#3545868 - 03/27/12 09:37 AM
Lift theory. Were we wrong all along???
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AD
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This may not be news to some of the rivet counters that inhabit these parts, but apparently the generally accepted theory of lift (using the bernoulli principle) is a complete fallacy. Don't take my word for it. NASA says so. http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.htmlAs I (don't) understand things, it seems that flow turning (is this the same as the coanda effect??) is now considered the accepted theory for how an aerodynamic surface creates lift. Or is NASA going crackers? As a teacher I HATE teaching the wrong thing, and teaching my students the various theories is out of the question. Cheers
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#3545873 - 03/27/12 10:17 AM
Re: Lift theory. Were we wrong all along???
[Re: AD]
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Speedo
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I don't think they're saying that Bernoulli's principle is wrong. Really all Bernoulli's principle says is that increasing the speed of a fluid decrease's its pressure. I read it as taking issue with texts that claim airfoils generate lift by having the top surface longer than the bottom surface. They're pointing out symmetrical airfoils (same length top and bottom) can generate lift as well as designs where the bottom surface is longer. Of course, I'm still half asleep right now, so I could be misreading something.
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#3545895 - 03/27/12 11:47 AM
Re: Lift theory. Were we wrong all along???
[Re: AD]
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Ant
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The 'debunking' is a bit of a nit pick. Although the original theory of air travelling faster over the top to meet the air coming from the bottom has been extremely widespread I don't remember specifically learning that when I studied Aeronautical Engineering at college in the 80s. In fact we used a rotating cylinder to represent an aerofoil in most theoretical situations. The nit pick comes from the fact that they are now saying that the air going over the top doesn't meet up exactly with the air coming over the bottom, so that's not the exact reason that the air goes faster. Basically the air goes faster over the top because it's got further to go than the air at the bottom, that's still true, they're just saying that it's not racing to catch up with the air on the bottom. Hope that makes sense, typing in a bit of a hurry
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#3545922 - 03/27/12 12:46 PM
Re: Lift theory. Were we wrong all along???
[Re: AD]
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Gopher
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The use of Bernoulli's theory is inappropriate for explaining the creation of lift, such as why the suction surface is *so* much stronger than the lower surface, AoA effects, assumption of equal transit times, not to mention the lack of viscous effects.
AD, as you mention, it IS the *turning* of air (or rather, accelerating it around the curvature of the aerofoil) which creates lift through changing the surface pressure. If you think back down to Newton's laws, F=ma - by turning the mass of air, you are accelerating it in a different direction, which exerts pressure on the aerofoil surface, resulting in lift.
If it was as easy as applying Bernoulli, we wouldn't need (so many) supercomputers to analyse the aerodynamic design of aircraft.
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#3545935 - 03/27/12 01:01 PM
Re: Lift theory. Were we wrong all along???
[Re: AD]
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Dart
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Measured in Llamathrusts
Lifer
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I don't know why this would be a "new" theory. Wolfgang Langewiesche wrote what makes lift work (Newtonian physics) in Stick and Rudder back in 1944.
What makes the air flow around and then downward on the trailing edge of the wing (and forcing the plane "up" on the airfoil) is aided by Bernoulli, but I always knew the whole "sucking the airplane into the sky" thing didn't make sense long before I read the science on it.
The opinions of this poster are largely based on facts and portray a possible version of the actual events. More dumb stuff at http://www.darts-page.comFrom Laser: "The forum is the place where combat (real time) flight simulator fans come to play turn based strategy combat."
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#3545950 - 03/27/12 01:27 PM
Re: Lift theory. Were we wrong all along???
[Re: Dart]
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Gopher
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I don't know why this would be a "new" theory. Wolfgang Langewiesche wrote what makes lift work (Newtonian physics) in Stick and Rudder back in 1944. Mathematical physicists had a decent and general formula long before then as well, so none of the underlying knowledge is new at all. I blame media ignorance for this whole Bernoulli thing.
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#3545958 - 03/27/12 02:11 PM
Re: Lift theory. Were we wrong all along???
[Re: AD]
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oldgrognard
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Lifer
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Does this mean my airplane will no longer fly ?
Good people sleep peaceably in their beds at night only because rough men stand ready to do violence on their behalf.
Someday your life will flash in front of your eyes. Make sure it is worth watching.
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#3545959 - 03/27/12 02:14 PM
Re: Lift theory. Were we wrong all along???
[Re: AD]
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Urban Furball
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Call me dumb if you wish, but as a question for lift ... Would not angle of attack have a considerable bearing of contributing to such as well as airfoil shape and camber?
Only asking this as I have built numerous wings for model aircraft and all work with consideration that remaining forces required for flight is within scope. (Drag, thrust, weight)
Not trying to deter from this subject in any way, but you can have an airfoil equal in surfaces and lift can still be achieved.
And I probably don’t know anything anyway…. Just my thoughts.
One Flash.......and ur Ash!!
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#3545977 - 03/27/12 03:05 PM
Re: Lift theory. Were we wrong all along???
[Re: AD]
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Gopher
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So does a plane with symmetrical aerofoils (for example a stunt plane) have to fly at a positive AoA to generate lift? Yup. Keep in mind that aeroplane wings may already be "rigged" at a positive AoA, so that a nose-level attitude may by default put the wings at a positive AoA to the oncoming air.
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#3545983 - 03/27/12 03:19 PM
Re: Lift theory. Were we wrong all along???
[Re: AD]
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Antares
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AD: Yep. As Dart said, it's all to do with Newtonian physics. (I'm about to explain the 'downwash' theory of lift. Feel free to debunk ) (1) Force = mass x acceleration (F = ma.) (2) Every action has an equal and opposite reaction. In this case, the airfoil simply changes the direction of the airmass, providing an acceleration. (Note the acceleration occurs because velocity is actually made up of two bits - direction and speed. Acceleration is simply the rate of change of velocity, so, if you change the direction of the airflow, you accelerate it just as if you changed its speed.) So we've got an acceleration of the airmass. Now when it's the right way up, the airfoil, due to shape and angle of attack, accelerates the airmass down. By accelerating the airmass downwards, we must be exerting a downwards force on the airflow, since F = ma. That last sentence is the key. It brings us to (2): Since every force has an equal and opposite reaction, the airflow creates an equal and opposite force acting upwards on the airfoil. Of course, that upward force is LIFT and it's the reason noble knights of the air such as ourselves exist. Cheers R P.S. This explains why symmetrical airfoils must fly at a positive angle of attack; at zero angle of attack, they don't throw the air down, so they produce no lift. But cambered (curvy, non-symmetrical) airfoils do: so they can fly at zero angle of attack. But cambered airfoils normally produce more drag than symmetrical ones, so you don't get them on fast movers. And, of course, they don't work nearly as well upside down as they do the right way up, which is why we select symmetrical airfoils for aerobats.
So the Thunderhawk would come screaming out of the sky at mach ohmygod! and as soon as the struts hit the ground they would rip off. You're trying to apply scientific principles to 40k. Do you not see the problem with that?
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#3545985 - 03/27/12 03:24 PM
Re: Lift theory. Were we wrong all along???
[Re: AD]
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ArgonV
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"Go Fly A Kite!" -Jason R. FS-WWI Project Leader FS-WWI Plane Pack SiteIntel i9 10900k Gigabyte Z490 Aorus Elite AC 64GB Corsair DDR4 2933 Vengeance RGB Pro AMD XFX 7900 XTX Merc310 Black Edition LG UltraGear 38GN95B-B 38" monitor Corsair HX1200 PSU 1TB EVO 980 Pro M.2 PCIe x4 SSD 2TB EVO 980 Pro M.2 PCIe x4 SSD Two 2TB EVO 860 SSDs Sound Blaster ZxR Win 10 x64 Pro HOTAS Cougar #4069 w/Uber II Nxt mod #284 & UTM bushings
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#3546023 - 03/27/12 04:40 PM
Re: Lift theory. Were we wrong all along???
[Re: BeachAV8R]
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ArgonV
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..and more importantly..will the airplane still lift off from the treadmill..?? African or European? And will coconuts be involved?
"Go Fly A Kite!" -Jason R. FS-WWI Project Leader FS-WWI Plane Pack SiteIntel i9 10900k Gigabyte Z490 Aorus Elite AC 64GB Corsair DDR4 2933 Vengeance RGB Pro AMD XFX 7900 XTX Merc310 Black Edition LG UltraGear 38GN95B-B 38" monitor Corsair HX1200 PSU 1TB EVO 980 Pro M.2 PCIe x4 SSD 2TB EVO 980 Pro M.2 PCIe x4 SSD Two 2TB EVO 860 SSDs Sound Blaster ZxR Win 10 x64 Pro HOTAS Cougar #4069 w/Uber II Nxt mod #284 & UTM bushings
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