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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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Ok, here I go again. L. Adamson will cringe when he sees I'm stepping into this. We've been around this block before on other forums for years :-) First of all, we do understand lift. The fact that there are millions of airplanes flying around is direct experimental evidence that we have a pretty good idea of what nature is doing in this regard. Second, Bernoulli IS Newton. There is no disagreement as Bernoulli derived his relations FROM Newton's laws. However, Bernoulli's relations apply most aptly in the cases of fluid flow in tubes; that is not what is happening with a wing. Bernoulli is really a restatement of energy and momentum conservation applied to fluids moving inside tubes Third, the criticism that Newton is wrong due to Einstein is unfounded. Newton is not wrong, just not 100% correct ;-). Newton is perfectly capable of providing a very accurate description of nature as long as your speed is below about 10% that of light. We seem to be able to target very small places to land spacecraft on other planets just by using Newton. Fourth, lift is really due to momentum conservation of the wing-air interaction. The wing changes the velocity vector of the oncoming air. The change in air momentum must be balanced by the change in the wing's mometum; what we call "lift". If you want to see a very detailed explanation of what is happening, check out this link: http://www.aa.washington.edu/faculty/eberhardt/lift.htmor better yet, get Eberhardt's book. There's a great section of calculations showing what a C172's wing cross section would look like if lift was entirely due to pressure differentials by Bernoulli rather than direct mometum conservation. He also shows how a barn door can fly; which by the way is a fairly good approximation to the wing cross section on most modern fighters ;-). Read the book and decide for yourself. Mike (PhD in physics, BTW)

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>http://www.aa.washington.edu/faculty/eberhardt/lift.htm>>or better yet, get Eberhardt's book. There's a great section>of calculations showing what a C172's wing cross section would>look like if lift was entirely due to pressure differentials>by Bernoulli rather than direct mometum conservation. Ok, this is actually a good example how Mr.Eberhardt is "butchering" the Bernoulli principle and is a perfect illustration of what the article in Plane&Pilot is talking about. He says that Bernoulli principle "relies on equal transit time" which is simply not true, Bernoulli never made such statements and I doubt he ever concerned himself with wings in XVIII-th century. Once you start attributing false premises to Bernoulli principle you will no doubt get false results. In fact Bernoulli's law can very well compute the pressure differential and then the actual lift provided you won't use it compute the air velocity. Given air velocity over the wing you can use Bernoulli's law to get pressures and then integrating over the wing - the lift.Just to quote from the original article: According to NASA, lift can be calculated by

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"According to NASA, lift can be calculated by

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> It's just that the Bernoulli>contibution is small compared to the momentum contribution.It is the reverse. The momentum contribution is minimal. Your barn door will fly by the momentum change but such lift force would be on the order of 5% of the lift if you shaped it to be a true airfoil.Mr.Eberhradt defintely butchers the Bernoulli principle if he claims that "equal transit time" is part of it. His whole credibility is ruined by this single statement.Also I don't see how Mr.Eberhradt's (or you) agree with the NASA statement. NASA statement doesn't have any 'qualifiers' about different contributions and about Bernoulli's contribution being small. NASA's statement is as straigtforward as it can be - you add forces based on Bernoullis law and you get the lift (ok, but don't use it for the 'barn door' - there is little of laminar flow there). By the way you chose to twist things around and change wording in the NASA's statement. It is not "adding ... to the aerodynamic force(?) on the body" (which is highly ambiguous) but instead "adding pressure variation to determine aerodynamic force on the body" - a substantial difference.But ultimately as the original article in Plane&Pilot correctly states both Bernoulli and Newton's approaches at looking at the lift are equivalent since Bernoulli was derived from Netwon. But like I also stated above neither is an adequate or appropriate tool for sudying lift - Kutta-Zhukovsky theorem is the correct starting point. And no matter which you would chose - your dp/dt would not be enough to make your barn door turn into a respectable wing.Michael J.http://www.precisionmanuals.com/images/for...argo_hauler.gifhttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

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Ok, if as you claim, momentum change is but 5%(do you have a calculation to back this up as Eberhardt has for his case?), please answer the questions I posed: How does a symmetrical wing fly? How does a non-symmetrical wing (so-called standard airfoil) fly inverted? Show me *precisely* how the Bernoulli contribution explains both of these and I'll be convinced. Mike

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Ah,ha......No definate conclusions yet!Just as I expected! :D L.Adamson

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> How does a symmetrical wing fly?... how ...., how ....Like I said before - Kutta-Zhukovsky theorem is what you need (and read further below). I am amazed that you as a physicist (Ph.D. ?) would allow yourself to be trapped by this Newton-Bernoulli controversy which is primarily creation of dilettants trying to understand everything by 'naive' mathods. But anyway, just by accident I found this interesting 'critique' of Mr.Eberhard's paper (and book): http://www.av8n.com/fly/lift.htmThe authors point out a number of significant errors in his paper. They finish with this statement: I agree that the paper makes two or three valid points about common misconceptions. Alas, it goes on to replace those common misconceptions with a string of other misconceptions. I suggest you read it in its entirety (3-4 pages) - they go point by point explaining all his misconceptions.At the end of this review you will also find a link: http://www.av8n.com/how/htm/airfoils.html which to me is one of the few honest, error-free popular writings about lift generation. I knew about this particular source before, it was neat to find it included as a counterbalance to Eberhard's claims. It is a rather elaborate document of 20-30 pages but is a must read for anyone ready for a fresh start in this field with no preconceived agenda. Inverted flights, symmetric airfoils ... it is all there.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,Very interesting read! Read every word. The critique and the long document.And how would you like your eggs? Poached, scrambled or boiled? :-)That's about what the whole article amounts to. All the different theories have valid points to make and mathmatically they can all be used to calculate lift. In all honesty I hadn't heard of the Kutta - whatshisname concept before, but don't see it as any sort of stand out theory to debunk all others. For me the most informative concept to explain lift is still Bernoulli's. All the Circulation theories fail miserably for supersonic flight where there is no ciculation (unless someone can explain to me how air goes backwards through a shockwave? - I'm willing to listen ) but using good old Bernoulli's principle I can understand, perhaps not mathmatically, but at least conceptionally, the production of lift in high speed flight.So I'm still a happy chappy.Cheers,Roger

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>Very interesting read! Read every word. The critique and the>long document.I am glad you liked it.>And how would you like your eggs? Poached, scrambled or>boiled? :-)As long as you don't think of air as 'bullets' hitting the underside of the wing and generating lift - you can have your eggs anyway you want ;)> In all honesty I hadn't heard of the Kutta - whatshisname>concept before, but don't see it as any sort of stand out>theory to debunk all others. For me the most informative>concept to explain lift is still Bernoulli's. And you are OK with Professor Bernoulli. But neither Bernoulli nor Newton can explain the distribution of velocity around the wing - Kutta condition comes to rescue. Once you get the distribution of velocities you can use either Bernoulli or Newton's law to calculate local pressures and hence the lift.Michael J.http://www.precisionmanuals.com/images/for...argo_hauler.gifhttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

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>And you are OK with Professor Bernoulli. But neither Bernoulli>nor Newton can explain the distribution of velocity around the>wing - Kutta condition comes to rescue. Once you get the>distribution of velocities you can use either Bernoulli or>Newton's law to calculate local pressures and hence the lift.Ok, now that we all agree on Bernoulli and Newton, the next question is: why the air does not turn around the trailing edge (Kutta condition)? :-lolMarco

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