3 ms·

"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

"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...
  - SonicScrub 5y ago
    I too would like another word, because I can tell you first hand that the confusion between total, static, and dynamic pressure is the source of many headaches amongst aerospace engineering students. It doesn't help that many textbooks often refer simply to "pressure" and expect the readers to intuit which they are referring to given the appropriate context. A book will say: "In low-speed aerodynamic flow, pressure is constant along a streamline" and then one chapter later say "Pressure changes with a change in velocity along a streamline" The first references to total pressure while the second refers to static, but at first glance they seem contradictory. However I would fully avoid the word "compression" since implies that we are squeezing more air into a fixed volume (a.k.a. an increase in density) which is NOT what is happening in a low-speed air flow. The definition of compression strictly applies to total pressure. Although most people don't learn this, since compression is often used in relation to stationary flows to begin with (such as in pressure vessels) where the dynamic pressure is 0, thus static pressure equals total pressure.

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...
- SonicScrub 5y ago
  I too would like another word, because I can tell you first hand that the confusion between total, static, and dynamic pressure is the source of many headaches amongst aerospace engineering students. It doesn't help that many textbooks often refer simply to "pressure" and expect the readers to intuit which they are referring to given the appropriate context. A book will say: "In low-speed aerodynamic flow, pressure is constant along a streamline" and then one chapter later say "Pressure changes with a change in velocity along a streamline" The first references to total pressure while the second refers to static, but at first glance they seem contradictory. However I would fully avoid the word "compression" since implies that we are squeezing more air into a fixed volume (a.k.a. an increase in density) which is NOT what is happening in a low-speed air flow. The definition of compression strictly applies to total pressure. Although most people don't learn this, since compression is often used in relation to stationary flows to begin with (such as in pressure vessels) where the dynamic pressure is 0, thus static pressure equals total pressure.