NPR's explanation of lift
01-07-2008, 05:15 PM
#32
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Originally Posted by Cubdriver
Rick, the problem with your calculation is that it does not measure suction as an abstraction from anything else. Bernoulli's equation is a type of energy conservation equation applied only to incompressible (such as slow) and adiabatic (without heat added) flows. It utilizes the laws of principles of conservation of mass and conservation momentum. The latter is a consequence of the well known law, F=ma. Relating the Bernoulli equation to lift is a very tricky problem, but it was solved in about 1905 by the use of vortices as models to develop estimations for circulation around an airfoil. From that we get lift via the Kutta Jakowski theorem (lift= density x freestream velocity x circulation). You may know about JavaFoil to calculate pressure distributions on low-speed airfoils, which uses vortex math and geometry to find the velocities and pressures. Vortex models can be extended to the third dimension to analyze wings. CFD is used when a more accurate estimation of lift is required, and it uses truncated Navier Stokes equations to accounts for things like gravity, body forces, compressibility, boundary layers and viscosity.
01-08-2008, 03:41 AM
#33
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My post was a bit verbose but what I meant was that Bernoulli's Law is based on Newton mechanics and cannot be separated from it since it is based on it. Bernoulli's Law requires conservation of mass and conservation of momentum. The conservation of momentum law is a consequence of F=ma. A debate about suction versus Newtonian forces is meaningless, although it is true that pressure coefficients on top of the wing are more negative than anywhere else, and it would seem that Bernouilli is operative on top and not the bottom. Lift is the result of the wing altering the entire flow field around it and it can't be divided into pieces since it's a complete package of simultaneous physical events happening all around the wing to quite a distance away. In fact most of the pressures are negative, even on the bottom. The only positive pressures are those found at the leading edge. Breaking it into pieces like suction and reaction force may help understand supporting principles, but in the end they are intertwined. It is true that one can calculate the lift on a wing by summing the coefficients of pressure around it like Barry Schiff. Just keep in mind this in no way illustrates Bernoulli versus Newton.
Last edited by Cubdriver; 01-08-2008 at 05:07 AM.
01-08-2008, 07:42 AM
#34
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Originally Posted by Cubdriver
Rick, the problem with your calculation is that it does not measure suction as an abstraction from anything else. Bernoulli's equation is a type of energy conservation equation applied only to incompressible (such as slow) and adiabatic (without heat added) flows. It utilizes the laws of principles of conservation of mass and conservation momentum. The latter is a consequence of the well known law, F=ma. Relating the Bernoulli equation to lift is a very tricky problem, but it was solved in about 1905 by the use of vortices as models to develop estimations for circulation around an airfoil. From that we get lift via the Kutta Jakowski theorem (lift= density x freestream velocity x circulation). You may know about JavaFoil to calculate pressure distributions on low-speed airfoils, which uses vortex math and geometry to find the velocities and pressures. Vortex models can be extended to the third dimension to analyze wings. CFD is used when a more accurate estimation of lift is required, and it uses truncated Navier Stokes equations to accounts for things like gravity, body forces, compressibility, boundary layers and viscosity.
I am not trying to show how many books I own just that it is silly to claim lift is due to X% Newton and X% Bernoulli. I admit I fall into the same trap because it is tempting to oversimplify to reduce the brain cells involved to gain a true understanding. Most of what we read is so oversimplified as to be comical.
I am not trying to show how many books I own just that it is silly to claim lift is due to X% Newton and X% Bernoulli. I admit I fall into the same trap because it is tempting to oversimplify to reduce the brain cells involved to gain a true understanding. Most of what we read is so oversimplified as to be comical.
I have read up on most of that, and I know the entire Bernoullui equation. You are correct that Bernoulli vs. Newton is a simplification. For purposes of this discussion, I think everyone was trying to contrast the differences between what happens on top of the wing vs. below the wing.
Same fluid flow, different geometry. I also agree that the flow adjacent to the boundry layer on the bottom could very well produce a negative (below ambient) pressure at that point...the previous discussion used the term differential pressure when discussing the "suction" on top.
The "newton" effect on the bottom may well occur some distance below the wing in the airmass...ie air "bouncing" off the air flow closer to the boundry layer.
I think we are arguing over macro characterization of micro events, but I'm sure we agree on the nature of the micro events.
01-22-2008, 10:26 PM
#35
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Originally Posted by daytonaflyer
I don't agree with your plywood example since that is more of an example of how drag works than lift. The reason it's hard to hold onto the piece of plywood is due to increased form drag as you accelerate, not increased lift.
01-29-2008, 11:39 AM
#36
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BTW - NASA also says Bernoulli's Principal provides only a minority of lift, with most of provided by the downward deflection of air.
http://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html
http://www.grc.nasa.gov/WWW/K-12/airplane/right2.html
Furthermore, they claim that what I have been taught and also taught myself, about lift, is WRONG:
http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html:
01-29-2008, 01:25 PM
#37
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flow turning
I could have saved several posts on this thread earlier simply by using the term "flow turning". That is precisely what I was thinking. A wing causes flow turning which is a more central concept even than Bernoulli.
1. Suction on the topside of a wing is inseparable from the operative principle of flow turning. Even Bernoulli's Law serves this principle, and Newton mechanics is the more relevant approach to understanding.
2. Lift is the byproduct of flow turning and is a result of the complete flow field around a wing. This point is aimed at Ira Flato's original incorrect statement that a wing does not possess lifting properties on the bottom.
Once again, Bernoulli's Law is important but it does not explain suction on top of a wing; the larger picture is that of a complex flow-field best understood by Newton mechanics and most easily summarized as flow turning. We tend to stick to Bernoulli as a crutch because the accurate picture is not so easy. Lift is a topic better minds in physics struggled with into the 1940s, and as a topic it gets kind of prickly.
1. Suction on the topside of a wing is inseparable from the operative principle of flow turning. Even Bernoulli's Law serves this principle, and Newton mechanics is the more relevant approach to understanding.
2. Lift is the byproduct of flow turning and is a result of the complete flow field around a wing. This point is aimed at Ira Flato's original incorrect statement that a wing does not possess lifting properties on the bottom.
Once again, Bernoulli's Law is important but it does not explain suction on top of a wing; the larger picture is that of a complex flow-field best understood by Newton mechanics and most easily summarized as flow turning. We tend to stick to Bernoulli as a crutch because the accurate picture is not so easy. Lift is a topic better minds in physics struggled with into the 1940s, and as a topic it gets kind of prickly.
Last edited by Cubdriver; 01-29-2008 at 07:10 PM.
02-05-2008, 09:41 AM
#38
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I think this site does a good job explaining what I believe Cubdriver is laying down. One of the best explanations I've seen.
http://www.allstar.fiu.edu/aero/airflylvl3.htm
http://www.allstar.fiu.edu/aero/airflylvl3.htm
03-20-2008, 12:16 PM
#39
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Throwing a curve ball into the discussion....
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