Talk:Lift (force)

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)

The animation would seem to be original research. It is a model that does not correspond with reality, and is clearly in need of speedy deletion. Jack 203.106.160.221 (talk) 02:05, 28 April 2013 (UTC)

Please see the policy on original images. This image is within that policy, since it represents the same ideas as presented in the source material. I do not know what you mean by " does not correspond with reality" since it is an accurate representation of the airflow according to the potential flow model. Mr. Swordfish (talk) 11:50, 28 April 2013 (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.

Distribution of pressure around a lifting airfoil

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 Wikipedia 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 Wikipedia 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)

Doug,

Thanks for the stimulating and very interesting article. I always enjoy reading different perspectives on this fascinating subject, and I particularly like your idea that a mutual interaction between pressure and velocity is what sustains the pressure difference. I would like to incorporate that into the article, and hopefully when I obtain a copy of your book we can add that with a proper citation to meet wiki standards.

But while I find your take on lift interesting and a very good read, I do not think it is an improvement on the current article. The current article's structure is not haphazard; instead it owes a lot to educators who have written about the pedagogy of explaining lift. As the American Association of Physics Teachers states:

"At least for an introductory course, lift on an airfoil should be explained simply in terms of Newton’s Third Law, with the thrust up being equal to the time rate of change of momentum of the air downwards."

The decision to present the material the way it is currently ordered was informed by the AAPT and other peer-reviewed articles addressing pedagogy by Hoffren, Smith, Weltner, Babinsky amongst others.

Here are some links to the articles:

Hoffren: http://corsair.flugmodellbau.de/files/area2/LIFT.PDF (see especially section 4)

Smith: http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PHTEAH000010000008000451000001&idtype=cvips&doi=10.1119/1.2352317&prog=normal

Weltner: http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=AJPIAS000055000001000050000001&idtype=cvips&doi=10.1119/1.14960&prog=normal

Babinsky: http://iopscience.iop.org/0031-9120/38/6/001/pdf/pe3_6_001.pdf

Not only that, wikipedia style guides recommend proceeding from the simpler to the more complex within an article, and avoiding assumptions that the reader is already familiar with the material. While I enjoyed your version of the article, I can't imagine someone who isn't already familiar with the material fighting their way through it and getting much out of it.

All that said, I do think there is material that can be incorporated into the article under the "more detailed physical description" section. As Dolphin points out, citing your own book is problematic, so I think it is best if you let other editors do that integration. — Preceding unsigned comment added by Mr swordfish (talk • contribs) 16:10, 1 February 2013‎

Doug,

Wikipedia articles are expected to be written in an impersonal, business-like tone appropriate to an encyclopedia. Wikipedia is not a textbook, instruction manual or guide-book. See WP:NOTGUIDE.

Much of your proposed text at User:J Doug McLean/sandbox is written in a tone that is not particularly appropriate to an encyclopedia but would be appropriate to a student textbook or guide-book. Following are some examples:

• Imagine riding on an airplane as it flies through the air
• One way to imagine that lift happens is to think of the airfoil shape and angle of attack as working together so that the airfoil pushes downward on the air as it flows past.
• But we still have to explain how the moving air is able to push back. As we saw above, it pushes back in the form of a pressure difference
• But to understand how the pressure difference is produced, we must understand what the flow does over a wider area
• Newton's second law tells us that a force causes air to accelerate in the direction of the force.
• To be sure to see the speed difference in the animation correctly, you must take care to keep track of corresponding columns of markers on the upper-and lower-surface streamlines. Over the length of the airfoil the upper markers nearly catch up with the lower markers one column ahead, which can be confusing.
• The pressure difference can exist only if something is there to “push back.”

All text in an article should be in a tone appropriate to an encyclopedia. Guidance is provided at WP:TONE.

Wikipedia’s desired standard in content, quality and tone can be seen in the daily Featured Article – this is presented each day on the Main Page. All Wikipedia’s Featured Articles are catalogued HERE. Dolphin (t) 05:34, 3 February 2013 (UTC)

This discussion illustrates why this topic is so difficult. Here we have three people who have obviously thought a lot about the issue, and we still have significant areas of disagreement.

Dolphin, you say "I think we agree that the lift on a wing can be explained using two steps", first kinematics to infer increased speed over the upper surface and then Bernoulli to infer lower pressure. In the next paragraph you go on to say that the preferred way to infer the increased speed is through a combination of circulation and Kutta-Joukowski, with possible help from knowledge of the Kutta condition. Assuming you intend this as a qualitative explanation for a nontechnical audience, I think this general approach has two serious disadvantages:

1) Whether it's intentional or not, the general implication of this explanation is one-way causation from increased speed to reduced pressure. The real cause-and-effect relationship is reciprocal.

2) Circulation and Kutta-Joukowski are mathematical abstractions that belong in the "Mathematical theories" section, not in an explanation for a nontechnical audience. Besides, the direct physical cause of the increased speed is the low pressure. Why not appeal to the direct physical cause when we know what it is?

I think I understand the rationale for wanting to start with other aspects of the flow and then to deduce the pressure difference. Pressure is the quantity most directly associated with lift, and by arriving at the pressure difference last, it feels more like we're predicting the existence of lift. But qualitative explanations in aerodynamics can't actually predict; the best they can do is explain. And since explaining is what we're really doing, why not start with the quantity most closely tied to the lift, i.e. the pressure, and show how the other things support it?

Your comments on the tone of my draft are well taken. I've revised it to try to fix these issues.

Mr swordfish, thanks for the comments and the links to articles on pedagogy. My book references the papers by Hoffren, Weltner, and Babinsky and discusses their lift explanations.

Hoffren's paper, by the way, does not really qualify as peer-reviewed. While AIAA Journal papers are subject to rigorous peer review, AIAA meeting papers are not. AIAA meeting papers are accepted based on short abstracts, which are generally too cryptic to support a rigorous review and are used just to decide what meeting sessions the papers belong in. As long as your abstract sounds relevant to the subject of the meeting, the AIAA will accept your proposed meeting paper. Then you can go ahead and write just about anything you like. So with regard to reliability, an AIAA meeting paper might as well be self-published. Hoffren's paper offers an interesting explanation of lift that is correct as far as I can tell, but his supporting discussion of potential-flow theory makes significant errors that would probably have been caught by a peer review.

In terms of the progression from simpler to more complex, the current article is well organized. But I think there are some key things that it doesn't quite manage to get across, such as that the quantitative theories are well established and have great predictive power, while the qualitative explanations have essentially none. This is the kind of thing I'd look for in an overview, but the current "Overview" merely defines a couple of key terms. My proposed new section, "The scientific understanding of lift versus qualitative physical explanations or descriptions" tried to do this, but I can see now that I made it too long, including discussion that belonged in the "Mathematical theories" section. In the currently posted version, I've tried reorganizing it, integrating this background material into the "Overview" and trying to keep it short enough to fit there. I also moved the definition of an airfoil from the "Overview" to the explanations section because it seems to me to fit better there.

Having the explanations section start with a downward-turning explanation and then Bernoulli, as the current article does, makes sense if you regard these as alternative explanations, each one satisfactory in its own right. However, I think I've made a strong argument that hadn't occurred to the authors of the pedagogy articles you cite, i.e. that as opposed to alternative/satisfactory explanations, these classics are really complementary/partial explanations. Neither one makes it clear that lift requires both downward turning and changes in flow speed, or that the interaction between pressure and velocity is mutual. So I would ask whether it still makes sense to lead off with them.

In the revised version of my draft comprehensive explanation, I've rearranged the early part so that action-and-reaction and flow deflection are pretty much up front, and someone with a short attention span could stop reading there. But I still propose leaving the details of the older deflection and Bernoulli explanations until after mine, given their incomplete nature.

My draft also suggests replacement of some of the farther-down sections, but if you end up keeping the old versions there are some problems that I think should be addressed:

Under "Angle of attack", flow separation at the stall is blamed on the flow becoming "turbulent". Actually, a turbulent boundary layer is a necessity in the pressure-recovery region under attached-flow conditions because it resists separation better than a laminar boundary layer. When separation does happen, it's not a result of turbulent flow.

The discussion of Bernoulli's principle misses some key points. First, it says "Bernoulli's principle does not explain why the air flows faster over the top of the wing; to explain that requires some other physical reasoning." I think that's true only if you don't know anything else about the flow. If you know that the flow speeds up, Bernoulli's principle explains precisely why: It speeds up because of reduced pressure. Bernoulli is really just a specialized integrated form of F = ma, and when we see that something has accelerated, F = ma tells us that the cause was a force.

Under "Limitations of explanations based on Bernoulli's principle", the failure of Bernoulli to apply in the case of an airplane in level flight is attributed to "adding energy to the flow". Actually, in the reference frame of the airplane, the drag doesn't add energy to the flow. In the reference frame of the air mass, energy is being added to the flow, but that's not the reason Bernoulli fails in this reference frame in the irrotational parts of the field. The usual form of Bernoulli's equation is valid only for steady flow, and the flow around an airplane is unsteady in the reference frame of the air mass. Think of the classical potential flow around a circular cylinder. That flow has zero drag, so that no energy is being added to the flow in any reference frame. Yet in the reference frame of the air mass, that flow violates the steady form of Bernoulli. For example, the pressure at the stagnation points is higher than the pressure in the farfield, but Bernoulli says it should be lower. I don't see Bernoulli's limitation to steady flow mentioned anywhere in the current article.

The section on the NS equations doesn't mention that the effects of turbulence cannot ordinarily be computed directly, but must be modeled, as in the RANS equations. This is an essential distinction in practical calculations of lifting flows.

The current description of the Kutta condition seems overly cryptic to me, so in my current draft I've added one with more detail on why lift in potential flow is indeterminate in the first place. Also, describing the Kutta condition in terms of the stagnation point introduces a new term that is unnecessary and not always applicable. An airfoil with a cusped trailing edge doesn't have a trailing-edge stagnation point, so in that case the Kutta condition doesn't just move the stagnation point to the TE, it makes it disappear. Better just to say that the right amount of circulation is the amount that results in the flow leaving the TE smoothly.

I think the quote by John D. Anderson gives the misleading impression that there are disagreements over the science itself, not just the qualitative explanations. And I think his statement that "the debate centers on which explanation is the most fundamental" is also a bit off the mark. I had an email exchange with him a couple of years ago, and he stated his criterion in a stronger form, i.e. that in choosing what physical principle to start with in an explanation of lift, the most fundamental principle is to be preferred. He preferred starting with conservation of mass because he regarded it as more fundamental than conservation of momentum. I begged to differ, saying that the criterion should be which physical principle is most relevant to the phenomenon we're trying to explain. Lift is a force, and what could be more relevant to a force than conservation of momentum? But I don't think I convinced him.

Thanks again for the thought-provoking discussions. J Doug McLean (talk) 20:08, 6 February 2013 (UTC)

I hear what you're saying - three editors, four opinions! That's par for the course for this topic. That said, I think the three of us can agree that when an airfoil is generating lift the following phenomena occur:

1 The foil experiences a net force

2 The air follows a path that is curved with a net deflection downward

3 There is a region of lower pressure above the foil

4 The air speeds up in this region of low pressure

These four things occur simultaneously, so no one is the cause of the others in the sense of "first A happens and then B happens as a result". Instead, all four phenomena must be present - you can't have any one without the other three.

Where we seem to disagree is how to present it. In the article, we're trying to explain item 1. My take (and it's not mine alone, but is reflected in numerous reliable sources) is that it is easy for a human to see item 2 from the geometry of an airfoil at non-zero angle of attack. One intuitively expects the air to follow a curved path and be deflected downward, and if smoke is injected into the airstream one can see this directly. Likewise, the downwash can be readily observed. So a simple explanation that starts with something readily observable and then applies a simple physical principle (newton's 3rd law or conservation of momentum, both of which are within the experience of most readers) to explain the lift force is readily digestible by almost anyone.

By contrast, we can't directly observe regions of low pressure or regions of higher speed. One needs specialized instruments to detect those phenomena directly. If air changed color or something when the pressure dropped then using pressure as a starting point might make sense; instead pressure is an abstract concept not directly detectable. This makes it harder for a lay-reader to internalize a pressure-based explanation. Moreover, attempts at simple pressure-based explanations are often butchered to the point of being actually false (eg equal-transit-time) This is why the current article starts with the deflection/flow turning instead of a pressure oriented approach. It's why NASA and AAPT advocate this approach. Of course, the simple deflection /flow turning explanation is, well, simple and not the full story; I think the current article is quite clear about this, and it goes on to present more complete explanations. So, to answer your question about whether it makes sense to lead the article the way it is now, the answer is yes.

The approach taken in your book - start with the NS equations and proceed from there - is a good one for it's intended audience. But it's not a good approach for the general lay reader of wikipedia. Our job as editors is to explain in terms that the average reader can follow, and the truth is that as soon as you say 'partial differential equations' you lose 99% of readers. That doesn't mean we can't include the mathematical treatment of the subject, but starting there is not a good approach. From WP:NOTGUIDE:

A Wikipedia article should not be presented on the assumption that the reader is well versed in the topic's field. Introductory language in the lead and initial sections of the article should be written in plain terms and concepts that can be understood by any literate reader of Wikipedia without any knowledge in the given field before advancing to more detailed explanations of the topic. While wikilinks should be provided for advanced terms and concepts in that field, articles should be written on the assumption that the reader will not or cannot follow these links, instead attempting to infer their meaning from the text.

So, while I think the latest draft in your sandbox is an improvement over the last one, I do not think it is an improvement over the current article. I do think there are parts that can be applied to the current article and that you have made some helpful criticism of items in the current article in your comments above. I don't have time to address them right now, but I do plan on addressing them at some time.

In sum, the structure of the current article was arrived at through a weeks-long consensus-building discussion involving multiple editors and is based on many reliable sources. I think it's a good one, and not something that we should change as a result of one new book that takes a different approach. I know that it is your book, but as wikipedia editors that shouldn't factor into the decision of how to balance the various reliable sources. Mr. Swordfish (talk) 16:16, 7 February 2013 (UTC)

I agree that we should start with the concrete (i.e. deflection and flow turning) and proceed to the more abstract later. By way of an analogy, there is often a confusion in the language in the opening sentences of articles, and from what I can understand Wikipedia articles are not about the word or words of the title (such an example here would be if the article started, "Lift is a term that is used to describe..."), but about the thing to which the words refer. Following this logic into technical articles, I have seen other debates about whether the article is about the phenomenon, or about the mathematical model or models that describe the phenomenon. Many will argue, for example, that the difference between positive and negative feedback is entirely down to the sign of the loop gain. I would argue that the opening of an article should describe the concrete phenomenon (in everyday language as far as possible) whenever that is possible, and then proceed to introduce mathematical models and their concomitant equations later. Analogously, articles do often include an etymology section about the words of the title, but they do not begin with it. --Nigelj (talk) 21:23, 7 February 2013 (UTC)

Mr swordfish,

Thanks again for more comments. I agree with your list of four phenomena that accompany lift and that none can exist without the other three. The four phenomena are obviously related, but you seem to be saying that the relationships between them aren't cause-and-effect because the phenomena are simultaneous, not sequential, and the relationships aren't one-way. But cause-and-effect relationships don't have to be sequential or one-way. For example, in Newton's second law the force causes a simultaneous acceleration. That's what's happening here, with the added twist that it isn't one-way. The non-uniform pressure field causes the accelerations in the flow field per Newton's second law, and the existence of the pressure gradients is supported by the accelerations, as I explained in my first post.

I understand the principle behind the ordering of the items in the current article, so I agree that in one sense the article is well organized. But I think you'd have to agree that the article reads like a shopping list of separate items (Try scanning just the headings, and I think you'll see what I mean). It offers very little in the way of explaining how things fit together in a big-picture sense, such as the difference between the mathematical theories and the physical explanations, and between prediction and explanation. To get any sense of the big picture a reader would have to read the entire article, and even then she'd have to work most of it out for herself. That's the void I'm trying to fill with my expanded "Overview" section. I'm not proposing that we "start with the NS equations and proceed from there". My proposed overview doesn't mention "NS equations" or "partial-differential equations"; that's left to the "Mathematical theories" section. My overview just explains what the mathematical theories represent and what their status is as science. If I were a lay reader, that's the kind of big-picture stuff that I'd want to know, and I wouldn't find it in the current article. I think my proposed overview does it in terms any "literate reader" can understand, so I think it's completely in line with the WP:NOTGUIDE paragraph you quote.

I don't think that bringing the mathematical theories in at the overview level is premature, as Nigelj seems to. The what of lift is pretty basic: It is a force. It depends on airfoil shape, angle of attack, and airspeed. But once you get into explaining how or why it happens, you're no longer just describing the phenomenon; you're talking conceptual models, and you should make it clear that the bedrock of our understanding in that sense is the mathematical theories. — Preceding unsigned comment added by J Doug McLean (talk • contribs) 06:56, 8 February 2013 (UTC)

I've gradually come around to not objecting to putting the two popular explanations ahead of mine, provided we expand the overview as I've proposed. So in my revised draft I keep the expanded overview, make "Airfoil shape and angle of attack" an introductory section to the explanations, go to the popular explanations and their limitations, and then go to my explanation. The citations would be essentially the same as in the current article, with citations of my book added in the appropriate places.

Of course my book is just "one new book". But I think technical issues should be settled on the merits of the arguments, not by voting or counting citations. I think I've made strong new arguments regarding the deficiencies of the older explanations and how to remedy them. Specifically, my main points, i.e. that it's important to consider the extended flowfield, that both downward deflection and changes in flow speed are essential, and that the interaction between pressure and velocity is reciprocal, make for a much-improved physical explanation of lift. So my explanation isn't just a "different approach"; it's a substantial improvement over the older approaches. If you disagree, show me specifically where I've gone wrong.

My current draft User:J_Doug_McLean/sandbox retains nearly all of the content and ordering of the current article, and addresses some of its deficiencies, in addition to adding my new explanation. I think that at this point its a substantial improvement over the current article, not just because it includes my explanation, but because it adds a much-needed overview. What do you think? If you don't have time to do the edit, and no one else volunteers, I'll do it.

By the way (and this addresses a point also raised by Nigelj), I think the idea that flow deflection is somehow more "concrete" than pressure (or force?) is a stretch. True, you can make flow deflection directly visible with smoke. But you can also make pressure visible, albeit graphically, by drawing the isobar pattern. Anyway, how many readers will have seen a smoke-flow visualization first-hand? Most will only see pictures, and most of those will be drawings of streamlines, as in the current article, not photos of smoke streams. As pictures, I don't think streamline patterns are fundamentally more compelling than isobar patterns. I think that pressure, being the thing that actually pushes on the airfoil, is quite concrete.

Thanks again for the discussion. J Doug McLean (talk) 06:38, 8 February 2013 (UTC)

Doug, If you are still around, be assured that I have not forgotten or ignored this discussion; I do intend to incorporate some of your suggestions into the article. This may take some time. Please be patient. Thanks. Mr. Swordfish (talk) 19:12, 20 March 2013 (UTC)

I'm still around, and I'm not in a hurry. I've made further changes to my suggested revised article in my sandbox User:J_Doug_McLean/sandbox. Instead of an "Overview" following the introductory section, I now lead off with the "what" of lift in "Lift depends on airfoil shape, angle of attack, air density, and airspeed" and then explain how the theories and explanations are related in "The understanding of lift as a physical phenomenon", which I think would go a long way toward avoiding the kind of misunderstanding raised by Westwind273 in the new string below. That leads into the popular explanations and then my more comprehensive one. I've installed some graphics, but not all, and only a few of the links and citations. J Doug McLean (talk) 01:20, 6 April 2013 (UTC)

where they got the idea of that airfoil shape

Nice article.

I was thinking - how about grounding the article in terms of where they got the idea of that airfoil shape. I discovered that sailors have been talking about lift and that airfoil shape since Egypt ruled, it's really a basic principle of sailing. For wings you just point the lift up, in sailing the lift goes to the side. I think I'm going to copy this to the wiki reference on wing as well.

When I started looking into this I thought these guys like George Cayley were pretty esoteric thinkers to just sit there with Bernoulli's Equation in the 1700's and come up with the airfoil. If you look at it, he was just describing a long-known phenomenon in the lab. In fact I'm a little shocked at how long it took to develop the airplane wing, historically speaking. We've known this for a real long time. This may be obvious you folks on the coast, but it wasn't obvious to this land lubber.

Just a sentence in the intro like...

Pb8bije6a7b6a3w (talk) 21:00, 12 February 2013 (UTC)

It would really help if you had a link to a reference where it says that "sailors have been talking about lift and that airfoil shape since Egypt ruled". Then we could look at this in a little more detail. --Nigelj (talk) 23:36, 12 February 2013 (UTC)

Improvements

I visited this site recently. It's improved fantastically since a couple years ago, from the point of view of those who view Wikipedia's mission as the advancement of honest curiousity, science, and reason. Back then we were losing the battle. All who are responsible deserve our thanks. — Preceding unsigned comment added by Mark.camp (talk • contribs) 03:20, 6 March 2013 (UTC)

Limits of Current Human Knowledge

Looking over the article and all the archived discussions, one basic thought occurs to me: Humans do not fully and clearly understand why airfoils generate lift. There are various competing explanations, and there is no broad, general, and complete consensus among scientists as to why airfoils generate lift. This is an astounding fact. I think this aspect of current human understanding should be mentioned in the introduction to the article. It is remarkable with the wide-spread use of airfoils throughout history (age of sail, age of flight) that humans do not fully and clearly understand why the airfoil works. --Westwind273 (talk) 18:06, 20 March 2013 (UTC)

I rather strongly disagree with the assessment that "there is no broad, general, and complete consensus among scientists as to why airfoils generate lift." In fact I would say just the opposite, that the science and engineering of airfoils and lift is a mature subject that is well understood and "settled" science. If you have gotten the opposite impression from the article, I apologize.

Where there is disagreement is in how to explain these rather complicated and sophisticated ideas in layman's terms. This disagreement over pedagogy is quite distinct from any controversy over the underlying science.

I am now more inclined to agree with J Douglas McLean's statement upthread that "the quote by John D. Anderson gives the misleading impression that there are disagreements over the science itself, not just the qualitative explanations" and will remove that material. Mr. Swordfish (talk) 19:02, 20 March 2013 (UTC)

I agree that when read by a layman, the article certainly gives the impression that lift is not "settled" science. To the layman, it certainly reads as if there are various competing explanations for the underlying science. For example, this introductory section definitely gives the impression of various competing explanations of the underlying science: "There are several ways to explain how an airfoil generates lift. Some are more complicated or more mathematically rigorous than others; some have been shown to be incorrect. For example, there are explanations based directly on Newton’s laws of motion and explanations based on Bernoulli’s principle. Either can be used to explain lift." Does the Newton explanation conflict with the Bernoulli explanation? Newton and Bernoulli are not alternative explanations of lift; rather they are both in effect simultaneously to create lift, aren't they? Then why present them as two alternatives? It is relatively easy for the layman to understand the Bernoulli effect, but the key question that the article never explains clearly to a layman is: Why does the air on the top flow faster than the air on the bottom? This is the key point. Later the article says "Explaining lift while considering all of the principles involved is a complex task and is not easily simplified." To me, this is a cop out as far as Wikipedia is concerned. The article is basically saying "We're going to punt on any attempt to explain this to a layman, and instead divert into scientific mumbo-jumbo that you will never understand." Is it really so hard to explain why the air on top is flowing faster than the air on bottom? Overall I am quite disappointed in this Wikipedia article. --Westwind273 (talk) 19:29, 21 March 2013 (UTC)

I think the whole concept of "multiple explanations" is the wrong way to approach this topic. It leads to deep confusion in the layman. If it is settled science, there should be one explanation which can be presented in greater or lesser levels of depth. --Westwind273 (talk) 19:38, 21 March 2013 (UTC)

I think that the problem being described here is the one about the way science is taught (in some schools). First there are phenomena in the real world, then there are scientific theories and mathematical models that attempt to explain the phenomena. There's nothing after that - no point at which physical phenomena stop being what they were and start following the dictates of the mathematical models, rather than their own inscrutable processes. That does not mean that science is wrong or badly understood, but that the scientific models and equations we have, are all we have, apart from the mysterious wonders of nature itself. People try to use similar arguments to rubbish climate science - "If this is all just a theory, then let's wait until it's been sorted out", etc. Lift is settled science - jet airliners fly every day to within tolerances very close to those their designers intended - but there isn't one simple explanation. We just have to get used to that. --Nigelj (talk) 20:35, 21 March 2013 (UTC)

>... there should be one explanation which can be presented in greater or lesser levels of depth.

Alas, the world is not that simple. Almost every scientific principle admits multiple explanations. For instance, there are at least nine ways to prove the fundamental theorem of algebra, and this is an area that has been "settled" for centuries. As another example, many problems in Newtonian physics can be approached in multiple ways - conservation of energy, conservation of momentum, direct application of newton's laws, the principle of least action, etc. These are just different ways of looking at the same problem.

Much of the controversy surrounding lift is due to some individuals thinking that there is "one true explanation" that is correct and therefore all other explanations must be incorrect. Unfortunately, one of the most common explanations actually is incorrect; were this not the case I surmise that the differing explanations would simply sit quietly side-by-side as they do in most topics.

I agree that the sentence "Explaining lift while considering all of the principles involved is a complex task and is not easily simplified." adds little to the article, and we have contemplated removing it. Maybe its time has come? Opinions form other editors?

Finally, to answer your question "Why does the air on the top flow faster than the air on the bottom?" there's a very simple answer:

When air follows a path that is curved, the pressure is lower on the inside of the curve than on the outside. So, there's a region of lowered pressure on the top side of the wing. When the air flows from atmospheric pressure into this lower pressure region, there is more pressure behind than in front resulting in a net force on the air which speeds it up. (the last sentence is basically Bernoulli's principle in a nutshell)

Perhaps something like this should be added to the article. I've never considered "why does the air speed up" to be a "key point" since lift can be explained (albeit incompletely) without even mentioning the speed changes. But if our readers are coming here looking for the answer to that question, perhaps we should present it. Mr. Swordfish (talk) 20:42, 21 March 2013 (UTC)

I agree with Mr Swordfish, and I disagree with Westwind273 when he writes Humans do not fully and clearly understand why airfoils generate lift. I disagree for two reasons. Firstly, science does not address questions about why the universe operates the way it does. That is a question for theology. Science merely observes the universe and attempts to determine principles and laws which describe these things. As a simple example, scientists understand and apply Newton's first law of motion but science does not bother with the question of why a force is necessary to cause an object to accelerate. Theologians might explain that a force is necessary because God dictated that it would be so, but scientists find Newton's observations, and his laws of motion, to be entirely adequate. Similarly, Bernoulli observed the flow of fluids and the interchange of speed and pressure and described it all in his famous principle; but he didn't bother to include any speculation about why it is so. That would not be science.

Secondly, Westwind273 has grasped John Anderson's statement that scientists and engineers disagree over what is the most fundamental way to explain the phenomenon of aerodynamic lift. Westwind273 has misunderstood Anderson to be referring to some remarkable mystery about aerodynamic lift. If it is true that humans do not fully and clearly understand these things, then that is equally true of any scientific phenomenon. Westwind273 should also be writing that humans do not fully and clearly understand magnetism, electricity, meteorology, thermodynamics, and so on. There is nothing specially intractable about aerodynamic lift which is a splendid application of Bernoulli's principle, Kutta's observations, Newton's third law of motion, the principle of momentum and so on. Each person contemplating the phenomenon of aerodynamic lift, whether that person is a student pilot, college student, professional engineer or research aerodynamicist, must choose whichever of these scientific principles is most satisfactory for him. There is no one true explanation for any scientific phenomenon, and certainly not aerodynamic lift. Dolphin (t) 23:28, 21 March 2013 (UTC)

I think it's understandable that Westwind273 got the mistaken impression from the current article that there is a lack of consensus on the science of lift. And I largely agree with the responses by Mr. Swordfish, Nigelj, and Dolphin, but with some quibbles below. I have a suggested revised version of the article in my sandbox User:J_Doug_McLean/sandbox that I think would avoid the mistaken impression that Westwind273 got. It sets the record straight on the science and also answers the question of why the flow over the upper surface speeds up a little differently from the way Mr. Swordfish did. I'd appreciate it if Westwind273 would read my suggested version and provide feedback.

The answer by Mr. Swordfish to the question "Why does the air on the top flow faster than the air on the bottom?" is a good start, but it needs to be added that the cause-and-effect relationship between the pressure field and the velocity field is circular, as explained in my suggested revision. Both flow curvature and changes in flow speed are caused by differences in pressure, and the differences in pressure are sustained by the changes in flow direction and speed.

The issue of cause-and-effect and "why" has come up before, and I still don't entirely agree with the hard-line take on that topic by Dolphin. I agree that for some fundamentals like Newton's second law we don't understand the "why". But at other levels, I think it's perfectly reasonable to talk about causation and "why", as in saying that a force causes an acceleration. So "why" does something accelerate? Because of a force. "Why" are force and acceleration related the way they are? We don't know. J Doug McLean (talk) 02:07, 6 April 2013 (UTC)

My opinion is that this article has long suffered from the fact that we editors are sometimes too close to the subject matter and get caught up in meta-discussions about the material instead of just presenting the material in a srtraightforward manner. We've all read the great Bernoulli/Newton debates of the 90s, and while this is interesting to folks who already know quite a bit about the subject I do not think it is helpful to re-hash that debate here. Perhaps it belongs in its own wikipedia article, but the purpose of this article is to introduce the basic ideas in a manner understandable to the lay reader. It seems to me that when we write about how we're we're going to explain the actual subject, we've made the artcle harder to understand by encapsulating it in a second layer of meta-analysis.

The result is that some readers, such as Westwind, get the mistaken impression that there is a lack of scientific consensus. Doug has suggested that we add language to correct that impression, eg; The mathematical theories are scientifically rigorous, are supported by empirical observations, and have been agreed upon by the scientific and engineering communities since the early 20th century. My take is that this should go without saying, of course the theories are rigorous and well accepted - having to state it explicitly seems like we "doth protest too much".

To put a finer point on it, everything in science has multiple explanations; all science is rigorous (otherwise it wouldn't be science). There is no need to apologize for choosing one explanation over another, or to have to make assurances that the science is actually science.

So, I would advocate editing the article to downplay the Bernoulli/Newton "controversy", and to avoid giving the impression that these are two or more competing theories. As an example, take a look at the Bernoulli's_principle article itself - it states that BP can be derived from either conservation of momentum or by directly integrating newton's 2nd law. To my eyes, it does so without implying that one is correct and the other is wrong, or that there's any controversy over which derivation is correct. The subtext is that both are correct; somehow that subtext seems to be missing in this article. I do think that this article should present both explanations, but I think we can do so without making such a fuss over it. In fact, the more I think about the subject, the more I am persuaded that it's really one big explanation that fits together in harmony, rather than multiple competing theories.

By way of analogy, this subject reminds me of the blind men and the elephant. It's a great parable, but here we're writing an article describing elephants, not the arguments of the blind men. Statements like "some say the elephant is like a snake and some say it is like a wall" doesn't really advance the article.

In other matters, Doug has pointed out a definite shortcoming of the current article - the airfoil affects the flow over a wide area around it - and this fact along with the reasons why it occurs (ie the self re-enforcing interplay between pressure and fluid motion) should be incorporated into the article. I'll take a swat at this in coming days, most likely just stealing Doug's text. Mr. Swordfish (talk) 21:20, 8 April 2013 (UTC)

The blind men and the elephant is spot on. Avoid the arguments between the blind men, and ignore those who ask "Why is there an elephant in the room anyway?" This is an article about lift, and the main conceptual and mathematical models that enable pilots and engineers to understand and manipulate lift. People who want to know why, or how did we get here, are looking for different topics, for which there may or may not be articles at this time. --Nigelj (talk) 22:33, 8 April 2013 (UTC)

I understand the urge to simplify things by downplaying the "meta-analysis", but I think it would be a mistake.

If we were the only ones who had ever discussed alternate explanations of lift, and if the controversies were limited to our little talk-page circle, then I'd agree that those discussions would be out of place in the article. But that's not the situation. The public folklore on this topic is full of misconceptions and erroneous explanations. And the controversies have been out there in public view for decades. I'll bet many of the potential readers of this article have already read a Bernoulli explanation (likely based on longer path length) or deflection explanation of lift and have probably also read somewhere that one or the other is wrong. And they're also very likely to have already gotten the impression from somewhere that the science is unsettled.

So the fact that the science on this particular topic is in good shape doesn't speak for itself. It needs to be spelled out. And given what's out there in the popular culture, we'd be short-changing the reader if we left out discussion of the pros and cons of the alternate explanations (and the outright errors in some versions). Whether we like it or not, these issues are now part of the topic of lift. Just the straight facts will not be enough to enable a reader to see through the fog.

Mr. Swordfish (Mr. Swordfish), you suggest that we should present "both explanations" of lift in the same way that the Bernoulli's_principle article presents its two alternate derivations. I don't entirely agree. Both derivations of Bernoulli are actually correct and self-sufficient. Our two simpler explanations of lift are both correct to some extent but also have significant shortcomings. The shortcomings shouldn't be swept under the rug.

Mr. Swordfish, you also say you're persuaded that the two simpler explanations really amount to "one big explanation that fits together in harmony". If you follow that idea to its logical conclusion and try to write it out as an explanation of lift, including the ideas of flow influence over a wide area and mutual interaction between pressure and velocity, what you arrive at is my "more comprehensive explanation" in my proposed revised version in my sandbox User:J_Doug_McLean/sandbox. The hard work is already done, including integrating the "one big" explanation with the simpler ones in the pedagogically favored order.

My proposed revision also attempts to put the mathematical theories and the various qualitative explanations in perspective in the new sections "The understanding of lift as a physical phenomenon", "Popular physical explanations of lift" and "Shortcomings of the popular explanations". Not all the material in these sections is new, but I think organizing it in this way makes things clearer. It explains the science in a way that would prevent misunderstandings like the one expressed by Westwind273, and it provides the necessary "meta-analysis" of the qualitative explanations. J Doug McLean (talk) 22:33, 17 April 2013 (UTC)

Recent additions by ‎Completeaerogeek

There are many problems with the material recently added.

1) Completeaerogeek has provided no sources.

Completeaerogeek:(First of all I apologise for an formatting errors I may have made but I am new to editing Wiki).

However, the above is not correct. I have cited Babinsky source material which was put into the public domain by the author and as such is fair use for educational purposes. material on Wikipedia must be cited byreliable sources; material lacking this is subject to immediate removal.

2) The physics is identical whether one expresses it in the reference frame moving in the direction of the foil or the reference frame of the air. This is basic Newtonian physics. While I agree that this sometimes causes confusion, the nature of the confusion is not what Completeaerogeek represents it to be.

Completeaerogeek:I am clearly referring to conceptual understanding and not mathematical explanations.Most discussion of lift generation in the public domain occurs at the conceptual level. I lecture in this subject at University so I see this regularly.Further, if confusion is not that great, please explain why so-called experts are still quoting Equal Transit Time as the source of lift. If that is not confusion I am not sure what is.'

Completeaerogeek:The point clearly here was that erroneous assumptions arise from attributing kinetic energy to the static air.

3) Completeaerogeek's comments on Babinsky's photo constitute original research which is prohibited in Wikipedia.

Completeaerogeek:The comments directly relate to observation of the video content and the events contained within and as such does not constitute original research.Anyone viewing the content will see the evidence and as such it is not an interpretation much as "The Eiffel Tower is in Paris" is unlikely to be disputed if you are in Paris standing in front of it.

4) Completeaerogeek's comment "the air does not accelerate" is simply false, and contradicted by every reliable source.

Completeaerogeek:Watch the Babinsky video and if you can show me where it accelerates in relation to the static streamlines I will withdraw the comment.

5)The edits have been marked as "minor" when they are anything but. Please see wikipedia's definition of minor edit.

I do think there is some material worth adding - in particular the notion that angle of attack is more of a factor than foil shape - but statements like this must site reliable sources.

Completeaerogeek:That is why I cited NASA's FoilSim where this can be demonstrated to be the case. I believe that NASA is a reliable source.

Happy to discuss further if you like.

We can't just write whatever we feel like writing. So, let's work to improve the article via discussion here on the talk page and avoid further edit warring. Please do not make major changes to the article without first reaching consensus on the talk page. That's the process that has made wikipedia the success that it is.

Other comments? Mr. Swordfish (talk) 12:52, 22 April 2013 (UTC)

Hello, Completeaerogeek,

Welcome to Wikipedia! Contributions by editors knowledgable in their fields are always welcome on wikipedia.

Wikipedia can be a confusing place at first, and it is not uncommon for new editors to have trouble understanding the rules, standards, and conventions here. Editing wikipedia is a process that is fairly well established and has produced very good results when it has been allowed to work. I hope that you will take some time to familiarize yourself with this process and the conventions so that your edits are better received in the future.

One of the pillars of the process is discussion on the talk page. Please see the help section on talk pages and try to follow the guidelines outlined there. In particular, it is not a good idea to edit other people's material on the talk page. Instead, place your replies below, use the indentation conventions, and optionally insert quotes to make clear what you are replying to. This is preferable to responding in-line with bold text.

The objective of discussion on the talk page is to reach consensus. Please follow that link to read up on how that process works. A key point is that if a change will modify a matter resolved by past discussion the editor should propose that change by first discussing it on the talk page rather than simply editing the article. For an article like this with a long history of discussions and consensus, it helps a lot to be familiar with past discussions. Please spend some time reading the talk archives for this article, I think you'll find that many of the issues you raise have been discussed and resolved. That doesn't mean you can't re-raise an issue here on the talk page with the objective of reaching a new consensus, but please avoid editing the article without first checking the archives to see if it goes against the current consensus.

One way the process can work is the BOLD, revert, discuss cycle. That is, try an edit. If another editor reverts your edit, then the next step is to go to the talk page to resolve it through discussion. Simply re-reverting is frowned upon as edit warring.

Here are my replies to your recent comments:

>I have cited Babinsky - yes, you have provided a link to a video, but you have added several statements that are not supported by the link. For instance the following is unsupported by a link to a reliable source: "Confusion about the nature of flow over a wing is sometimes created by representation of air 'moving' over a static wing..."

>please explain why so-called experts are still quoting Equal Transit Time as the source of lift There are no experts in the field of aerodynamics advocating the Equal Transit Time Fallacy. This incorrect notion appears in popularizations, but never in "expert" writing other than to debunk it. Agree that there is a lot of confusion here, but it is unrelated to the choice of co-ordinate system as you assert.

> erroneous assumptions arise from attributing kinetic energy to the static air. I don't understand this comment. Kinetic energy is dependent on the reference frame. In the coordinate system stationary with respect to the air, the air has zero kinetic energy. But in any other coordinate system the air will have non-zero kinetic energy. There is no error in using a coordinate system other than the one stationary with respect to the air. Anyway, unless you can provide a reliable source stating this it can't go into the article.

>Watch the Babinsky video and if you can show me where it accelerates in relation to the static streamlines I will withdraw the comment Watching a video and drawing your own conclusions is original research. Claiming that the Eifel Tower is in Paris because you've been there personally and seen it is a classic example of original research. Please read that section of the help pages and try to assimilate it. If you want to assert "the air does not accelerate", you need to find a reliable source making that claim and provide a cite.

>That is why I cited NASA's FoilSim where this can be demonstrated to be the case. I believe that NASA is a reliable source. Yes, NASA is a reliable source, and their foil sim app is an excellent way to experiment to learn about lift. However, playing with foil sim and drawing your own conclusions is original research.

I hope you can now see why your edits were problematic and why I reverted them. If you don't follow me, please refer to reliable source, original research, verifiability, and verifiability, not truth which explain it better than I can. I'm going to put the article back to where it was on 18 April. We can continue to discuss your proposed changes here. Thanks for your understanding. Mr. Swordfish (talk) 12:22, 23 April 2013 (UTC)

This article is still deeply flawed.

I see that the editors of this article still don't "get" how lift works.

1. Why still all the different "explanations" for what is essentially the same phenomena? Instead of giving a good understanding of the fairly straightforward Mechanics of lift, all these wishy-washy "blind men describing an elephant" explanations suggest a deep misunderstanding.

2. Bernoulli vs Newton? Same thing!

3. Why so much talk about "downwash" behind the wing, but so little mention of the everpresent "upwash" in front of the wing? (Note the terribly misleading AoA Figure. And, BTW, why no Figure numbers?)

4. Why no mention of Lanchester's "wave of sustenance" to explain the abovementioned "upwash"? Was he not a good enough source?

5. In "Flowfield Formation", why still no mention of Unsteady Bernoulli? And why still the unrealistic "impulsive" start to the flow, and then the flawed conclusion of "no lift or drag" initially?

6. Coanda effect!? Why mention this at all (other than, perhaps, to immediately dismiss it)?

7. Why not a simple and accurate account of fluid dynamic lift as given by Lanchester, Kutta, Zhukovsky, and Prandtl more than a hundred years ago?

8. Why is the interweb and Wikipedia so intent on degrading the knowledge base?

And a whole lot of other things... But perhaps the most inexcusable flaw:

9. Why no mention, right at the top of page, that birds, bats, insects, fish, dinosaurs, and even falling plant seeds, have usefully exploited fluid dynamic lift for millions of years!?

Zapletal — Preceding unsigned comment added by 101.170.42.150 (talk) 02:13, 18 July 2013 (UTC)

There is no such thing as the editor, or editors, of this article. It is the result of a collaborative effort by a large number of people, all of whom give their time freely. There are no constraints on who may contribute or what qualifications are necessary in order to contribute. This is an encyclopedia, not a peer-reviewed journal or a text book. It is an encyclopedia so we do not add text simply because we believe it to be true. An encyclopedia is an assembly of information taken from reliable published sources, not an assembly of information that individual contributors believe to be true. If there is a flaw in mankind's knowledge of a particular subject, that flaw should also be reflected in encyclopedia articles on that subject.

You are obviously knowledgeable about this subject. Presumably you have access to a number of reliable published sources on the subject. You are welcome to contribute to the article by adding information you have in your reliable published sources. You can start immediately. I recommend you begin by discussing your plans right here on this Talk page. That way, everyone with an interest in this article can comment on your plans and even offer to assist. Welcome to Wikipedia. Dolphin (t) 06:21, 18 July 2013 (UTC)

(Zapletal writes....) Dolphin,

It is quite clear that there are three main editors of this article (ie. "Dolphin", "Swordfish", and "Crowsnest"). Likewise, it is clear that should anyone else change the article in a way that you, the three main editors, disagree with, then you will revert the changes. There may then follow a long period of "edit-warring" that benefits no one, and is biased towards the "more equal" of the supposedly equal editors (ie. you). Therefore, although I would like to see this article improved, I have no intention of wasting everyone's time by editing it.

Furthermore, the notion that an Encyclopedia is simply "an assembly of information taken from reliable published sources" is codswallop. There are today countless "reliable published sources" offering many and varied explanations of fluid dynamic lift. Unfortunately, a great many of these explanations are pure bulldust. A random sampling of these sources does not constitute an encyclopedia. Worse yet, a biased sampling, which is perfectly acceptable under your definition, is a disservice to society in general.

In short, the editors of good encyclopedias filter out the nonsense. If this does NOT happen, then very soon we will have to believe that the world is flat, voodoo is real, and the star signs predict our future. That is, any and all of the urban myths and superstitions that can be found in some "reliable published source" somewhere, will quickly spread throughout society. This is because these myths are usually easier to understand than the more difficult truths, so are more often repeated. Soon after, the difficult truths become outnumbered, and eventually disappear.

This particular article is a good example of the above process. The "Bernoulli vs Newton" explanations, the "Coanda effect", and "lift is because of downwash", are all examples of the dumbing-down of this phenomena. An appropriate quote by Theodore Von Karmann is; When you are speaking to technically illiterate people you must resort to the Plausible Falsehood instead of the Difficult Truth.

I believe this article could be significantly improved by grouping all these "plausible falsehoods" together in a small section (with ETT), and then adding the above quote, and perhaps a few more words, to immediately dismiss them. I am not going to waste my time doing this because I am not interested in entering into the very likely, and long-winded, abovementioned "edit-warring" process.

Nevertheless, should you, the current main editors, really wish to improve this article, then here are some further suggestions for changes.

1. The notion that "downwash", IN ITSELF, is a necessary cause or consequence of lift (via NIII) was shown to be flawed by Lanchester as early as 1894 (in a paper read to the Birmingham Natural History and Philosophical Society). Briefly, Lanchester noted that if lift was only associated with a downwash, then after millions of years of birds flying overhead the air at ground level must be extremely dense, and the air above the birds extremely rarified. That is, common sense (or "Conservation of Mass"++) suggests there must be as much "upwash" associated with lift as there is "downwash". This should be noted throughout the article whenever "downwash" is mentioned.

2. The above considerations led Lanchester to postulate that the initial (unsteady, 2-D) motion of an aerofoil generates a motion in the air (or any fluid generally) that he described as a "supporting wave". This wave has energy and momentum, and travels through the bulk fluid as an entity, with the individual fluid particles being only temporary guests of this wavelike motion. As a result, and in short, a 2-D aerofoil can generate lift with no drag. Shortly after, Kutta, Zhukovsky, and Prandtl all mathematically described this "supporting wave" as the "bound vortex" that travels with the aerofoil. In the case of 3-D (finite-span) wings, Lanchester described a pair of "vortex trunks" that must necessarily spring from the wing tips, and Prandtl quantified the effects of these mathematically (ie. he calculated the loss of lift, and the "vortex drag").

3. All of the above is, of course, just the "vortex", or "circulation", theory of lift that is taught at tertiary level these days, but usually very poorly explained. The editors of this Wikipedia article could go some way to improving this situation by studying the writings of the abovementioned researchers (who are, hopefully, "reliable" enough?).

4. Furthermore, a small change to the animated figure in the article (coloured dots flowing past aerofoil, made by Crowsnest?) would also help. If the animation is changed, or another added, to be in the reference frame of the stationary bulk fluid, with the aerofoil moving from right to left at Vo (= "onset velocity", which is subtracted from current fluid velocities), then the motion of Lanchester's "supporting wave" becomes quite apparent. Showing more of the surrounding fluid (ie. with smaller aerofoil) would help with this visualisation. Adding a coloured pressure map (eg. red = high pressure, blue = low pressure), and selected "particle pathlines", would further help explain cause and effect of the fluid motions.

5. The above can be improved even further by showing the very different pressure map when the aerofoil is accelerated wrt the inertially stationary bulk fluid. This simply requires the addition of the d(Phi)/dt term in the unsteady version of Bernoulli's equation (Phi = Velocity Potential). This shows that even when the streamlines appear as "potential flow with zero circulation" there is most definitely a lift+drag force acting on the aerofoil (ie. an equal and opposite force from-aerofoil-to-fluid is required to set the fluid moving). Note that the theoretical background for all this was established as far back as the middle 1800s by Stokes, Helmholtz, Thompson (aka Kelvin), and others. It is hardly "original research"! The only thing that might be added to what was known 100++ years ago are the colourful animations. These would be merely visualisations of the 100++ year old maths, something that was more difficult to produce back then.

6. In "Talk Archive 6, Fundamental Physics", Nelsonpom asks, What makes the air come down? Is it "drawn" down? If so is this due to intermolecular attraction as you get with water? I doubt this. There follows a long discussion about "cause and effect", that, frankly, suggests poor understanding of the phenomena by some of those involved. Interestingly, Nelsonpom gives the correct answer to his question immediately before he asks it; There is going to be a void on the upper surface if air does not move down to fill it. Nigelj also gives the same answer; If it did not, there would be a vacuum from the leading edge backwards. Clearly the air 'rushes in' from above to fill this ... I don't know why people have so much trouble visualising this. Maybe I was lucky that no one tried to explain it to me at school using badly formed theories. And, indeed, Lamb has the same answer in Article 79, Chapter IV, of his "Hydrodynamics". This rather obvious explanation should appear in all of the sections mentioning "Coanda effect", "air is sticky", etc., and be used to immediately dismiss them.

7. In the same Talk section, you, Dolphin, say; My answer is that it is due to the vortex in the fluid surrounding the airfoil - in particular the bound vortex... ... the vortex system induces a velocity at every point... ... the fluid above the airfoil accelerates due to the vortex system... (my emphasis). You then go on to say (regarding Bernoulli); It is tempting to contemplate which is cause and which is effect, but that is not a valid line of thought... In other places you suggest that science is NOT about determining causes, but just about observing relationships... (I can't find the appropriate quotes right now, too many...). Firstly, the pursuit of understanding of Nature is most definitely about finding, or at least postulating, the "causes" of phenomena (see, for example, Newton's First "Law" (= Axiom), and his "Rules of Reasoning in Philosophy", in "Principia".). Secondly, your implicit assumptions of cause and effect in the above quotes are backwards. The mathematical concept of a pointlike core at the centre of a "free vortex" is most definitely NOT something that "causes" the flow around it. Rather, any low viscosity fluid that is forced to flow in curved paths, perhaps by solid boundaries doing the forcing/causing, will do so in such a way that the curved fluid motions are around "centres" (perhaps distributed) that can be modelled as "free vortices" (ie. roughly speaking the fluid velocities are inversely proportional to distance from the "centres" of the curved paths).

8. You also say (in same Talk section); This bound vortex exists because of the angle of attack and the sharp trailing edge of the airfoil - if there was no angle of attack, or the trailing edge of the airfoil was rounded like the leading edge, there would be no lift., and, Any attempt at a complete explanation of fluid dynamic lift that doesn't incorporate:

               1. the necessity for a sharp trailing edge,
               2. the bound vortex necessary to achieve the Kutta condition,
               is doomed to ultimately fail.

(You also seem to suggest that the Kutta condition is somehow the "cause" of the bound vortex.) These assertions are clearly proved false by the well-known effect of a circular cylinder experiencing (oscillating) "lift forces" when in a cross-stream. The nature of this lift is very similar to the establishment of lift on a more conventional aerofoil, except that the flow around an aerofoil stabilises after the shedding of the first "starting vortex", whereas the cylinder keeps shedding (similar) vortices from alternating sides. Spinning the cylinder about its axis, as per the "Magnus effect", stabilises the otherwise unsteady flow, and thus develops quite conventional lift WITHOUT A SHARP TRAILING EDGE. Also, toy polystyrene gliders, with very rounded TEs, generate reasonable amounts of lift. And, of course, a cambered aerofoil at ZERO AoA generates good lift...

9. Much more to say, but this post is already too long. So, at the very least, this article would be improved if it included; ... right at the top of page, that birds, bats, insects, fish, dinosaurs, and even falling plant seeds, have usefully exploited fluid dynamic lift for millions of years... (End Zapletal....)