This is an archive of past discussions about Lift (force). Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page.
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 started a more complete physical description of lift production. Until I find a good general reference, it's unreferenced and almost certainly has some problems. It would also benefit from diagrams showing the stages of lift production, which might save some of the verbiage. I acknowledge that it may be overly technical at the moment, which I'll work on once I'm sure it's correct. It think it is, at least, better than NASA or The Straight Dope, and who seem content to conclude "Why the flow moves faster over the upper surface is complicated, don't worry about it."
The "Newton" and "Bernoulli" explanations of lift should be added to the common misconceptions section, perhaps as “Newton vs. Bernoulli”, and/or to a section on ways to measure lift. If you know the details of the flow field (or are making measurements in a wind tunnel), then sure either
the “Newton” way: a direct measurement of the force (via a force balance) or
the “Bernoulli” way: integrating the pressures (via pressure taps on the surface of the airfoil)
will give you a reasonably accurate value for lift. But neither answers the real question a physical explanation should address: “Why does a wing generate lift?” Michael Belisle (talk) 00:38, 20 April 2008 (UTC)
We must bear in mind that Wiki is a general encyclopaedia and not a technical treatise, it should be written in language and style that is accessible to all. After all, the formulae are derived from principles not the other way round. The article, in my view, should not primarily be a description of the detailed physics or of the derivation of mathematical models but sections on these can be included. I agree that the physical explanation has to focus on addressing the question "why does a wing generate lift" but it should first do it in layman’s terms.
The value of many Wiki articles are effectively destroyed by us technicians expounding nuances that are absolutely irrelevant to the general user.
Your idea that aspects of the Newton and Bernoulli debate would sit very well in the misconceptions area is an excellent one.
I believe it is relatively straight forward to explain why the flow moves faster over the upper surface to a layman and without once mentioning Bernoulli or conservation of energy. I will give it some thought. Rolo Tamasi (talk) 08:24, 20 April 2008 (UTC)
John D. Anderson says “You cannot get more fundamental than this, conservation of mass and Newton's second law” (JDA, Introduction to Flight, p. 355). I use JD Anderson as a reference because, as Curator of Aerodynamics at the Smithsonian and a standard in Aerodynamics texts, he is quite possibly the most authoritative person on the subject. His explanation cannot be presented on equal footing with eskimo.com.
Bernoulli's principle, as applied, comes from Newton's second law while conservation of mass (in 2-D) is simply is a constant. In the equal transit-time explanation, people have no trouble accepting that that air moving faster has lower pressure than slower moving air, which is an application of Bernoulli's principle. True, the form of Bernoulli's equation applied is actually “Bernoulli's equation for inviscid, incompressible, steady flow along a streamline”, but in some ways that's more information than is necessary. It's the most common form of the equation and the only way to explain lift without mentioning Bernoulli is to not give the principle a name.
That said, I admit that what I put in there is temporarily not written in laymen's terms. I tried to first to put something in there that is correct and referenced (what was there was understandable but misleading). We can go back and make simplifications as appropriate, especially adding pictures.
After I wrote the section on viscous flow and lift startup (from memory), I searched for references in books by reputable aerodynamicists and I came across JD Anderson's explanation, where he explains lift in steady-state terms with conservation of mass and Newton's second law, neglecting viscosity. Technically, this is correct: if you started up a viscous Navier-Stokes solver and solved only for the steady-state flow field, you would never see the starting vortex. You could then describe the flow field as he has done. So I threw in a sloppy version of his explanation. JD Anderson also explicitly describes downwash (i.e. Newton's third law) as an effect of lift and circulation as a mathematical model of lift. His reasoning there is sound.
I have since located a good explanation of the stages of lift production in K Karamacheti, Principles of Ideal-Fluid Aerodynamics and clear diagram in FM White, Fluid Mechanics. The stages are inclusive of the foregoing. I think a good argument could be made to move the stages to another article. But, if I had looked up lift before I was an Aerospace Engineer, I would have wanted to know about viscosity. Failing to mention the starting vortex or viscosity is hand-waving. “The generation of [lift] depends on entirely on the nature of viscous flow past certain bodies.” (Karamacheti) Michael Belisle (talk) 22:18, 20 April 2008 (UTC)
I think what we need to do here is rewrite the article in a way that it conforms in some way to Wikipedia:Make technical articles accessible, without loss of information. It is unavoidable that, like special relativity, a variety of readers will come here, including aerospace engineers who don't understand how a wing produces lift. (Strange but true: I can guarantee that this article will show up in undergraduate lab reports on airfoils. Back when I was a TA, I mostly solved this problem by taking off points for citing Wikipedia because it was often wrong: for example, see Image:LiftCurve.svg or “At a zero angle of attack, no lift is generated,” which was in this article yesterday. I'd prefer not to have to do that.)
Lift, on its own, isn't significant enough to warrant its own article like Introduction to special relativity, but there is certainly a way to organize it in such a way that laymen don't feel it's overly technical, while aerospace engineers don't feel it's overly simple. Perhaps we need something like an Introduction to flight article, but that should involve a team effort within WP:AVIATION about what to include and how to present it.
The physics community is way ahead of aeronautics in communicating technically correct ideas to both general and technical audiences. I think it's a good model to follow. Michael Belisle (talk) 03:36, 21 April 2008 (UTC)
Wow, being a layman I read this article and understood nothing. Then I read the The Straight Dope page as mentioned above and I'm feeling happy again. Even if I don't understand fully and the Straight Dope has cut corners (which it probably has), at least I feel I understand 'lift' now. So can someone please make this page make me feel like that? Then I'll truly be a happy (lay)man.... Malick78 (talk) 16:49, 9 October 2008 (UTC)
I honestly think that a 'lay explanation' based around action-reaction is more intuitive to most people. The problem we experience when explaining lift is that one group of people try to explain how you would calculate lift, which almost always goes the pressure route; and the other group is trying to "explain" lift, without much focus on ease of calculation. Pressure integrals are used to calculate lift because it is easier to use the well-defined boundary of the wing, but it is possible to calculate lift from the deflection of the air as well. In my experience with students (I am a lecturer at a university), the explanation that wings experience an upward and backward force by deflecting air downwards gels with their experience of things like fans. The question of why airfoils are shaped the way they are then largely becomes related to how to deflect the most air, which involves delaying flow separation on the top surface. The pressure integral approach leaves some very interesting questions about why the air is moving faster on the top surface than the bottom one. Of course these two things are different ways of looking at the same phenomenon, but the flow deflection argument requires less build-up about Bernoulli. Chthonicdaemon (talk) 05:05, 4 December 2008 (UTC)
It might be more "intuitive" to some readers, but others will disagree. Like you said, “these two things are different ways of looking at the same phenomenon”; WP:NPOV therefore says we should include both. This article tries to write one explanation that does that, encouraging readers to recognize that the Bernoulli and Newton are equivalent and related. I disagree that airfoil design is a question of how to deflect the most air. You still have to deflect the air in a certain way. The related fact that a rotating cylinder in a free stream generates lift has nothing to do with deflecting the most air. Michael Belisle (talk) 18:49, 9 January 2009 (UTC)
A summary of where I think the physical explanation should go
I've done some thinking, examined some more sources, and reviewed a little of the prior Bernoulli v. Newton debate here. Of the debate, I see that pretty much everything has already been said.
My current thinking (which is still evolving) is that the final value for lift comes from an equation where, in physical terms, often acceleration is on the left while the pressure gradient, viscosity, and gravity are on the right: effectively ma=F. Which terms are your favorite? (I like viscosity, pressure, and then acceleration. There's no love for gravity from me.)
We should develop one cogent, understandable explanation that develops the whole picture, not obscure parts under the guise of accessibility. Michael Belisle (talk) 01:43, 22 April 2008 (UTC)
The Bernoulli Principle is rather one dimensional as it describes conservation of forces along a streamline (static pressure plus dynamic pressure equals stagnation pressure).
In reality, Newtons Laws of Motion must be valid for each degree of freedom. For the 2 dimensional case presented here, Newtons Laws must be true in two orthagonal axes, whether it is represented as a global x and y basis, or a local streamline and equipotential direction basis.
Bernoullis Principle ignores the forces transverse to the streamlines which act in the direction of net lift.
Also, Bernoullis Principle fails to explain the curvature of streamlines which happen to be caused by applying Newtons Laws of Motion in the transverse streamline direction. "Particles of fluid remain stationary or travel at constant velocity in a straight line unless acted on by an external force". If Newtons Laws are going to be applied they need to be fully considered. —Preceding unsigned comment added by 58.106.38.84 (talk) 14:50, 17 June 2008 (UTC)
The thing that needs to be understood here is that bernouilli explanations aren't wrong- the lift on the wing really is partly due to bernouilli. Neither is the Newtonian stuff wrong- the lift really does involve deflecting air to give lift. Nor is the Coanda effect not involved. The Coanda effect is why the flow curves around the wing in the first place. They're ALL correct. It's like the blind men with an elephant, they're each just a piece of the overall picture. You can look at this different ways and particular features stand out.- (User) WolfKeeper (Talk) 15:46, 17 June 2008 (UTC)
Wolfkeeper is right (except that I think the definition of the Coanda effect should be limited to what Coanda considered, a method for a high-lift device).
Saying that "Bernoullis Principle ignores the forces transverse to the streamlines" is nonsense. Pressure has no direction: it's a scalar. Variations in pressure generate forces (e.g., the net pressure difference between the upper surface and the lower surface). Bernoulli lets you use the pressure at one point to find the pressure at another point, which in turn can be used to find the aerodynamic forces. That's all; it doesn't explain why the flow is the way it is.
But Newton's third law, by the same token, doesn't explain lift either. It just considers an effect of lift (the downwash) and tells you how much lift there is, analogous to what Bernoulli does. The curvature of the streamlines is not explained by Newton's third law. Only in a hypersonic flow does Newton's third law have some direct influence on the amount of lift generated.
A more complete picture of why an airfoil generates lift doesn't start to appear until you consider viscosity. Viscosity is complex, but there is no lift, no pressure difference, and no momentum deflection without it. Michael Belisle (talk) 06:20, 27 June 2008 (UTC)
Michael, the following is my understanding of the logic behind your statement "there is no lift...without viscosity". Is it correct?
Newton's 3rd law is a differential equation. As such, it has an infinite number of solutions. This infinite family of solutions doesn't explain why lift occurs, because each solution gives a different value for lift. So, any imaginable direction or amount of lift is possible.
To get a specific solution for the lift which corresponds to the real world, and thus an understanding of why lift occurs, one must consider viscosity. Doing this allows one to determine that the aft stagnation point must be at the trailing edge. With this added boundary condition, now Newton's law can be used to predict and explain why and airfoil generates lift.
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)
Euler solver and Lift
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)
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)
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)
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)
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)
I reverted the edit of Ccrummer, which does it make appear that the Coanda effect is considered as a serious alternative candidate for the explanation of lift. Moreover, it used Fluid Mechanics of F. White as a reference stating that "... It (the Coanda effect) happens as a result of the viscosity of the fluid as it shears past the curved surface", while I cannot find anything in the 4th edition of White's book on the Coanda effect. -- Crowsnest (talk) 23:47, 12 October 2008 (UTC)
This subsection, and the other one on the "equal transit-time" both lack reliable secondary-sources (see WP:PSTS and WP:RS), i.e. peer-reviewed scientific papers (since this is a scientific subject) in support of these views. They mostly seem to appear in some popular explanations of lift. Further these theories are only qualitative, i.e. there are no reliable sources giving accurate quantitative predictions of lift. Nor are there reliable sources available on scientific experiments in their support. So, these theories do not deserve undue weight, and claims associated with them need some proof before they can be incorporated into the article. -- Crowsnest (talk) 01:24, 13 October 2008 (UTC)
Alternate physical description of lift on an airfoil
Forward speed and an effective (lift producing) angle of attack of an airfoil results in a coexistant pair of forces pependicular to the directon of travel relative to the orientation of an aircraft: a downwards force exerted by a wing onto the air and an upwards force (lift) exerted by the air onto the wing (in accordance with the third law of Newton's laws of motion). The force exerted by the wing onto the air coexists with a downwards acceleration of air (in accordance with the second law of Newton's laws of motion). The acceleration of the aircraft perpendicular to the direction of travel depends on the sum of the lift force and gravity. In level flight, lift force exactly opposes gravity and is equal to the weight of an aircraft. There are also coexistant forces in the direction of travel, the aircraft exerts a forwards force onto the air, and the air exerts a backwards force onto the aircraft (this component is called drag), and these forces coexist with a forwards acceleration of air.
The forces and accelerations of air and wing coexist with pressure differentials in the vicinity of the wing: lower pressure above (and behind), and/or higher pressure below (and in front). Air accelerates in all directions from higher pressure areas to lower pressure areas, except that a wing, being solid, blocks upwards (and backwards) flow, so the result is a net downwards (and forwards) acceleration of air.
In the case of a flat airfoil, mechanical interaction between air and the bottom surface of the air foil deflects air downwards. If the angle of attack isn't too high, the air will also tend to somewhat smoothly follow the upper surface downwards due to "void" effect (from wing: a low pressure region is generated on the upper surface of the wing which draws the air above the wing downwards towards what would otherwise be a void after the wing had passed.). At the front of the air foil, the air seperates into two flows, and because of the pressure differential (lower above, higher below), some of the air is diverted upwards, lowering the seperation point to below the leading edge of the air foil. If the leading edge angle isn't too steep, or too sharp, then a Coanda like combination of surface friction and vicosity allow the air to bend around the leading edge, and this bending also corresponds to a net downwards acceleration of air, and the coexisting pair of vertical forces (air upwards on wing, wing downwards on air).
Outside of mechanical interactions, there is no net work done on the air, and when no net work is done, then there is a coexistant relationship between acclerations, forces, pressures, and velocities that follow Bernoulli's principle.
The acceleration of air results in an increase in the kinetic energy of the air. An efficient airfoil shape generates most of it's lift force via reduction in pressure in the vicinity above the wing and minimal net mechanical interaction below, which allows a Bernoulli like conversion of pressure energy into kinetic energy, reducing the amount of power required to produce lift. The other way to make a wing more efficient is to reduce the amount of kinetic energy required to produce lift by increasing the size of the wing, because a larger mass of air accelerated at a lower rate consumes less power. It turns out that it's more efficient to make the wing span longer than than the wing chord wider. Jeffareid (talk) 18:26, 13 September 2008 (UTC)
Void theory (as opposed to Coanda effect) is already cited in the artile on wing. Bernoulli principle relates pressure and velocities in a work free environment, and these relationships don't hold in the immediate vicinity of a wing, specifically the pressure jump zone where work is done, but Bernoulli does apply outside this zone. This article on propellers explains what I'm getting at: But at the exit, the velocity is greater than free stream because the propeller does work on the airflow. We can apply Bernoulli's equation to the air in front of the propeller and to the air behind the propeller. But we cannot apply Bernoulli's equation across the propeller disk because the work performed by the engine violates an assumption used to derive the equation.propeller analysis. Note that a wing, similar to a propeller, operates in it's own induced wash, often ignored in streamline diagrams. As far a Newton goes, the 3rd law, forces only exist in pairs, explains why downforce from a wing coexists with upforce from the air which pretty much explains lift. Newtons 2nd law relates forces to mass and acceleration. incoporate This section was added as food for thought, I'm not sure how to reword it for the actual article, but it's good enough for the discussion section. Jeffareid (talk) 01:04, 28 September 2008 (UTC)
As far a Newton goes, the 3rd law, forces only exist in pairs, explains why downforce from a wing coexists with upforce from the air Okay. which pretty much explains lift. No. That's just one piece of the big picture. “Several physical principles are involved in producing lift. Each ... is correct .... [But] do we get a little bit of lift because of Bernoulli, and a little bit more because of Newton? No, the laws of physics are not cumulative in this way. There is only one lift-producing process. Each of the explanations ... concentrates on a different aspect of this one process.” http://www.av8n.com/how/htm/airfoils.html#sec-consistent
Bernoulli's principle absolutely applies in the context of lift on a wing. If it didn't, every aerodynamicist might as well just quit now. Apparently we've been wasting our lives measuring pressures to determine lift. (There are other ways, of course. In a wind tunnel, we can directly measure lift using a force balance. But we don't generally determine lift by measuring the downwash simply because it's not easy to measure.) While a propeller shares some things in common with a wing, it's really a different problem that requires a different approach.
My awkward wording is why I didn't suggest this as a proposal for inclusion in the main article. I didn't intend to make this a Newton versus Bernoulli debate. Obviously both principles are involved in lift, Newton explains the relationships between force, mass, and acceleration. Bernoulli explains the relationship between pressure differentials and acceleration of air. It would seem that an explanation of lift should focus on how those pressure differentials are created (angle of attack and air speed), then continue to explain the coexistant relationship between forces, pressure differentials, and acceleration of air.
There is that nagging issue that the final result is a change in total energy of the air, which means that during the process of producing lift, a non-Bernoulli like interaction between air and airfoil occurs, and how this occurs should be described, although I haven't seen a good description of this process, just a note that work was done. This is more obvious in the case of a propeller because a propeller is typically less efficient than a wing. Jeffareid (talk) 11:18, 28 September 2008 (UTC)
The way to reword it for the article is to use a citation to back up any claims with a citation. I see the word “void” in the wing article, but it is rather poorly written and not cited. I had never heard the term “void theory” until you mentioned it. Google suggests that you are the only person to use that term when discussing lift. If you have seen the term somewhere, then we need that source to verify what you say. If not, then there's no way to work it into the article. Michael Belisle (talk) 05:18, 28 September 2008 (UTC)
"Void theory" is my terminology, but I don't think I was the first to use it as such, and it was someone else that used it in wing. In the case of landbased vehicles, it's often explained that most of the drag is due to the void at the rear of the vehicle. Void theory seems to be a much better explanation of why air follows the upper surface of a moving inclinded plane than Conada effect which to me implies an effect related to friction and viscosity. Void theory - air molecules are colliding with surface (or the surrounding boundary layer) of a moving airfoil, but as the uppper surface passes by, it leave a downwards and forwards moving "void" that requires that the air molecules travel farther before they collide with the surface, resulting in a net downwards (and forwards) acceleration of air. I'm not aware of another term used to decribe the interaction between air and movment of a surface "away" from the air.
Void citations are rare, but I found another one in addition to the wiki article on wings:
draws the air above the wing downwards towards what would otherwise be a void after the wing had passedwing
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:
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)
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)
Old discussion: attack on the use of the Bernoulli equation as a partial explanation of aerodynamic lift
This is an attack on the use of the Bernoulli equation as a partial explanation of aerodynamic lift.
Bernoulli's equation for the conservation of energy is commonly used to explain aerodynamic lift, however the equation assumes a steady flow, i.e. flow in which particles follow velocity streamlines. A stream line is a line of constant velocity in the flow. When the particles themselves do not follow the streamlines, they are changing velocities, that is, they must cross the streamline bundle. Shear flow is an example of non-steady flow.
The flow near the airfoil surface is shear flow because of the existence of the boundary layer, a layer of air in which the velocity changes across the flow; the layers of air shear past one another, faster layers toward the ambient flow and slower layers near the airfoil. This is non-steady flow and Bernoulli's equation is therefore not applicable in this region. (See pg. 9ff. Fluid Mechanics. Landau, L. D. and E. M. Lifshitz. (1959.) Addison-Wesley: Paris.
Here is the beginning of a correct explanation of this part of lift:
Upwash (from the bottom of the wing at angle of attack) joins with the main flow at the leading edge to envelop the top of the wing. The combined flow interacts with the boundary layer on the curved part of the top of the wing and, for sufficiently small angles of attack, depletes the boundary layer population there, i.e. the pressure on the top is decreased, by the "blowing" of particles away from the wing's surface as it curves away from the flow. It is the curvature of the wing that is crucial in this component of lift. This decrease in pressure on the top of the wing adds to the lift. It can be seen in the Coanda effect, i.e. a smoke stream near the curved surface is deflected toward the surface by the higher pressure in the flow (the ambient pressure). The difference between the lower pressure at the top surface and the higher pressure under the wing results in lift. —Preceding unsigned comment added by Ccrummer on 27 January 2007.
I have just found these comments published by Ccrummer in January 2007. There is universal agreement with Ccrummer that Bernoulli's principle is not applicable in the boundary layer because BP is stated to be applicable to inviscid flow (or flow that closely resembles inviscid flow) and the flow in the boundary layer is clearly not inviscid flow. However, the great majority of air affected by an airfoil is not part of the boundary layer and its flow closely resembles inviscid flow.
Ccrummer’s "correct explation of this part of lift" looks like original research. There is no reference or citation to indicate where the ideas come from, or whether these ideas have ever been published by someone. Wikipedia is not the place for our original research or our personal views on what we believe is true. See Wikipedia:No original research.
Ccrummer’s stated views are at odds with statements by most (all?) specialist authors in the fields of fluid dynamics and aerodynamics. For example, in the excellent book Aerodynamics, L.J. Clancy has written “When a stream of air flows past an airfoil, there are local changes in velocity round the airfoil, and consequently changes in static pressure, in accordance with Bernoulli’s Theorem. The distribution of pressure determines the lift, pitching moment and form drag of the airfoil, and the position of its centre of pressure.” See Clancy, L.J. (1975), Aerodynamics , Section 5.5, Pitman Publishing Limited, London. ISBN0 273 01120 0.
Ccrummer has written “steady flow, i.e. flow in which particles follow velocity streamlines. A stream line is a line of constant velocity in the flow.” I have read many books on fluid dynamics, but I haven’t ever read of a “velocity streamline”. Also, when Ccrummer writes “A stream line is a line of constant velocity in the flow” he is at odds with all the authors I have read in this field.
In my view, as "an attack on the use of the Bernoulli equation as a partial explanation of aerodynamic lift" these comments have failed completely. Dolphin51 (talk) 12:43, 25 May 2008 (UTC)
The article at present makes an attempt to include Scott and Eberhardt (which can be improved), but their view is not really at odds with anything that's written (except when they invoke the Coanda effect as a surrogate for viscosity). The third link appears to just be some guy writing with a giant blue font. It's not an appropriate reference. See WP:SPS.
I left out a link. A Physical Description of Flight. Via correspondence with one of the authors of this web page, I got the (perhaps false) impression that Bernoulli was an issue. As I mentioned before, it was beyond me, but assuming the goal here is to provide accurate information, perhaps someone could correspond with these guys, who are appparently AE's, to get their feedback. Jeffareid (talk) 16:23, 12 October 2008 (UTC)
Unclear sentence
This sentence...
"Relatively speaking, the bottom of the airfoil presents less of an obstruction to the free stream, and often expands as the flow travels around the airfoil, slowing the flow below the airfoil."
...seems to be ungrammatical. The subject of "expands" is "the bottom of the airfoil", which is probably not what the writer intended.
I'm not able to suggest a correction because I do not know what the writer was trying to say. First, I'm not able to form any mental picture of the "bottom of the airfoil" creating "less of an obstruction" (than the top?). (What does it mean to speak of the top of the foil presenting an obstruction? ) Secondly, I am unable to think of anything in the actual physics of aerodynamics that would support this explanation, and I've never encountered this concept in any text books or articles on the subject.
Mark.camp (talk) 00:24, 27 December 2008 (UTC)
I agree Mark. The sentences you have identified represent a poor attempt to explain the variation of fluid speed in the flow field around an airfoil. No source has been cited so it is reasonable to assume that the editor who added this text was using his own impression of things, and his own ideas to explain them. I have deleted the offending sentences. The original editor, and others, are of course free to re-insert these ideas but they should be expressed in a clear fashion and preferably supported by a citation of the source from which the ideas were taken. Dolphin51 (talk) 09:57, 27 December 2008 (UTC)
I agree that the wording could be clearer, but the idea comes from Anderson's Introduction to Flight, starting on p. 353 (as stated at the start of the section “following the development by John D. Anderson in Introduction to Flight”). I put the sentences back in because they're important to the section and I'll rework them to be clearer later. The “relative” obstruction can be seen in the streamline diagram. Michael Belisle (talk) 22:40, 2 January 2009 (UTC)
Michael, I just read the section you cite in Anderson, and unfortunately it is no clearer to me what he was trying to say than in the extract you've included in the article.
In fact, one quote of his was very disturbing: "The airfoil is designed with positive camber; hence the bottom surface of the airfoil presents less of an obstruction..."
It is clear from that sentence that Anderson is laboring under a form of the misconception that was univerally published in high school and undergraduate physics texts until recent years, that the non-zero circulation of the flow outside the boundary layer, and the resulting pressure difference, is caused by the camber of a typical airplane wing, rather than by the Kutta condition, etc.
I'm sure if you asked 100 pilots (as opposed to aeronautical engineers) today, 99 of them would use the same fallacious explanation, based on camber, that Anderson does.
Because of this, I feel strongly that it's important to remove Anderson's explanation from the article altogether, rather than try to repair it, and replace it with a scientifically sound explanation.
I recognize your opinion about camber, but it's not critical to his explanation. Even so, of course camber generates lift. It works in concert with the Kutta condition, which does not generate non-zero circulation on its own. For example, if we have a symmetric airfoil in a free-stream at zero angle of attack, there is no lift. There is still, however, an infinite number of valid potential flow solutions. The Kutta condition allows us to choose the correct one (i.e., that there is no circulation and hence no lift). If we add positive camber or increase the AoA, then lift is generated and the Kutta condition allows us to determine how much circulation is generated. All these concepts work with each other; no one concept explains lift on its own. If you do wish to argue the problems with camber and lift, then of course you need a citation for it.
Regardless, I disagree with the idea of removing it. We could go back to the old edition if you prefer, but I think that'd be a mistake. A recurring problem with this article is that editors would repeatedly insert their personal favorite explanation of lift. This current explanation is an improvement over what preceded it and it's better than someone continuing to think that Equal Transit Time explanation is correct. But it obviously still needs some work.
The solution is not to “throw the baby out with the bath water” and say that because this explanation is imperfect, it shouldn't exist. WP:WIP is just an opinion, but it's one that I sympathize with. And there is no universally accepted, perfect solution of lift. There are different ways of looking at lift, but there's really only one explanation. I think this article should strive to be inclusive without favoring one over the other, and in particular explain how the different explanations complement (not contradict) each other. Michael Belisle (talk) 04:43, 9 January 2009 (UTC)
Michael, do you agree with this sentence: "A wing with positive camber will generate negative, zero, or positive lift, depending up on the angle of attack, and the same is true of a wing with zero camber or negative camber?"
I do. The sentence implies that camber is not critical to generating lift, whereas the Kutta condition definitely is.
A couple of things you say above about the importance of camber in generating lift make me wonder if we agree on the sentence. If we don't, then we first need to come to agreement on it.
Reason: if my statement is false, then Anderson's explanation of lift, which depends on camber being critical, is fine and my objection to it is invalid.
Note: I do realize that the amount of camber, negative or positive, is very important in determining at *which angles of attack* you will get zero, negative, or positive lift. For example, an airplane with a positive-cambered wing has to fly at a larger angle of attack when flying upside-down (when it essentially has a wing with negative camber) than it does flying right-side up.
I also am not ignorant of the fact that the reason for positive camber in airplane wing design is that a positive-cambered wing has a higher angle of attack at stall, which is important feature not only for safety but because it also results in a higher maximum co-efficient of lift.
Sure, and this article mentions the Kutta condition where appropriate. But you'll notice that this WP article doesn't mention camber (which is an oversight, but never mind that for a moment). The article uses AoA to achieve the same result as Anderson. (I chose AoA arbitrarily when I made the reference diagram. It could just as easily be changed to an airfoil with camber.) Anderson just uses camber because that was a simple case he chose to explain the concept of lift: An airfoil with positive camber at zero angle of attack generates lift; hence, Anderson's explanation is correct. But nothing he nor this article says tries to imply that camber is the only way to generate lift. Also, remember that the Kutta condition is not necessary for lift: where do you apply the Kutta condition on a rotating cylinder in a free stream [1]? Or on the examples mentioned above? Michael Belisle (talk) 19:17, 9 January 2009 (UTC)
Michael, I will answer one of your questions. The Kutta condition is undefined for a rotating cylinder. Therefore, it cannot be used to imposed the boundary conditions needed to find a unique solution to the potential flow equation, and thus the lift. For potential flow, there IS no unique solution--sans friction, you have to impose boundary conditions arbitrarily to get a specific solution to the differential equation. To find the real-world solution requires that you take friction into account (exactly what Kutta did for the case of airfoils!). In fact, if you look on the web, you will find animated diagrams of the potential flow around a cylinder, or even a NACA wing section, where you can impose boundary conditions by moving the mouse. A continuum of solutions, each with its own flow and pressure fields, and its own value of lift, is shown. In no case does the air know what the "angle of attack of the flow" or the "camber of the object" or the "chord line of the object" is. These are human descriptive conventions with no mathematical or physical significance, and they aren't even defined terms for objects other than wings. If they had anything to do with explaining lift, then in most physical cases, the wind would shrug it's shoulders and say something "this boulder has an undefined chord, so my angle of attack is undefined, so I don't know how much lift to create". Apologies to Anderson, but that's the way it is. He was brought up on the same physics textbook rubbish as the rest of us, and some of it lingers in his mind. (IMHO ;-)
But you raise more points than I can answer. One of us has so many subtle misconceptions about fluid dynamics that it is not possible to have a meaningful discussion without a blackboard, a beer, and about three hours to kill. I will assume that it's I who misunderstands, as I'm not a scientist, just poor working stiff, and will take my leave for now. I've enjoyed the debate and I thank you for it.
On 27 December 2008 Mark.camp drew our attention to some unclear sentences in Lift (force) (see above). I agreed that the offending sentences did nothing to add clarity or valuable information to the article so on 27 December I deleted the sentences. On 2 January Michael Belisle restored the sentences, saying he would rework them to be clearer later. I have no objection to Michael reworking material so that it is clearer, but considering how unclear these sentences are, I see no point in retaining them while Michael does his rework.
These sentences do not simply fail to add anything - they confuse and mislead. The article is improved when they are no longer present. Here is my analysis of the offending sentences:
Relatively speaking, Not a good start. These words are redundant. It is not clear what relationship, or relativity, is intended. Beginning a sentence with the words “Relatively speaking,” is a bit like beginning “By the way,”. It might be fashionable but it is not good written expression.
"Relatively speaking" means the top and bottom relative to each other.
the bottom of the airfoil presents less of an obstruction Less than what? Less of an obstruction than the top of the airfoil? If that is what is intended it should be stated explicitly. However, this would probably make Wikipedia the only document in existence that talks about the top surface and bottom surface of an airfoil presenting obstructions to the free stream, and the two obstructions being different. Does John D. Anderson really explain circulation in terms of the airfoil obstructing the free stream? I doubt it. Wikipedia already explains circulation about an airfoil in terms of the Kutta condition.
No. We are not discussing circulation here. He considers circulation shortly after this explanation explaining that it's a mathematical concept, not a physical explanation of lift. But, Anderson does indeed talk about lift in terms of obstructions; see the bottom of page 353: "As stream tube A flows to the airfoil, it senses the upper portion of the airfoil as an obstruction and stream tube A must move out of the way of this obstruction. ... the bottom of the airfoil presents less of an obstruction to stream tube B, and so stream tube B is not squashed as much as stream tube A...."
and often expands as the flow travels around the airfoil Taken at its face value, this is saying the bottom of the airfoil is expanding as the flow travels around the airfoil! What was intended was probably that the flow often expands as it travels around the airfoil. High quality English expression is very important in any encyclopedia, and especially in Wikipedia, so this sentence must disappear, at least until it is cleaned up.
That's supposed to refer to the streamtube.
(Contrary to the equal transit-time explanation of lift, In any good explanation of lift, no purpose is served by diverting sideways to assert that the equal transit time theory is not a good explanation of lift.
If we say that the flow slows down as it goes over the bottom and speeds up over the top, then I think it's possible that some people will think that the molecules are going to meet because the Equal-Time Theory is so pervasive . It's important to highlight here that this is not the case.
I will again delete the offending sentences. I would appreciate it if their content is not re-instated until it is reworked into high quality ideas and high quality English expression with appropriate citation of sources. Dolphin51 (talk) 01:58, 7 January 2009 (UTC)
I respectfully disagree. If something is unclear, the solution is to make it clearer, not to delete it (i.e., I agree with WP:WIP). I have addressed your specific issues. It sounds like you haven't looked at the reference; if you had, you might have been able to identify what the passage was trying to say and improved upon it. Michael Belisle (talk) 04:26, 9 January 2009 (UTC)
Michael, thanks for leaving some comments on this Talk page. I have access to Anderson's Fundamentals of Aerodynamics, but not Introduction to Flight so I am grateful that you have quoted some actual text from the book. However, with expressions like "so stream tube B is not squashed as much as stream tube A...." we are clearly not looking at a serious book on fluid dynamics. (The opening sentence in this article establishes that Lift is a concept in fluid dynamics. Nothing is said about Flight.) I won't say Introduction to Flight should not be used as a source in Lift (force) but I will question whether it is a suitable source to set the strategic direction for a scientific article about fluid dynamics.
Wikipedia must make sense to all readers, not just those who have a copy of the source document beside their computer so they can work out what the Wikipedia article is about. The burden is on the contributor to ensure additions are accurate and clearly expressed. Where users feel additions are inaccurate or poorly expressed they are encouraged to be bold and delete (unless they can improve, of course.) Keep up your good work. Dolphin51 (talk) 06:30, 9 January 2009 (UTC)
Introduction to Flight is a serious book; it's irrelevant whether it's a “fluid dynamics” or a ”flight” book since the introduction might as well say “Lift is a concept in flight”, but I think fluid dynamics encompasses “flight”. The book is written for freshman in Aerospace Engineering, so it's written in language to be clear and understandable to people who know nothing about aerodynamics. And like you said, “Wikipedia must make sense to all readers”: although it should be technically accurate, it should be written in a language that's understandable. I think that's exactly what makes Introduction to Flight an excellent reference for Wikipedia. It's not like John D. Anderson, currently Curator of Aerodynamics at the National Air and Space Museum, is some charlatan.
In reference to "not just those who have a copy of the source document", that makes sense for (some) readers. But I feel that editors should check the source document if they feel something is unclear or misinterpreted. That's why we require Wikipedia to be verifiable, which requires reading the source document to verify what's written in Wikipedia:
“Any material lacking a reliable source may be removed, but editors might object if you remove material without giving them sufficient time to provide references, and it has always been good practice, and expected behavior of Wikipedia editors (in line with our editing policy), to make reasonable efforts to find sources oneself that support such material, and cite them.”
If you just delete something because you don't understand, it makes it hard for someone else to improve it later (because now it's somewhere in the history, and one has to go back, search through the history, revert, revise the sentence, etc.). In this case, I was busy for the past few weeks, but I specifically said I would come back to it and revise it when I had time. There is also this in the list of WP:MISTAKES:
“Deleting useful content. A piece of content may be written poorly, yet still have a purpose. Consider what a sentence or paragraph tries to say. Clarify it instead of throwing it away. If the material seems mis-categorized or out of place, consider moving the wayward material to another page, or creating a new page for it. If all else fails, and you can't resist removing a good chunk of content, it's usually best to move it to the article's "Talk page", which can be accessed using the "discussion" button at the top of each page. The author of the text once thought it valuable, so it is polite to preserve it for later discussion.”
My difficulty with the current text is Relatively to the top of the airfoil, the bottom presents less of an obstruction to the free stream. I will refer to this as the Differential obstruction theory. Apparently this is sourced from Anderson’s Introduction to Flight so I have arranged to obtain a copy of that book to check exactly what Anderson has written. (I acknowledge that Anderson is not a charlatan, but have a look at Appeal to authority.)
Work by early aerodynamicists and mathematicians such as Joukowski and Kutta observed that the flow around one side of a lift-generating airfoil was significantly faster than around the other. As you would expect of professional scientists, these people did not get distracted trying to explain why it was so. We know that the difference in speed of the flows around the two sides of a 2-D body is dependent on the body having a cusped (sharp) trailing edge. Mathematically, it is also dependent on irrotational flow. Apart from those two considerations, scientists are happy to observe the difference in speed around the two sides of an airfoil, and they don’t offer an explanation as to why. (In the same way, Newton’s Laws of Motion state what has always been observed, but without attempting to explain why.)
Newcomers to aviation often ask why there is a difference in speed around the two sides of the airfoil. There have been numerous attempts to provide simple, easy-to-understand explanations of why there is a difference in speed. The Equal Transit Time Theory is perhaps the best known. It is simple and easy to understand, but unfortunately it is incorrect. If one asks “Why must the two streams transit the airfoil in equal times?” there is no satisfactory answer. Consequently, we don’t tolerate credibility being given in Wikipedia to the Equal Transit Time Theory, even though there are many books that can be cited as the source of this Theory.
I am keen to see if Anderson provides an answer to my question How do you know the bottom of the airfoil presents less of an obstruction to the flow than the top of the airfoil? I suspect the Differential obstruction theory is no better than the Equal Transit Time Theory. If that is so, I will be advocating that Lift (force) simply states that the flow speeds around the two sides of a lift-generating airfoil are different, without attempting to explain why, in the same way that Wikipedia’s treatment of Newton’s Laws of Motion makes no attempt to explain why.
Anderson's "differential obstruction", "equal transit-time" and "Air ESP" are all motivated by the intuitive need to explain the BEHAVIOR at the front by CONDITIONS at the front--the camber is this, the angle of attack is that, the air "feels an greater obstruction", etc. Or in the case of equal transit time explain the BEHAVIOR above and below the foil by the CONDITIONS there: the curvature and thus the path length.
Why this need? Because our understanding of causation, which is based on discrete analysis of a small number of interacting bodies, rebels against the suggestion that the behavior of every parcel, even those in front of the wing, is causally dictated by the conditions at the TRAILING edge. "How could causation flow backward? Doesn't a parcel first reach the leading edge, and later the trailing edge?"
Of course, that is exactly what does happen: the Kutta condition--THE CONDITIONS AT THE BACK--completely determine the flow everywhere. Even in front of the wing! (Of course, one must also assume Newton's law P'(x,y)=F(x,y) where P is Momentum vector, plus the values of the F being given by the Bernoulli equation. But these two laws are NOT usually the problem for the student. He has already taken the time to understand them.)
No-one forces the student to confront this seeming contradiction. Therefore, he assumes that he must have stumbled down the wrong path, or that he is too stupid to understand this, and he gives up. Then someone comes along with an "explanation" which doesn't require him to face the paradox. Now, it is logically necessary that this new "intuitive" explanation violates Newton's laws or the Kutta condition (or Bernoulli), but the student never realizes this--he read it in a book, so it must be true. Wikipedia's rules are of no help because they only require an author to find a reputable source, not a correct source. Since the first false theory of this type came out in a 1920's physics text (if I remember the history right) and was copied in a thousand text books, there is always a reputable source for the false explanations.
Hi Mark. Yes, I agree with all you have written. In thermodynamics there is a challenging concept called entropy. Wikipedia has an article on Entropy, but in addition it has a supporting article called Introduction to entropy. Perhaps Wikipedia needs Lift (force) to contain rigorously correct information, and a new article called Introduction to lift to contain suitable information for newcomers to what is a rather challenging concept.
I suggest that when you reply to another user's post you add it at the end, rather than interleaving it amongst the other user's text. By interleaving your additions, sense can only be made of the other user's post by reading it from the History tab. I have rectified this situation immediately above. Dolphin51 (talk) 22:30, 14 January 2009 (UTC)
I think Wikipedia needs an article called Introduction to Flight (maybe Flight is that article) or Introduction to Aerodynamics, of which Lift would be one topic covered. Lift, on its own, doesn't have enough content to warrant the attention of its own introduction article, since that can be done just as well in the introduction to the present article.
As I said before, everything works together: the Kutta condition, aifoil thickness, angle of attack, camber, etc. With regard to Mark.camp's overemphasis on the Kutta condition, consider truncating an airfoil at the 50% chord: the upstream flow would be largely unchanged, but the drag would be enormous. Nothing here violates the Kutta condition, Newton's laws, or Bernoulli, so I'm not sure what is meant by saying that a simplified explanation necessarily violates the truth.
How do we know that the top presents more of an obstruction than the bottom? The answer is geometrical based on looking at the picture of an established flow. But I think there's a problem with bringing it up in this section when it's trying to be a section that explains lift in an established flow. It properly belongs in the next section, which answers the question "How did the flow get to be this way?" (which notably includes a mention of the role of the Kutta condition). I took out the mention of obstructions in this section, but It should be incorporated into the next section.
The most import thing to remember when writing this article, I think, is that there is no universally accepted explanation of lift. You may think your explanation is “correct” and you may find some knowledgeable people who agree with you. But you'll also find knowledgeable people who disagree. As long at the topic is “controversial”, the Wikipedia article can't pick sides. Michael Belisle (talk) 00:24, 15 January 2009 (UTC)
Michael, since you feel that I "overemphasize" the Kutta condition, here is an explicit statement of what I think the role of the Kutta condition is in explaining lift. I will choose one of the standard fluid dynamics derivations which are used to prove that an airplane wing must develop lift, namely 2D potential flow theory.
This derivation assumes a representative airfoil section and orientation. From that it develops a boundary value problem from Newton's second law. A family of solutions is found, from which an infinite range of values of lift is proved. Thus, Newton's law is sufficient, under the simplifying assumptions of 2D potential flow theory, to prove that it is POSSIBLE for an airplane wing to create positive lift, but nothing more. Lift is not yet explained. Next, the boundary value problem is solved, meaning a unique solution is found, by imposing the Kutta condition. Finally, one proves that this unique solution has positive lift.
As you can see, in this derivation, (a) lift is NOT proved until the Kutta condition is imposed, and (b)once it IS imposed, lift is proved immediately. This is exactly how important I believe the Kutta condition is to the above proof of lift of an airplane wing: it is necessary.
I'm not sure what you mean by "proving" lift. What you have presented is a simplified, mathematical description of lift. Nature does not solve a boundary value problem. It does not calculate a result from the NS equations. These are but tools to simplify the real picture.
The Kutta condition is not the cause of lift. It is a consequence of the underlying physics, useful in situations where certain approximations are made. For example, given a set of conditions, there is not an infinite set of solutions unless you neglect viscosity as one does in potential flow. If you solve the viscous Navier-Stokes equations directly, there is only one solution and it arises naturally without imposing the Kutta condition. (I should say that there is typically only one solution. The uniqueness and existence of solutions to the NS equations over the whole domain is still an open question. No one has yet found a non-unique solution, but that's not to say that there isn't one out there. Engineers don't really care about that question because the equations work in the domain of our typical design space.)
Did you know that in the real world, a slender ellipse at an angle of attack generates lift? It's like D'Alembert's paradox for the Kutta condition, though less interesting and not very useful. Michael Belisle (talk) 19:35, 15 January 2009 (UTC)
Yes, I did know that a slender ellipse at a positive angle of attack generates positive lift. I believe that this lift is not predicted ("explained", "calculated", "demonstrated", "proved",...) by 2D potential flow theory, if no ad hoc assumptions are made. Am I correct?
Michael, when I speak of the importance of the Kutta condition, I'm referring only to the explanation of lift given in the article section we are discussing--a typical NACA airfoil with one sharp edge aft. To explain this simple case to the reader is enough of a challenge, perhaps even impossible!
To discuss more general cases like the one you brought up, or even the familiar case of holding one's hand, or a flat thin sheet of metal, at an angle out the car window, requires much more complex mathematics. The Kutta analysis has to be generalized: it needs to be shown that when there is more than one sharp edge, they are not all equal in their power to control the circulation. For example, in the case of a thin flat surface at positive angle of attack, if we try to simply extend the Kutta argument for the NACA section, we would perhaps apply a "Kutta condition" at the sharp leading edge, and say that the forward stagnation point MUST be there.
I think you know what the result would be. You could then TRULY say that someone was overemphasizing the Kutta condition ;-) If we solve Newton's equation with that condition, we will discover that there is downwash in front of the wing, upwash behind it, the air will be flowing faster over the bottom surface than the top, and there will be negative lift. We know that this is not the case in the real world. When there is more than one sharp edge, there is another mathematical principle involved. (It is the same math that explains why a fan cools your face if you sit in front of it, but not if you sit the very same distance behind it.).
I suggest that we not try to explain that in this brief introductory section.
In a physical explanation of lift, as this section is titled, it's hand waving to say "It's the Kutta condition." That's why the Kutta condition is mentioned where it arises physically (at the end of the “Stages of Lift Production” section) and again later when some discussion of potential flow theory occurs (although this later mention is a bit out of place). I added an earlier mention of when describing the two streamtubes and the dividing stagnation line. But it shouldn't be used as though its the explanation of lift. It's a way to get the right mathematical answer, but it's not a physical explanation.
(Also, a flat plate does not require more complex mathematics. An airfoil with a sharp trailing edge has one singularity in the conformal mapping of the airfoil surface. A flat plate has two. Same math, different geometry. See NASA GRC's explanation of Conformal Mapping.) Michael Belisle (talk) 22:52, 15 January 2009 (UTC)
Agree that a flat plate doesn't require different math. I meant that one can easily explain, at a high level, why the Kutta condition occurs in an a real (viscous) flow when there is only one sharp edge, at the trailing end. One cannot as easily explain why a sharp trailing edge has MORE influence over the stagnation points (and thus, the circulation) than an equally sharp leading edge.
But first, would you as a student of the subject please confirm my facts: in a real, viscous flow, air is happier to pick a forward stagnation point away from a sharp leading edge--even if that means it has to back up and round that edge--than it is to pick an aft stagnation point some distance from a sharp trailing edge--and be forced to back up and round a sharp TRAILING edge. I picked this little bit of knowledge on the web somewhere, and it may be a dangerous thing.
Michael, thanks for deleting the sentence about the differential obstruction of different sides of an airfoil. Under the circumstances, that was the most appropriate thing to do.
I now have a copy of Anderson’s Introduction to Flight, fifth edition. I have examined Section 5.19 closely.
I think the Section begins very well, leading up to the excellent statement The answer is simply that the aerodynamic flow over the airfoil is obeying the laws of nature, namely, mass continuity and Newton’s second law. Unfortunately, after that it goes downhill as Anderson resorts to intuition and naïve language: stream tube A is squashed to a smaller cross-sectional area ...
I was also disappointed with his peculiar explanation using the notion that the upper and lower surfaces of an airfoil provide different levels of obstruction to the stream. I acknowledge that this book is an introduction to the subject, but Anderson’s use of this notion is not consistent with the level of authority and rigour I see elsewhere in the book.
This part of Anderson’s explanation hits rock-bottom where he considers stream tube B: The airfoil is designed with positive camber; hence, the bottom surface of the airfoil presents less of an obstruction to stream tube B, and so stream tube B is not squashed as much as stream tube A in flowing over the noise of the airfoil. Why does Anderson restrict his comment to airfoils with positive camber? I think he is saying the lower surface of the airfoil is flatter than the upper surface, and a flatter surface presents less of an obstruction than a rounded surface! This is a tragedy. Anderson is aware that an airfoil with a negative camber generates lift, because that is the subject of his example 5.22 (page 359). He would also be aware that symmetric airfoils generate lift. Explaining lift in terms of positive camber, as Anderson has done, has been exposed as a fallacy to the same extent as the Equal Transit Time Theory.
Anderson’s alternate explanation appears to be an appeal to intuition to help beginners get one foot on the bottom rung of the ladder that leads to acceptance of airfoils generating lift. I suggest that the Wikipedia article on lift should not give Anderson’s alternate explanations more importance than he intended. Dolphin51 (talk) 03:05, 21 January 2009 (UTC)
Sure. That's why there are two sections in the explanation. The first is the basic, introductory description. The second gets into the details.
Like I said before, Anderson probably chose positive camber as a simple example, but his analysis can be generalized quite readily beyond this specific case. The "obstruction" is a pure geometrical argument, nothing more. If the wing has positive camber and is at zero angle of attack, then the stagnation streamline (which intersects the chordline at the leading edge in the figure at right) divides the upper and lower streamtubes. But, because of the positive camber, the airfoil is more bulky above the stagnation line than below it. This can be seen clearly by comparing the airfoil areas above and below the chordline here.
In the simplest terms, a symmetric airfoil would present an equal obstruction to each streamtube. Now, if you break the symmetry with positive camber, then clearly the upper stream tube has a greater obstruction than the lower one. Negative camber will have the opposite effect. I don't understand what your confusion about the relative obstruction of the airfoil to the upper and lower streamtubes. Michael Belisle (talk) 21:14, 21 January 2009 (UTC)
Hi Michael. Thanks for the prompt acknowledgement. You have written that a symmetric airfoil would present an equal obstruction to each streamtube. I assume you are writing about a symmetric airfoil at zero angle of attack. What comment would you make about a symmetric airfoil at non-zero angle of attack? (If the two surfaces of a symmetric airfoil present equal obstructions at all angles of attack then, according to the obstruction theory, a symmetric airfoil would never generate lift.)
You have also written that negative camber will have the opposite effect. I assume you are writing about a negatively cambered airfoil generating zero lift. (If negative camber has the opposite effect at all angles of attack then, according to the obstruction theory, it could not be used to generate lift when it is flying upside down, contrary to what Anderson explains in his example 5.22 on p.359.)
I'm not confused by the obstruction theory. Obstruction is not defined in aerodynamics or fluid dynamics and so is lacking in rigour. I think the notion of obstruction (which appears to be peculiar to Anderson) is intended to be entirely intuitive and is useful only as a means of introducing beginners to the notion of lift. It is simple, easy-to-understand and does not necessitate any math, but it is no better or worse than the Equal Transit Time Theory and has no place in an encyclopedia. Dolphin51 (talk) 22:00, 21 January 2009 (UTC)
For a symmetric airfoil at nonzero angle of attack, I would make the comments that were previously in the article. I am not talking about a negatively cambered airfoil generating zero lift. I was discussing the effect of camber, ceteris paribus: a negative cambered airfoil at zero angle of attack will generate negative lift while a positive cambered airfoil at zero angle of attack will generate positive lift and a symmetric airfoil at zero angle of attack will generate no lift. (Also, in all cases considered, the flow conditions are unchanged, the airfoil geometry is unchanged except for camber, the laws of physics still apply, and 1+1=2.)
The word “obstruction” is defined in fluid dynamics as it's defined in the dictionary: “ a thing that impedes or prevents passage or progress; an obstacle or blockage”. Example: “An airfoil presents an obstruction to a flow.” The explanation has a place in a general-audience encyclopedia because 1) you have not yet conclusively explained why anything that is said is wrong and 2) it strives to use clear and understandable language. I removed it not because it was wrong, but because it was in the wrong place. Michael Belisle (talk) 23:02, 21 January 2009 (UTC)
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)
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)
Unclear sentence about flow "naturally following the shape of an airfoil"
I am not able to understand the following sentence:
"If one assumes that the flow naturally follows the shape of an airfoil...then the explanation of lift is rather simple..."
I hope that someone who is knowledgeable about fluid dynamics can make this sentence easier to understand for the typical reader.
The sentence seems to assume that there is only one flow that "naturally follows the shape of the airfoil".
For me, this assumption doesn't seem to be true.
First, to me, ANY flow meeting these conditions seems more or less "natural", for an ordinary airplane wing:
(1) The flow splits exactly once, near the front, and rejoins at the trailing edge
(2) At the limit of great distance in front of the foil (directly in front, and also above and below), the flow is in a single direction (opposite the direction of the aircraft) at a single speed (the speed of the foil).
(3) Near the foil the flow is tangent to the surface of the foil.
Second, and more importantly, the flow that seems MOST natural to me is not the actual flow. I suspect that most lay people assume the same thing that I did about the "natural" flow:
(1) The aft stagnation point is in accordance with the Kutta condition (this part we lay people get right, though we've never heard of poor Kutta, let alone his condition).
(2) The forward stagnation point is the front of the foil.
The second bit is what we amateurs guess slightly--but critically--wrong.
We learn this when the experts show us the experimental results from the wind tunnel. Or, we may equally well learn of our error when they show us the results from applying plain old Newton, with the assumptions of no friction, no variation of density, time-invariant velocity, and the Kutta condition.
Counter to our intuition, there is an updraft ahead of the wing, and as a result, the flow splits not at the very front of the wing, but slightly lower and thus slighly aft of that point. The shocking consequence is that the air briefly flows the wrong way along the wing--"into the wind"!
The idea behind that that sentence is that if you take an airfoil and throw it in a wind tunnel at some specified conditions, there will only ever be one flow pattern. The question answered by this section is “Why does that flow pattern generate a lift force?” If you read the entire section in context, it attempts to explain and show with a diagram what that "natural" flow pattern is, including your mention of the flow going the "wrong way" (although it doesn't say so explicitly). It can still be improved, of course.
Thanks, Michael. You point out that there is only one flow which occurs if we take an airfoil and throw it in a wind tunnel at some specified conditions.
I agree. Here is where I got confused, however.
The sentence in the article refers to the flow or flows which "naturally follows the shape of an airfoil". When I read it, I interpreted it as "the one flow that would occur naturally to the reader of the article", not as "the one result which experts report from lab experiments".
Do you think that I misinterpreted the sentence? If so, was it a misinterpretation which others might fall into? If so, then can you suggest a rewording of the sentence to prevent this misreading?
I've not heard back so I assume we are in agreement that the text is intended to explain lift with the starting assumption being the actual flow. As written, it appears to start instead with an appeal to the reader's intuition about what that flow would be.
Therefore, if no objection, I will re-order the sentences and trim one of them, as follows, to make it clear that the starting point is the calculated potential flow, and that we are deferring for the moment the question of WHY this flow occurs:
That's reasonable. I made part of that edit, but I didn't reorder the sentences. In the future, be bold (WP:BOLD). There doesn't have to be consensus before you make an edit. If someone doesn't like it, they'll let you know. (So yeah, I agree that silence implies consent: WP:SILENCE.) Personally, I rarely have a problem with someone rewording what I (or any other editor) says. I'm not always the clearest writer. But I might complain if someone deletes something that I felt was important. Michael Belisle (talk) 05:00, 9 January 2009 (UTC)
Re the ability of air to sense the "top of the airfoil" at a distance
Presently, the text states that the air in front of the airfoil is able to "sense" the upper surface of the airfoil some distance, and that it moves accordingly, thereby making airplanes fly.
I'm pleased to see the scientists lose their monopoly on this page, and New Age philosophers have a chance to give their views at last.
Although the discovery of air's ESP is admittedly the most striking, in fact the entire introductory section has become a hodge-podge of amazing facts from outside the stodgy and stifling world of mathematically proven, experimentally validated science.
Of course the upstream air is able to sense an obstruction and adjust at subsonic speeds. This is perhaps one of the most important points in aerodynamics. Disturbances propagate upstream at the speed of sound, causing the streamlines to deflect far upstream of the obstruction. It's one of the reasons why Newton was wrong when he tried to explain lift (and consequently, why his explanation works pretty well at hypersonic speeds, when the oncoming flow has no idea what's coming). Michael Belisle (talk) 23:45, 14 January 2009 (UTC)
Does this phenomenon--"air sensing an obstruction"--have a scientific definition?
Yes, I linked to the basics of it. In short, an object in a free stream generates a pressure field around it; see Section 3.5.1 of Fundamentals of Compressible Fluid Dynamics. It's clearer, common, and not incorrect to say that the flow “senses” an obstruction (which that source does in section 3.5.3). Michael Belisle (talk) 19:53, 15 January 2009 (UTC)
Pressure on bottom of wing????
I'm pretty sure that the article is not due weight. So far as I am aware as (most, but not necessarily all) wings go through the air they form a high pressure area under the wing, and this pushes the wing and generates lift. This doesn't seem to be in the article right now. Further, my understanding is that the push effect is actually more important, by and large-but not all the time, than the Bernouilli effect on top of the wing.- (User) Wolfkeeper (Talk) 03:23, 21 January 2009 (UTC)
Hi Wolfkeeper. I suggest you find a good textbook on aviation or aerodynamics and brush up on the theory. When a wing is generating lift there is very little increased pressure on the underside of the wing. The pressure under the wing is approximately the same as the pressure in the freestream. Conversely, the pressure on the upper surface of the wing, particularly near the leading edge, is significantly lower than the pressure in the freestream.
The relative speed of the flow past the underside of the wing is approximately the same as the speed of the wing; but the relative speed of the flow past the upper surface, particularly near the leading edge, is significantly faster than the speed of the wing. This is all exactly as one would expect, considering Bernoulli's principle. Dolphin51 (talk) 05:25, 21 January 2009 (UTC)
Yes, sorry, I was totally unclear/wrong. Wings work in several different ways simultaneously, but I really meant that this was another effect which wasn't mentioned. I think the degree of the effect (and in the examples showed above it's a much smaller effect than the low pressure above the wing) depends on the shape of the wing and the angle of attack. IRC (which I may not) it's more pronounced at higher angles of attack. And I still think it's well worth mentioning, even just to say it's usually a smaller effect.- (User) Wolfkeeper (Talk) 11:51, 21 January 2009 (UTC)
Pressure coefficient of NACA 0012 airfoil at 11° angle of attack. You are correct that it depends on the wing and the angle of attack. The pressure is always higher than the freestream pressure at the stagnation point (or attachment line, in the case of a swept wing), which is typically near to the leading edge, as this is the location where the velocity is at a minimum.
I'm not sure what Dolphin51 means when he says that the pressure is lower than the freestream pressure near to the leading edge. For the example diagram in the article, a NACA 0012 at 11° AoA (intentionally high to exaggerate the difference in streamtube areas), the pressure is higher than the freestream pressure for the entire length of the bottom of the airfoil, evidenced by the positive pressure coefficient. (Note that it's customary to plot -, so that the lower curve is the lower surface and the upper curve is the upper surface.)
I'm not sure about how close to the leading edge, but if you reduced the AOA attack of that NACA 0012 airfoil, or perhaps with a different airfoil at low AOA, the pressure of air under the wing at the aft end (exit point) can be below ambient. I don't know how far forwards on the under surface of a wing that the pressure could be reduced below ambient (the pressure above would be reduced further still). I would assume that an airfoil where the diverted flow pressure is below ambient pressure would improve efficiency Jeffareid (talk) 03:28, 31 January 2009 (UTC)
Regardless, it's not the high pressure alone that “pushes” the wing; it's the difference in pressure between the upper surface and the lower surface that creates an aerodynamic force. The area between the upper and lower curves is the amount of lift. If you find a good citation that explains this fact, feel free to work something into the article that makes this clearer. Michael Belisle (talk) 21:05, 21 January 2009 (UTC)
Just for fun my father once built a glider with triangular cross-section wings. IRC all the lift came from the undersurface and the top was flat to the airflow and so would have given no low pressure zone. The L/D was probably horrible but it did fly.- (User) Wolfkeeper (Talk) 01:26, 22 January 2009 (UTC)
Alternative explanations
The article Lift (force) contains a section called Common misconceptions addressing the so-called Equal Transit Time Fallacy, and the Coanda effect. The tenor of this section is very judgemental, reflecting more the personal views of various editors than what has been written in the cited sources. For example, the Equal Transit Time theory is always described as a Fallacy when in fact it is an alternative theory able to be supported by numerous references that could be quoted as citations.
One of the objectives of Wikipedia is that it should reflect a Neutral Point of View. The section called Common misconceptions does not reflect a neutral point of view at present — it reflects some of the passion of various editors. I am about to amend this section, firstly by changing the section heading to Alternative explanations of lift, and secondly by presenting the Equal Transit Time model as simply an alternative explanation of lift which some authors have identified as inaccurate.
At present, the sub-section on the Equal Transit Time Fallacy contains one strongly judgemental statement that is probably just the personal prejudice of the editor. I will attach a "Fact" tag to that one to see if it can be supported by a citation. Dolphin51 (talk) 02:24, 22 January 2009 (UTC)
I'm all for the title change to “Alternative explanations”. I did that once before (inspired by Anderson), but someone objected and I didn't press the issue.
However, I would recommend being careful when rewording the ETT section to make it “NPOV”. What you describe here is not an NPOV issue. There is scientific consensus that the explanation is flawed, which makes it fall under WP:FRINGE. It is readily observable (RealPlayer video, fast forward to 5:29) that the explanation has no basis in reality. There's no scientific support for it. The moon is not made of cheese. See also WP:V#Reliable sources.
As for Coanda, I think it's pretty NPOV as it is. It explains who says it's Coanda, who says it's not, why they disagree, and has an sufficient quantity of citations throughout. And here again, science (with few exceptions) agrees that it's not Coanda.
Thanks Michael. I approve of reducing the title of the new section to simply Alternative explanations.
Looking back at the old version (Common misconceptions) to see what it was in Equal Transit-Time (ETT) that seemed clearly non-NPOV I see there were multiple uses of the word fallacy. This is a highly judgemental alternative to the neutral words model, theory etc. Clearly, there are citable authors who have used the ETT in good faith, and other citable authors who have ridiculed ETT. The number of authors who have used ETT in good faith is significant — possibly greater than the number who have ridiculed it, so it was not giving appropriate balance to the two groups if only the judgemental approach was reported. If there are editors who want to challenge the new NPOV appearance of ETT I am happy to debate them. Wikipedia's mission is to objectively report everything that is out there in citable sources, and not to judge which of two or more views is superior.
Similarly, to use the word misconception in the heading also seemed to reflect what the editor believed, rather than being the most appropriate word given that the ETT receives as much good faith useage as ridicule. I think the section on ETT is now much improved in that it is less judgemental and more NPOV.
A relevant direction to editors is the one found at WP:Verifiability: The threshhold for inclusion in Wikipedia is verifiability, not truth. As you know, what editors believe to be true is irrelevant. It is what is verifiable in citable sources that is relevant. (My words.) In areas that are likely to be challenged, those sources must be cited. Dolphin51 (talk) 04:48, 23 January 2009 (UTC)
Doesn't "Proposition X is a fallacy" simply mean, roughly, "Proposition X is incorrect, because of an internal flaw of logic or fact"? If so, why is a text of the form "X is a fallacy" which meets Wikipedia standards necessarily any less acceptable than a statement "X is true"?
Hi Mark. If fallacy and incorrect are truly synonymous then there would be no difference which of the two words is used in Wikipedia. However, I believe they are not truly synonymous. I believe incorrect is objective. It implies nothing more than what you see at face value. I would argue that fallacy, in the context in which it was used in Lift (force), is not an objective word. It is a dysphemism so has a negative implication. (There is a Wikipedia article on the Bohr model of the atom, even though this model is known to be incorrect. How do you feel about this article being re-titled Bohr fallacy?)
If all (or most) citable sources concur that the ETT is a fallacy then it would be reasonable for Wikipedia to use fallacy in its description of ETT. However, there are many sources that use ETT in good faith. Presumably the authors were striving to explain lift in simple, easy-to-understand terms, for commendable reasons. Those authors would legitimately object to an encyclopedia describing their description as a fallacy (or even a myth), but they might readily concede that, technically, it is incorrect.
The fact that some citable sources concur that the ETT is incorrect is not sufficient to allow a Wikipedia editor to use subjective terms, especially if those terms are not used by a suitable proportion of those citable sources. It is Wikipedia's role to report the existence of both points of view, and to report that the ETT is technically incorrect. This can only be done with objective language. Dolphin51 (talk) 01:24, 24 January 2009 (UTC)
Wikipedia does have a criteria for judging the relative weight of two views that is applicable to this case, which is that reliable sources are favored. It is not simply good enough to have just any source: “as a rule of thumb, the greater the degree of scrutiny involved in checking facts, analyzing legal issues, and scrutinizing the evidence and arguments of a particular work, the more reliable it is. Academic and peer-reviewed publications are highly valued and usually the most reliable sources in areas where they are available, such as history, medicine and science.” The most reputable and peer-reviewed sources agree that ETT is based on a fallacious argument. (And yes, I will appeal to authority when the authority is a recognized expert in the field in question. The only source that the WP "Appeal to Authority" article cites says one important thing: "This fallacy is committed when the person in question is not a legitimate authority on the subject." Like I said once before, we can't present eskimo.com on equal footing with John D. Anderson.)
The immediate difference between ETT and the Bohr model that I see is that the Bohr model is commonly presented with the disclaimer that it's wrong and followed up with the more complicated and more accurate theory. The Bohr model occasionally gives useful results. It's probably not called a fallacy because it's taught as a historical note about what was once an accepted theory. ETT, however, is almost never is presented in this manner. It gives no useful results because it's never correct and there is no scientific basis for its principal premise. I don't know that ETT ever carried any serious weight except maybe in the days of alchemy and the aether. Michael Belisle (talk) 07:17, 24 January 2009 (UTC)
There seems to be universal agreement that the ETT is incorrect. No-one is arguing in favour of it being presented in Wikipedia as a seriously technical explanation of lift. All that remains is how to describe it. We have a citation from John D. Anderson Jr (Introduction to Flight) saying This is simply not true. Using this, I would be happy to see the ETT described as untrue, or incorrect, or any other objective term.
If Wikipedia is to use a more emotional term, or a dysphemism, it should be a term used in at least one of the citable sources. We have seen fallacy, and currently myth. Is either of these terms used in a reliable source? If not, perhaps we are looking at a dysphemism chosen by a Wikipedia editor primarily because it matches his (or her) passion on the subject of ETT.
I am in favour of describing ETT either in the same way as in reliable sources, or using terms that are scrupulously objective. Dolphin51 (talk) 07:38, 24 January 2009 (UTC)
The explanation of the theory above (Belisle, 21-Jan-09) says that, in the case of a positive camber section at zero angle of attack, the forward stagnation streamline intersects the airfoil at the leading edge. Could someone confirm this fact? (My understanding was that intercept would be on the bottom surface.)
Mark.camp (talk) 14:45, 24 January 2009 (UTC)
With a symmetric airfoil there is an axis of symmetry joining the trailing edge and the leading edge, so the notion of the chord line is very straight-forward. (The angle of attack can be considered the angle between the chord line and the vector representing the velocity of the airfoil relative to the atmosphere.) With a cambered airfoil it is not so straight-forward because there is no axis of symmetry and the orientation of the chord line is arbitrary. (If two people locate the chord line at slightly different orientations they will measure slightly different angles of attack.) To eliminate the arbitrariness of the chord line on a cambered airfoil it is useful to replace it with the zero lift axis. If the angle of attack on an airfoil is measured relative to the zero lift axis the lift is always zero when the angle of attack is zero.
In answer to your question — a cambered airfoil at zero angle of attack measured relative to the (arbitrary) chord line is at positive angle of attack relative to the zero lift axis, so it is generating lift. Because it is generating lift, I would expect the stagnation point (where the dividing streamline contacts the airfoil) to be located below the point where the zero lift axis intersects the leading edge. So I agree with your understanding.
On an airfoil with positive camber, the zero lift axis intersects the leading edge of the airfoil above the point where the chord line intersects it. So at one particular lift coefficient the stagnation point would coincide with the point on the leading edge where the chord line intersects the leading edge. This matches Michael's comment. Dolphin51 (talk) 00:20, 25 January 2009 (UTC)
The chordline for an airfoil is defined as the line from the leading edge to the trailing edge regardless of symmetry. Yes, at zero angle of attack (with respect to the chordline) the forward stagnation point is at the leading edge. (Some simple calculations using conformal mapping or Xfoil can confirm this.) At the angle of zero lift, an airfoil with positive camber has and hence the stagnation point would be above the leading edge.
Remember that this is a forum for discussion about improving the article, not for general discussion about lift or aerodynamics.
Michael, I agree. My question was in response to a post in this talk page stating that the forward stagnation point was at the leading edge (in fact, it was your own post, I think).
One respondant (Dolphin51) said that my understanding was correct and that the post (your post?) was incorrect. You said the opposite: "Yes, at zero angle of attack (with respect to the chordline) the forward stagnation point is at the leading edge."
I am, at least to the extent the stagnation point is “very near to the leading edge". The location is indistingishable from the leading edge in an Xfoil calculation of a NACA 2412. The reason I mentioned the talk page guidelines is that we can argue points relevant to the article, but we already agreed to take out the obstruction description and there's no mention of stagnation points on a cambered airfoil in the article. Michael Belisle (talk) 04:48, 26 January 2009 (UTC)
Right, the question's now irrelevant. Hadn't noticed that the text was deleted.
Question about flow and pressure over a cambered airfoil
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)
