Bernoulli or Newton: who's right about lift?

[mnmon 0]

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)

[michal 44]

>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

[mnmon 0]

"According to NASA, lift can be calculated by

[michal 44]

> 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

[mnmon 0]

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

[LAdamson 240]

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

[michal 44]

> 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

[Roger Mazengarb 0]

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

[michal 44]

>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

[Murmur 3,940]

>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

[mnmon 0]

"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." Why are you engaging in ad hominem attacks? I haven't done that to you. It's beside the point and irrelevant. I'm very familiar with John Denker, and have already read what he has done many times,and I don't disagree with his criticisms of Eberhardt. My main point is one of his acknowledged valid points of Eberhardt's paper. I quote from Denker: "Valid pointsThe paper makes several valid points. These include 1) The notion that the air passing above and below the wing must do so in equal time is totally false. 2) The notion that a wing must be curved on top and flat on the bottom is totally false. 3) Given the velocity field near a wing, we can calculate the lift by considering how the air is deflected and using the principle of conservation of momentum." Point #3 is what I have been emphasizing, that's it. Do you disagree with Denker's point #3? If not, then we are in agreement. Mike

[Murmur 3,940]

>3) Given the velocity field near a wing, we can calculate the>lift by considering how the air is deflected and using the>principle of conservation of momentum.">> Point #3 is what I have been emphasizing, that's it. Do you>disagree with Denker's point #3? If not, then we are in>agreement.I'm very surprised Denker makes that statement, because either1) he's wrong;or (much more probable)2) the sentence is poor phrased hence leading to misunderstanding.Actually,.if we take two imaginary infinite planes perpendicular to free stream velocity, one upstream and the other downstream of the wing;.then we integrate velocity field on each of the two;.then we calculate the difference of the two integrated results;we will find that the change in momentum of the air is exactly equal to the lift produced by the wing, in perfect accordance with Newton's law.Marco

[michal 44]

Marco,Frankly I don't understand why you need those "planes". Denker's statement is very clear and it isn't wrong. You have to integrate over the whole curvature of the wing (under and above) and you are integrating vectors, not scalars. You can't integrate just what is in front or behind - you would lose all the important velocity distribution around the wing and most importantly at the end you would only get the horizontal force. Go back to the formula that lift equals "circulation x ...." - you can't get this circulation unless you integrate around the wing.Mike,I guess I agree with you.Michael J.http://www.precisionmanuals.com/images/for...argo_hauler.gifhttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

[Murmur 3,940]

>Marco,>>Frankly I don't understand why you need those "planes".>Denker's statement is very clear and it isn't wrong. You have>to integrate over the whole curvature of the wing (under and>above) and you are integrating vectors, not scalars. You can't>integrate just what is in front or behind - you would lose all>the important velocity distribution around the wing and most>importantly at the end you would only get the horizontal>force. Go back to the formula that lift equals "circulation x>...." - you can't get this circulation unless you integrate>around the wing.Hi Michael,as was stated previously in this thread, both Bernoully and Newton are equally valid equations to describe in a quantitative manner the lift produced by wings.As you correctly said, if you integrate the pressures over the entire surface of the wing, you obtain the lift produced by it. The pressure variation is described with Bernoully theorem, so in this respect Bernoully is a valid way to describe lift.But, another equally valid way is to use the momentum change theorem by Newton. Infact, if you integrate the vertical component of the velocities on a plane (perpendicular to free stream velocity) and placed _ANYWHERE BEFORE_ the wing, and then you integrate the vertical components of velocities on another plane placed _ANYWHERE AFTER_ the wing, and then make the difference of the two, you will see that the vertical momentum of the air has changed. That is, the air before the wing has some upward momentum, while the air after the wing has some downward momentum. According to Newton's law, this change in momentum of the air must correspond to a force in the opposite direction applied on the body that caused it (in this case, the wing), and that force is exactly equal to the lift produced.The misunderstanding about Bernoulli and Newton is that neither of the two explains "WHY" a wing produces lift. And that's because physics just describes HOW things happen, not WHY they happen. One could say "A wing produces lift because it causes a change in momentum of the air around it", or one could say "A wing produces lift because it causes a changes in pressure distribution around it".But then one could ask "Why does it cause the change in momentum?" or "Why does it changes pressure distribution around it?", and here we go ad infinitum. :)Marco

[michal 44]

Marco,Your "infinte planes" perpendicular to free stream is just another "loop" around the wing (see 3.10.4 in their paper about quantifying circulation). As it happens you can get circulation no matter what path you pick around the wing. So if you think about it there is actually no conflict between your methods of integration or alternatively making a tight loop around the wing's airfoil - they all going to produce the same result. So in the end there is nothing wrong, misleading or poorly phrased in Denker's statement. It seems we ALL have been on a wild-goose chase - all roads lead to Rome. ;)I hope we all have learnt something from this thread, I certainly did.Michael J.http://www.precisionmanuals.com/images/for...argo_hauler.gifhttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg