Talk:Lift (force)/Archive 2

message: The "physical description" of lift reads like an old argument by another name. There's only one right answer: both.  Neither momentum nor pressure explain lift on their own, yet are both tremendously important to it. This should be obvious, since each section talks about the other.
Unfortunately, an accurate physical description of lift is not a trivial task and not even perfectly understood by anyone. A more correct description might talk about viscosity, the starting vortex, pressures, and momentum. Mostly it all comes out of the Navier-Stokes equations (which come from Newton's second law).
The mathematical models section has similar problems. The three “models” are convoluted and not explained very well:
*There is no such thing as the "Lift coefficient  model". It's just a nondimensionalization for convenience, allowable thanks to the Buckingham π theorem. I temporarily deleted this, but a link to Lift coefficient would be a good addition somewhere since it comes up frequently.
*Bernoulli's principle is not a model of lift, but it does factor into lift calculations: for example, it's used when deriving the Kutta-Joukowski Lift Theorem. The integrals used here to calculate aerodynamic forces do not follow from Bernoulli's equation and should not be labeled as such, so I retitled this section to “pressure integration”, but there's probably something better. This section seems to confuse a variety of concepts.
*The Kutta-Joukowski Theorem section needs to be cleaned up and better explained. 
I'll return to this when I have time. Mbelisle (talk) 05:00, 19 April 2008 (UTC)
thread_title: This article is a start, but still has problems

message: In the literature are there any examples, however idealized, where lift can be calculated exactly; i.e., without approximations in the math? I would like to find such an example cited for it would help in understanding lift. Renede (talk) 00:07, 8 May 2008 (UTC)
thread_title:  Exact calculation of lift 

message: When you solve the Euler equations with a finite differences or finite volumes solver to simulate the flow around a wing or a profile, the computed lift matches theory and experimental results. If lift requires viscosity to be generated, where does this paradox comes from: numerical viscosity? Jmlaurens (talk) 12:42, 3 August 2008 (UTC)
replies: text: This is a good question that may hint at a problem with the treatment in the article. There is no drag in potential flow (D'Alembert's paradox) and no lift without imposing a circulation. You're right that inviscid numerical solvers (like Fluent) don't have this problem. I'll try to reconcile this so that the article addresses this question. Michael Belisle (talk) 17:26, 1 September 2008 (UTC)
replies: text: Numerical viscosity may be the source, but it looks like it's not an easy answer. Rizzi and Eriksson (1983) consider some possibilities. I think the topic is beyond the scope of the current Lift article. Michael Belisle (talk) 07:55, 21 September 2008 (UTC)
thread_title:  Euler solver and Lift 

message: The Coanda effect section has been rewritten numerous times, switching between the version  that I think is NPOV and the current version by Ccrummer, which favors one viewpoint without adequate citations. I think it's important to remember that some consider it important to lift in a general sense while others limit it to a jet impinging on a curved surface. I'm in the latter camp and have tried to write the article so considers both viewpoints and uses citations to support any claims.
In its current form, "A common misconception about the causes of aerodynamic lift is that the cause of the Coandă effect is not one of them or at least that it is a negligible component." needs a reputable citation. Nobody I've seen has referred to this as a misconception; the misconception is universally the other way around. Raskin uses it incorrectly, while Scott Eberhart and David Anderson mention it in passing (in a manner different from most texts on the subject of lift, who don't mention it at all), and then proceed to talk about viscosity. Others, for example JD Denker explain that using the Coanda effect to explain lift is a misconception since it's properly limited to effect of a jet impinging on a curved surface that generates additional lift. It was an effect used to generate additional lift by Coanda and never was intended to explain lift in general.
The citations in the section now don't support many of the claims:
*The "real and powerful" effect supported by references 17 and 18 is that of the Coanda effect as a method for a high lift device, which nobody disputes. The dispute is whether or not the Coanda effect should be used to explain lift in general.
*Raskin never really explains lift: he demonstrates the Coanda effect (as a jet exiting from a straw impinging on a curved surface) and then waves his hands saying that he's proved lift. It is not a reputable citation, except to explain his misconception regarding lift. (see  comments by JS Denker and the paper by Auerbach),
*Eberhart and Anderson make no such claim that "the Coandă effect should not be ignored in a comprehensive explanation of lift". They merely mention the existence of the effect, and then explain that viscosity causes the fluid to follow the surface. (Which is a false link: viscosity is not enough to explain the most dramatic examples of the Coanda effect and the Coanda effect is not interchangeable with viscosity. They should be two separate topics.)
Additionally, this section grows each time it's revised; there is now a fair amount of information that is here and not in the main article. Properly, this should be a summary of the main article, not an standalone description of the effect. I rather like the main article, in fact.
Maybe the title "Common misconceptions" isn't the best title for the section. By and large, the Coanda effect is something that only comes up as a general explanation of lift in non-technical texts, so maybe something like "Alternative" or "Popular Explanations" would be better, so that it doesn't have to be limited to misconceptions.
At this point, it's more or less a futile edit war, so I'd like to talk about it before I make any more changes. I'd like some more info on what the other camp's reasoning is so that we can come to some agreement. Michael Belisle (talk) 21:13, 5 August 2008 (UTC)
replies: text: Hi Michael.  A debate about lift, and misconceptions related to lift, raged on Bernoulli's principle for a few months.  Each time we mentioned that Bernoulli’s principle could be seen in action in the lift on a wing, one user or another would delete it, saying 'It is a fallacy to use Bernoulli to explain lift'.  These claims were never accompanied by any citation, reference or source.  I posted a “Citation needed” flag against a claim that Bernoulli had no place in the explanation of lift and a kind user added a citation of Anderson and Eberhardt’s book Understanding Flight.  The citation didn’t identify a chapter or page so I obtained a copy of the book.  That book also led me to Langewische’s book Stick and Rudder.  I wrote a section about these two books and their relevance to explaining lift, and posted it in Bernoulli's principle.  Eventually that section was transferred to Bernoulli’s Talk page and you can still see it, complete with citations, HERE.  It is relevant to this discussion on Lift (force).  Dolphin51 (talk) 23:59, 5 August 2008 (UTC)
replies: text: Since there was no answer in support of the current version, which was just a POV rewrite with no new citations, I reverted the section back to the version dated 23:37, 31 July 2008. The old version can certainly be improved. I'll look more in detail at some of the stuff you wrote for the Bernoulli article when I have time (hopefully in the next few weeks). I think this issue about the Coanda effect stems from a misunderstanding that "Coanda effect" implies "Newton was right", which it doesn't. Even if the Coanda effect were properly applied to why the flow attaches to the wing, we could still use Bernoulli's principal to calculate the amount of lift. In fact, we do use Bernoulli's principle to calculate lift. Michael Belisle (talk) 19:53, 17 August 2008 (UTC)
thread_title:  Coanda Effect 

message: The upper stagnation point continues moving downstream until it is coincident with the sharp trailing edge (a feature of the flow known as the Kutta condition). If this statement is true, then where is the sharp trailing edge on these pre-shuttle prototype lifting bodies? No thin sharp trailing edge, plus they have flat tops (tapered tail but not sharp) and curved bottoms. Being potential re-entry vehicles, I'm sure the lift to drag ratio was poor (my guess between 5:1 to 10:1), but they worked:
M2-F2 glider with F104 .jpg
M2-F3 rocket powered with B52 .jpg
The M2-F3 had a top speed of mach 1.6, so its drag wasn't excessive. Jeffareid (talk) 15:30, 27 September 2008 (UTC)
replies: text: The statement is true when you, as the article says, "consider the flow around a 2-D, symmetric airfoil at positive angle of attack," pictured as a NACA 0012 (which of course has a “sharp” trailing edge). But the article never says that such a trailing edge is the only way to generate lift. Try not to, ummm, deny the antecedent. Michael Belisle (talk) 22:43, 28 September 2008 (UTC)
replies: text: Well I got the impression that the article was implying the sharp edge and Kutta condition as requirements for lift. I read too much into that, sorry. Jeffareid (talk) 02:44, 29 September 2008 (UTC)
thread_title:  Stages of lift production 

message: How would "obstruction theory" or "Kutta condition" explain these lifting body airfoils with "humps" on the bottom and "thick" trailing edges? M2_F2_glider.jpg M2_F3_rocket_powered.jpg Jeffareid (talk) 09:28, 30 January 2009 (UTC)
replies: text: There is a little relevant information at Lifting body.  I think the key to understanding these lifting bodies is that they are intended to operate at VERY high speeds — hypersonic (as occurs at re-entry), supersonic and high sonic.  Their maximum lift coefficient doesn't need to be particularly high so they don't employ classic sonic or even supersonic airfoil shapes.  Notice that, in both photographs, there appears to be a trailing edge flap deployed.  Presumably this is to increase maximum lift coefficient, both during the launch from the B-52, and during landing.  Notice that landing gear is not yet extended in the landing photograph.  The gear is possibly deployed only when extra drag is needed, or just prior to touch-down.  Dolphin51 (talk) 10:08, 30 January 2009 (UTC)
replies: text: Nice link. Thanks for making this a separte section. It appears that the M2-F2 is unique compared to the two other old lifting bodies (those look more "normal"), but maybe ahead of it's time as most modern hypersonic models also have relative flat tops and all the protusions like engines and lifting surfaces below. I realize the purpose was hypersonic flight (re-entry), but it did glide (or fly) reasonably well at subsonic speeds (once they added the third vertical stabilizer), and it would seem that this is a wiki article on lift in general, not just effecient sub-sonic lift. Compare the apparent angle of attack with the F104 chasing it in the first photo, it's not bad. If nothing else, it looks cool, and at least would seem to be a good example to dispute equal transit theory. I'm a bit curious as to who and how it was figured out that such a design would work in the first place. Jeffareid (talk) 10:58, 30 January 2009 (UTC)
thread_title: Lifting bodies

message: What are the aspects of the Coanda like airflow across the top of a cambered air foil, especially near the leading edge? It would seem that there is an "inwards" (downwards) acceleration of air over the top of cambered airfoil with a significant component of acceleration perpendicular to the flow, so without much change in speed (magnitude of velocity) and therefore without much change in kinetic energy of the air flow near the wing (although the low pressure and viscosity would accelerate the surrounding air into that diverted flow). Jeffareid (talk) 11:22, 30 January 2009 (UTC)
thread_title:  Question about flow and pressure over a cambered airfoil