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Picture with dotted stream line

Hello, very nice article. I think that in the picture with the dotted moving streamline there is an error. On the upper surface, the black dots that are near the airfoil shall move faster than those of the free stream, thus resulting on a curve of the opposite direction. The pressure near the upper part of the airfoil is smaller than the pressure of the stream, indicating that the air travels faster. isn't this correct? — Preceding unsigned comment added by Stelios19781111 (talk • contribs) 08:22, 16 January 2012 (UTC)

If you watch the picture carefully, you will see that the dots on the top of the airfoil speed up as they pass over the wing and momentarily exceed the free-stream velocity. However, they first slow down as they approach the wing and the the momentary increase in speed is not enough to compensate for the slowdown as they approach. Thus, all the air in the vicinity of the wing is delayed. Mr. Swordfish (talk) 16:16, 16 January 2012 (UTC)

The animation is a massive failure. The very top line and the very bottom line do not track together, thus, above the wing the whole top half of 'air' is moving faster than the whole bottom half of the picture when in reality the only influenced air should be near the aerofoil. Its actually so wrong that is will make students confused! — Preceding unsigned comment added by 115.70.80.179 (talk) 17:43, 29 July 2012 (UTC)

The animation correctly depicts the fact that air flow is affected some distance from the wing, not just that "near" the airfoil. A rough rule of thumb is that air flow is affected to about a distance equal to the wingspan. Mr. Swordfish (talk) 14:03, 30 July 2012 (UTC)

The animation was made by User:Crowsnest, lets just see if we can get him to make an animation that shows points far enough away that the most outer points do track together — Preceding unsigned comment added by 198.82.93.203 (talk) 21:07, 4 December 2012 (UTC)

Difference in areas redux

There was quite the spirted discussion of the "alternative" explanation "In terms of a difference in areas" a little over three years ago on the talk page. I'd like to revisit that, since I don't think is was resolved successfully.

In particular, a Google search for '"difference in areas" lift' turns up zero relevant hits other than this article (and the numerous wiki clones out there on the web) and shudder articles citing this article as their source. As this seems to be the only source for that terminology it would seem to violate the prohibition on OR. So at the very least, we should retitle the section with something more appropriate. JD Anderson uses the term "squashes" to describe the reduced stream tubes. NASA uses the term "constriction". The Smithsonian Air and Space Museum uses the term "squeeze". I do not currently have access to the other two sources, but have them on order and will peruse them in coming days.

Once I get my hands on the Smith and Brandt titles I'll take a stab at crafting a better treatment in my user space. Meanwhile I'd appreciate any pointers to other articles that explain lift in this manner - Googling for it is difficult since there appears to be no standard name for the explanation. Mr. Swordfish (talk) 14:08, 7 June 2012 (UTC)

Ooh, yes, that section does grate. What is that about 'zero angle of attack', then going ahead and getting lift anyway? And it goes straight for Bernoulli's principle too, without there being any deflection, turning or circulation of the flow. It is a sub-section under the heading 'Other alternative explanations for the generation of lift', alongside 'equal transit-time'. What's not clear in that whole section, is are these subsections all right? wrong? helpful? confusing? We can't have a big section including discredited theories without making the distinctions completely clear. --Nigelj (talk) 22:23, 7 June 2012 (UTC)

I've now read the relevant sections of Brandt et al and Smith. I've also turned up several other expositions of this explanation. What all of them have in common is lack of anything that I recognize as physics or engineering - there's no math or anything quantitative in any of the treatments - and the treatments are all quite brief. For instance, Brant et al devote all of two short paragraphs to it in a book spanning over 500 pages.

NASA's Glen Research Center is quite critical of this explanation, calling it flat-out "wrong".

My takeaway is that this is not a "scientific" theory, but rather a crude analogy that gives the right answer in some situations, while leading to misconceptions in others. For instance, while it makes intuitive sense that a conventional airfoil (i.e. flat on the bottom and curved on top) will "obstruct" more air on the top than the bottom, this idea cannot explain why lift occurs for flat plates, sailboat sails, symmetric airfoils, or for cambered airfoils that are upside down. It also fails to explain why lift increases with angle of attack, since as the AOA increases it's the bottom that's presenting the larger obstruction. Moreover, it might give one the false impression that a wing with a hump on top will generate more lift since it presents a bigger obstruction.

So, what to do about the article? This idea shows up twice, once at the end under "alternative" explanations, but it also appears under "A more rigorous description of lift" in the subsection "Lift in an established flow". I think this is misleading, since there's nothing rigorous about this idea. To be clear, it is rigorous to say that smaller streamlines imply faster flow and that faster flow implies lower pressure. What is not rigorous is to deduce smaller streamlines from "constriction" or "obstruction". To be fair, the section doesn't actually claim this, but a cursory reading might give some readers the wrong impression. I think some clarification is in in order and I'll try to take a stab at it in coming days.

Actually, after rereading the entire article several times, I think it has a serious flaw in its presentation: we say early on that lift can be explained by either Newton or Bernouilli, but don't ever take the bull by the horns and explain it with Bernoulli in a straightforward manner. We should. I think a good structure would be:

Intro (more or less what's there now)

Newton explanation (more or less what's there now)

Bernoulli explanation in layman's terms (which would be a new section)

The more rigorous explanation w/ math

Alternate explanations

Again, I'll try to craft something in my user space for review. The new section could conceivably encompass the "obstruction" explanation meaning we could remove the "Difference in areas" section altogether. But I'm not wed to that idea. Other thoughts?Mr. Swordfish (talk) 17:43, 17 June 2012 (UTC)

I agree that the difference in areas explanation of aerodynamic lift is a crude analogy. When explaining the concept of aerodynamic lift it has the advantage that it is intuitively satisfying, probably because it enables the kinematics of the flow field around the airfoil to be presented in a simple, easily comprehended, visual format.

It is not a fundamental observation. The fundamental observation is that the fluid surrounding the airfoil exerts a resultant force on the airfoil. From that, we can deduce that the mean pressure of the fluid on one side of the airfoil is different to the mean pressure on the other. From that, we can apply Bernoulli's principle and deduce that the mean speed of the fluid is different on one side of the airfoil than on the other. From that, we can apply the continuity equation and deduce that the mean separation of streamlilnes adjacent to one side of the airfoil is different to the mean separation adjacent to the other.

Your offer to write some words to incorporate the Bernoulli explanation in layman’s terms is appreciated. Go for it. I look forward to seeing it.

Here is a brief summary of a personal view of mine, although I know there are many others who share it. Science does not attempt to explain things, or to say why things happen the way they do. Science observes, always looking for patterns, repeatability, predictability etc. so that principles, theorems, laws can be formulated to describe what is observed. Once a principle (or theorem or law) has been formulated we can make other observations and say they are consistent with the principle. Science doesn’t say the universe is good or bad, or attempt to explain why the universe is the way it is. Science merely observes and describes. For this reason, science observes that aerodynamic lift can occur on a body moving relative to a fluid, and science can point to a number of principles (or theorems or laws) and say the phenomenon of lift is consistent with those principles, and therefore an example of each of those principles in action. The phenomenon of aerodynamic lift is consistent with a number of principles so it is unsound to contemplate which is the correct one – they are all correct. Our objective with this article should be to show that the phenomenon of aerodynamic lift is consistent with a number of scientific principles – Bernoulli’s principle, Newton’s laws of motion, Joukowski’s circulation theorem and so on. We shouldn’t be attempting to explain lift, or say why it occurs. Dolphin (t) 23:26, 17 June 2012 (UTC)

I now have an alpha version of the revised article in my user space: http://en.wikipedia.org/User:Mr_swordfish/Lift I invite comments and suggestions. I realize that much of it is uncited at this point - that will change before it goes live, but it may take me a few weeks to get it to the point where it meets wiki standards for sourcing. I'd also like to get some more pictures and diagrams into the article. Let me know your thoughts about the new material and organization. Mr. Swordfish (talk) 21:13, 26 June 2012 (UTC)

lift

please move lift-section to dedicated article. -paul — Preceding unsigned comment added by 188.25.109.9 (talk) 16:25, 21 June 2012 (UTC)

I need to verify final results in lift-section.Let someone verify it thx!

188.25.109.9 (talk) 16:28, 21 June 2012 (UTC)

July 2012 Article reorg

Hello,

Per the discussions under Difference in areas redux, I have been working on a reorganization of the article in my user space. It's at the point now where it's a release candidate. http://en.wikipedia.org/User:Mr_swordfish/Lift

Since this is a substantial overhaul, I'm going to allow a week for comments and suggestions. Since it's an almost live wiki article I have no objection to edits in place before transferring it to the live site. Have a go at it - no point in waiting until it's live.

In about a week I'll transfer it to the live site. Mr. Swordfish (talk) 21:47, 17 July 2012 (UTC)

I have no objection to the reorganised version going live. Just one observation - the diagram Airstreams around an airfoil in a wind tunnel is used twice. If that was a deliberate decision I have no problem with it. Dolphin (t) 04:26, 18 July 2012 (UTC)

Yes, this is deliberate. One reason is that I don't want to make the user scroll up to see the diagram; I want it to be right next to the text. Another reason is that using the same picture but interpreting it differently re-enforces the primary a major theme of the article that there are multiple ways to explain lift. If you look at the picture with Newton's 3rd law in mind you see the air being deflected; if you have Bernoulli in mind you'll see the size of the streamtubes. It's almost like one of those optical illusions where some people see a vase and others see two faces. A third reason is that I don't have ready access to another picture - this is probably the biggest reason. I'm not wedded to the idea of reusing the same picture, so if someone can come up with a better graphic I won't object. Mr. Swordfish (talk) 13:23, 18 July 2012 (UTC)

I agree with your reasoning. Dolphin (t) 01:55, 19 July 2012 (UTC)

Sails

I know you guys have done, and are doing a good job here. Can I drawn your attention to the comments I just made at Talk:Forces on sails#Expert attention? I don't know if any of you have the time/inclination/expertise to want to help there? --Nigelj (talk) 13:04, 12 August 2012 (UTC)

A more complete explanation of lift and suggested major revision of the article

The current article presents explanations based on downward deflection of the flow and on Bernoulli, and states that either can be used to explain lift. I would argue that neither of these is complete by itself and that a complete explanation not only requires both downward deflection and Bernoulli, but also a more detailed discussion of the flowfield and of the interaction between pressure and velocity. My proposed explanation has some novel elements, but it has a citable source. Bear with me while I explain the physical basis for this more complete explanation.

Two questions have been asked but not answered in earlier pages of this discussion:

1) What causes air passing above the airfoil to be deflected downward to follow the downward-sloping upper surface?

2) What causes air passing above the airfoil to accelerate to higher speed?

The answer to both questions is to be found in the nature of the pressure field around the airfoil, and when this is followed to its logical conclusion it suggests a more complete way to explain lift.

Both the downward deflection and the increase in speed reflect accelerations of fluid parcels in a vector sense. Newton's second law (F = ma, where the force F and the acceleration a are vectors) tells us that the proximate cause of any acceleration must be a net force. (I regard F = ma as a cause-and-effect relationship. F can always be thought of as causing a, though the causation needn't always be one-way.)

So what is the force that causes these changes in velocity? Outside the thin viscous boundary layer and wake, the viscous and turbulent stresses are negligible, so that the pressure is the only force of any significance in most of the flowfield. And to exert an unbalanced (net) force on a fluid parcel, the pressure must be non-uniform (i.e. it must have a nonzero gradient, in math terms). When pressure is non-uniform, a fluid parcel experiences an unbalanced force in the direction from higher pressure to lower pressure (i.e. "down" the pressure gradient).

The flow is in the continuum domain, where the fluid flows as if it were a continuous material that deforms and changes course to flow around obstacles instead of just flying into them. The airfoil affects the velocity and the pressure over a wide area. In the flow around a lifting airfoil there is generally a diffuse cloud of low pressure over the upper surface, and if the airfoil is thin enough there will be a diffuse cloud of (usually weaker) high pressure under the lower surface. I've sketched the gross aspects of these clouds using notional isobars in the field (The minus sign doesn't mean the pressure is negative in an absolute sense, only that it is lower than ambient). The differences from ambient are generally largest somewhere on the airfoil surface and die away gradually away from the surface.

Fluid parcels passing through different locations in this non-uniform pressure field (the low-pressure and high-pressure clouds) experience unbalanced forces in the directions indicated by the block arrows in the sketch. The result is that flow above and below the airfoil is deflected downward, flow above the airfoil is speeded up, and flow below is slowed down, as seen in the current article's flowfield animation. Thus the answer to both our questions above is that all the changes in flow direction and speed in the flowfield are directly caused by the non-uniform pressure field.

Dolphin argues that these changes in vector velocity are "due to" or "induced by" the bound vortex and that they can be calculated by a "precise mathematical relationship." Well, yes, the Biot-Savart law allows you to infer velocity from vorticity, but calling the relationship "induction" is a misnomer in this case. Biot-Savart is just a vector-calculus relation between a vector field and its curl. When it is applied to an electric current and a magnetic field, it reflects actual physical cause and effect, for which "induction" is the appropriate term. When it is applied to vorticity and velocity in fluid mechanics, it is just kinematics, not dynamics, and thus doesn't reflect cause and effect. If you want to explain physically how a velocity change comes about, you have to get into the dynamics, which means identifying the force that causes the acceleration. Biot-Savart and the idea of "induction" in aerodynamics have caused a great deal of confusion, with many commentators, including some of the sources cited in the current article, promoting the erroneous idea that vortices cause changes in velocity. Nelsonpom is right in saying that the vorticity is not a cause of the velocities elsewhere, but a result.

So the changes in flow speed and direction are caused by differences in pressure. But what causes the differences in pressure? This part is more difficult for our intuition to grasp. In the mathematical theory, the cause-and-effect relationship between pressure and velocity in steady aerodynamic flows is implicit, described by multiple partial-differential equations (conservation equations for mass, momentum, and energy in the case of the NS or Euler equations) that must be satisfied everywhere in the field simultaneously. The only way I know to explain this nonmathematically is to say that the cause-and-effect relationship between pressure and velocity is mutual, or reciprocal. Pressure differences cause the accelerations in the flowfield, and the pressure differences are sustained by the combination of the accelerations and the inertia of the fluid, in a manner consistent with Newton's second law. One intuitive way to look at it is that a pressure difference can exist only if something is there to "push back," and what pushes back is the inertia of the fluid, as the fluid is accelerated by the pressure difference.

The pressure field and the velocity field thus support each other in a mutual interaction. This circular cause-and-effect is not "something for nothing" or "perpetual motion." The details of the pressure field and the velocity field are dictated by the combination of the airfoil shape and angle of attack and by the fact that Newton's second law must be satisfied throughout the field. As long as the flow doesn't separate ahead of the trailing edge (i.e. as long as the airfoil has a reasonable shape and the flow isn't stalled), the flow next to the surface naturally follows the airfoil contour. The continuum nature of the fluid then requires that the pressure and the speed and direction of the flow are affected over a wide area. The mutual interaction between the pressure field and the velocity field is just nature's way of making it all happen.

To me, it's clear from the above that sustaining the clouds of non-uniform pressure requires sustaining pressure differences in both the vertical and horizontal directions. This requires accelerations of the flow in both the vertical and horizontal directions. Thus sustaining the pressure differences requires both downward turning of the flow and changes in flow speed according to Bernoulli's principle.

As opposed to just a "Newton" or "Bernoulli" approach, the above arguments lead to what I would call an "Euler" approach to explaining lift. For a 2D flow, the Euler momentum equation is a vector equation with two components that must both be satisfied. And, after all, an airfoil flow is at least a 2D flow, not 1D, and we shouldn't expect a 1D approach ("Downward deflection" or "Bernoulli") to suffice. A complete explanation really needs both.

So the stance taken by the current version of the article, i.e. that things can be explained adequately with either "downward deflection" or "Bernoulli" by itself, isn't quite right. The idea that either of these very-different-sounding explanations can be correct and complete by itself is something many people have been uncomfortable with, and it has been a source of a lot of unnecessary controversy. I think the recognition that a complete explanation needs both downward turning and changes in flow speed solves this problem. And explaining the spread-out nature of the pressure field and explaining that the cause-and-effect relationship between pressure and velocity is reciprocal would also be helpful additions.

The physics behind these arguments isn't new, but this particular way of combining the arguments into an explanation of lift seems to be novel, which raises the question of a citable source. As far as I know, the only one is my own book, Understanding Aerodynamics -- Arguing from the Real Physics, recently published by John Wiley and Sons. It contains a long section devoted to physical explanations of lift. In it, I critique all the existing explanation approaches I could find and present my own explanation, based on more detailed versions of the arguments above.

In my personal sandbox User:J_Doug_McLean/sandbox I have posted a proposed draft of the text for a revised version of the article that attempts to meet Misplaced Pages content and style guidelines (It still needs to have citations and graphics added). This draft applies only minor changes to the introductory paragraph and the Overview, but it makes substantial changes to the sections on physical explanations and the mathematical theories. I'll leave it there a while for feedback before I attempt any editing of the article itself.

J Doug McLean (talk) 00:53, 31 January 2013 (UTC)

Doug, thanks for taking the time to read our article Lift (force) carefully, and to propose ways of improving it. After reading your post immediately above I can make a couple of comments that might help you and others anticipate what reaction you will receive from other readers.

In referring to a complete explanation of lift, you appear to be alluding to the existence of One True Explanation Of Lift. There is no One True Explanation Of Lift. Many contributors to this Talk page promote their favourite explanation of lift as the correct one, and then conclude that all other explanations must be at least partly incorrect. Different people will find different explanations of lift to be satisfactory – an explanation of lift that is satisfactory for a student pilot will be different to one that is satisfactory for a professional aerodynamicist, and vice versa, even though both explanations may be scientifically sound. The questions for the Misplaced Pages community are: what level of complexity is appropriate for an encyclopedia, and do reliable published sources exist to support each element of the explanations provided in Lift (force)?

You have made many references to a cause-and-effect relationship. Be aware that even though many situations can be described accurately by identifying a cause and an effect, there is no scientific principle that says all situations can be described as a cause-and-effect relationship. We have seen inconclusive debates on Bernoulli's principle about whether changes in pressure are the cause, and changes in velocity the effect, or vice versa. (I would argue that Bernoulli correctly identified the relationship between static pressure and dynamic pressure but he was wise enough to avoid speculating about which was the cause and which the effect.) The notion of cause-and-effect is not a scientific principle so it should not be used in any attempt to explain phenomena in the field of science.

You have written Dolphin argues that these changes in vector velocity are "due to" or "induced by" the bound vortex. Yes, there is an advanced mathematical model based around the notion that fluid motion is induced by a vortex field. (This notion is essential if we are to make use of the Kutta condition when quantifying lift.) The language accompanying this model talks of the flow around a wing being induced by a bound vortex and at least two trailing vortices. Anyone who finds this language unconvincing, or the mathematical model too complex, should simply ignore the model and find another explanation that satisfies their needs.

You are proposing citing as your source a book written by yourself. This presents a potential problem. There is a conflict of interest when the author of a book cites his book as a reliable published source. See WP:COI and WP:SELFCITE for guidance.

I hope to add more ideas in the days to come. Dolphin (t) 07:00, 31 January 2013 (UTC)

Doug,

Can you post a link to your draft of the changes to the article? I can't find it on your user page and I'd like to read it first before commenting further. Mr. Swordfish (talk) 13:13, 31 January 2013 (UTC)

Done. See last paragraph of my posting. --J Doug McLean (talk) 17:10, 31 January 2013 (UTC)

Dolphin, thanks for the thought-provoking comments, some I agree with and that I hope we can use to improve my draft, and others indicating that we have philosophical differences.

I didn't mean to imply that my proposed explanation is the last word on the subject, the "One True Explanation Of Lift". We should be careful that nothing we put in the article implies that it is. That said, however, I think value judgments are possible and appropriate. Some explanations are more complete than others (assuming "complete" can be a matter of degree), and we shouldn't hesitate to say so. In the heading of my talk post I refer to my proposed explanation as "more complete", and the heading in my proposed text refers to it as "A comprehensive explanation", using "A" rather than "The" on purpose to avoid implying that it's the final word. If more care is needed, I'd welcome suggestions.

Any one of the simpler explanations, Bernoulli only, for example, might be satisfactory for some people's purposes, but I think it's still fair to call it incomplete if it leaves a physically necessary part of the phenomenon unexplained. For lift to exist, a pressure difference is physically necessary. For a pressure difference to exist on a finite body, pressure gradients in both the horizontal and vertical directions are physically necessary. A Bernoulli-only explanation doesn't explain how the vertical gradient is sustained and is thus incomplete by this standard.

Given the history of lift explanations, I think an encyclopedia article should survey the whole landscape of published explanations, or at least the major categories. My new explanation is the most comprehensive published so far, as far as I know, and thus I think it belongs in the article along with the older ones. As far as a reliable source is concerned, I think my book is at least as reliable as many of the sources already cited in the article. It is self-written, but it is not self-published. Wiley submitted it to its usual review process and sent sample chapters to several academic experts. The sample chapters included the one with the lift explanation. The experts made many comments that chapter, but not one criticized the lift explanation, in spite of its novelty. WP:COI says that citing yourself is acceptable if it is relevant and conforms to content policies, which I think my book is and does.

Still, I must admit to feeling a bit awkward having to cite my own work, which is one of the reasons I'm seeking the buy-in and help of this community.

I agree that "there is no scientific principle that says all situations can be described as a cause-and-effect relationship." But I disagree with your statement that cause-and-effect "should not be used in any attempt to explain phenomena in the field of science." Fluid mechanics involves many relationships, some reflecting direct physical cause-and-effect and some not. When constructing a physical explanation of a fluid phenomenon one should always try to make the cause-and-effect relationships clear. And if there is a choice of different ways to explain something, an explanation that is based on direct physical cause-and-effect is preferable to one that isn't.

And the cause-and-effect question brings me back to "induction" of velocity by vorticity. It's not that the term "induction" is "unconvincing"; it's that it's misleading. To me, to induce something is to cause it, and that's not what's happening here. Several of the classical sources (e.g. Milne-Thomson, Theoretical Aerodynamics, Dover, 1966) talk about how the causation implied by the term "induction" isn't real. So I would say that the use of Biot-Savart in fluid mechanics is based on the notion that the velocity field is associated with the vorticity field, not induced by the vorticity field. I think it's an important distinction. J Doug McLean (talk) 23:10, 31 January 2013 (UTC)

Doug, thanks for your prompt and well-considered reply.

You have written A Bernoulli-only explanation doesn't explain how the vertical gradient is sustained and is thus incomplete by this standard. I would say Bernoulli's principle doesn’t explain any of the pressure gradients around a wing – vertical or horizontal. Bernoulli merely relates changes in static pressure to changes in speed. Information about changes in speed must come from knowledge of the kinematics of the flow field.

I think we agree that the lift on a wing can be explained using two steps – firstly we must consider the kinematics of the flow field around the wing; and secondly we must use Bernoulli’s principle to translate changes in flow speed to changes in pressure acting on the wing, resulting in a net upward component of aerodynamic force which we call lift. The second of these two steps, Bernoulli, is relatively simple. The first, kinematics of the flow field, is relatively complex. I believe the reason most literature in the fields of aviation and aerodynamics focusses almost exclusively on Bernoulli when explaining lift is because Bernoulli’s principle is relatively simple whereas the kinematics of the flow field is not.

A number of well-informed contributors to this Talk page, and other similar forums, have asked the question “Why does the air flow faster across the upper surface of the wing than across the lower surface of the wing?” A wide variety of attempts have been made to explain this aspect of the flow field, including the notorious Equal Transit Time Theory. In my opinion, most of these attempts fail to satisfactorily explain why the air flows faster across the upper surface than the lower surface. One satisfactory explanation makes use of the Kutta condition and the concept of the horseshoe vortex to identify the strength of the bound vortex. Either the Kutta-Joukowski theorem or the Lanchester-Prandtl Lifting-line theory can then be used to determine the velocity of the flow field at any point. Whether we say the flow field is induced by the vortex line, or is associated with the vortex line, is unimportant. I think to accept, in a rigorous way, that air flows faster across the upper wing surface, we need to have an understanding of the Kutta condition and the horseshoe vortex. Anyone who takes the view that Kutta and the horseshoe vortex are too esoteric or complicated, and who seeks to explain the kinematics of the flow field from a more elementary perspective will end up with an unsatisfactory explanation, even though many readers might find it attractive. So that is the reason I have written about the bound vortex inducing a flow field around the wing. I have no objection to the word “inducing” being replaced by another word, providing it is supported by the cited sources. Dolphin (t) 07:23, 1 February 2013 (UTC)

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