questions-and-answers/n-figure-see-pressure-distribution-computed-xfoil-airfoil-flight-re-note-xfoil-plots-airfo-q19145238 https://www.chegg.com/homework-help/questions-and-answers/n-... Just to explain the chart a little bit, in aerodynamics, pressure is usually simplified to a Pressure Coefficient (CP) value. A CP of 0 is when static pressure equals atmosphere. A CP value of 1 occurs at the stagnation point (where velocity is 0, therefore static pressure equals total pressure). Note how this type of chart has an inverted y-axis (a common convention so that the wing upper surface is at the top). Notice how the static pressure on the lower surface is roughly atmospheric, while the upper surface pressure suction peak is high. In this case roughly equal in magnitude to the dynamic pressure. This is a typical pressure distribution for most airfoils, with the suction peak increasing in magnitude as angle of attack increases. This plot can be obtained mathematically using some sort of potential flow scheme (see: XFOIL for 2D airfoils), or experimentally using pressure taps on a wind tunnel model. The area between the upper and lower surface curves is directly proportional to lift. The larger the difference between upper and lower surfaces, the more lift.
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

by nas 5y ago

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.
nas 5y ago
> Surely you mean density? The ideal gas law applies, at least nearly enough. So PV = nRT. By saying the density is equal between the top and bottom, you are also saying the pressure is equal. The air around the wing is having it's momentum changed, not it's pressure. At least, at sub mach speeds.
- SonicScrub 5y ago
  You're confusing static, dynamic and total pressures. Static pressure is the pressure of a fluid on a body when the body is at rest relative to the fluid. Dynamic pressure is the velocity created pressure. Total pressure is the sum of the two, and is what is used in the ideal gas law. To compute lift force static pressure is what is integrated around the wing surface. Total pressure remains constant in the fluid flow for low Mach numbers. Static pressure can and absolutely does change significantly as it accelerates through a streamline such as in low-speed aerodynamics. I understand the semantics on the different kinds of pressure can be confusing. But you should know that when aerodynamics refers to "pressure" as it applies to lift generation, they are referring to static pressure. I should also mention that this pressure absolutely does change a lot over the flow field, and is commonly used to experimentally and mathematically quantify lift. The following is an image of the pressure distribution of a NACA 2412 airfoil at low speeds. https://www.chegg.com/homework-help/questions-and-answers/n-figure-see-pressure-distribution-computed-xfoil-airfoil-flight-re-note-xfoil-plots-airfo-q19145238 https://www.chegg.com/homework-help/questions-and-answers/n-... Just to explain the chart a little bit, in aerodynamics, pressure is usually simplified to a Pressure Coefficient (CP) value. A CP of 0 is when static pressure equals atmosphere. A CP value of 1 occurs at the stagnation point (where velocity is 0, therefore static pressure equals total pressure). Note how this type of chart has an inverted y-axis (a common convention so that the wing upper surface is at the top). Notice how the static pressure on the lower surface is roughly atmospheric, while the upper surface pressure suction peak is high. In this case roughly equal in magnitude to the dynamic pressure. This is a typical pressure distribution for most airfoils, with the suction peak increasing in magnitude as angle of attack increases. This plot can be obtained mathematically using some sort of potential flow scheme (see: XFOIL for 2D airfoils), or experimentally using pressure taps on a wind tunnel model. The area between the upper and lower surface curves is directly proportional to lift. The larger the difference between upper and lower surfaces, the more lift.

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