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#71
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visualisation of the lift distribution over a wing
Jim Logajan wrote:
Alan Baker wrote: Because the push is caused by the impact of countless air molecules with the surface of wing. If those collisions fall to zero (i.e. in a perfect vacuum) then there is zero push. I don't see what a change in air density (such as taking the extreme case of a vacuum) has to do with lift. I'm going to build a thick wing, real thick! About 10,000 feet thick. While the bottom surface of the wing sits at 29.92" sea level atmospheric pressure, the upper surface will be *way* up there, in a lower-pressure area. This airplane is gonna to lift off the ground at zero airspeed with no power. |
#72
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visualisation of the lift distribution over a wing
In article ,
Scott wrote: Alan Baker wrote: In article , Scott wrote: Alan Baker wrote: Observation can lead you astray: and that is clearly the case here if you actually think that air can *pull* on a surface. Why can't air PULL on a surface? Air is made up of molecules. Molecules have mass. Anything with mass can attract anything else with mass, can't it? Gravity? You're not serious. Anti-gravity in this case. If air can push something, why can't it pull something? It does, but the tiny resulting forces are completely swamped by things like van der Waals force, static electric charges, magnetic fields, and any number of things that are usually quite happily ignored by us in normal life. |
#73
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visualisation of the lift distribution over a wing
In article ,
Beryl wrote: Jim Logajan wrote: Alan Baker wrote: Because the push is caused by the impact of countless air molecules with the surface of wing. If those collisions fall to zero (i.e. in a perfect vacuum) then there is zero push. I don't see what a change in air density (such as taking the extreme case of a vacuum) has to do with lift. I'm going to build a thick wing, real thick! About 10,000 feet thick. While the bottom surface of the wing sits at 29.92" sea level atmospheric pressure, the upper surface will be *way* up there, in a lower-pressure area. This airplane is gonna to lift off the ground at zero airspeed with no power. Only if you make it from monatomic Unobtanium. Only thing light enough for the job. |
#74
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visualisation of the lift distribution over a wing
In article ,
Jim Logajan wrote: Alan Baker wrote: Because the push is caused by the impact of countless air molecules with the surface of wing. If those collisions fall to zero (i.e. in a perfect vacuum) then there is zero push. I don't see what a change in air density (such as taking the extreme case of a vacuum) has to do with lift. Unless you are claiming density change as a requirement? I believe lift can be reasonably computed using inviscid _incompressible_ flow theory (e.g. as far back as Kutta's 1902 dissertation,) so I don't see why any change in _density_ - much less the vacuum edge case - needs to be invoked. Any change in pressure is *by definition* a change in the number of particles in the fluid that are impacting the surface. I never mentioned density. -- Alan Baker Vancouver, British Columbia http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg |
#75
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visualisation of the lift distribution over a wing
Alan Baker wrote:
/snip/ I have never noticed the fabric lifting on my wings, however I have seen the fuel siphon out of a wing tank due to an improperly applied fuel cap. And greater pressure in the tank than outside of it... Right, but in a sealed metal tank, is all that other fuel PUSHING the fuel out of the vent since air can't PULL it out? First of all, the tank is not completely sealed. If it were, the fuel pumps would soon have difficult pumping the fuel out of the tank. So, yes, the greater pressure inside the tank is pushing the fuel out. Hmmm...it probably goes more like this: there's a 100 mph? wind past an open port, with some venturi effect certainly, but plenty of turbulence. If you beat up the surface with a gusty blow, it gets wavelets which can lap the filler and blow out the fuel. Which reminds me of that trick that suction pumps use for high lift. As you probably know, if you pump down even to a vacuum above a tall 3water pipe, the water will not rise more than about 30 ft - (if it were mercury, it would not rise more than 29.92 inches on a standard day, remember?) Anyway, the mine engineers who want to pump up water MORE than 30 ft, say 40 ft without placing a force pump at the foot of the head, blow air into the water column which has the effect of reducing the density of the mix. If the relative density goes down from 1.0 to 0.5 they COULD pump up to nearer 60 ft. How bout that! Brian W |
#76
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visualisation of the lift distribution over a wing
Steve Hix wrote:
any number of things that are usually quite happily ignored by us in normal life. EXACTLY!! I don't have to know if air pushes or sucks (although I'm quite sure it SUCKS in L.A.) to control the airplane to do what I want it to do. |
#77
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visualisation of the lift distribution over a wing
Alan Baker wrote:
In article , Jim Logajan wrote: Alan Baker wrote: Because the push is caused by the impact of countless air molecules with the surface of wing. If those collisions fall to zero (i.e. in a perfect vacuum) then there is zero push. I don't see what a change in air density (such as taking the extreme case of a vacuum) has to do with lift. Unless you are claiming density change as a requirement? I believe lift can be reasonably computed using inviscid _incompressible_ flow theory (e.g. as far back as Kutta's 1902 dissertation,) so I don't see why any change in _density_ - much less the vacuum edge case - needs to be invoked. Any change in pressure is *by definition* a change in the number of particles in the fluid that are impacting the surface. That assertion is incorrect. You are no dummy so I'm sure you'll correct it when you realize the errors. I never mentioned density. Sorry, but you used the word "vacuum." The notable characteristic of a vacuum is that its density is zero. |
#78
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visualisation of the lift distribution over a wing
In article ,
Scott wrote: Alan Baker wrote: In article , Scott wrote: Alan Baker wrote: Observation can lead you astray: and that is clearly the case here if you actually think that air can *pull* on a surface. Why can't air PULL on a surface? Air is made up of molecules. Molecules have mass. Anything with mass can attract anything else with mass, can't it? Gravity? You're not serious. Anti-gravity in this case. If air can push something, why can't it pull something? Quote: "There are two things you need to know to be an engineer: 1) F = M x A 2) you can't push on a rope." Your question involves a violation of rule #2. |
#79
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visualisation of the lift distribution over a wing
Robert Bonomi wrote:
Quote: "There are two things you need to know to be an engineer: 1) F = M x A 2) you can't push on a rope." Your question involves a violation of rule #2. Hehe... If you push downwards on a tight rope carefully enough, you can qualify as a tightrope walker?? :-) Brian W |
#80
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visualisation of the lift distribution over a wing
In article ,
Jim Logajan wrote: Alan Baker wrote: In article , Jim Logajan wrote: Alan Baker wrote: Because the push is caused by the impact of countless air molecules with the surface of wing. If those collisions fall to zero (i.e. in a perfect vacuum) then there is zero push. I don't see what a change in air density (such as taking the extreme case of a vacuum) has to do with lift. Unless you are claiming density change as a requirement? I believe lift can be reasonably computed using inviscid _incompressible_ flow theory (e.g. as far back as Kutta's 1902 dissertation,) so I don't see why any change in _density_ - much less the vacuum edge case - needs to be invoked. Any change in pressure is *by definition* a change in the number of particles in the fluid that are impacting the surface. That assertion is incorrect. You are no dummy so I'm sure you'll correct it when you realize the errors. Sorry, but it's not. Pressure is created by particle collisions. I never mentioned density. Sorry, but you used the word "vacuum." The notable characteristic of a vacuum is that its density is zero. That is *a* notable characteristic. -- Alan Baker Vancouver, British Columbia http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg |
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