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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 maj

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.
- twic 5y ago
  > I’m sure smarter people than I wrote the article I doubt it. This daft idea that wings are hard to explain is kept alive by pop science writers because it's a reliable source of money for old rope, that's all.
  - SonicScrub 5y ago
    The experts cited in the article are some of the most well-known researchers in the field of aerodynamics. John D. Anderson is the author of 7 reference textbooks, all of which are commonly used in the field. His research took him from the US Airforce Aerospace Research Laboratory to the becoming the Chief of the Hypersonics Group at the US Naval Naval Ordnance Laboratory. He was Chairman of the Department of Aerospace Engineering at the University of Maryland. He is a member of the National Academy of Engineering for aerospace engineering. His name is attached to enumerable scientific journals that I encourage you to search through if you think that an average Hacker News user knows more about aeronautics than him.
jameshart 5y ago
"how is that air moving from the upper surface to the lower surface of the wing?" What? Of course it isn't moving from above the wing to below - just that it's being compressed below the wing, and decompressed above it. "Why can’t they separate from it and fly straight back?" Because... they are parcels of gas, full of molecules flying in all directions at high speeds, bouncing off one another and things nearby, and if that parcel of air flies straight back it will find itself above a bit of space that contains nothing at all, and the molecules which are going in that direction will find they are able to do so unopposed (until they hit the wing) - so some of them will do so. As a result, the mass of the gas will spread out into a larger volume, the number of molecules colliding with the surface of the wing per unit of time will drop (as they are more diffuse), and the pressure will drop. This doesn't seem very mysterious.
- SonicScrub 5y ago
  This doesn't explain why a higher pressure on the lower surface should necessarily equal a lower pressure on the upper. I posted that quote to focus on the person I was addressing who stated that "pressure is lower on top of the wing and higher below because the air is being pushed downwards by the wing". The above commenter was stating that somehow the higher pressure on the lower surface somehow causes the lower pressure above. The quotation I posted refutes that point. Static pressure in a flow is not a conserved value. It does not have to come from anywhere. This can be easily demonstrated with an airfoil shape with a relatively flat bottom such as the NACA airfoil series. Static pressure on these airfoils' lower surfaces tends to roughly equal atmospheric. Yet the upper surface still creates suction. In the majority of airfoils there is way more suction than there is pressure increase on the lower surface. Why? This is not adequately explained by the above commenter's statement. Also just a minor point of pedantry: wings don't compress air. At least not in low Mach number flows. The static pressure changes as a result of the relationship between pressure and velocity. Compression is when the total pressure (static + dynamic pressures) changes. Total pressure in a low-mach number flow remains constant.
  - jameshart 5y ago
    Maybe there's a different word than 'compression' that means 'to cause an increase in pressure'. It seemed like the logical choice, but I'd love to know what other word is preferred. Because I'm pretty sure wings cause an increase in air pressure. I guess the idea of 'dynamic pressure' is 'pressure that is caused by colliding with air just because you're moving relative to it'. But surely in the moving reference frame of the wing, that looks, locally, quite a lot like compression...

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.

twic 5y ago
> I’m sure smarter people than I wrote the article I doubt it. This daft idea that wings are hard to explain is kept alive by pop science writers because it's a reliable source of money for old rope, that's all.
- SonicScrub 5y ago
  The experts cited in the article are some of the most well-known researchers in the field of aerodynamics. John D. Anderson is the author of 7 reference textbooks, all of which are commonly used in the field. His research took him from the US Airforce Aerospace Research Laboratory to the becoming the Chief of the Hypersonics Group at the US Naval Naval Ordnance Laboratory. He was Chairman of the Department of Aerospace Engineering at the University of Maryland. He is a member of the National Academy of Engineering for aerospace engineering. His name is attached to enumerable scientific journals that I encourage you to search through if you think that an average Hacker News user knows more about aeronautics than him.

jameshart 5y ago

"how is that air moving from the upper surface to the lower surface of the wing?" What? Of course it isn't moving from above the wing to below - just that it's being compressed below the wing, and decompressed above it. "Why can’t they separate from it and fly straight back?" Because... they are parcels of gas, full of molecules flying in all directions at high speeds, bouncing off one another and things nearby, and if that parcel of air flies straight back it will find itself above a bit of space that contains nothing at all, and the molecules which are going in that direction will find they are able to do so unopposed (until they hit the wing) - so some of them will do so. As a result, the mass of the gas will spread out into a larger volume, the number of molecules colliding with the surface of the wing per unit of time will drop (as they are more diffuse), and the pressure will drop. This doesn't seem very mysterious.

SonicScrub 5y ago
This doesn't explain why a higher pressure on the lower surface should necessarily equal a lower pressure on the upper. I posted that quote to focus on the person I was addressing who stated that "pressure is lower on top of the wing and higher below because the air is being pushed downwards by the wing". The above commenter was stating that somehow the higher pressure on the lower surface somehow causes the lower pressure above. The quotation I posted refutes that point. Static pressure in a flow is not a conserved value. It does not have to come from anywhere. This can be easily demonstrated with an airfoil shape with a relatively flat bottom such as the NACA airfoil series. Static pressure on these airfoils' lower surfaces tends to roughly equal atmospheric. Yet the upper surface still creates suction. In the majority of airfoils there is way more suction than there is pressure increase on the lower surface. Why? This is not adequately explained by the above commenter's statement. Also just a minor point of pedantry: wings don't compress air. At least not in low Mach number flows. The static pressure changes as a result of the relationship between pressure and velocity. Compression is when the total pressure (static + dynamic pressures) changes. Total pressure in a low-mach number flow remains constant.
- jameshart 5y ago
  Maybe there's a different word than 'compression' that means 'to cause an increase in pressure'. It seemed like the logical choice, but I'd love to know what other word is preferred. Because I'm pretty sure wings cause an increase in air pressure. I guess the idea of 'dynamic pressure' is 'pressure that is caused by colliding with air just because you're moving relative to it'. But surely in the moving reference frame of the wing, that looks, locally, quite a lot like compression...