Talk:Lift (force)/Archive 7

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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....) — Preceding unsigned comment added by 101.170.255.224 (talk) 04:22, 17 August 2013 (UTC)

Done. See my diff.

Can you identify any edit made by me (or Swordfish or Crowsnest) that erased worthwhile or valuable text? (You can identify any such edit by posting the diff or even by nominating the date on which the edit was made.)

It is easy to see a listing of all the edits made by me and Swordfish and Crowsnest by examining our list of contribs. In contrast, it isn't possible to see a reliable listing of all edits made by users who contribute, as you do, from an IP address. Please consider registering as a User. It costs nothing, provides greater security than contributing from an IP address, and allows high-quality communication and interaction between like-minded users. Dolphin (t) 06:33, 17 August 2013 (UTC)

(Zapletal writes ...) Dolphin,

Thank you for your quick reply, and for adding the section that acknowledges that "birds, etc..." mastered Lift long before we humans. Also I apologize for my very slow reply. I don't have much time to spend on these things, but nevertheless I would like to see this article improved. So below are some more suggestions for you and the other main editors to consider. Note that I won't be making any direct changes to the article, or registering as a User, as I am having enough difficulty formating this text here (whatever happened to WYSIWYG?).

I would like to suggest for your consideration a complete rewrite of the article. There is no offence intended here, and I accept that you will want to take some time to think about this.

My reasoning is that the current article seems like a mish-mash of the many popular but ill-informed explanations that have appeared in various textbooks, papers, pdfs, etc., on the subject over recent years. Just one example is:

Reference 44. - "... this flow regime is a consequence of viscosity, and cannot be explained without recourse to the Coanda effect, circulation, and the curvature of the wing..." Gordon McCabe...

As with many writers on the subject, McCabe misses the most important feature of Lifting flows, and then muddies the waters with unjustified recourse to things like "consequence of viscosity" and "Coanda effect". As mentioned earlier, I think editors of good encyclopedias are obliged to filter out these unjustified embellishments.

As mentioned earlier, IMO the most important feature of Lifting flows, which also happens to be the feature most often left out of popular explanations, is Lanchester's "sustaining wave". This view of Lift being dependent on a "wavelike" motion set up in the fluid was also used earlier by the Lillienthal brothers in a practical way, and later by the main developers of the mathematical "vortex theory of Lift" in Kutta, Zhukovsky and Prandtl. I stress again that this is all 100++ year old knowledge, so not "original research".

As mentioned in my previous post this wavelike behaviour is best seen by some simple modifications to the animation currently in the article. Also, when seen this way the unjustifiable assertion that "viscosity is necessary for Lift" is more clearly seen to be flawed (more on this below - see Lighthill...).

~o0o~

So, suggested REWRITE:

INTRODUCTORY SECTIONS. - Here I think that a greater emphasis should be put on distinguishing "Fluid Dynamic Lift" (ie. this article) from things like fluid static lift (= buoyancy), shockwave lift (= supersonic), rarefied gas lift (= hypersonic), planing lift (= between different density fluids), and any other types of "fluid lift" (eg. in "plain bearings" where viscosity has a significant influence). Specifically, this article is about the Lift force of a body completely immersed in, and moving wrt a large volume of fluid, with the fluid considered to be incompressible and inertially stationary. Note the change in emphasis from "fluid moving past a stationary body" to "body moving through a (mostly) stationary fluid". This makes a significant difference to understanding, even though the "maths" are about the same.

Considering the chronological usage of Lift, perhaps the order should be "fish, insects, dinosaurs, birds, bats .... and very recently humans have managed to benefit from Lift..."

~o0o~

HISTORY OF THEORETICAL UNDERSTANDING - Presenting the "theory" this way may help with the readers' understanding. It may also be a good way of dismissing some of the currently popular, though flawed, explanations. A very good reference for this is Division D "Historical Sketch" by R. Giacomelli+ (written in 1930's), in Volume 1 "Aerodynamic Theory: A General Review of Progress", edited by W. F. Durand.

Very briefly listed here, but this section could cover some of;

• Ancients - Aristotle++.

• Pascal - Static fluid exerts pressure equally in all directions.

• Newton - Laws of Motion (includes NI = Galileo's Law of Inertia). Newton's attempted calculation of Lift of a flat plate at low AoA greatly underestimates the force. Flaw is that only downwash is considered, and the fluid consists of NON-interacting particles (ie. not a "continuum").

• d'Alembert - Did many practical experiments, but his lifetime of theoretical calculations all suggested "resistance is absolutely zero"! (= "d'Alembert's Paradox").

• Bernoulli - Integrates Newton's Second Law.

• Euler - Further develops the "continuum" equations.

• Navier - Adds internal shear stresses, BUT THESE NOT RELEVANT HERE.

• Stokes - Calculates fluid forces in unsteady "Potential Flows" (= "added mass", but this is misleading name.).

• Rankine - Develops useful Potential Flow methods.

• Helmholtz (& Kirchoff) - Vortex Laws, and concept of "surfaces of discontinuity" and their resulting drag that resolves d'Alembert's Paradox.

• Thompson (aka Kelvin) - Proof that Potential Flow is minimum energy solution for given boundary conditions, and Theorem of Constancy of Circulation in inviscid fluids.

• Magnus - Studies rotating cylinders that develop Lift.

• Lillienthal brothers - Experimentally find that curved plates give much better Lift and less drag than flat plates.

• Lanchester - Describes "sustaining wave" in 2-D, and "vortex trunks" in 3-D.

• Zhukovsky - From consideration of Magnus effect, finds that "Lift = Rho-Vee-Gamma".

• Kutta - From consideration of Lillienthal's curved plates, and assumption that flow streams smoothly off TE (= "Kutta Condition"), finds as above (L=RVG).

• Prandtl - From similar considerations to Lanchester's "vortex trunks", calculates the 3-D loss of Lift and vortex drag.

All of the above prior to about 1910, but most recently,

• Lighthill - ("On the Weis-Fogh Mechanism of Lift Generation." J. Fld. Mech., 60(1)pp 1-17,1973.) Describes mechanism whereby generation of Lift does NOT depend on viscosity. Animation depicting this mechansim would help here (ie. simple PF development of a "doublet", = source+sink, or 2 x counter-rotating vortices). Note here that countless sources claim "viscosity is necessary for Lift", but not a single one that I have yet seen provides a convincing argument. A few, like Landau and Lifschitz, never make the claim either way. IMO the "viscosity is necessary" claim could usefully be "filtered out" of this article. I know we have discussed this before, but the claim is unsupportable.

~o0o~

SOME CURENTLY POPULAR, THOUGH INADEQUATE, EXPLANATIONS OF LIFT. - This short section could start with Von Karman's quote in my last post. It might then cover:

• Lift is dependent on "downwash" - Even in a "continuum" the downwash only gives half the value of lift, with the other half coming from the upwash of the "supporting wave".

• Newton vs Bernoulli - Essentially the same thing. Newton is in terms of "changes of momentum" of fluid particles (or "parcels" = Euler), while Bernoulli integrates these in terms of "kinetic/pressure/gravitational energies" of the parcels. Both accurately explain "small-picture" effects, but miss the "big-picture" (= the wave motion set up in the fluid by the body's initial movement).

• Equal Transit Time - Plain wrong, as seen in wind-tunnel "smoke-line" experiments (which above LZKP theory correctly predicts).

• Coanda Effect - Totally irrelevant, because it refers to a high pressure "jet" of air injected into otherwise stationary fluid. Note that nowadays the definition of "CE" seems to have changed to mean "any fluid flow that follows a curved surface", in which case it is now a meaningless "circular definition"!

• Air "comes down back of aerofoil because it is sticky" - Again, nonsense. Here can mention Lamb Article 79 (ie. = fluid comes down because it is under a net ambient pressure, else a vacuum would form). Photos of real cavitating hydrofoils would help explain this.

Not all of the above have to covered.

~o0o~

EXPERIMENTAL RESULTS - Mention that these all support LZKP "circulation/bound-vortex" theory to a high degree.

Introduce the "Aero-(Lift)-Force = Half-Rho-Vee-Squared.CL.Area" equation here. Note that the Force, Rho (= fluid density), and Velocity are all measurable quantities in the experiments, but the Area is determined arbitrarily (can be any "Length" dimension squared, or even Volume^(2/3)), and therefore "CL" is also arbitrary. That is, this equation is used to "fit a curve" to some given experimental results. In this sense it is less fundamental than Lift = Rho-Vee-Gamma.

Some mention could be made of maximum achievable values of Lift (eg. Handley page getting CL = 4+ in ~1910) , L/D (= 40+ for gliders), the fact that the CL-AoA slope is ~2.Pi (for 2-D aerofoils...), very different behaviour when near other bodies (eg. in "ground effect"), and so on.

Maybe note that real results differ from LZKP theory mainly when viscous forces become more significant wrt the inertial forces in the fluid (ie. at lower Reynolds numbers). So insect wings resemble "flat-plates", but the flow is modified by a "laminar recirculation bubble" on their upper surface just aft of LE, which gives overall flow pattern similar to a conventional "curved teardrop" wing. Also maybe note that at very low Re WINGS DO NOT WORK AT ALL. So microbes rely on "corkscrew" propulsion, whereby a cylinder moving through a relatively "high viscosity/density" fluid has a greater resistance to motion perpendicular to its axis, than parallel to its axis.

~o0o~

As a final note I would like to restress that a good Encyclopedia should ABSTRACT THE MOST IMPORTANT features of the given subject, and NOT DILUTE the explanation by mentioning minor issues, or total irrelevancies.

So IMPORTANT to Lift is that it is a force experienced by a body moving through stuff that can be thought of as a "continuum", with that stuff also having "mass" (ie. with "inertial" reaction to change of motion), and being under a net pressure, and resisting volume changes in compression, but having no resistance to tension or shear (ie. the stuff is a "perfect fluid"). LESS or IRRELEVANT are the small compressibility of real fluids, the small viscosity, the gravitational forces on the fluid if it is in a gravitational field (= bouyancy, and note difference between gravitational and inertial forces), the small inter-molecular attractive forces (= surface tension, or "stickiness"), and, indeed, the supposed atomic nature of "real fluids" (eg. are atoms really billiard-ball like "particles", or more like "waves" in Quantum Mechanics?). If any of these are considered necessary for a good explanation of Lift, then should the "smell", or "colour", or ???, of "real fluids" also be necessary?

(... End Zapletal (= Grumpy old man disappointed in gradual degeneration of the knowledge base, which is like re-photocoping a page multiple times, with no knowledgeable corrections to the little errors that creep in, so that ultimately you just have a messy sheet of grey paper...) — Preceding unsigned comment added by 101.170.42.155 (talk) 03:59, 18 September 2013 (UTC)

Lift force formulas for multiairfoil and multivortex flow

This is a new section in the article. Does it belong? My take is that it adds little and makes the article harder to read and understand so I would advocate removing it. what do others think? Mr. Swordfish (talk) 14:35, 2 October 2013 (UTC)

My first objection to the new section is that it begins by confining itself to two-dimensional flow, but then in two places it refers to "lift and induced drag". Lift-induced drag does not exist in two-dimensional flow. It may be possible to rectify this problem, and the result may be a legitimate comment about the lift on multiple airfoils in close proximity but I doubt it warrants a place in our article on lift because it is too specialized. The article on lift should confine its attention, as far as possible, to lift in general; lift as experienced by all airfoils, including lift generated by birds and other flying animals. The details about particular airfoil shapes and the variety of airfoil combinations and configurations should not be incorporated in our general article on lift. It may be possible to create separate specialist articles on the most noteworthy of these various combinations and configurations. I'm in favor of this new section being removed. Dolphin (t) 22:44, 2 October 2013 (UTC)

The section has been extensively rewritten, but I don't see much if any improvement. It's highly specialized, and makes little sense to someone not already familiar with the material. I've read the cited sources, and the gist is that the Kutta-Joukowski theorem can be extended and generalized to apply to multiple airfoils. That's interesting, but do we need to say much more than that, assuming we need to say anything at all? Mr. Swordfish (talk) 13:53, 5 October 2013 (UTC)

This section is essentially correct, though terribly written (possibly, and excusably, because Chinese authors?). Induced drag is indeed possible in 2-D flows (contrary to Dolphin's claim that - "Lift-induced drag does not exist in two-dimensional flow."), and was described by Wagner in 1920s, and by many others since (and probably also earlier by Lanchester, Prandtl, +++). The section explaining forces in unsteady flows as being dependent on change in Velocity Potential is also correct (though poorly worded). This last issue (understood ~150 years ago!) should be greatly expanded and clarified, since it is key to understanding Fluid Dynamic Lift. (Signed - Zapletal) — Preceding unsigned comment added by 129.78.233.211 (talk) 05:23, 15 October 2013 (UTC)

The question remains.

Asked by an anonymous contributor (I moved this from the main article):

The question remains. Why does an increase in velocity result in a decrease in pressure? Is it possible that velocity increases because the pressure increases due to a compression\expansion cycle formed as a fluid such as air does when passing over a cambered surface. When compression (increasing pressure) is applied to one third of a surface and expansion (decreasing pressure) is applied to two thirds of the same surface will result in a net reduction in the pressure acting on the entire surface. The compression/expansion cycle causes the reduction in pressure because a gas can be compressed faster than it can rebound resulting in a lower working pressure acting on a retreating surface. This action is much like a piston in a cylinder. No matter how fast a piston is pushed into a cylinder, compressing the air inside, it will always rebound at the same rate and when the piston is withdrawn faster than the rebound rate a reduced pressure will be applied to the piston. The leading or up slope of a wing can be viewed as the piston moving into a cylinder and the trailing or down slope surface as the piston withdrawing from a cylinder. Therefore camber produced lift results in a mechanical advantage somewhat greater than the 1:1 thrust to lift conversion of deflection lift (angle of attack).

-Mr. Swordfish (talk) 13:28, 15 November 2013 (UTC)

Why is a small increase in velocity accompanied by a similar small decrease in pressure? This is Bernoulli's principle which is the fluidic analogue of the principle of conservation of mechanical energy (PCME). The PCME explains the oscillations of things like the pendulum and the elliptical orbit of comets, satellites and planets. A small increase in the kinetic energy of a pendulum is accompanied by a similar small decrease in the potential energy, and vice versa. The dynamic pressure at any point in a fluid is a measure of the kinetic energy per unit volume at that point; and the static pressure at any point in a fluid is a measure of the potential energy per unit volume at that point. Just as the sum of kinetic and potential energies of a pendulum are constant (except for long-term decline due to air resistance and friction), so the sum of dynamic pressure and static pressure at a point in a fluid flow are constant. When one increases, the other must decrease. Dolphin (t) 09:54, 16 November 2013 (UTC)

A major revision proposed to be installed soon

Some of us have been saying for some time that this article needs revisions, to clarify the relationship between the mathematical theories and the qualitative physical explanations, to make clear the incomplete nature of the popular explanations, to offer a better explanation that emphasizes the spread-out nature of the flowfield and the reciprocal nature of the interaction between pressure and velocity, and so on. Over a year ago Mr. Swordfish offered to take a "swat" at incorporating these last two points into the article, but it hasn't happened yet, so I'm proposing adoption of my own revisions, which do essentially the same thing Mr. Swordfish proposed, among other things.

A draft of my proposed revised version is in my sandbox User:J_Doug_McLean/sandbox. I've just made major revisions to it, and I think it's now worthy to replace the current article, but I hope others will suggest corrections and improvements, so I'll leave it for a few weeks before I install it in place of the current version.

The reasons revisions are needed have been discussed at length on this page, by me and several others (Zapletal recently made some very good points), so I'll touch on them here only in summary form.

The proposed new version is longer than the current one, but I would argue that the additional material is needed to clarify the issues and avoid confusion on the part of the reader. If you don't think this kind of clarification is needed, just read the discussion on this page. The typical reader of the article is likely to be puzzled by some of the same questions that puzzle the participants in this discussion. Questions such as why the flow speeds up over the upper surface, why the flow follows a curved path, and why the pressure changes in the ways that it does. My draft attempts to make the answers to such questions clearer than they are in the current article. Read my draft carefully. If you find it leaves you with questions unanswered, let me know, and I'll try to fix it.

The proposed new version cites my own book, but I think the citations are relevant and not excessive, and thus in keeping with Wikipedia guidelines.

The proposed revision preserves some later sections of the current article, with minor changes: "Pressure integration", "Lift coefficient". and "Lift forces on bluff bodies". Otherwise, the changes are major, and the headings are new.

The title is proposed to be changed to "Lift (aerodynamic force)" to be less ambiguous. The introductory section and a new section, "Lift is a result of pressure differences and depends on airfoil shape, angle of attack, air density, and airspeed", describe what lift is and its general behavior, i.e. the "what" of lift, but without explanation of "how" or "why".

Next, "The understanding of lift as a physical phenomenon" is an all-new section that tries to establish a key distinction that isn't clear in the current article, i.e. the settled status of the science compared with the less settled status of the qualitative physical explanations, to set the stage for going into the details in following sections.

"Popular physical explanations of lift, and their shortcomings" covers much of the same ground as the current article, but the organization and most of the words are new. It integrates the explanations and their shortcomings into a single section to make the shortcomings more prominent. The "blind-men-and-the-elephant" problem is part of the folklore of explaining lift and thus should not be swept under the rug. I think this section, combined with the discussion in "The understanding of lift as a physical phenomenon", puts it all in perspective and equips the reader with enough background to avoid the common misunderstandings.

"A more comprehensive physical explanation" is a shorter version of the explanation in my book, but with different graphics to avoid copyright issues. It does not incorporate Zapletal's suggestion to adopt Lanchester's "wave of sustenance" metaphor, because I don't regard the wave idea as helpful. To understand Lanchester's "wave", you have to go into the details of the interaction between the pressure and velocity fields, as my explanation already does. Once you've done that, I don't think the "wave" idea adds anything. The resemblance of an airfoil flow to a wave is superficial, not fundamental, it seems to me.

"Mathematical theories of lift" attempts to make the nature of the theories clearer without going into too much detail. It does not incorporate Zatletal's suggestion to get into the history of fluid mechanics. That might be a good topic for another article, but it would be out of place in this one. In this article I think it's appropriate to explain the physical principles involved, as currently understood, not the history of their discovery.

"Lift of three-dimensional wings" is an all-new section that tries to remedy the current article's lack of information on lift in 3D.

"Viscous effects: Profile drag and stalling" explains how viscosity produces profile drag and limits the lift curve (stalling) in more detail than the current version. "Coandă effect is not relevant to explaining lift" takes a firmer stand on the relevance of Coandă and debunks the idea of a role for viscosity in the flow's ability to follow the upper surface. It also debunks the idea that the flow "sticks" to the upper surface and is "pulled down" toward it.

The "Further reading" and "External links" sections have been shortened by the deletion of a few items that were less than helpful for one reason or another or already in the reference list.

I look forward to constructive feedback and to getting these revisions installed.

J Doug McLean (talk) 19:19, 23 May 2014 (UTC)

My apologies for not following through my promise to incorporate your proposed changes. I have not forgotten, but other matters have garnered my attention and this one went on the back burner.

I have not had a chance yet to review your latest revision. I will do so in the coming week and give my feedback. Thanks for contributing; I think it is great that we have someone with your knowledge and expertise on the edit team. I am optimistic that we can come to a consensus that improves the article. Mr. Swordfish (talk) 22:27, 24 May 2014 (UTC)

Doug,

I have now had a chance to read the proposed revision in some detail.

I think the revision is much improved from the one from last January and it is clear that you have put quite a bit of thought and effort into it. However, I still do not think it is an improvement on the current article, and would not support changing it out wholesale. Apologies for repeating myself, but the current article's structure is based on the work 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. My strong preference is to state early and explicitly that "lift is a reaction force" and explain it in terms of Newton's 3rd law. Any revision that presents a more complicated explanation first will not have my support.

I do recognize that a pressure based approach is favored by aerodynamic engineers, and surely that approach should appear in the article. We just need to start with the simpler Physics 101 explanation such as provided in Halliday and Resnick.

You have written much that is of value that should be incorporated into the article. I have been remiss in not following through with my promised integration. I hope to have a first pass up in my sandbox in a week or so. Mr. Swordfish (talk) 21:45, 30 May 2014 (UTC)

I think the proposed change has lost the focus on explaining what lift is and why it happens - the proposed changes seem to focus more on giving a critique of various "simplified explanations". While there might be a case for having an article on that topic, putting it at the top of this article seems likely to add to the confusion of the typical reader of this article, rather than reducing it. Djr32 (talk) 22:43, 31 May 2014 (UTC)

First for the response from Mr. Swordfish (talk). Thank you for the encouraging words. But I don't see the basis for your objection that my draft "presents a more complicated explanation first". I don't think it does. In "Lift is a result of pressure differences and depends on airfoil shape, angle of attack, air density, and airspeed" I do say that lift is a result of differences in pressure, but that's just part of describing what lift is; it's not explaining lift in terms of the physical principles involved. The first explanation presented in my draft is "Flow deflection", the same one advocated by the AAPT.

In the quote from the AAPT, they advocate explaining lift "simply in terms of Newton's third law", but then they equate the force to a "time rate of change of momentum". This is the domain of the second law, not the third. When you use both laws you should explicitly mention both, as my draft and the current article do.

So it seems to me that we've both followed the AAPT recommendation in spirit, if not to the letter.

Regarding the structure of the article, I think it's important to present a sufficient description of "what" lift is before presenting explanations of "how" it works, i.e. how the principles of physics apply to it. The fact that lift is exerted in the form of differences in pressure is a key part of an adequate description, independent of any physics-based explanation as to how the pressure differences come about.

The current article muddles the distinction between description and explanation by presenting its explanations under headings that bill them as "description", i.e. "Description of lift on an airfoil" and "A more detailed physical description".

Pedagogically speaking, describing something adequately before you present the physics-based explanations is just common sense. I doubt that the educators at the AAPT would object to the way I've structured things.

As for explicitly stating that lift is a "reaction force", I don't think it's a good idea. It seems to me that the term is either misleading or practically meaningless, depending on how it's couched and understood. The first sentence of the current article's "Deflection" section, i.e. "Lift is a reaction force—an airfoil deflects the air as it passes the airfoil", is misleading because it implies that causing an acceleration is part of the definition of a "reaction force". After this misleading introduction, one has to study the subsequent sentences to get the straight story. Of course the correct definition of a reaction force has nothing to do with causing acceleration, as is clear in the article "Reaction (physics)" to which the reader is redirected in that first sentence. When "reaction force" is properly understood as simply a force that has an action/reaction partner in accordance with Newton's third law, the statement "Lift is a reaction force" is practically meaningless because the same can be said of any other force. If we were to keep this section of the current article, I would at least delete the first clause, "Lift is a reaction force". Then you'd have something close to my draft, but I think my draft is more clearly worded.

So it doesn't seem to me that your objection to my proposed structuring of the article is justified, or that you've suggested any real improvement to my draft, at least so far. I think my draft represents a marked improvement over the current article, in both structure and content. Unless you or someone else suggests improvements soon, I'll go ahead and make the replacement. BTW, I've changed the proposed title to "Lift (fluid-dynamic force)" to be consistent with identifying "aerodynamic force" as a special case, as is done in the current article.

I disagree with the response by Djr32 (talk). Just focusing on a straightforward explanation sounds appealing in its simplicity. But given the history of multiple, mostly flawed explanations that have been presented to the public over the years, just presenting yet another explanation, without discussion of the earlier ones, would cause confusion. Given the history of this topic, context-setting is essential if confusion is to be avoided. Again, just look at the confusion manifested in the discussion on this talk page. My proposed revisions are intended to reduce this kind of confusion by providing context.

J Doug McLean (talk) 22:35, 3 June 2014 (UTC)

J Doug McLean wrote: Unless you or someone else suggests improvements soon, I'll go ahead and make the replacement.

I'm sorry, Doug, but that is not the way wikipedia works. The objective is to arrive at a consensus that the proposed changes to the article are an improvement. Thus far, I have not seen any evidence of such a consensus here on the talk page, either with this iteration or the one you proposed last January. To issue an ultimatum that you will "go ahead and make the replacement" unless certain terms are met is at variance with the wikipedia processes.

I agree with Djr32's criticism above.

Moreover, the proposed revision has other problems at the moment:

• Major sections have no citations at all. (e.g. A more comprehensive physical explanation)
• Other sections have insufficient citations.
• Controversial assertions are made in a somewhat opinionated manner, with the only reference being to your book. This violates both WP:NPOV and WP:COI (e.g. Coandă effect & viscosity)
• In many places it reads more like a personal opinion than a neutral encyclopedia piece.

I think it is a good article, but I don't think it is a good wikipedia article. I would like to continue to work towards incorporating some of it, and I'm hoping that you, I, and the other editors can work together to improve the current article .

At this point I'd like to solicit the opinions of other editors. What's your take? Mr. Swordfish (talk) 14:44, 4 June 2014 (UTC)

Thanks for the response. I'm sorry about the ultimatum. I am appropriately chastised and won't let it happen again. Still, there's a lot about the current article that needs changing, and I'm eager to get things off top-dead-center. And I still intend to push for large parts of the article to be replaced, either by me or by others.

I can see now how I got on the wrong side of WP:NPOV regarding Coanda and viscosity, and possibly on the popular explanations of lift. I've reworked those parts to take a more neutral point of view in the writing, while still making clear that there are reliable sources whose position is that some arguments are incorrect. When there are two opposing views on something, there is by definition a "controversy". But it doesn't follow that both views should be labeled "controversial". When one view is supported by the science (and can be verified as such by reliable citations), and the other is not, the physically sound view isn't controversial, to my way of thinking. In the case of the Coanda effect, the primary issue is the questionable use of a term, so I've labeled it "controversial". In the case of the purported role of viscosity in flow turning, it's not just semantics. On this one the science is clear, so I've labeled the "pro" statement a "misconception" and provided a citation of Milton Van Dyke, a leading authority on fluid-flow theory, whose mathematical analysis convincingly supports the "con" statement, i.e. it shows that viscosity plays no significant role in supporting flow curvature in boundary layers. In contrast, the sources for the pro side of this argument (Anderson and Eberhardt, and Jeff Raskin) provide only arm-waving statements with no mathematical support. In a case like this, it seems to me that not all sources should be given equal weight.

I don't see how I've violated WP:COI. As I've said, I think the citations of my book are relevant and not excessive. Boeing, with which I now have no relationship, holds the copyright, and I have no financial interest in the book's sales. And my book is not the only source I cite for the argument that Coanda is not relevant to lift. There are three others. So I think the citations are OK there, but I have revised that section to take a neutral point of view.

You wrote "Major sections have no citations at all. (e.g. A more comprehensive physical explanation)". I'm sure there are places where more citations would be good, but I found no section that had none at all. "A more comprehensive physical explanation" cited my book in the opening paragraph. In one sense that should be sufficient, since everything in the section can be found in that reference. This explanation of lift is my original work, but it is not "original research" by Wikipedia's definition because it has a citable source. I know of no other source that has put all of these ideas together in one explanation, so if we're to include this explanation I think we have no choice but to cite my book. The constituent ideas (e.g. the spread-out effects of the airfoil on the flow, the reciprocal relationship between pressure and velocity, and the acceleration of fluid parcels by the pressure gradient), on the other hand, are all well established in fluid mechanics, and there are other sources that can be cited for them. I've added some and continue to look for more. If you see other places that could benefit from more citations, I'd appreciate it if you'd point them out specifically.

J Doug McLean (talk) 22:48, 5 June 2014 (UTC)

I now have a first pass at integrating the proposed changes into the current article. It's at https://en.wikipedia.org/wiki/User:Mr_swordfish/Lift

I don't claim that it is perfect, or even ready (yet) to replace the current article. Since it is a mash-up of two different articles the writing style varies a bit. It would be nice to address that. In particular, the cites from the current article are very "web friendly' with hyperlinks where available and brief quotations of the relevant supporting text. In the main article space the reader can mouse-over the footnote and see for him or herself the supporting material without having to go find a book in a library (unfortunately, the mouse-over feature doesn't work in a sandbox). It would be nice to flesh out the citations in the new material to take advantage of this feature.

Regarding WP:COI, I did not mean to accuse anyone of ethical lapses or attempting to profit off our efforts here. Merely that Wikipedia's COI policies are rather strict - an author citing his own work and unilaterally editing an article over the objections of other editors would be a problem. As long as it is a group effort with other editors involved we should be in the clear.

Not everything from both articles made it into the integration. Arguments in favor of "restoring" certain sections cheerfully accepted. But when combining two rather long articles it's inevitable that some material would be cut. Perhaps more should be cut or moved to a separate article.

Some material is repeated. Since we're not making a set of mathematical axioms or constructing a normalized database, repetition in itself is not a problem. But there are redundancies that probably should be removed.

So, I'm soliciting comment on this draft. Whether that happens here on this talk page or the talk page in my sandbox shouldn't matter. Since the proposed revision is a release candidate, edits in place are welcome. I'll continue to make edits over the next few days, then I'm on vacation for a while. I'm thinking maybe a target date of July 1 to get it in order and replace the current article, subject to reaching consensus here, of course. Mr. Swordfish (talk) 21:11, 10 June 2014 (UTC)

Integration of McLean's proposed changes and current article

The integration is up in my user space at https://en.wikipedia.org/wiki/User:Mr_swordfish/Lift. Please make general comments here, and please start a new section for each specific topic so that we can discuss unrelated issues one at a time. Mr. Swordfish (talk) 21:49, 10 June 2014 (UTC)

After spending only a few minutes looking at in-line citations I noticed that some merely nominate an author and a year; no page or section number, no title, no publisher etc. That standard does not meet Wikipedia's guidelines. For complex referencing tasks, I recommend Wikipedia:Harvard citation template examplesDolphin (t) 01:08, 11 June 2014 (UTC)

Note that the author/date citations in the "Notes" section refer to entries in the alphabetized "References" list that give the title and publisher. True, some of them need to have a page or section number added. This is a format I've seen in the article on Navier-Stokes equations and in the Journal of Fluid Mechanics. I think the alphabetized list makes some things much easier for the reader, such as answering questions like "does this article cite Lanchester?" The "Notes" section is now a mixture of this new format and the original format. In the original entries that include quotes out front, author information is deeply buried and thus not so easy to find in a quick visual scan. A change that would be helpful, but would take some work, would be to put all the notes in a name/date/section/quote format and put all the all the other information on the references in the "References" list. J Doug McLean (talk) 20:01, 20 June 2014 (UTC)

Proposed change to limitations of Bernoulli

Under limitations of Bernoulli, I propose deleting the third bullet item because it implies that the assumptions behind the steady incompressible Bernoulli equation are invalid for "real world airfoils" in general. Actually, for low-Mach-number steady flow outside the boundary layer in the reference frame of the airfoil, the Bernoulli equation is highly accurate, contrary to what this paragraph and its sources imply. Significant limitations to the Bernoulli equation apply only in circumstances different from those of the usual airfoil explanations (e.g. accelerating flow or high-speed flow). The arguments in the four cited sources are seriously flawed, and I can give you my detailed reasons if you wish to see them.

I look forward to feedback on this proposed change. J Doug McLean (talk) 20:01, 20 June 2014 (UTC)

I have no objections to this. I've implemented this proposed change in my version of the draft. https://en.wikipedia.org/wiki/User:Mr_swordfish/Lift#Limitations_of_explanations_based_on_Bernoulli.27s_principle Mr. Swordfish (talk)

Angle of attack

For an asymmetric foil, "angle of attack" can only be defined by arbitrary convention. (The concept is physically meaningless. It is physically meaningful only with respect to the trailing stagnation point, which is justifiably considered the aft point of the chord). The forward stagnation point is not completely defined by the geometry of the foil, but rather by the flow under specified conditions. The point defined, arbitrarily, as the forward point of the chord, has no physical meaning, only an intuitively simple significance.)

I edited two sentences accordingly. Each of these implicitly treated "angle of attack", incorrectly, as a physically meaningful concept in the general case, irrespective of symmetry.75.185.66.0 (talk) 03:07, 10 February 2015 (UTC)

Except, the first source I opened disagrees. It defines the "chord line" through from the centres of curvature of the leading and trailing edges, and uses this as the baseline for the angle of attack. Yes it is arbitrary in a theoretical sense, but not in an engineering sense. Also, your agonies of pedantry are not a style of writing that sits well with readability. It is easier to start again, so I am undoing your edits. — Cheers, Steelpillow (Talk) 09:55, 10 February 2015 (UTC)

Different sources define the chord line differently, and so the angle of attack will also vary depending on how one defines the chord line. Is this variance in the strict definition of "chord line" important? I don't think so, especially at this point in the article where it would be premature to go into too much technical detail. Clancy's definition is fine, but it's not the only one and I'm not sure that we are fairly representing the sources by using it without mentioning the others. Since I don't think the reader would be served by going onto a tangent about different definitions of "chord line", my preference is to simply link to the article Chord_(aeronautics) and remove Clancy's definition. I think the diagram to the right is sufficient to get the idea across. I've made the edit - let me know what you think. Mr. Swordfish (talk) 18:19, 10 February 2015 (UTC)

Yes, thank you, that is a definite improvement. — Cheers, Steelpillow (Talk) 20:32, 10 February 2015 (UTC)

75.185.66.0, I agree that "angle of attack" can only be defined by arbitrary convention and that different authors use different definitions. But the place to present that information is not in an introductory section aimed at lay readers in an article about aerodynamic lift. The minor differences among the differing definitions would be a distraction from the discussion and would not foster better understanding by the intended audience. There are articles on the angle of attack and chord line - either would be a better place for this level of detail. Mr. Swordfish (talk) 18:41, 10 February 2015 (UTC) Mr. Swordfish (talk) 18:41, 10 February 2015 (UTC)

I did not explain my concern very well.

Please be assured that I understand that chord has a conventional definition, or more than one, which is/are very useful in engineering, even though they only apply to common commercial configurations (not a Flettner rotor, for example, or a blob, or a foil with a sharp edge somewhat aft but not aftmost). My problem with it is NOT that it's not practically useful, say in an engineering handbook or product catalog. But this article is trying to explain lift, conceptually. Introducing it in the introduction is not just harmless but irrelevant, but I think highly misleading, because it amplifies a common, highly intuitive tendency to misunderstand lift. It reinforces the view that lift is like a billiard ball striking another off-center. In the billiards case, it is correct to believe that the forward-most point of the struck ball is a physically meaningful chord end-point.

We know that there is a common misconception that lift results from air striking the "bottom" of a "tilted" plane--with the "leading edge" being the farthest point forward and the trailing edge being the farthest aft, just as in the case of a billiard ball or a thin planar surface. The intuitive belief is that the airflow must split at this point, and that all points aft of this on the bottom are obstructing the flow, and thus creating high pressure, and all points aft of this on the top being in a condition of vacuum.

We know that this conception is completely without theoretical justification, and is factually incorrect (in the case of a barn door, even if we use the simplistic complex analysis model, the airflow splits well aft of the end of the "chord"!)--it is the misconception that we are trying to replace with a correct account of the actual pressure and velocity fields, and a correct account of why they occur. The aft stagnation point indeed happens to be the aftmost point on an ordinary commercially available wing, but the intuitive conception that it is the stagnation point BECAUSE it is the aftmost point is completely incorrect; in fact, the Kutta condition is the explanation, and the sharpness of the point, and not the fact that it is aftmost, is the relevant fact. And the idea that the foremost point is a stagnation point is not only based on completely spurious reasoning, it also happens to be completely incorrect experimentally and also inconsistent with every physical model from complex analysis up to and including the most complete differential equation.

My concern is that presenting an arbitrary engineering convention about how to define "tilted" in the intro, as if it were somehow related to the subject of lift, strongly encourages this misconception. ~~ — Preceding unsigned comment added by Mark.camp (talk • contribs) 02:51, 13 February 2015 (UTC)

"it is the misconception that we are trying to replace with a correct account..." - stop right there. No we are not. This is one of the commonest misconceptions by PoV editors as to what Wikipedia is about. We are trying to build and encyclopedia according to the WP:FIVEPILLARS and other goodly policies and guidelines. Most relevant here, we seek what is often called "verifiablility not truth". Where are the reliable sources (WP:RS) that claim this is a common misconception that must be addressed, and that the standard introductory texts do in fact explicitly debunk it? — Cheers, Steelpillow (Talk) 15:38, 13 February 2015 (UTC)

I'm not quite sure how this discussion sprung from the description of "angle of attack" in the article. It's a simple geometric definition, not a description of the physics.

From a pilot's perspective, AOA is very important to the subject of lift. In Stick and Rudder, Langeweische spends most of the book talking about it. The Lift coefficient is dependent on AOA. If it gets too high, the aircraft stalls.

When you say "tilted", are you referring to, "when an aircraft is climbing, descending, or banking in a turn the lift is tilted with respect to the vertical"? That sentence has nothing to do with angle of attack, but arises from the fact that lift is, by definition, perpendicular to the relative airflow over the wing. Burninthruthesky (talk) 16:07, 13 February 2015 (UTC)

I think what Mark is trying to say is a)it is not necessary to introduce the notion of chord line at this point in the article, and b) doing so is misleading because it may re-enforce by implication some common misconceptions.

I agree with point a) to some degree. The article used to say the angle of attack is the angle between the foil and the oncoming air; recently the definition was made more precise by introducing the notion of chord line and linking to that article. My take is that it was fine before, but that it's a bit stronger with the more precise definition.

As for point b), I suppose one could imply all those incorrect notions of which Mark speaks from our addition of four little words (" the chord line of"), but it would take quite a bit of imagination. I think all of the misconceptions mentioned by Mark are either directly addressed and corrected by the article, or at least not re-enforced. Perhaps material could be added elsewhere to address Mark's concerns? for instance, we don't cover the "skipping stone" misconception - perhaps we should. Mr. Swordfish (talk) 16:45, 13 February 2015 (UTC)

Mr Swordfish, I am sorry to say, expresses what I was trying to say just a wee bit better than I did. So I will start by taking his cue and asking this: what is the purpose of introducing the notion of chord line (which is useful only in establishing an arbitrary point of reference for purposes of communication, for example in operator's manuals and marketing materials) at this point of the article, which is not intended to be an operator's guide for some commercial product, but a general explanation on lift ? Mark.camp (talk) 04:40, 14 February 2015 (UTC)

I agree that for a cambered airfoil, the concept of chord line is arbitrary. In many analytical situations we are interested in the change in angle of attack rather than the absolute value, and in these situations it doesn't matter how the absolute values are determined. In analytical situations where we want to avoid the arbitrariness of a chord line it is customary to measure the angle of attack relative to the zero-lift line. Dolphin (t) 05:37, 14 February 2015 (UTC)

The main article on angle of attack covers these various reference lines in more detail. If it's felt that mentioning the "chord line" is confusing, I've no objection to a simpler description in this article. In that case perhaps an explicit "main article" link would be helpful. Burninthruthesky (talk) 08:23, 14 February 2015 (UTC)

So, let's see how reliable sources treat this subject. Here are the three elementary treatments that I have handy:

1. Kermode; Mechanics of flight, 1972 edition, pp 75-76: Titles a section "Chord line and angle of attack", then discusses the chord line before turning to the angle of attack.
2. Clancy; Aerodynamics, 1975, p 56: introduces the chord line and then, lower in the same section, defines the angle of attack in terms of the chord line.
3. Simons; Model Aircraft Aerodynamics, 1978, p 10: Introduces the angle of attack, then defines it with respect to the chord line, then defines the chord line - all within a single paragraph.

That list is uncensored - I didn't pull any books that disagreed with a PoV or anything.

It is clear to me that if we do not know what the angle of attack is, then any statement about the angle of attack of a cambered aerofoil is well nigh meaningless. A Wikipedia article is expected to provide, as a minimum, the information to make some kind of sense of its narrative, and to link to more detailed explanations where appropriate. As an example of the conceptual semantics involved here, the accompanying illustration finds it necessary to depict the chord line in order to give adequate realisation to the angle of attack. The text needs to do the same. As you can see, the reliable sources I found all support that view. None is followed immediately by addressing any confusion over the stagnation points, though Kermode discusses the more direct confusion over the difference between angle of attack and angle of incidence. Unless anybody can find sufficiently weighty sources to the contrary, these sources show that we need to introduce both topics in close association and not get distracted by indirect confusions over other topics. Therefore, rather than deleting all mention of the chord line, we should actually reintroduce its definition. — Cheers, Steelpillow (Talk) 11:58, 14 February 2015 (UTC)

Momentum transfer at microscopic level

long discussion unrelated to improving the article

If I understand well, there is a positive net transfer of (upward) momentum from the particles striking the foil. Is this correct? Mark.camp (talk) 19:12, 6 February 2015 (UTC) There is no transfer of momentum from anything to the foil. The net force on the foil is zero (unless the machine is manoeuvring). In a billiard-ball model, the balls are deflected downwards in accordance with L=dp/dt. The balls gain downward momentum but the foil does not gain upward momentum. The planet beneath gains the upward momentum as gravity attracts it towards the heavier-than-fluid foil, but the effect on the airflow and on the foil is negligible. Of course, net momentum change of planet + balls = 0. — Cheers, Steelpillow (Talk) 19:31, 6 February 2015 (UTC) @Steelpillow, you wrote: "There is no transfer of momentum from anything to the foil. Are you saying that the collisions result in no transfer of vertical momentum? So, are you saying that the downward momentum transfer from air particles striking upward-facing surface subareas is equal in sum (over long enough time period) to the upward transfer of air particles striking downward-facing subareas from below? " In a billiard-ball model, the balls are deflected downwards in accordance with L=dp/dt. The balls gain downward momentum but the foil does not gain upward momentum. The planet beneath gains the upward momentum as gravity attracts it towards the heavier-than-fluid foil, but the effect on the airflow and on the foil is negligible. It is true that the foil does not gain upward momentum, but that is irrelevant because it is not disputed. The effect on the airflow and on the foil is irrelevant because it is not disputed. The question raised is only about whether there is a momentum exchange between foil and air, not about anything else. To believe that collisions with the air particles create lift equal to the net momentum exchange is not to say that either experiences a change in momentum, nor to say anything about the effect on the airflow or the foil. Your statement that I've asserted that the balls gain downward momentum is correct if it is interpreted to mean that the balls (the body of air as a whole) gain downward momentum across the foil-air boundary. Obviously, the balls striking the upward-facing surfaces gain not downward but upward momentum. Obviously, in the whole system, the balls experience no change in vertical momentum: they gain in upward momentum from collisions with earth just what they gain from collisions with the foil. Mark.camp (talk) 02:27, 18 February 2015 (UTC) The net force on the foil is zero (unless the machine is manoeuvring). In a billiard-ball model, the balls are deflected downwards in accordance with L=dp/dt. The balls gain downward momentum but the foil does not gain upward momentum. The planet beneath gains the upward momentum as gravity attracts it towards the heavier-than-fluid foil, but the effect on the airflow and on the foil is negligible. Of course, net momentum change of planet + balls = 0. — Thanks for the clarification. Is there a transfer of vertical momentum to the foil when a single particle with nonzero vertical momentum collides with the foil? Mark.camp (talk) 00:18, 7 February 2015 (UTC) (Zapletal writes ->) Mark, "Is there a transfer of vertical momentum to the foil when a single particle with nonzero vertical momentum collides with the foil?" I recall this being discussed before, but can't remember where. The short answer is that there is a big difference between a "Kinetic Theory of Gases" description of Lift, and a "Continuum Mechanics" description. Both give the same end result, but in different ways. In yours and the KToG descriptions, very soon after your particle bounces downwards off the underside of the foil it hits another particle below it, bounces back upwards off it, and then continues to bounce up-and-down between the underlying particles and the foil (this, of course, is greatly simplified...). There are a great many (squillions!) of these exchanges of momentum, and their net result, summed over the whole surface of the foil, is the Lift force. However, in the CM description, these "collisions of particles against foil" are simply called "pressure". There are more frequent collisions on the underside of the foil, so "greater than ambient pressure" there, and less frequent collisions above, so "lower pressure". By definition, the Continuum cannot consist of individual particles, so the only way it can interact with other Bodies is via these "pressure" forces acting at the mutual boundaries. (This ignores Gravitational, EM, etc., interactions, and strictly speaking the interactions are via a "stress tensor", which also models "friction = viscosity".) (End Zapletal) 101.171.255.254 (talk) 04:47, 7 February 2015 (UTC) I think you are saying the answer is yes, there is a transfer of vertical momentum to the foil when a single particle with nonzero vertical momentum collides with the foil. Thanks, this confirms what I remember being taught. When you say that the net result, summed over the whole surface of the foil, is the lift force, (which is non-zero) are you saying that the net result (the net vertical momentum transferred to the foil) must be non-zero? Mark.camp (talk) 23:37, 7 February 2015 (UTC) Mark, There is no net transfer of momentum to the foil in steady flight - that is pretty much the definition of steady flight. At an individual level, each ball imparts a small momentum to the foil depending only on where it strikes and the component of velocity at right angles to the surface of the foil at that point. In an elastic collision, which we assume here, perhaps surprisingly the direction of travel of the ball is not relevant. Meanwhile gravity provides a steady opposition to these brief bursts of lifting force. So although lift and gravity cancel each other out overall, there is a certain "noise" to the steady state. HTH. — Cheers, Steelpillow (Talk) 11:47, 7 February 2015 (UTC) @Steelpillow, you wrote that there is a certain noise to the steady state. Noise is random by definition. Are you saying that the collisions are random, that they do not sum to an upward transfer of momentum? If the collisions are random, then over time the lift from the collisions, which is exactly equal to the average rate of momentum transfer from the collisions, is zero. But isn't it true that the lift is non-zero? Mark.camp (talk) 02:50, 18 February 2015 (UTC) Thanks, Steelpillow. You reference brief bursts of "lifting" force. But I think, to nitpick, you mean the net result of brief bursts of lifting force and brief bursts of downward force (particles impinging from above the foil surface at the point of impact). This is how I picture it, and I want to confirm that my picture is correct. I am trying to reconcile the micro and macro descriptions of lift in my mind. It seems that they should yield consistent answers about momentum transfers. There is no change in momentum to the foil in steady flight, I think. True, one possibility is the one you mention, that there is no net transfer of momentum from particle collisions to the foil. But there is another, that there is a net transfer of momentum from these collisions, and that it is cancelled by another net transfer, from gravitational interaction. Mark.camp (talk) 00:36, 8 February 2015 (UTC) (Zapletal Writes ->) Mark, Your last sentence pretty much nails it (in terms of this simplified "billiard ball" model...). For an even simpler version, consider an aerofoil moving at constant horizontal velocity through a vacuum, and above a large massive body such as the Earth. The gravity force between Earth and foil pulls them together, and gives them both equal time-rate-of-change of inwards momenta (ie. d(m.V)). Of course, the lighter foil has the higher dV here, so follows a more curved path (roughly parabolic, concave down). Now picture something like a football bouncing up and down "elastically" (!) between the two bodies. Each time it hits the Earth and bounces back up, it "exchanges" twice its vertical linear momentum with the Earth, and gives it some downward dP (but of very tiny dV). Similarly, each time it hits the underside of the foil and bounces back down, it gives the foil the same upward momentum dP. So answer to your original question is, indeed, YES. End result is that the foil travels along a path a bit like "mmm". The bouncing ball in this model is roughly equivalent to Lanchester's "pillar" from page 9 of his book, ie., "As a whole, the fluid, in the previous section, does not gain or lose momentum any more than does a cast-iron pillar supporting a load." . Adding more detail to the model has many more balls bouncing every which way, which add, in Continuum Mechanics terms, an "ambient pressure" everywhere. But, for this BB model to work there MUST be a time-averaged increase in the number of these balls bouncing between each other in the zone between the Earth and foil. This "increased pressure zone" MUST ALSO travel steadily with the foil, much as the single football does above, or as does Lanchester's "cast-iron pillar". (End Zapletal)101.171.127.235 (talk) 03:01, 8 February 2015 (UTC) @Zapletal: Thanks, good explanation! Mark.camp (talk) 19:56, 8 February 2015 (UTC) @Mark, yes, "lift" can be negative if the ball strikes the upper surface. In such a thin medium where this model works, the foil has to be angled so that more of them strike the lower surface. But it is not directly transferable to a dense medium where the balls are a jumble constantly bouncing off each other, because then the balls start fighting each other for personal space and Bernoulli's analysis becomes significant. A carefully-shaped foil can generate lift through Bernoulli's principle, even when not angled upwards. At a microscopic level though, we are still simply summing the balls that hit from different directions, it's just that Bernoulli gives the balls above the foil attitude so they strike less often than one would otherwise expect (they prefer to hurry on past and don't have the time to). And in all cases, when we sum the net rate of change of momentum of the balls striking the foil it equals the lift. In fact, it is only because L=dp/dt that Bernoulli's principle works. — Cheers, Steelpillow (Talk) 10:23, 8 February 2015 (UTC) @Steelpillow, you thought I was saying that "lift can be negative". I realize that it can, but that is not relevant to my point. I was saying that, even in the case of positive lift, the air particles striking much of the wing--all of the upward-facing parts--are imparting downward momentum. Mark.camp (talk) 21:58, 17 February 2015 (UTC) @Steelpillow--Thanks much, again. I'm starting to try to think now of the air as a single body comprising all the air particles. It has a single momentum, equal to the sum of the particle momenta. It is bounded by surface between the foil, the upper boundary of the air, and the earth. Is it correct to say that the average rate of vertical momentum transfer to this body from the foil equals the average rate of vertical momentum transfer to the air particles striking the foil, i.e., the lift? Since the air as a single body cannot have a continuous net change momentum, I guess there would have to be also a continuous transfer of vertical momentum from the earth to the body of air as well, via collisions of air particles with the earth. So if this is correct, then lift = rate of downward momentum imparted to particles striking the wing = rate of upward momentum imparted to the wing by particles striking the wing = rate of downward momentum transferred to the craft by gravitational attraction to earth = rate of upward momentum imparted to the air particles striking the earth. Is all of this correct? If so, then I think I have for the first time a good picture in my mind of the microscopic, momentum-based view of lift. Thanks, Mark.camp (talk) 19:56, 8 February 2015 (UTC) I would say yes, that is about right, as long as we remember that some of these are changes and not absolute values. You can round off the picture by adding that these also equal the rate of downward momentum imparted to the Earth by the air particles = the rate of upward momentum imparted to the Earth by the gravitational attraction of the foil. — Cheers, Steelpillow (Talk) 23:07, 8 February 2015 (UTC) Thanks @Steelpillow. Good additions. Mark.camp (talk) 20:37, 9 February 2015 (UTC) (Zapletal Writes ->) Mark, I feel obliged to add a clarification to the above discussion. Fluid Dynamic Lift, of the type that is presented in the main article, is a subject that was developed in, and best belongs in, the field of Hydrodynamics. This is the study of idealised, inviscid, INCOMPRESSIBLE, fluids similar to water (hence "hydro"). The main article is currently written mostly from the point of view of Aerodynamic Lift. But, because gaseous air moving in what is commonly called "low-speed, sub-sonic flow" behaves very much like the incompressible liquids of Hydrodynamics, the original Hydrodynamic explanation of FDL works fine here. But an important distinction needs to be made. Whereas gaseous fluids can be modelled as Billiard Balls that are mostly flying freely through a vacuum and only briefly bouncing off each other, in liquids the modelled BBs are ALWAYS in intimate contact with other BBs. (So picture the difference between 10 x BBs bouncing around a big billiard table, and 1,000 x BBs all in contact in a group on the table, but with this group freely deformable, or "fluid".) Thus the BBs in the hypothetical liquid can exert a force on a Body indefinitely, without the BBs ever moving, and without anything ever "exchanging momentum" (eg. "hydro-static" pressure forces). Note that the hypothetical particles that are assumed to make up a "real" liquid (ie. atoms, etc.) are thought to be in a constant "jiggling" motion related to their "heat energy". But so too are the atoms of a solid, and nobody pretends that the atoms of a cast-iron pillar ever need to "exchange momentum" to support a load. Neverthless, the "particles" that make up a hypothetical Hydrodynamic fluid (continuously divisible, so not atoms!) are assumed to possess Inertial mass, so they DO require a force impressed upon them to change their "quantity of motion" (= Newton's term for "momentum"), as per Galileo's Law of Inertia, aka NI. So it takes a force acting over a distance to get this fluid moving, and the fluid then gains momentum and kinetic energy. But, again, these same hypothetical fluid particles can also transmit forces with NO MOTION, or NO "exchange of momentum", whatsoever. Picture a heavy boat floating in a quite pond of water. Or the same boat floating in a pond of very slippery, but stationary, billiard balls. Same-same. (End Zapletal)101.171.213.77 (talk) 02:19, 9 February 2015 (UTC) Thanks. Your comments are very thought-provoking. The other cases you bring up (liquid statics, liquid dynamics, cast-iron pillar) baffle me...I am not able to come up with a unified picture in my mind that makes sense. But for now I will focus on this article, and explore whether there are possible suggestions for improvement coming out of the above discussion. I am aware that I'm barging into the middle of a long-running discussion by very knowledgeable people, so for the moment I am not ready to make any suggestions. In fact, I still have a question about the airfoil case. Do the above simple conclusions about momentum transfers between the three bodies--earth, wing, and air--which apply to the discrete collision view (microscopic view) apply to the continuous fluid model? Mark.camp (talk) 21:22, 9 February 2015 (UTC) They should be good for a continuous gas model, as the molecules remain well separted. As I remarked earlier, the jostling between balls invokes Bernoulli's principle but the same transfer mechanism applies. A liquid usually contributes static lift as well as dynamic, the "iron pillar" effect, but let's ignore that and focus on the dynamic. Because the balls are all in contact with each other, the momentum transfers are less easy to track, one can think of transfer "through" a ball "pushed by" the foil rather than "to" a ball "striking" it, and the momentum tends to disappear into the crowd, but the underlying principle is the same. I think it would not be a good model for liquid flow to describe on Wikipedia though, unless it can be well sourced. — Cheers, Steelpillow (Talk) 23:14, 9 February 2015 (UTC) So, I think you are saying that even in the fluid dynamics view, not just the microscopic view, there is a non-zero rate of momentum exchange between the three bodies, equal in magnitude to the lift. Wing up, air down (associated with net force from air pressure); air up, earth down (associated with forces of gravity and air pressure); earth up, wing down (associated with force of gravity between wing/vessel and earth). (In the fluid dynamics view, it becomes not the average of many random transfers, but rather a continuous rate of transfer). Is this correct? So, even though (in the fluid dynamics model) the wing has zero change in vertical momentum, it is exchanging vertical momentum with the air as a single body. Here I get lost though. In the continuous model, there is a force between earth and vessel, but there is, unlike the microscopic model, no change in momentum of earth or vessel/foil. So the wing is getting upward momentum but there is no cancelling momentum. I am missing something still. A momentum can cancel a momentum, and a force can cancel a force, but how can a force cancel a momentum????Mark.camp (talk) 23:53, 9 February 2015 (UTC) (Zapletal Writes ->) Two points to cover here. 1. Semantic quibble first. This "Lift" subject is part of a nested hierarchy, roughly; Classical Mechanics = Rigid-Body-Mechanics + Fluid-Mechanics +..., Fluid-Mechanics = Fluid-Statics + Fluid-Dynamics +..., Fluid-Dynamics = Hydrodynamics + Aerodynamics +..., etc. The explanations of "Lift" in this article come mostly from the field of Hydrodynamics, but they are couched, unfortunately IMO, mostly in Aerodynamic terms. So, the article uses the word "air" too much, rather than the more general "fluid". Note that anything that "flows" is a fluid. Anyway, "Fluid-Dynamics" covers both the individual-particle model (whether the particles be of liquid or gas) and also the continuum model (again, of both incompressible liquids and compressible gases). But "Hydrodynamics" generally only refers to the incompressible continuum model. To stress it again, the Circulation Theory of Lift (ie. the main one presented in this article), is a Hydrodynamic theory. ~o0o~ 2. Mark, I think you are asking, "[in both bouncing-particle and continuum models] there is a non-zero rate of momentum exchange between the three bodies [wing, fluid, ground], equal in magnitude to the lift. ... So, even though (in the fluid dynamics [continuum] model) the wing has zero change in vertical momentum, it is exchanging vertical momentum with the air as a single body[???]" Simple answer, NO. There is NO "exchange of momentum" between wing<->fluid, or between fluid<->ground, in the CToL continuum model (which happens to be the best model available for last ~120 years). But you are not alone in this misunderstanding. Certainly most of this Talk page, and some of the others, are devoted to arguing this issue. In brief, in the CToL model of steady flight of a wing, the NET FORCE acting on the wing, namely the sum of downwards-forces (eg. gravity) + upwards-fluid-pressure forces, is ZERO. The force system acting on the wing is in EQUILIBRIUM. Hence "steady" flight. Same for the ground. However, there ARE changes-of-momentum of the various different parts of the continous fluid. These are a result of the pressure field that travels with the wing and supports it. This pressure field terminates at the fluid's boundaries, which include the wing and ground surfaces. The above misunderstanding is, IMO, a result of "fluid" being slippery stuff that offers little resistance to being pushed, so it simply moves out of the way whenever you push it. But then, because of the fluid's slipperyness combined with the MOMENTUM it picked up when you first pushed it, it manages to circle around behind you, and push you forward so you fall flat on your face! <- This is a very non-technical description, but it is the gist of how the CToL maintains the pressure field so it travels unchanged with the wing. (End Zapletal)101.171.127.229 (talk) 05:15, 10 February 2015 (UTC) @Zapletal, are you saying that the surface integral of vertical component of momentum transfer across the air-wing boundary is zero? If so, then there must be some other interaction between air and foil other than these collisions that accounts for the non-zero lift. I cannot think of any, other than by taking into account the fact that there is some interaction being ignored in saying that air particles are precisely elastic particles, with no distant interactions. But this is a small error. Am I missing something? Here is my new thinking: You wrote: "There is NO "exchange of momentum" between wing<->fluid, or between fluid<->ground, in the CToL continuum model (which happens to be the best model available for last ~120 years). But you are not alone in this misunderstanding." You heard what I did not say. I don't think that there is any exchange of momentum in the CToL contuum model. If there is a continuous force between air and foil, and no change in momentum of foil or air, then it follows that there is no change in momentum. It is certainly true that CToL continuum theory requires that there is no momentum exchange across this surface. It is certainly true that the momentum of the air is constant, in both the continuum and the discrete particle models. But it doesn't follow that there is no momentum exchange across the air-wing surface, only that if there is, then there must be something in the approximation that is made in deriving CToL continuum from the discrete model which results in the former giving an incorrect result for momentum transfer. (And of course, there must be an opposite transfer, which we agree on at the earth-air boundary.) This is what seemed impossible to me at first. If anything is inaccurate about CToL, it would have to be infinitesimal, since the difference between the theories is infinitesimal. It vanishes with larger and larger numbers of collisions. It also seemed unlikely that there would be a consensus here to the contrary, with so many knowledgeable contributors and so many references from the literature. But since it seemed unavoidable, I started to think about how, at a microscopic level, the two theories could produce such startlingly different results. It turned out to be easy (just a week or so of agonizing day-and-night thinking) to see, once I looked at the other "force" involved. Gravity is a continuous Newtonian force. I did a thought experiment. What if there were two opposing continuous forces, say gravitational and electrostatic) on a rigid motionless body, call it "X"? Would there be any momentum transfer between the earth and X? Would there be any momentum transfer between the charged body which was the source of the lift force and X? No! So, in terms of momentum transfer, there are two kinds of physical phenoma producing net forces over time: particle collisions, and continuous forces. The former type results, if there are many collisions over time, in continual (but not continuous) momentum exchanges which integrate to a potentially non-zero value; the other does not predict any momentum transfer at all, at any instant nor over time. Both produce net force over time. So, as soon as we approximate the effect of many real collisions with an imaginary continuous force (from air pressure) we change from a model which predicts momentum transfer to one which predicts none! No matter how long you integrate the momentum for in the continuous model, you will never get any momentum because the momentum exchange was continuously precisely zero. Would like your thoughts. Mark.camp (talk) 19:35, 17 February 2015 (UTC) @Mark, Zapletal here sets up his favourite straw man - an opponent who does not exist - in order to evangelise his own mantra. We all agree that there is no overall transfer in a steady-state condition, despite Zapletal's protestations nobody has ever begged to differ. What I describe as "disappearing into the crowd", he describes as "changes-of-momentum of the various different parts of the continous fluid". His pressure field is of course just the jostling of the crowd seen as a whole. Please do not be misled into thinking that our explanations differ. — Cheers, Steelpillow (Talk) 11:14, 10 February 2015 (UTC) @Steelpillow, this debate is technically over my head for now, though I am eager to study it later. I am still stumbling in the dark, and taking one step at a time. On another subject, making posts more pleasant to read: A trick that works well for me, when I think of it, is to let my notes cool overnight before sending them. Your post seems a bit confrontational for a Wikipedia post, to be candid ;-) Mark.camp (talk) 17:11, 11 February 2015 (UTC) @Zapletal, here is my tentative conclusion as of the moment. 1. Fluid dynamics is a statistical approximation of the kinetic theory of gases. 2. Question: "What is the average rate of positive vertical momentum transfer across the closed boundary between foil and air, due to mechanical interaction between air and foil, if the lift is 10,000 N?" Answers: Per precise theory: 10,000 N Per approximate theory: 0 N Which is correct, with respect to physical reality? My current thinking: the more precise theory must be correct than the approximation. How to resolve the apparent contradiction with what you said about no momentum transfer? Perhaps you meant "per CToL theory, but not in physical reality". Or perhaps there is an error in my logic or facts. Pls comment or correct any errors in my thinking? Mark.camp (talk) 17:11, 11 February 2015 (UTC) (Zapletal Writes->) Mark, Steelpillow's and my explanations most certainly DO differ. Steelpillow, my explanations are based on a long study of the works of the many people who founded this body of knowledge, together with careful checking of the implications of those works to determine how well their predictions correlate with measured observations in the real world. Your explanations, from what I have seen so far, are based on a narrow ideology (eg. "TS"), that is founded on a meager understanding of fundamental principles, that gives NO quantifiable predictions, but which you nevertheless promote by attempting to suppress all alternative viewpoints. I could also mention that your tone is decidedly "uncivil", you seem to be "not-here" to educate or build a better encyclopedia, etc. But I doubt it will make any difference... Doug, I must apologise to you that I am finding "Wiki" a thoroughly futile exercise. Thank you for your (excellent!) efforts so far. But, s jwe;ifgvyubwe w;adly, I think good education is impossible under these conditions. (End Zapletal) 101.170.127.248 (talk) 02:55, 11 February 2015 (UTC) @Zapletal, above you wrote to me that your and @Steelpillow do indeed disagree. I think you may be confusing my question with some other. I didn't make a statement to the contrary, nor ask a question about this subject. I don't have any knowledge or opinion about it. Currently I have just asked for comments and corrections to my most recent conjecture about the comparison between conclusions of kinetic theory of gases model (billiard ball) and fluid dynamics. Mark.camp (talk) 16:01, 12 February 2015 (UTC) @Mark, my apologies for coming across a bit strong at being accused of certain incorrect statements, but I had my reasons. Sadly, there is a long history behind it. I will tolerate no more of Zapletal's accusations of ideology and suchlike ranting (I use the term advisedly - I'll post the diffs of this guy's first two contributions to this page if you need convincing) and I leave the above remarks in place only for your education. If Zapletal posts again, I will see it summarily deleted and if necessary I'll ask for the page to be protected. — Cheers, Steelpillow (Talk) 19:13, 11 February 2015 (UTC) @Steelpillow: no problem! but thanks, I very much appreciate your apology. I am not familiar with the history, sorry. My opinion, as a newcomer: I would try to separate the two things--my public posts on the technical subject at hand, and my administrative appeals or approved editing actions to seek to enforce Wikipedia standards of conduct. I believe that all of us would do well to respect the rules against personal attacks very strictly in our posts, even if we believe that others have not. Mark.camp (talk) 16:01, 12 February 2015 (UTC)

The talk page is not a forum for general discussion

I would like to remind the participants of relevant wikipedia policy:

• Discussion forums. Please try to stay on the task of creating an encyclopedia. You can chat with people about Wikipedia-related topics on their user talk pages, and should resolve problems with articles on the relevant talk pages, but please do not take discussion into articles. In addition, bear in mind that talk pages exist for the purpose of discussing how to improve articles. Talk pages are not for general discussion about the subject of the article, nor are they a help desk for obtaining instructions or technical assistance. Material unsuitable for talk pages may be subject to removal per the talk page guidelines. If you wish to ask a specific question on a topic, Wikipedia has a Reference desk, and questions should be asked there rather than on talk pages.

See WP:FORUM. I would ask that participants restrict discussion to that related to editing the article's content and that lengthy discussion outside the context of how to improve the article should be moved to user talk pages. Mr. Swordfish (talk) 21:26, 17 February 2015 (UTC)

I would heartily endorse Mr. Swordfish. Please can users confine off-topic chat either to their own user talk pages or entirely off-wiki. — Cheers, Steelpillow (Talk) 21:39, 17 February 2015 (UTC)

For me this section "Angle of Attack" shows not as a separate article but as a continuation of another section, "Momentum transfer at microscopic level". Anyone else have this problem? I checked the syntax and it seems that the same tokens, leading and trailing instances of "==", are the same. Mark.camp (talk) 01:39, 18 February 2015 (UTC)

Mark, are all your latest questions aimed at improving this article or just for your own satisfaction? If they are not aimed at improving the aticle then it is time to move the convesation elsewhere. — Cheers, Steelpillow (Talk) 09:36, 18 February 2015 (UTC)

Suggested text on momentum transfer

Re:

"Some of the air passing the airfoil has downward momentum imparted to it at a rate equal to the lift."

The text complicates the discussion by dividing the air arbitrarily into two bodies, each of which must then be defined and analyzed. The subject gets extremely complex and difficult to explain.

Why not just directly apply Newton's second law, expressed in terms of rate of momentum change between two bodies, to the two bodies we are discussing, the foil and the air? I suggest this text.

"There is an exchange of vertical momentum between the foil and the air."

This raises some issues.

First, it is a controversial assertion here at the moment, and the controversy would first need to be resolved. I attempted to prove the assertion in a subsequently collapsed Section. I was waiting for a response correcting or accepting my proof. (I assume that that thread can still be responded to. Otherwise, I will duplicate the proof in this Section.)

Second, it would need authoritative references.

Third, assuming the assertion is correct, the article would need to address, somewhere, a very difficult and subtle issue: why does the continuum theory, which is critically important in the settled science and the article, and otherwise correct, imply that the momentum exchange is zero? I attempt to show the source of the incorrect result in detail in the collapsed post.

Fourth, the remainder of the article would need to be made consistent with the sentence.

Fifth, the momentum balances of the foil or craft, air and earth would need to be addressed in terms consistent with the above.

Mark.camp (talk) 23:16, 18 February 2015 (UTC)

I'm not gone, yet at least. Back to Mark's question.

I assume Mark originally brought his question up not just as a matter of general interest, but because he thought it might impact what should be included in the article. So it seems to me to have been a bit arrogant to judge the whole "Momentum transfer at microscopic level" section to be "unrelated to improving the article".

That said, Mark's apparent contradiction between the microscopic and continuum views isn't really a contradiction. Both ways of describing the flow are correct and consistent with physical reality. We have the appearance of a contradiction only if we use the word "momentum" indiscriminately. It is resolved when we note that "momentum" doesn't generally refer to the same thing in the continuum description as it does in the molecular description.

In the microscopic view, "momentum" is associated with the motions of molecules, including the random thermal part of the motion. The random thermal motion has no preferred direction, and in gas flows at low Mach number, the thermal motion predominates, with the "flow" looking like a relatively small directional "drift" superimposed. At any stationary solid surface, the average (continuum) velocity of the gas goes to practically zero, so sufficiently close to the surface of an airfoil the only molecular motion we see is random thermal motion. The bouncing of individual molecules against the surface is diffuse, not specular, and individual bounces are not elastic (an individual molecule can either lose energy to the surface or gain energy from it), so that incoming and outgoing perpendicular momentum are not usually equal-and-opposite for any one molecule. But on average over many molecules, incoming and outgoing perpendicular momentum are equal-and-opposite, provided the surface and the gas are in thermal equilibrium. So the surface pressure is the result of many molecules per unit time having the perpendicular component of their thermal momentum reversed, on average, in their collisions with the surface, and the thermal momentum fluxes of incoming molecules and outgoing molecules account for half of the pressure each.

"Momentum" in the continuum description is based on the continuum velocity of the fluid. As noted before, all components of the continuum velocity near the surface are practically zero, and there is no change or "exchange" of continuum momentum taking place as a result of exerting pressure on the surface. The pressure in the continuum description is just a force per unit area, and nothing needs to be said about the details of how molecules produce the force. Whether pressure consists of forces transmitted between molecules in direct contact with their neighbors, as in a liquid, or is produced by bounces of isolated molecules, as in a gas, doesn't matter in the continuum description. The representation of the pressure as a force per unit area in the continuum momentum and energy equations is the same regardless of whether the fluid is a liquid or a gas.

If we take the molecular view of upward lift on an airfoil, the lower surface is imparting a downward change to the thermal momentum of molecules at a greater average rate than the upper surface is imparting an upward change, so there is a net downward imparting of thermal momentum (Mark's "10,000 N") to molecules impacting the surface. On the other hand, nothing is happening to the vertical continuum momentum locally at either surface, so the net rate of change of vertical continuum momentum is practically zero for the fluid very close to the upper and lower surfaces (Mark's "0 N"). In the actual flow field, vertical momentum is imparted to the flow over an extended region around the foil, and practically none of it is associated with molecules that acquired it directly through collisions with the surface. So Mark's "10,000 N" and ""0 N" refer to two different things, and are both correct. There is no contradiction.

Further points worth noting:

• For atmospheric flight at ordinary scales, the molecular mean free path is extremely short, and the continuum theory is highly accurate. The molecular description offers practically no advantage in fundamental accuracy.

• If you know the density and temperature locally, the molecular theory can tell you the pressure. But the molecular theory is practically useless for predicting the density or pressure variations in ordinary aerodynamic flow fields. The continuum theory is the only choice for actual predictions. And for gasses at low Mach numbers, or for liquids, the continuum theory can predict the pressure differences without having to deal explicitly with the density differences.

• The continuum approach is also the only workable choice for qualitative explanations at the flow-field level. So if Mark was thinking that the microscopic description should perhaps play a role in explaining lift in the article, I think the correct response would be "No, it shouldn't". The molecular description is practically useless for understanding why ordinary flow fields behave as they do. This makes sense when you realize that at a given Reynolds number the flow field described in dimensionless terms is the same regardless of whether the fluid is a gas at low Mach number or a liquid. At the flow-field level, the only properties of the fluid that matter are the density and viscosity, which are macroscopic quantities. The details of the molecular motions have no effect on the global motion and thus provide no help in understanding it.

I agree with Mark that the current quantitative statement unnecessarily "complicates the discussion". But his suggested fix misses on two counts: 1) There is no vertical momentum imparted to the foil in level flight, and 2) To be correct in general, his version of the statement has to be referring to the microscopic version of momentum, which for the reasons I discussed above isn't how aerodynamics analyses and explanations are constructed. Citable sources that do it that way, and do it correctly, are nonexistent.

So no wholesale reworking of the article is in order. But Mark's problem with The Statement would be alleviated, and the article would be improved (as I've said repeatedly) if the current quantitative version were replaced by a qualitative version, as in Langewiesche. I don't think we're done with that discussion. More later.

J Doug McLean (talk) 02:36, 19 February 2015 (UTC)

Thanks much, Doug. I will need to study your latest to understand it, but unfortunately will not be replying as I'm excusing myself from the discussion. This community has become a very unpleasant place for conversation, and I admire your persistence in remaining engaged in the hopes of improving the article for its users.

Mark.camp (talk) 23:00, 19 February 2015 (UTC)

To reiterate a point made many times: Wikipedia does NOT document what its editors believe to be true. Wikipedia documents what reliable sources tell us is true (see WP:VERIFICATION and WP:RS). This conversation is paying no attention whatever to sources and as such it is mere off-topic chat masquerading as an edit discussion. If folks don't shut up I'll take this to ANI in my own way. — Cheers, Steelpillow (Talk) 07:41, 19 February 2015 (UTC)

@Steelpillow, I did not alter the article to agree with what I believe to be true, so you are preaching to the choir. This conversation is based on what I as an ordinary (non-expert) consumer find to be confusing about the current text, and until that is clarified by the experts here, and the text improved if that is deemed necessary, it is perfectly legitimate. Wikipedia policy doesn't require a person who finds article text confusing to provide any sources in the context of a Talk page...otherwise, no user would ever be permitted to raise such an issue--who would ever have written a paper about the fact that some Wikipedia text is confusing to one individual? This discussion is very much on topic, and my intention is not to masquerade about anything.

However, since you and others seem to find my attempts to help offensive, and you question my motives rather than just my words, I will not pursue either of the two discussions I started any further.

Mark.camp (talk) 23:00, 19 February 2015 (UTC)

Mark, if I misunderstood your motives, then I must of course apologise. But it makes no practical difference. The interminable edit discussion on the Newtonian model was closed. It should not be reopened gratuitously unless new sources are found to support revisiting the consensus finally reached. WP:DISRUPTION explains that "Editors may be accidentally disruptive because ... they lack the social skills or competence necessary to work collaboratively. The fact that the disruption occurs in good faith does not change the fact that it is harmful to Wikipedia." Also, "In some cases, editors have perpetuated disputes by sticking to an allegation or viewpoint long after the consensus of the community has decided that moving on to other topics would be more productive. Such behavior is disruptive to Wikipedia." This is where we are now on the matter of Newtonian lift. You are of course welcome to pursue these discussions on your own user talk pages, such as User talk:Mark.camp or User talk:J Doug McLean, and to invite other editors to contribute. Unless Zapletal creates themself a user account, they will have to make do with someone else's, as their IP address is not static (Even there, excessive abuse of other editors may lead to more formal sanctions such as IP blocking). If anybody would like some help on using Wikipedia's user pages more effectively, I will be glad to do what I can. — Cheers, Steelpillow (Talk) 10:37, 20 February 2015 (UTC)

J Doug McLean,

The reason I collapsed the discussion is that I did not see anything concrete regarding how the article might be changed. I'm sure you've been to meetings where discussion goes off on a tangent - at some point someone needs to bring it back on-topic. In structured meetings, it's required that some specific motion be on the table before any discussion takes place. I don't think we need that level of structure here, but long posts that don't relate to actual proposed edits don't get us anywhere - they are fine on user talk pages, but let's try to stay focused here.

You suggest replacing the current quantitative version with a qualitative version as in Langewiesche. Can you provide sample text?

Mark.camp,

I do not take offense at your participation, and think your input is valuable. In particular, if you find the text of the article confusing we should take that criticism seriously. It is not enough that the article simply be technically correct, it needs to be written in a way that ordinary non-experts can understand.

My apologies if you took my recent attempts to focus the discussion the wrong way.

As a housekeeping note, I'll be away from the internet for a while, so don't take it the wrong way if I don't respond immediately. Mr. Swordfish (talk) 20:40, 20 February 2015 (UTC)

Mr. Swordfish and Mark.camp:

A qualitative version of the flow-deflection explanation has long been part of the candidate text in my sandbox User:J_Doug_McLean/sandbox. I think this kind of explanation avoids the problem identified by Mark.camp, but I'd be interested to hear his opinion.

Steelpillow's assertion that "This conversation is paying no attention whatever to sources" is unfounded. Finding that one set of sources is not consistent with other more authoritative sources is a perfectly valid line of argument. Along that line, Zapletal's take on the physics is spot on, as is his position that The Statement and the firehose model, as presented by the AAPT authors and Clancy, represent a small-minority view that is at odds with the mainstream understanding. He's also right that giving this view the prominent place it has in the current article is misleading. In my opinion it's also inconsistent with Wikipedia policy. See in particular Due and undue weight.

Carrying this further, I would also cite Fringe theories. Among the sources supporting The Statement, only Waltham and Clancy present actual analyses, and those analyses are based on the firehose model. Is the firehose model a fringe theory? Steelpillow has described it as "a model that is still in the textbooks", implying it has wide support in the literature, but Clancy's and Waltham's works are the only places I've seen it. At most, the percentage of books and papers on aerodynamics that it appears in is very small. The other standard to apply is that of scientific rigor. As described in Fringe theories, fringe theories "characteristically fail to adhere to scientific standards and methods." In general, flow models used in aerodynamics should be consistent with the equations of motion locally (the potential equation, at least), and simplifications should be based on rigorous asymptotic analysis. The classical control-volume analyses of lifting flows meet these standards, but the firehose model does not. Thus I think the firehose model satisfies Wikipedia's definition of a fringe theory.

Regarding the idea that this issue has been "settled by consensus", I know I gave in and agreed to the new version of The Statement. I did so only because I thought it was the least-bad option I could get. I still think it degrades the article. The new wording ("some of the air") was intended to avoid saying anything that is outright false, and it succeeds in a legalistic, mathematical sense, at least in the body of the article. But when the average reader reads that "some of the air has downward momentum imparted to it at a rate equal to the lift", he's going to infer that all of that downward momentum actually shows up downstream of the foil, when, in fact, only half of it does. Thus it is still misleading unless the reader follows the link to the new section "Momentum balance in lifting flows", and reads it carefully. And if the reader follows footnotes 26, 28, and 29, The Statement in unqualified form (no "some") is there in a prominent place, in violation of Due and undue weight.

I have long maintained that a qualitative version of the deflection explanation would be more appropriate in this section of the article. And now it seems to me the "consensus" insisting on including a quantitative dp/dt = -L has eroded. Mr. Swordfish was originally in the pro camp, but in December he was finding The Statement "problematic". And now we have Mark.camp, who finds The Statement unduly complicated. And we have Zapletal, who has ruffled some feathers but is technically knowledgeable and still has a right to be heard. Even Burninthruthesky said in December that he could live with a qualitative version, as in Langewiesche. I'd like to see a fresh straw poll on the issue of whether we should keep the current quantitative statement or replace it with a qualitative version like the one in my sandbox.

J Doug McLean (talk) 02:58, 5 March 2015 (UTC)

No new source has been put forward in the above. In all the endless discussions that have gone on, not once has an independent and reliable source been produced that explicitly refutes the Newtonian model. To those of you who remain confused about the whole thing, I would ask you to think hard about why, in all this time, two highly motivated and knowledgeable people - one of whom has himself published the only known rebuttal - have been utterly unable to produce any independent corroboration of their attack.

@Mark.camp:, since you opened this discussion, do you have a view on where it should be going, bearing in mind that WP:CONSENSUS requires us to "Limit article talk page discussions to discussion of sources, article focus, and policy"? — Cheers, Steelpillow (Talk) 12:10, 5 March 2015 (UTC)

The firehose model is clearly not realistic, but it's only supposed to be a simple scientific model. As the linked article says, all models are simplified reflections of reality, but, despite their inherent falsity, they are nevertheless extremely useful. That's somewhat different from pseudoscience.

We agreed in January that the language in the current article is supported by Lissaman's classical analysis. Taken out of context, it is also consistent with the firehose model, but the remaining text and diagrams are clearly not. An argument against that model presented as an argument against the article is a form of straw man argument, therefore it fails. Burninthruthesky (talk) 15:45, 5 March 2015 (UTC)

Steelpillow again mischaracterizes the situation when he claims that opponents of The Statement have been "unable to produce any independent corroboration of their attack". I've already cited the mainstream sources for control-volume analyses that contradict The Statement, and they are all now cited in the article as well (more on this below re Berriman). The idea that some "new source" must now be put forward makes no sense. And his assertion that I have "published the only known rebuttal" is absurd. Besides, even if my work were the only known rebuttal, the fact remains that it's a citable source under Wikipedia policy.

Burninthruthesky: I agree that it's permissible for a scientific model to take liberties with reality. But a model is "useful" only if it 1) makes useful quantitative predictions, or 2) aids in arriving at a correct understanding of the phenomenon in question. The firehose model makes no prediction (It contains a free parameter, the depth of the assumed affected stream, which is in effect adjusted to produce the desired downwash angle), and the understanding it provides is, as Zapletal said, an unbalanced half-view of the true behaviour. I stand by my assessment of it as a fringe theory that falls short of accepted scientific standards.

Burninthruthesky has made the "straw man" accusation before, and it still doesn't hold water. The current version of The Statement is consistent with Lissaman in a legalistic way, but it is still misleading for the reasons Zapletal and I have explained at length. And the article still cites Clancy and Waltham in support of the statement, and thus criticizing the model on which they based their support of The Statement is not a "straw man" argument. J Doug McLean (talk) 00:36, 7 March 2015 (UTC)

(Zapletal Writes->) Doug, your qualitative version of the "Flow-deflection..." section is much better than the current, live, version, so I support making the change. However, I would actually prefer to go much further with these changes. Given the huge disruptions currently going on, this will likely be a long-winded process. So here are just some rough draft ideas that you might like to consider.

1. Delete the whole "2. Simplified.." section. The article is already very long and the "4. A more comprehensive explanation" covers it all anyway (and in a simple enough way). The "2..." section also gives the impression that there are two competing explanations, namely Newton vs Bernoulli, which IMO reinforces a longstanding, but WRONG, misunderstanding. I think this has been covered before, but I wanted to give my support here for a major deletion. Any "Newton XOR Bernoulli" debates can be covered under "misconceptions" (ie. "Bernoulli explanation" is simply "Newton" integrated).

2. The "2.1 ... Newtonian ..." section alone could perhaps be completely reworked so that it is given in the reference frame of the bulk-fluid, with particle-pathlines used instead of streamlines. There are abundant RSs giving these pathlines, including gold-standard RSs like Lanchester and Lamb. These pathlines, which typically look like roller-coaster "inverted-loops", give a good picture of how the fluid particles are first accelerated upwards, then downwards (at top of loop), then upwards again at the end of their motion. This approach also gives the "Newtonian" explanation in terms of a Lagrangian model, and the "Bernoulli" explanation in terms of a Eulerian model, which I think is a helpful introduction and distinction to give to students of the subject.

3. Two small points. I am particularly troubled by the all too widespread, though very wrong, view of "... horizontal onset flow in front of wing, then downwash behind wing". So...

3.1 - Rather than the flow being described as "curved downward", I think "concave down" might be better. I think "curved downward" allows the "horizontal flow, then downwash" view to perpetuate, because the average reader naturally assumes an undeflected, hence horizontal, flow in front of wing (ie. because no explicit denial is given of this wrong view). "Concave down", perhaps also with some emphasis on the UPWARDLY deflected upwash in front of the wing, may help dispel this misconception.

3.2 - There are several Figures (unnumbered) in the article showing an aerofoil cross-section with a horizontal arrow, labelled "flow direction" or similar, pointing rightward. I think this again misleadingly reinforces the above misconception of horizontal onset flow with no upwash. I think it would be better for the arrow to point leftward, and be labelled "direction of aerofoil movement" (or "direction of wing motion relative to the bulk fluid", or similar).

I have many more detailed suggestions on how to improve the article, but sadly too much time spent trying to cope with the disruptive editors! (End Zapletal, for now) 101.170.255.225 (talk) 03:50, 9 March 2015 (UTC)

(Zapletal Writes->) Further to above. The very last line of the article, under "External links", has this link - "One Minute Physics How Does a Wing actually work? You Tube video" . And, yet again, this gives the "Horizontal-onset-flow/Wing/Ever-downwardly-departing-flow" picture in all its absurd glory, supposedly as an accurate account of how real Lift works (ie. "because of Newton/TS"). This very wrong, but very ubiquitous, picture will need a great deal of explicit refutal before wider society can grasp the essence of Lift. (End Zapletal) 101.170.255.225 (talk) 04:54, 9 March 2015 (UTC)

Berriman

I just added a cite to Berriman's 1913 book on Aviation on the matter of Newtonian lift. In the quotation I cite, "Thus, the wing in flight continually accelerates a stratum of air downwards, and must derive a lift therefrom", the italicisation of "must" is his own. Berriman was a respected authority of his day, and learned theoretical writings of his published in Flight through 1912-13 make it abundantly clear that he was well aware of Lanchester's work. His book also received positive reviews. I have found not the slightest hint or suggestion of any historical controversy over the quantification of Newtonian lift at this time, least of all between Berriman and Lanchester. — Cheers, Steelpillow (Talk) 16:55, 2 March 2015 (UTC)

(Zapletal Writes ->) Steelpillow, you say "I have found not the slightest hint or suggestion of any historical controversy ... between Berriman and Lanchester."

As I have already pointed out (and RSed), the Cambridge School, including such authors as Berriman, did NOT accept the CToL until very late in the 1920s. So, once again and briefly, Berriman's Flat-Earth theories of Lift were exactly what Lanchester had to spend three decades battling against, at least in the UK. On the other hand, the Germans grasped Lanchester's ideas and ran with them almost immediately, from early 1900s. Hence "Kutta-Zhoukowski" theory, etc. You should think deeply about why you are so insistent on reintroducing those long discredited Flat-Earth theories. (End Zapletal) 101.171.213.65 (talk) 02:14, 5 March 2015 (UTC)

I see more than a "hint or suggestion" of inconsistency between Berriman and Lanchester. My dictionary defines stratum as "a horizontal layer of any material, especially one of several parallel layers arranged one on top of another". So Berriman's quote implies that one horizontal layer of air is being accelerated downward and that layers above and below it are not. And later in the same passage he quantifies the relationship as equivalent to dp/dt = -L. Berriman's implied model is thus similar to the firehose model, and identifying the affected air as a stratum puts him at variance with Lanchester, who makes it clear that to find dp/dt = -L in a real lifting flow field the control volume must be a column, not a stratum. Furthermore, we know from later sources such as Lissaman that a stratum that is wide compared to its vertical depth (a pancake control volume) contains just as much upward acceleration as downward acceleration, and a total dp/dt = 0.

Berriman's book may have been a fine book for its time, but this particular passage is not consistent with other more authoritative sources and should not be given weight. The quoted passage is misleading and negatively impacts the article. I have reverted the change. J Doug McLean (talk) 03:08, 5 March 2015 (UTC)

Doug, my key point is all about citing the sources. I have gone to the trouble of explicitly quoting and referencing the material that I find. You really do need to do the same - vague assertions to the contrary are not more good enough now than they ever were. Where are these citations of sources that explicitly refute Berriman's statement of the Newtonian principle? (And I don't mean just provide a different analysis using a different control volume, I mean explicitly refute the principle he states.) I am prepared to be educated about Berriman vs. Lanchester - many of my leads (mostly for work unrelated to this discussion) come from the pages of Flight, at a time that Berriman was its technical editor, so it is possible that I would pick up a selective view of his relationship with Lanchester. Meanwhile, one cannot remove a citation to a reliable source simply because one disagrees with it. Worse for you, it would break the terms of editing where there is a conflict of interest and also continue your pattern of disruptive editing. I would caution you not to remove the citation. This page is indeed the correct place for an editor with a conflict of interest to agree such details. — Cheers, Steelpillow (Talk) 10:29, 5 March 2015 (UTC)

Zapletal, this is a final caution about your abusive posts such as this. Stop the abuse or face sanctions. Rather than delete your rant on this occasion I sanitised it in order to make the point that my summary deletions are about abusive language, not about the technical arguments. If you cannot grasp even this simple point about civilised conversation, then you are not welcome on Wikipedia. I shall not sanitise any more, but shall return to wholesale deletion of abusive IP posts and if that fails to stop the abuse I shall seek page protection. — Cheers, Steelpillow (Talk) 10:29, 5 March 2015 (UTC)

Two sources are Lissaman (1996) and my own book, which paraphrases Lissaman's results, with attribution, of course. Berriman asserts that dp/dt = -L for "a stratum of air", which is contradicted by Lissaman's finding of dp/dt = 0 for a flat, horizontal control volume. It's not a matter of a "different analysis" or a "different control volume"; Lissaman's is an authoritative quantitative result for the same control volume, and it directly contradicts what Berriman says. And Lanchester finds dp/dt = -L only for the case of a tall column, which, in a way, is also in contradiction to Berriman.

Lissaman does not explicitly say "This result contradicts Berriman's explanation of 1913", but a direct quantitative contradiction should be enough. If you disagree, please point out specifically where Wikipedia policy requires "explicit refutation" for a source to be given reduced weight or no weight.

I'm not arguing that the citation should be removed "simply I disagree with it". I've made my case based on how this source stands relative to the more-authoritative sources, and I've pointed out in detail why those other sources should be regarded as more authoritative. As to whether I can remove the citation or not, I'd say that by adding the citation, you "bolded" a proposed change to the article. I have now reverted that change, which is an acceptable thing to do (and not "disruptive"), if I read the policy correctly. Before you reinstate your proposed change, the onus is on you to seek consensus in support of it.

Regarding my having a "conflict of interest", you've made this accusation before, and it is unfounded. I have said before that I receive no royalties or other financial reward for the publication of my book, nor does any friend or relative. And Wikipedia's definition of conflict of interest offers no other possibility that applies. So please stop making unfounded accusations. J Doug McLean (talk) 00:41, 7 March 2015 (UTC)

The section introduces the Newtonian theory of lift. Berriman's treatment is a particularly clear and unabmiguous introduction. He was also an acknowledged authority who around that time was technical editor of the Royal Aero Club's official journal Flight. This makes him an excellent source to quote in this section.

There is no contradiction, as you suggest there is. Lissaman does not contradict Newton. Both he and Berriman are consistent with the view that Newton's laws apply. Both are explicit that the force on the airfoil itself derives from pressure. Lissaman points out that different control volumes yield different distributions. Berriman does not define a control volume, so cherry-picking one of Lissaman's control volumes for comparison is wholly invidious. WP:FRINGE states that "For a fringe view to be discussed in an article about a mainstream idea, reliable sources must discuss the relationship of the two as a serious and substantial matter." There can be no doubt that the Newtonian theory of lift is a mainstream idea. On the other hand, the suggestion that there is some contradiction here is not mainstream: any discussion of such a relationship between the two positions is wholly absent from all of the many mainstream sources that have ever been cited in this year-long debate. You argue for such an interpretation of Lissaman's work, but the discussion is not there in Lissaman. You wrote that, "Lissaman does not explicitly say 'This result contradicts Berriman's explanation of 1913', but a direct quantitative contradiction should be enough." The above quotation from WP:FRINGE debunks that opinion of yours.

A conflict of interest is not about money. WP:CONFLICT is clear: "Any external relationship (any secondary role) may undermine that primary role, and when it does undermine it, or could reasonably be said to undermine it, that person has a conflict of interest." Your authorship of a book, whose unverifiable claim you promote to exhaustion on this talk page, is clearly within scope of that definition.

You have had enough policies thrown at you over the months to have figured this much for yourself, you may well have been told directly anyway. Your WP:WIKILAWYERING in an attempt to return to the fray is unacceptable. If you revert my addition of a perfectly appropriate citation once more, or take any other contentious action on this topic, I will deem the time ripe to take your behaviour to WP:ANI. @J Doug McLean: THIS IS YOUR FINAL WARNING. — Cheers, Steelpillow (Talk) 11:17, 7 March 2015 (UTC)

Your arguments here are not consistent with the physics, with the sources, or with Wikipedia policy:

Berriman says that dp/dt = -L for "a stratum of air". Lissaman analyzed three rectangular control volumes: a tall vertical column, a square, and a flat horizontal slab. I chose the flat horizontal slab because it matches the dictionary definition of a "stratum" and is thus the right one to compare with Berriman. Calling this "cherry-picking" is illogical. It's also illogical to say there's no contradiction when Berriman says dp/dt = -L, and Lissman finds dp/dt =0.

Regarding conflict of interest, I don't think my having written a book some years ago constitutes an "external relationship" or a "secondary role" that in any way compromises my role as an editor. You are misreading the letter and intent of this policy. J Doug McLean (talk) 20:03, 13 March 2015 (UTC)

At the top of this section, regarding Berriman's PoV, Steelpillow says "I have found not the slightest hint or suggestion of any historical controversy...".

No controversy? In Bloor's book, which I referenced in these Talk pages back in Dec 2014, the eminent academic G. H. Bryan is quoted giving this response to Berriman's view.

"... there is no such thing as "sweep" except in Newton's ideal medium of non-interacting particles satisfying the sine squared law. ... Mr. Berriman's "sweep" is, physically speaking, an IMPOSSIBILITY. If, however, "sweep" is defined as the depth of a hypothetical column of air, the change of momentum in which would represent the pressure on the plane, then the introduction of this new quantity is ONLY A USELESS AND UNNECESSARY COMPLICATION...".

(My added emphasis. "Sweep" = vertical thickness of "firehose-stream". This quote from G. H. Bryan's 1912 review of "The Dynamics of Flight" by A. G. Greenhill, Aeronautical Journal 16:264-67.)

So Bryan describes Berriman's version of TS as an "impossibility". This is the same view that Lanchester had a decade and a half earlier. This "impossibility" is nowadays so obvious that the majority of knowledgeable authors don't even bother mentioning it.

Conclusion: TS is a "fringe theory". (Zapletal) 101.170.42.154 (talk) 00:46, 18 March 2015 (UTC)

Lest others be misled: so here we have Zapletal's CAPITALISED SPIN on Bloor's report of Bryan's comment on Greenhill's report of one aspect of Berriman's analysis. And we are expected to believe that context has been preserved? Google Books shows the quote in question: go here and search for sweep. In my search, it was the second extract returned. Bloor first writes, "The basic formula is Force = Mass x Acceleration, but how is this formula to be applied? What is the mass of air that is involved? The original Newtonian picture [streams of Billiard balls - SP] must have underestimated this mass, hence the underestimation of the lift that can be generated. Berriman suggests that ... the wing exerts an influence on 'all molecules within an indefinite proximity to the plane; in other words a stratum of indefinite depth'"

The effecive depth for practical purposes is the "sweep" of the wing. Bryan was a mathematician and theorist. He pointed out only that the idea of sweep had no theoretical utility, he was not saying that it was wrong as such.

Thus, the discussion Zapletal quotes is essentially about how to identify "the air deflected downwards" (F = ma = -dp/dt), not how to confirm or deny it. Berriman's populist book Aviation, which began this thread, does not dig deep enough to mention "sweep" (as far as I can tell) but it does acknowledge "a cyclic or eddying disturbance around the leading edge of a wing in flight".(Page 101). It was that question of cyclic vs. eddying which lay at the heart of the controversy that Bloor was leading up to, not the TS bickering perpetrated above.

Bloor later recounts how Lanchester's theory effectively merged Berriman's "sweep" with the cyclic motion just mentioned, to develop his early circulation model. No controversy here, just a steady evolution of ideas.

Strip away the spin, and what we have left is an excellent source explicitly confirming TS and setting it in context. — Cheers, Steelpillow (Talk) 10:59, 18 March 2015 (UTC)

No. What is left is a hypothesis (= TS) which is "physically speaking, an impossibility", and is practically speaking "useless and unnecessary". Lanchester most definitely DID NOT "merge" Newton's idea of sweep (which, incidentally, is valid, but ONLY valid, in the Newtonian Medium) with the entirely different idea of cyclic hydrodynamic motion. Instead, he started by thoroughly refuting the ideas of "sweep" and "air deflected downward" with his Principle of No Momentun. He then developed the entirely different Circulation Theory of Lift, which to this day gives the most accurate description of Lifting flows. The two theories are as incompatible as Flat-Earth and Round-Earth theories. (Zapletal) 101.171.85.62 (talk) 00:56, 19 March 2015 (UTC)

Momentum theorem

Here is the version proposed by J Doug McLean (talk · contribs) for the text tagged for discussion:

For an extended region, Newton's second law takes the form of the momentum theorem for a control volume, where a control volume can be any region of the flow chosen for analysis. In the analyses described below, the flow is assumed to be steady, and the effects of gravity on the flow are assumed to be negligible. Under these assumptions, the momentum theorem states that the integrated force exerted at the boundaries of the control volume is equal to the integrated flux of momentum through the boundary.

Is this an improvement? — Cheers, Steelpillow (Talk) 19:57, 20 March 2015 (UTC)

Revisions to "Momentum balance in lifting flows"

Some time ago I proposed revisions to "Momentum balance in lifting flows" and posted the proposed new version in my sandbox. These changes simplify and clarify the section and align it better with the cited sources. Having heard no objection to these changes, I have installed them. J Doug McLean (talk) 21:29, 18 March 2015 (UTC)

Doug, just one tiny quibble. The very last words of that section are "...as was first noted by Lanchester in 1907." My understanding is that Lanchester first publicly presented his ideas in 1894, in a talk given to the Birmingham Natural History and Philosophical Society. He then expanded these ideas into a paper that he sent to two (?) scientific journals, including The Physical Society of London (1897), but was rejected. Meanwhile, the German school, including Kutta, Zhoukowsky (a Russian, but writing in German journals), and Prandtl, apparently got a whiff of these ideas and starting publishing many papers based on CToL in the very early years of 1900s. Many of these papers were in-school (Gottingen) memos, so exact dates are hard to establish. Anyway, the notion that, in Lanchester's words, "the fluid ... does not gain or lose momentum any more that does a cast-iron pillar supporting a load" was probably well understood quite a bit earlier than 1907. So I would suggest just dropping the word "first" from the above quote.

Also of historic interest is that "vortex/circulation lift" was used by Rayleigh in his 1877 paper "On the Irregular Flight of a Tennis Ball". Bizarrely, this idea was NOT taken seriously at the time, being considered only a flippant speculation regarding a trivial game. It is difficult to understand (beyond belief to me!) why it took the Cambridge school another 50 years to finally accept CToL. (Well, they were ideologically wedded to the idea of "Stokes flows", where viscosity is essential...) But then again, there are still people today who resist CToL. (Zapletal) 101.171.85.57 (talk) 02:17, 19 March 2015 (UTC)

Good point. Thanks. I made the change. J Doug McLean (talk) 17:34, 19 March 2015 (UTC)

Wait a minute. I think that it would have been more collaborative if you had discussed the changes 'before installing them. Robert McClenon (talk) 13:51, 20 March 2015 (UTC)

In particular, the momentum theorem had been tagged for discussion. Deleting the tag without reaching consensus here is not acceptable. That "Having heard no objection" does not stack up.

I am also unhappy about the addition of this paragraph:

Thus it is found that the change in momentum flux from upstream to downstream accounts for the entire downward force $-L'$ exerted by the airfoil only in the case of the tall, slender rectangular control volume. For control volumes of other shapes, the integrated pressure difference between the top and bottom offsets some or all of the $-L'$ exerted by the airfoil, and the change in momentum flux is between $-L'$ and zero.

I seem to recall from earlier discussions that if the firehose is cut off at a certain point about a chord's length behind the foil, the momentum change across the boundary equals the lift, so that "only in the case of the tall, slender rectangular control volume" is untrue. However the paragraph - whether corrected or no - adds nothing to the reader's understanding. Nor has any citation for it been provided. What I do recognise it as, is essentially an extract from the long-running spin against the Newtonian statement. I am now seriously considering whether this trips the wire and I should take J Doug McLean (talk · contribs) to WP:ANI and, if need be, WP:ARBCOM for his long-term WP:DISRUPTION. — Cheers, Steelpillow (Talk) 19:38, 20 March 2015 (UTC)

Robert McClenon must be new to this discussion. As noted in the first sentence of this talk section, these changes were proposed some time ago. That was in the context of a lengthy discussion on this talk page (in January of this year) that was archived by one of the other editors. In that discussion, valid questions were raised about the the article section in question, which was authored by me and was new at the time. The technical issues were discussed at length. I suggested revisions and posted them in my sandbox. No opposition to the proposed changes was expressed by any of the participants.

Steelpillow is not new to the discussion, but seems to have forgotten its history. The article's explanation of the momentum theorem was tagged for discussion, and it was subsequently discussed at length. I went to the trouble of drafting and explaining proposed changes to address the issues that were raised. Steelpillow and everyone else had ample opportunity to comment on the proposal, but no one did. And someone took it on himself to archive that section of the talk page before a resolution was reached. I think I exercised due diligence by participating in the discussion, making a proposal, and asking for feedback. Consensus and collaboration are supposed to be a two-way street. I finally took it on myself to make a bold edit. I think my revisions are technically correct and are supported by the sources cited.

The later paragraph that Steelpillow now is unhappy with is intended as a summary of the rather technical discussion in the paragraph preceding it, which describes the results of the published and cited control-volume analyses that used the uniform-flow-plus-vortex flow model, and as it stands it accurately reflects what the cited sources say. It is clear from the context that this paragraph refers only to the findings arrived at using the generally accepted uniform-flow-plus-vortex flow model. That a counterexample can be deduced from the firehose model is beside the point and is irrelevant to this section of the article. Besides, the firehose model was intended only to reflect the total integrated change in momentum flux. Even its originators would not have claimed that it has anything correct to say about the spatial distribution of the flux. The cited classical analyses, on the other hand, do accurately reflect the spatial distribution as well as the total.

I see now that Steelpillow has reverted my changes and has asked below for comments on the part dealing with the momentum theorem. We'll see what the response is this time around. So far, I'm the only one who has made a viable, constructive suggestion as to how to fix the passage in question. Regarding the paragraph he's unhappy with, I'd argue that it's a helpful summary and should be reinstated, but that leaving it out only harms the article a small amount. His contention that it is "untrue" is unfounded, as is his accusation of WP:DISRUPTION.

J Doug McLean (talk) 23:11, 20 March 2015 (UTC)