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michal

Bernoulli or Newton: who's right about lift?

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This happens to be the title of an article in the most recent magazine of Plane&Pilot (July 2006, page 74). I urge all those who like to resurect this subject once in a while on this forum to grab it and read it. It should finally settle this issue once and for all.It is impossible to retype this whole fun article but I can offer some quotes:When I asked Dr.Sheila Widnall, professor of Aeronautics and Engineering Systems at MIT, about this .... she responded, "It's scary there might be controversy about this issue. This is the basis of all subsonic aircraft design.".......The truth is that, among the Newton-Bernouli disputers, neither party is wrong. According to Dr.Jean-Jacques Chattot, professor of Mechanical and Aeronautical Engineering and director of the Center for Computational Fluid Dynamics at the University of California-Davis, the descriptions of lift advocated by Newton and Bernoulli "are actually the same thing, just from two different perspectives". How is this possible? ...Thankfully, Netwon's laws are failry straightforawrd and less confusing. Conversely, poor Bernoulli's concept tends to be butchered on a regular basis...Another widely held misconception about Bernoulli asserts that ...But if both Newton and Bernoulli are correct, how did so many pilots get so off track?...To add more fuel to the fire, prestigious academic institutions seem to favor one theory over another. Portions of Harvard's and Princeton's Websites discuss Bernoulli as though his ideas are the only ones available....Michael J.

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G'day Michael,Both Newton and Bernoulli are used to explain lift but for me Bernoulli gives a far greater understanding as to just exactly what's going on re- the boundary layer and also explains the stalling process far better. Newtons laws take a far more simplistic view, and for some strange reason I get the distinct impression that Americans tend to use Newtons laws :- just an observation, I could be wrong. In all my training here in Ausralia, and the rest of the English speaking world I have found Bernouli's principle is used. In all the textbooks I've used (even American) I've noticed that Newtons Laws are mentioned in a single sentence and then the rest of the Chapter is devoted to Bernoulli's principle.To be absolutely a 100% pedantic nerd, Newtons Laws are NOT correct. Einstein killed them when he proposed that TIME is not a constant. The differences are so infinitesimally small that to all practical extent Newtons laws work just fine.Cheers,Roger

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I actually regret that this article did not go a step further and said that neither Newton's laws nor Bernoulli theory is the right place to start investigating airfoils and lift forces. Newton's laws are too basic (they are at the root of it) and Bernoulli alone doesn't explain other things necessary to compute lift forces. It is the Kutta-Zhukovsky's circulation theory (derived of course from Netwon's laws) that is the basis of the modern aerodynamics. But the article addressed only the "popular" interpretations of lift theory and such popular interpretations clearly want to stay away from any complications/concepts that are inaccessible to general population. Most people with even high school education heard of Newton or Bernoulli but few heard of Kutta-Zhukovsky (or rather 'Jukowski').Michael J.http://www.precisionmanuals.com/images/for...argo_hauler.gifhttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

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G"day Michael,A great article BUT ...........There a few instances where the author displays a complete misrepresentation of the facts."Another widely held misconception about Bernoulli asserts that

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Hi, real or virtual PICs.Bernoulli or Newton? Who's right? Not THAT important ! What IS important about lift, is to knowthat a wing can stall at ANY **speed**. The most dangerous thing about a wing isan angle of attack too high. But that's some thing many pilots haveproblems to catch/understand. . . An example with a Cessna 172: How can I stallat 65 IAS, when in the POH they say50 kts, flaps up ???? What a pilot has to understand, is WHAT can force17* of angle of attack on his wing ? If he understands that, he will not make THEmistake, and will live & fly long ! B-) RIGHT ?+++++++++++++++++++++++++++ And we could say: A wing is nearly a half venturitube, when we fly fast, with an almost ZERO degree AoA. In that condition, the underside of the wing isalmost parallel to the relative air flow. So it's the top that does all the "work".The wing is the bottom part of the infinite venturi.Blue skies.

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G'day Michael,If I understand it correctly ( maybe I don't) you are refering to the airflow around the bound vortex that travels with the wing. What that gives is a doughnut of vortices consisting of the starting vortexleft and right wing tip vortex'sthe bound vortex around the lateral axis Whilst I agree that this circulation theory is the best explanation for accelleration and decelleration of airflow above and below the wing it doesn't explain, at least to me - I aint got no univesrety educashion :-) , the actual force of lift. Accepting that there is a difference in air velocities around an airfoil, then Bernoulli's principle explains the lift force.So for my money both of these concepts are needed.Cheers,Roger

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There we go again, the eternal misunderstanding of basic physics...Why do so many people (including those who are probably intelligent enough to pass a PPL exam) apear to be incapable of grasping the basics of physics?The prof who says both are right is of course correct, but apparently the author thinks he knows better than 2 persons with a doctorate in physics.

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>There we go again, the eternal misunderstanding of basic>physics...>>Why do so many people (including those who are probably>intelligent enough to pass a PPL exam) apear to be incapable>of grasping the basics of physics?>>The prof who says both are right is of course correct, but>apparently the author thinks he knows better than 2 persons>with a doctorate in physics.Well, one thing about it...The exact cause"s" of lift will still be argued for many years to come. Is the professor correct? Maybe, maybe notWould two people with doctorate's in physics have all the answers? Maybe, but most likely not!I've been keeping track of this subject for around 35 years now, and am well aware, that it's still not "fully" explained by a few laws of basic physics. We know how to use "lift" in design; however, there is still much reasonable disagreement on the cause.L.Adamson

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Personally I think nobody 'truly' understands the principles of lift, if they did wings on aeroplanes would be far more efficient. After all, a bird flies, and more importantly so does a Bumble Bee which according to Newton's laws, Cant.

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Planes don't fly. They glide; just like birds when they are not flapping their wings. When was the last time you see a bumble bee glide? Their lift is not generated the same way as the way the wings on airplanes do.

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they are far more likely to have the answers or at least the data towards finding the answers than some journalist...

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