Wikipedia talk:Requests for mediation/Lift (force)

Resolved:

Participants agreed on a resolution, as discussed here and in a separate RfC

This mediation case is closed. Please do not modify it. No further edits should be made to this case page.

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Mediation opening

@Robert McClenon, Burninthruthesky, Steelpillow, J Doug McLean, Mr swordfish, and SPACKlick: Hey all. I will be the mediator for this case. I have looked through the links provided to me, although I can't quite put my finger on the exact issue. Is it to do with the current open RfC (this), or is it a wider issue. Thanks all, Mdann52 (talk) 17:37, 1 April 2015 (UTC)

I think that the RFC is representative of the wider issue. The wider issue appears to be to what extent to present the quantitative interpretation and to what extent to present the qualitative interpretation of lift. There may be other related issues having to do with the complexity of the technical explanation. I think that some of the other editors can explain the issues in more detail. Robert McClenon (talk) 17:51, 1 April 2015 (UTC)

It is true that within the next few days, editors who are Jews or Christians may be advised to meditate on the significance of the events that are observed based on the lunar calendar. Other editors may follow their own spiritual practices if any. Robert McClenon (talk) 17:51, 1 April 2015 (UTC)

I think that a recap may be useful. There is a long-standing observation in the theory of lift that Newton's laws of motion apply. Firstly, in order to exert lift on a wing or foil, the fluid surrounding it must experience an equal downforce. A mass m of gas such as air cannot sustain such a single-direction force F and so gets accelerated downwards a according to F=ma. The momentum p changes as the gas accelerates, with the rate of change represented mathematically as dp/dt. We find that ma=dp/dt, therefore F=dp/dt. This expression of Newton's law is commonly used by engineers. If F is the lift, then the force on the air is -F and so you will sometimes find us here discussing F=-dp/dt.

An early model of lift, conceived by Newton himself, comprises streams of billiard-ball particles striking the underside of the wing and pushing it up. This and elaborations of it can explain some aspects of lift but not others such as why any particular wing has a given lift. One of the difficult questions has always been "well, exactly which air is accelerated downwards?"

Another approach was that of Bernoulli, whose analysis showed that the air above speeded up and that created a drop in pressure. This pressure model also explains some things but not others.

Later workers developed models in which the air circulates above or around the wing and these theories have gone on to underpin the theory of lift in modern fluid dynamics. A key finding was the "no momentum theorem" in which the wider analysis shows that overall, no momentum gets imparted to the air - what goes up must come down and it all cancels out. Something called "control volume" analysis draws a box round some of the air and watched how the air and its momentum flow through. Depending on the precise box chosen, different amounts of momentum are seen: in only a few special cases is it seen to be quantitatively equal to the lift.

The article on Lift (force) currently does what a lot of secondary and tertiary sources do, it introduces the two simplistic models before going on to develop a more useful analysis. Our specific bone of contention arises in editorial views on how to regard the "Newtonian" model. One school of thought is that the quantitative statement - that all the lift as such is created in reaction to the downward acceleration of air - is so widely given in secondary and tertiary sources, even mandated by one educational body, that it is important to include it. The other school regards this as technically incorrect and moreover so misleading that it should as a minimum be watered down to avoid implying all the lift. Opinions differ similarly as to which view certain primary and secondary sources really support as technically correct.

Does this help? If I have summarised the technical issue incorrectly, I am sure that someone will correct me. — Cheers, Steelpillow (Talk) 18:52, 2 April 2015 (UTC)

• Comment I agree with that summary as a general overview. SPACKlick (talk) 19:22, 2 April 2015 (UTC)

I also agree with the summary above, assuming the foil is not close to the ground. Burninthruthesky (talk) 10:03, 3 April 2015 (UTC)

Ok, thanks for the summary. So the issue is here to decide which one gets more time, or included at all? The obvious issue here is that, as there is an ongoing RfC, I don't want to overlap with that too much (it seems pointless rehashing the points made in that), so I'll try and discuss this in a different way.

One of the things I often look at as early as possible in any form of dispute resolution is a compromise. Would it be possible to include both in the article, either in the same section or different sections, explaining there are different ways of saying it (I'm not sure if or how this will be possible, just proposing it, you lot are probably more knowledgable in this and could explain how it could happen, or not as the case may be). Mdann52 (talk) 10:54, 3 April 2015 (UTC)

The issue is specifically about how to present the Newtonian model of lift (mathematically F=-dp/dt, often referred to by us as "TS" - The Statement). One camp wants to present its basic form first and then introduce the criticisms further on. As I understand the other camp (and here, I think they need to speak for themselves), they regard its basic expression as just so inappropriate that, despite its prevalence in many sources, it should not be given at all or at best presented only as a falsehood. At one time we thought we had an acceptable compromise but that broke down. The forms of words being discussed in the RfC are symptomatic of that rift. My own suspicion is that there can be no reconciliation at a technical level: we need to turn to reliable sources and to achieve some consensus on what they are actually saying as opposed to what each editor thinks they are trying to say. — Cheers, Steelpillow (Talk) 13:26, 3 April 2015 (UTC)

As stated in the RFC the question is quite specific. I'll recap it here for convenience. In the article section "Simplified physical explanations of lift ", subsection "Flow deflection and Newton's laws", there appears the passage:

Some of the air passing the airfoil has downward momentum imparted to it at a rate equal to the lift. (See "Momentum balance in lifting flows" for details) This is consistent with Newton's second law of motion which states that the rate of change of momentum is equal to the resultant force.

In article talk-page discussions this has been referred to as a "quantitative" statement (or TS, short for The Statement) because it states a mathematical equality (momentum is imparted at a rate equal to the lift, or dp/dt = -L, for short). The proposal being debated is whether to replace TS with the following qualitative statement:

The air flow changes direction as it passes the airfoil and follows a path that is curved downward. According to Newton's second law, this change in flow direction requires a downward force applied to the air by the airfoil. Then, according to Newton's third law, the air must exert an upward force on the airfoil. The overall result is that a reaction force is generated opposite to the directional change.

The case for replacement is based on three main arguments:

1) Article structure: Given its place near the top of the article, and the fact that it is billed as a "simplified physical explanation", a qualitative version is more appropriate than a quantitative one and is just as well supported by sources. The quantitative issues are subtle and are best left to the discussion farther down in the article under "Momentum balance in lifting flows".

2) The physics: The current simplified quantitative statement is misleading because it is actually untrue unless "some of the air" is defined in a specific way. The reader won't know this unless he follows the link to "Momentum balance in lifting flows" and reads it very carefully. The problem is that the "resultant force" on the air, which is required for a proper application of Newton's second law, isn't generally the same as the lift. For most of the possible ways of defining "some of the air", the pressure field produced by the airfoil exerts a significant force on the air, in addition to the force exerted on it directly by the airfoil. Thus TS is true only if "some of the air" is chosen in a specific way so that this additional pressure force vanishes, for which the only citable example is the air in a control volume whose vertical dimension is very large compared to its horizontal dimension, referred to in the talk-page discussions as the "tall sliver". This is supported by multiple citations from the mainstream aerodynamics literature.

3) The citable sources: The authoritative analyses of the momentum balance are found in the mainstream aerodynamics sources (Durand, Batchelor, and Lissaman) cited in the section "Momentum balance in lifting flows". They all use the standard, accepted flow model of a potential vortex superimposed on a uniform flow, which they rigorously show to be accurate where it is applied, which is far from the airfoil. What they show is that the rate at which momentum is imparted to the flow is affected by the pressure field and is different for different subsets of the air (control volumes of different shapes). The simple statement dp/dt = -L is thus shown to be misleading unless it is qualified. On the other side, five sources are cited in the current version in support of dp/dt = -L:

• Clancy: Here the analysis presented in support of the statement is based on the "firehose model", a model that doesn't model the flow accurately and doesn't actually substantiate the statement, but simply assumes it a priori. The "firehose model" also assumes, without justification, that the only force acting on "the air" is the force exerted on it by the foil.
• Three papers from one non-specialist journal (AAPT): Waltham also bases his analysis on the "firehose model". Swartz cites Waltham. Smith couches dp/dt in terms of m delta v/delta t, without saying how single values of m and v can be assigned to an infinite, non-uniform flowfield, unless what he has in mind is the "firehose model". All of these assume, without justification, that the only force acting on "the air" is the force exerted on it by the foil.
• Berriman: This citation is in dispute (see article talk page section "Berriman") because it makes an erroneous version of the statement, ascribing dp/dt = -L to "a stratum of air" (a horizontal layer), a body of air for which Lissaman showed that dp/dt = 0.

Conclusion: the sources in support of the simple dp/dt = -L do not meet accepted standards of aerodynamics analysis and should not be accorded the same weight as the mainstream sources, per Due and undue weight.

Steelpillow recently suggested an alternative to the "replace" proposal, which differs only by the addition of the words ", the lift," in the last sentence:

The air flow changes direction as it passes the airfoil and follows a path that is curved downward. According to Newton's second law, this change in flow direction requires a downward force applied to the air by the airfoil. Then, according to Newton's third law, the air must exert an upward force on the airfoil. The overall result is that a reaction force, the lift, is generated opposite to the directional change.

This could be the compromise we're looking for.

J Doug McLean (talk) 22:40, 3 April 2015 (UTC)

Ok, thanks, that makes it a bit clearer. What I was thinking of was presenting the 2 thoughts in the same section, under different headers (however, as the crux of the issue is going through an RfC at the minute, I think I'm going to achieve little here). Of course, my idea seems like it isn't going to work. Mdann52 (talk) 10:30, 4 April 2015 (UTC)

Sourcing

I note the issues raised above with the sourcing. In terms of on either side, without overly worrying about accuracy re. real life (ie. accurate models used), how do the sources line up in terms of supporting either point of view? It looks fairly even to me, although I may well be missing some. Mdann52 (talk) 10:30, 4 April 2015 (UTC)

There has been a great deal of disagreement about this: what the source actually said, what they meant by it, what was the context for their remark, how relevant is it anyway, what weight should we give it, etc. etc. So, while I anticipate that the answer to your question is key to unlocking this issue, it has so far proved anything but easy to settle. — Cheers, Steelpillow (Talk) 15:22, 4 April 2015 (UTC)

On the side of the Newtonian camp (changing my hat and becoming partisan), a good many secondary and tertiary sources have been found which either directly support the quantitative version or remain silent on which way they might fall. That silence should not be interpreted as consistent with the opposing claim, since no comparable secondary or tertiary sources have been shown to make that specific claim and so the silence is to all appearances an editorial one. The deeper technical textbooks are not concerned with such introductory ideas and focus on the more sophisticated models that come later in the article, so any critique to be found there belongs later in the article too. There is a fair selection of all these sources here. So far, nothing has been produced to support the anti- camp at this introductory level. — Cheers, Steelpillow (Talk) 15:22, 4 April 2015 (UTC)

I've read through all that, and I suppose it depends on what side you're on as to how you interpret those sources. To me, as an untrained amateur to the area, it looks fairly balanced in terms of sourcing, so that does not appear to be a route to go down - while we could debate the validity of the sources, as these two viewpoints are present, the existing analysis appears to be just each side rejecting the other. I note that a compromise has been mentioned above - are there any objections to using this, or at the very least discussing that here? Mdann52 (talk) 16:45, 4 April 2015 (UTC)

If a compromise is reached on the passage in question, it would surely change the game. But I am not so sure that it would change minds, and the concern must be that differences of opinion would erupt elsewhere in the article, when attempts are made to expand the compromise statement in more detail. For example if one were to ask editors whether the compromise statement were qualitative or quantitative in nature, one might get different opinions.

On the idea that each side is just rejecting the other, what else would one expect? I would suggest that there is a difference in emphasis in the arguments. The Newtonian camp are considering the sources more or less at face value. Some of us feel that the anti- camp are putting too much emphasis on truth and what the authority meant to say, leaning away from verifiability. For example the criticisms above of Clancy and Berriman are based on claims that their statements are wrong or making unjustified assumptions, not on their lacking verifiability. So I would ask that you consider our various justifications very carefully for their encyclopedic relevance. — Cheers, Steelpillow (Talk) 21:53, 4 April 2015 (UTC)

I don't think it's as simple as one side rejecting the other. The Newtonian camp accepts all the sources as true and non-contradictory. The anti- camp claims some of the sources are not good enough.

I've just added a specific quote to the article, from Anderson (a modern aerodynamics textbook), "L = time rate of change of momentum of airflow in the downward direction". He uses a similar model to Clancy's 5.15.

I found out why Clancy says, "The area of cross-section of the cylinder of affected air is 1/4πb²":

Prandtl also discovered that the hypothetical cross-sectional area, A', of the mass flow of air that is deflected downward is equivalent to a circular area of diameter equal to the wing span.
A' = πb²/4

— Perkins & Hage (1949), Airplane Performance Stability and Control p. 25

This is used in derivation of the induced drag coefficient for an eliptical wing, so it's not just an a priori assumption. Burninthruthesky (talk) 07:53, 14 April 2015 (UTC)

The firehose model is the firehose model, whether it's used by Clancy, or Anderson, or Waltham. And dp/dt = -L cannot legitimately be considered to be a result of the model because it is imposed as an a priori assumption, along with the implied assumption that the air experiences no force from the pressure field. The firehose model is definitely "not good enough" to say anything realistic about the spatial distribution of vertical velocity or of dp/dt.

The derivation of the induced drag of an elliptical wing based on A' = πb²/4 is just a secondary derivation showing that if you make the right assumption regarding the cross-sectional area of the flow affected by the wing you can get the same result as a real derivation based on either lifting-line theory or Trefftz-plane theory. The reasoning behind A' = πb²/4 is still just the firehose model, applied in 3D, and it still has all the shortcomings it had in 2D.

The anti camp's argument is that cited statements based on the firehose model should not be accorded the same weight as statements based on more-realistic models. That the firehose model makes "unjustified assumptions" is not just a "claim" of ours. It is a fact that is clear from a comparison with the mainstream sources that it is unjustified to assume that only the flow in a stream of finite depth is affected by the foil, that the effect is uniform throughout that stream, and that the pressure field exerts no force on that stream. The anti-firehose argument is not based on what the sources "meant to say", but on what the sources actually say, and our criticisms of the firehose are well supported by the mainstream sources. We are not "leaning away from verifiability". J Doug McLean (talk) 22:24, 7 May 2015 (UTC)

Compromise?

Discussion moved to Talk:Lift (force)#Compromise statement?, as it is directly relevant to the RfC. — Cheers, Steelpillow (Talk) 12:08, 7 April 2015 (UTC)

Cites as a test of consensus

With the RfC now closed and a text agreeable to people on both sides now on the table, there remains the issue of inline referencing. The current version cites six refs to support it, Doug's proposal cites two. The compromise proposal has not yet got any. Several of the current refs are explicit about the equality of lift with downward momentum change. I think we'd all agree that putting in all eight of the references would be overkill, but which ones could and should be dropped? An editor can't just remove a cite because they don't agree with it, we need a degree of consensus for any such removal. I think this question will highlight whether we have any significant bone of contention remaining with us.

First, here is the compromise text as it stood when the RfC was closed:

The air flow changes direction as it passes the airfoil and follows a path that is curved downward. According to Newton's second law, this change in flow direction requires a downward force applied to the air by the airfoil. Then, according to Newton's third law, the air must exert an upward force on the airfoil. The overall result is that a reaction force, the lift, is generated opposite to the directional change.

Here are the cites, reformatted as plain text:

Cites from Doug's sandbox

• "Lift is a force generated by turning a moving fluid... If the body is shaped, moved, or inclined in such a way as to produce a net deflection or turning of the flow, the local velocity is changed in magnitude, direction, or both. Changing the velocity creates a net force on the body.""Lift from Flow Turning". NASA Glenn Research Center. Retrieved July 7, 2009.
• Landau, L. D. (1987), Fluid mechanics, Course of Theoretical Physics, vol. 6 (2nd revised ed.), Pergamon Press, ISBN 0-08-033932-8, OCLC 15017127 {{citation}}: Unknown parameter |coauthors= ignored (|author= suggested) (help), pp. 68–69 and pp. 153–155.

Cites from the current version

• "Most of the texts present the Bernoulli formula without derivation, but also with very little explanation. When applied to the lift of an airfoil, the explanation and diagrams are almost always wrong. 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." Cliff Swartz et al. Quibbles, Misunderstandings, and Egregious Mistakes - Survey of High-School Physics Texts THE PHYSICS TEACHER Vol. 37, May 1999 pg 300 http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PHTEAH000037000005000297000001&idtype=cvips&doi=10.1119/1.880292&prog=normal
• Clancy, L.J.; Aerodynamics, Pitman 1975, page 76: "Thus the lift of the wing is equal to the rate of transport of downward momentum of this air."
• "...if the air is to produce an upward force on the wing, the wing must produce a downward force on the air. Because under these circumstances air cannot sustain a force, it is deflected, or accelerated, downward. Newton's second law gives us the means for quantifying the lift force: F_lift = m∆v/∆t = ∆(mv)/∆t. The lift force is equal to the time rate of change of momentum of the air." Norman F. Smith "Bernoulli and Newton in Fluid Mechanics" The Physics Teacher 10, 451 (1972); doi: 10.1119/1.2352317 http://dx.doi.org/10.1119/1.2352317
• Lissaman (1996), Section titled "Lift in thin slices: the two dimensional case"
• Berriman, A.E.; Aviation, Methuen 1913, Page 303: "Thus, the wing in flight continually accelerates a stratum of air downwards, and must derive a lift therefrom."
• "L = time rate of change of momentum of airflow in the downward direction" – Anderson, John D.; Introduction to Flight, 5th edition, McGraw-Hill 2004, pages=352–361, §5.19. ISBN 0-07-282569-3.

— Cheers, Steelpillow (Talk) 09:51, 14 April 2015 (UTC)

Discussion on which to keep/remove

For my part, I believe it important to give due weight to the number of sources which give the quantitative description, and to include one or two such. — Cheers, Steelpillow (Talk) 09:51, 14 April 2015 (UTC)

I would say that, if we could reach an agreement to include citations on both views, that would probably be best. Which ones will be best? Mdann52 (talk) 15:46, 15 April 2015 (UTC)

First, I note from here (since moved to Talk:Lift (force)) that J Doug McLean (talk · contribs) will be away for a few more days, and it is important to obtain his input on this issue.

Of the citaions listed, I would avoid anything too deeply learned, because at this level of the article we are dealing with a common introductory approach for the beginner, that basically means avoiding Landau & Lifschitz. The links to The Physics Teacher web site appear badly-behaved. I do not have access to the Lissaman essay, though I note that in the synopsis all three standard models (Newton, Bernoulli, circulation) are dissed. Berriman is antiquated and should only be resorted to in desperation (ahem - guess who did just that).

All of which leaves:

• NASA (from Doug's list)
• Clancy
• Anderson

I would be happy to lose any one of these if it helps. — Cheers, Steelpillow (Talk) 18:33, 16 April 2015 (UTC)

I agree. Landau & Lifshitz relates force to the difference in momentum flux passing two planes upstream and downstream of the foil. The average reader will struggle to find any direct support for the text above. I understand Lissaman's analysis is similar. Clancy/Anderson describe the same physics at some length using a simpler model, more suited to an WP:UPFRONT explanation. Burninthruthesky (talk) 07:13, 17 April 2015 (UTC)

I can live with the three citations listed above by Steelpillow, but I think different quotes from Clancy and Anderson would more directly support the new wording in the article. Here are the quotes I would choose:

Clancy (p. 76): "[The] lift force has its reaction in the downward momentum which is imparted to the air as it flows over the wing."

Anderson (pp. 356-7): "The wing deflects the airflow such that the mean velocity vector behind the wing is canted slightly downward, as sketched in Fig. 5.77. Hence, the wing imparts a downward component of momentum to the air; that is, the wing exerts a force on the air, pushing the flow downward. From Newton's third law, the equal and opposite reaction produces a lift."

J Doug McLean (talk) 19:16, 2 May 2015 (UTC)

To me, the reading of this proposed text and citations all suggest an equality between lift and momentum change, per Newton's Third Law, although nowhere is it now made explicit. But it is stated explicitly in the sources, and Doug's proposed modifications to the cites apear to have been carefully chosen - one might say censored - to avoid such explicit statement. In this, Doug's suggestion that his cites better support the proposed text is not quite right - they better support only his interpretation of the text. The sources make clear that there is a different, quantitative interpretation of major significance and this is airbrushed out by Doug's changes. I am therefore against these changes. As I have said, I will live with it if one cite must be watered down or removed to gain peace and quiet, but I cannot support the watering-down of both. — Cheers, Steelpillow (Talk) 13:13, 5 May 2015 (UTC)

If we are to quote our citations, I've no objection to the selection above from Anderson.

The Clancy quote above would provide good support for the third sentence of our proposed text. It would also support our second sentence if we include Clancy's next sentence as well: "[The] lift force has its reaction in the downward momentum which is imparted to the air as it flows over the wing. Thus the lift of the wing is equal to the rate of transport of downward momentum of this air." Burninthruthesky (talk) 13:40, 5 May 2015 (UTC)

Steelpillow wrote: The links to The Physics Teacher web site appear badly-behaved.

I do not think that has any bearing on whether to use them. They are scholarly articles published in peer-reviewed journals - that's pretty much the gold standard for cites. Like many journals, The Physics Teacher is not generally available for free; I work at a research university so I get free access to a lot of academic publications that are not generally available to everyone. If you don't have that access, the links won't work for you. But, just because something is not freely available on the internet is no reason to exclude it.

The Norman Smith article should definitely be kept as it is the first (to my knowledge) peer-reviewed article to explicitly debunk the "standard explanation" based on Bernoulli. It's a classic, must-read for any educator who's teaching about lift. The Schwartz cite is kind of a drive-by - my preference would be to keep it, but I won't fight for it. Mr. Swordfish (talk) 15:02, 5 May 2015 (UTC)

"I do not think that has any bearing on whether to use them." - of course not. But it does have a bearing on what I can say about them. I was listing the cites I wanted kept and I can hardly endorse what I have not seen. I have no problem with cites that I personally cannot verify, as long as somebody can. Based on your endorsement, I would be happy to see the Smith citation kept as well. — Cheers, Steelpillow (Talk) 17:07, 5 May 2015 (UTC)

Just to let you know, I am keeping an eye on this, although it seems you lot are working this out on your own, so my input isn't really needed :) Mdann52 (talk) 20:12, 6 May 2015 (UTC)

We've agreed to a revision of the article body that removes the explicit quantitative statement (TS) and replaces it with a qualitative version more appropriate to an introductory section titled "Simplified physical explanations ---". I have further suggested that the explicit TS be removed from the accompanying source quotes. This is not a "whitewash", as Steelpillow would have it. It is to avoid including a statement that is misleading when made in unapologetic form as I argued in my post here on 3 April. If TS is included in the source quotes, each instance should be accompanied by appropriate clarification, such as:

Other sources have shown that the mathematical equality of the lift to the rate of momentum change is only conditionally true, that is, it depends on a particular assumption as to what air is included in the calculation. See "Momentum balance in lifting flows" for details.

If Steelpillow and Burninthruthesky oppose including such a clarification, as they have before and I expect they will again, then I would suggest that they are the ones advocating a "whitewash", not I.

Smith's paper is valuable for its debunking of equal transit time, but his inclusion of an unapologetic version of TS is not helpful. And his equation F_lift = m∆v/∆t = ∆(mv)/∆t is objectionable because the variables m and ∆v have no straightforward definitions in an infinite, non-uniform flowfield.

J Doug McLean (talk) 22:14, 7 May 2015 (UTC)

I am disappointed to see the allegation still alive that I have ever somehow sought to airbrush out any criticisms. The article has a brief sub-section immediately following on the limitations of [the] deflection/turning model and I am not in any way suggesting its removal. I have also always said that the place for the main critique is further down the page in the more technical discussions. That is not whitewashing the critiisms, that is taking them seriously.

Returning to the point in hand, the simple and overriding fact is that many sources present the quantitative statement as an introductory principle and we would be failing in our duty as Wikipedians if we did not do so too. Whether it is right or wrong is immaterial. It is verifiable, and that's what matters.

[Footnote 1] I should also like to clear up one small thing, which I see as a misunderstanding by Mdann52 (talk · contribs). The "no" team are not arguing for an alternative paragraph/cites to put alongside it, they have only a censored paragraph/cites to replace it. We are not really choosing between forms of words; J Doug McLean (talk · contribs) has now made it clear that we are choosing whether or not to delete verified material from - frankly to censor - the one cited paragraph.

[Footnote 2] On Smith's equation, it might be worth clarifying that as near as makes no difference F_lift is just the lift, ∆v/∆t is the acceleration a, and mv is p so ∆(mv)/∆t is just dp/dt. Thus, Smith is writing that; Lift = ma = dp/dt, which is just the form that Doug takes such exception to.

All in all, I think this is at last bringing the heart of the dispute into the open here. — Cheers, Steelpillow (Talk) 16:46, 8 May 2015 (UTC)

Under the current proposal, quotes from two or three sources would include TS prominently and in unapologetic form. No hint of any qualification or criticism would appear until "Momentum balance in lifting flows", more than half way down in what is now a very long article. If the problems with unapologetic TS were just minor quibbles this would be okay, but in my estimation, given the fundamental nature of the problems, this burying of the caveats amounts to a "whitewash".

In case you missed it, in my post above I suggest an alternative to omitting the TS quotes, and that is to include a clarification with each instance. Another alternative would be to include a blanket clarification under "Limitations of deflection/turning". After the current first paragraph of that section we could add a new paragraph:

Note that along with the simple flow-deflection explanation some of the sources quoted above [Clancy, Anderson, Smith] also state that the lift is equal to the time rate of change of the momentum of the air. Analyses by other sources have shown that this mathematical equality is only conditionally true, that is, that it depends on a particular assumption as to what air is included in the calculation. See "Momentum balance in lifting flows" for details.

With this proposal I'm not advocating suppressing TS, but just accompanying it with the clarification it needs, to avoid misleading. So is something like this acceptable to the pro faction or not?

Yes, Smith's equation is equivalent to dp/dt = -L. But that doesn't solve the definition problem. How does Smith define dp/dt? He doesn't say. But the only rigorous way to do that in a non-uniform velocity field is by integration, which leads to control-volume analysis and calls for a realistic model of the flowfield (The firehose model doesn't qualify as realistic). The rigorous control volume analysis for a large circular volume (Durand) predates Smith by decades and found dp/dt = -0.5L. So Smith's equation gives the misleading impression that the momentum picture is simpler than it really is. It needs to be accompanied by a caveat as I suggest above. J Doug McLean (talk) 22:28, 8 May 2015 (UTC)

I am again disappointed at the continuing allegations about "burying of caveats". Let me repeat, there is no burying, there is only concern to find the right home for them. And one place they do not belong is in citation notes. Cites are there to verify the fact presented, not to discuss it. This particular thread is about the cites, so can I suggest that we keep it that way and address your question about the caveats in a different thread. — Cheers, Steelpillow (Talk) 17:08, 9 May 2015 (UTC)

This discussion is about the cites, in particular whether to keep the unapologetic TS quotes or not, and if they are kept, whether to accompany them with a caveat. The caveat question is directly related to the cites, so I see no need for a separate thread.

The idea that "cites are there to verify the fact presented, not to discuss it" makes sense if the "fact" in question is a simple one that doesn't need qualification, or if the citations provide all the needed qualification. Our TS quotes are different. TS is actually false unless a particular assumption is made, so TS in the unapologetic form in which it appears in these quotes is misleading. In this situation, I think that including a caveat in the citation notes, referring the reader to a much later section of the article, is perfectly appropriate. I've also proposed the alternative of putting essentially the same caveat close by in the "Limitations of deflection/turning" section. In either case, my position is that if the quotes are included at all, the caveat should appear in the same prominent position in the article, not "buried" several thousands of words later.

I have explained my concerns and made what I think are constructive suggestions for addressing them. If you oppose my suggestions, please give your reasons and suggest constructive alternatives. J Doug McLean (talk) 16:52, 12 May 2015 (UTC)

I have done so above. It is pointless to repeat myself. Now it is somebody else's turn. — Cheers, Steelpillow (Talk) 20:02, 12 May 2015 (UTC)

"[The] lift force has its reaction in the downward momentum which is imparted to the air as it flows over the wing. Thus the lift of the wing is equal to the rate of transport of downward momentum of this air." (Clancy). This is both verifiable, and true, with the qualification provided. Burninthruthesky (talk) 20:36, 12 May 2015 (UTC)

I don't see anything untrue in the quote from Smith in the current article, either. Burninthruthesky (talk) 21:02, 12 May 2015 (UTC)

By "qualification" I meant a stipulation as to what body of air "the air" refers to. I see no such stipulation in either the Clancy quote or the Smith quote. Given the actual wording, I'd say "the air" or "this air" could reasonably be interpreted as anything, including the atmosphere as a whole.

Burninthruthesky, when you say that the Clancy quote is "true, with the qualification provided", what words, specifically, are you interpreting as providing "qualification"? And in view of this "qualification", for what body of air, specifically, is the statement true? J Doug McLean (talk) 17:28, 13 May 2015 (UTC)

Smith clearly refers to air which is subject to a force of F_lift as it is accelerated downward by the force from the wing.

Clancy refers to a volume of air which has momentum imparted to it "as it flows over the wing".

I'd say "the air" or "this air" could reasonably be interpreted as anything, including the atmosphere as a whole. As I already mentioned, we all know the net rate of change of momentum of the atmosphere as a whole is zero. Published expert opinion is that this fact is "perfectly simple and obvious" (Lanchester p. 398).

These authors have chosen their words as they see fit. There is no evidence that anything the sources actually say are incorrect. Burninthruthesky (talk) 20:53, 13 May 2015 (UTC)

I'm talking about what the quotes in question actually say, not what we know from other sources such as Lanchester. And based on what the quotes actually say, neither of the "qualifications" you cite does anything to narrow down what might be meant by "the air" or "this air".

In the case of Smith's quote, the fact that "the air" has a force F_lift exerted on it does almost nothing to define what air he's talking about. F_lift is exerted on the air at the foil surface, which is just one boundary of any body of air that surrounds the foil. Identifying one boundary of a body of air doesn't define what air you're talking about. I stand by my previous assertion that based just on the words in the quote, "the air" could logically refer to any body of air that completely surrounds the foil, up to and including the atmosphere as a whole. The same goes for Clancy's quote and the air "as it flows over the wing".

TS has been shown to be false if "the air" is defined as a circular volume, a square volume, a pancake volume, or the atmosphere as a whole. What is actually said in either Smith's or Clancy's quote doesn't rule out any of these possible interpretations of what might be meant by the air, and so both quotes are open to possible false interpretation. The fact that TS is false for the atmosphere as a whole may be obvious to Lanchester and other experts in the field (and to us), but we shouldn't expect it to be obvious to readers of the article.

Your answers to my question support my argument: Neither of these quotes adequately defines what is meant by "the air", and qualification is needed. J Doug McLean (talk) 00:56, 14 May 2015 (UTC)

I never said Smith or Clancy precisely defined the body of air under discussion. Clancy, at least, is discussing a model which represents only the relevant deflection/force. We know only from other sources that in a realistic flow, such a body of air can be identified and that the cited facts can be rigorously proven.

None of the sources say that "the whole atmosphere" is deflected downward or flowing over the wing. IMO, this misinterpretation is clearly contradicted by the language in the sources. Surreptitious switching of an author's premise for one that makes their argument ridiculous is a gross misrepresentation of the source and an insult to the cited authors. This is completely unacceptable. Burninthruthesky (talk) 04:45, 14 May 2015 (UTC)

I think all this pedantry about "the air" is a red herring. Somewhere in previous discussions is a historical quote saying that the problem has always been to identify the particular air affected. But the principle, that air must be deflected downwards in equal measure to the lift according to Newton's laws, was not in dispute. It is rather trite to remark that if one chooses the "wrong" air, then its rate of momentum change will not equal the lift. Since the "right" air is so hard to find, one must expect all those easily-analysed shapes to be "wrong". Even finding a volume for which TS is true does not guarantee the uniqueness of the solution, it may just be a mathematical coincidence. This is not evidence against TS, it is evidence consistent with it. Likewise, any general principle such as TS or Bernoulli's, or circulation theory for that matter, is likely to be too crude to apply directly and will need refinement in practice. All this is going way beyond the citations being discussed here to verify a brief introduction to the general principle. If a complex and sophisticated theory must be bottomed-out in order to agree a cite for a simple statement of principle, there is something very wrong going on in this discussion. — Cheers, Steelpillow (Talk) 09:07, 14 May 2015 (UTC)

Burninthruthesky's response here seems to me to flout simple logic. He is conflating not saying X with contradicting X. The sources don't say that "the air" refers to the whole atmosphere, but there is nothing in the quotes that rules out (or contradicts) that interpretation. That "the air" could refer to the whole atmosphere would be a perfectly reasonable interpretation for a non-specialist reader to make, and it is not at all "contradicted by the language in the sources", at least not in the language in the quotes themselves..

I've in no way misrepresented what these sources say. I just maintain that their presentation of TS, as embodied in these quotes, is insufficiently specific and thus open to false interpretation. Yes, I'm criticizing their presentation of TS for lacking technical rigor, but I'm not making a personal attack.

Steelpillow's response misrepresents the physics issue. The "air affected" and the air for which dp/dt = -L are not the same thing. And the "principle, that air must be deflected downwards in equal measure to the lift" is in dispute. This is not a general principle. The real principle, Newton's second law, is that air must be deflected downwards in equal measure to the net force acting on it. The net force acting on the air is not generally equal to the lift, except in the limiting case of the tall, slender control volume. This in not "pedantry" or "refinement" or a "red herring". It is just the rigor required if one is to apply NII correctly. Even in a "brief introduction to the general principle" we should represent that principle correctly, and not oversimplify it in a way that is potentially misleading. Unapologetic TS is not "a simple statement of principle"; it is a sloppy and misleading statement unless it is accompanied by proper clarification.

If I'm not mistaken, Mr. Swordfish, as of last December, agrees with my take on unapologetic TS. I'd like to hear what he has to say now about my proposal to include a caveat, either with the cites themselves or under "Limitations of deflection/turning". J Doug McLean (talk) 23:21, 14 May 2015 (UTC)

'That "the air" could refer to the whole atmosphere would be a perfectly reasonable interpretation for a non-specialist reader to make, and it is not at all "contradicted by the language in the sources", at least not in the language in the quotes themselves.' Yes it is. Clancy clearly contradicts the idea that, "[The] lift force has its reaction in the downward momentum which is imparted to the whole atmosphere as it flows over the wing." Given that no wing is of comparable size with the whole atmosphere, this reinterpretation of what he wrote is absolute nonsense. We all agree such a statement is untrue, yet you keep trying to convince us that Clancy may have actually meant to say something so ridiculous. If that isn't a personal attack, I don't know what is. I've no doubt all the sources under discussion have been extensively reviewed before publication. If the mainstream literature is wrong, Wikipedia is not the place to right it.

If evidence is presented of a genuine flaw in the article I will happily discuss it. For the time being, I have nothing further to add. Burninthruthesky (talk) 05:25, 15 May 2015 (UTC)

I've looked through all this, and there appears to be some arguement over how precise we should be. As Wikipedia should just give an overview, as opposed to being too technical, could I propose the suggestion to use "the air" instead of being too precise would be the best option here, as opposed to having a long debate about the merits or not of various contradicting counter-proposals? Mdann52 (talk) 06:57, 15 May 2015 (UTC)

Burninthruthesky has misread what I've been saying. I have not said or implied anything about what Clancy or Smith meant to say or what they did or didn't understand about the physics. His allegation of a personal attack is unfounded.

What's important here is what the quotes in question actually say. The quotes state that dp/dt = -L for "the air", or "this air", or in a longer quote in Clancy's book, "the air as it flows over the wing". None of this is sufficiently specific about the size, shape, or location of the region of air to guarantee that the statement (TS) is true. And this is where what we know from other sources legitimately comes in: For TS to be true, the region of air must have a particular shape. This is not a matter of "how precise we should be", as Mdann52 would have it, as if we were homing in on the right answer to within the last few percentage points. It is a matter of being specific enough to avoid implying that dp/dt = -L for subsets of the air where it is actually -0.5L or even zero.

Mdann52 suggests that we should avoid being too "precise" or detailed. That was my suggestion as well in my earlier proposal to choose different quotes from these authors, quotes that make the statement in qualitative terms rather than in the quantitative terms that I find so problematic. But that proposal was rejected by Steelpillow and Burninthruthesky.

If we keep the quantitative quotes, as they insist, then basic technical accuracy requires that they be accompanied by clarification (Note that "accuracy" is not the same as "precision"). Steelpillow has already rejected including clarification in the citation notes, so I hereby propose that the following new paragraph be included in "Limitations of deflection/turning":

The rate at which downward momentum is imparted to the flow depends on what air is included in the calculation and is thus a more complicated issue than simple explanations of this kind indicate. See "Momentum balance in lifting flows" for details.

This is short and to the point, doesn't directly criticize any of the sources, and would be unobtrusive in the location I'm proposing. I think the only reasonable objection to it that anyone could make would be if it were untrue, and in light of the sources cited in "Momentum balance in lifting flows", I don't think anyone can make that argument. So would this be an acceptable compromise? J Doug McLean (talk) 18:21, 16 May 2015 (UTC)

I find this propsal convoluted and unnecessary, for the reasons given by Mdann52. It has other gross problems which would require more of the same to fix, but I'll let that pass. Because fundamentally, it is just batting the dispute back into the content in a local ping-pong match: do we resolve our differences in the content or the cites? The debate began in the article, moved to the talk page for a long time where it eventually grew an RfC, then moved here where the outcome of the RfC duly followed it and it proceeded to move to the cites for that outcome. Now, it is threatening to be batted back to reopen the content discussion which got closed by the RfC. When I opened this discussion on the cites, I suggested it might prove an acid test of our differences. I believe that this has been amply confirmed. — Cheers, Steelpillow (Talk) 20:58, 16 May 2015 (UTC)

In my view, a reasonable interpretation is an honest belief the author may have intended that meaning, but I appreciate the acknowledgement that the authors understood what they were saying. I cannot imagine a hypothetical reader who is aware of how to perform a control volume analysis of momentum flux, yet is simultaneously ignorant of basic mechanics to the extent they might believe the whole atmosphere is accelerated downward. Nor can I imagine any reader who would find the caveat above helpful rather than confusing. Adding a statement to a "simple explanation", just to point out there is also a more complicated one, is redundant and patronizing.

Clancy's qualification of the air clearly excludes air before and after it "flows over the wing", but suggests no limitation on the height of this air. His description is entirely consistent with the tall sliver we have discussed repeatedly. Looking again at the source, I see he only goes on afterwards to describe the simple model which is objected to. The quote under discussion is not based on this model, as implied earlier. Clancy evidently gave his explanation some thought, and I believe he's done a better job than we have. We should be focussed on improving our own presentation rather than criticizing the sources. Burninthruthesky (talk) 09:27, 18 May 2015 (UTC)

Steelpillow and Burninthruthesky seem to be straining here to find reasons to oppose my simple proposal.

"Convoluted"? Please. This is a simple, straightforward way to clarify a statement that is not as simple and basic as it seems.

"Unnecessary"? That's obviously a matter of opinion here. I think it is necessary. My proposal is intended as a compromise. Keeping the TS quotes is a compromise for me, and I'm asking you to compromise by agreeing to include the caveat even though you consider it unnecessary, unless you have a stronger objection to it. It is true, it is supported by sources, and it is just two short sentences. Or are you simply unwilling to compromise on this issue?

Burninthruthesky's hypothetical reader is very different from mine. In my post of 14 May, and in my earlier ones, I was referring to a non-specialist reader. I think my concern is valid that such a reader would be likely to arrive at an erroneous interpretation of unapologetic TS. My proposed caveat might be redundant (not patronizing) to an expert, but not to a non-specialist. And I'd expect non-specialists to be the vast majority of our readers.

Sure, Clancy's quote is consistent with the tall sliver, but that's not good enough. It's also consistent with a circle, or a square, or any compact volume confined to the neighborhood of the foil, for any of which TS is false. TS needs more qualification than the quotes contain, and "improving our own presentation" is precisely what my proposal aimed at. J Doug McLean (talk) 19:15, 19 May 2015 (UTC)

Applying policy to the truth

Following on from the above thread, I'd like to look at our differences from an encyclopedic perspective. The argument above about "the air" was not about the content, nor even about the quotations in the cites, it was actually a technical rabbit-hole designed to expose the perceived "truth" or otherwise of the quantitative statement. We should not be being dragged down it; Wikipedia does not give a damn about what we editors argue to be correct, only about what we can verify as having been said. The quantitative statement can indisputably be verified. And so one strand of debate becomes, do we need to caveat that, right in the introductory section, whether as content or cites? I'd like to answer this question and settle the level of information to be provided before I pursue any further what that information might be. I would submit that at this stage Wikipedia's policies and guidelines give a resounding "No" to either the embellishment or the sanitisation of the basic principle. Setting aside our irrelevant opinions about technical truths, does anybody have a problem either with Wikipedia's policy or with my understanding of it? — Cheers, Steelpillow (Talk) 20:58, 16 May 2015 (UTC)

Not at all, I think that's perfectly reasonable. There are sometimes occasions when we have to correct obvious mistakes from the sources. This simply isn't one of them. Burninthruthesky (talk) 09:27, 18 May 2015 (UTC)

If the TS sources were our only sources on this topic, and if the version of the truth that I'm pushing were just OR, I'd have to agree that your understanding of policy is correct. But that's not the situation we face, and I don't agree.

We have two groups of sources, one giving us a "simple" explanation (unapologetic TS), and the other giving us more rigorous analyses based on circulation and control-volume theory. These two sets of sources give us two very different pictures of the momentum balance in lifting flows, and the rigorous ones clearly show that there severe limitations to the truth of the "simple" ones. In a case like this, where truth is supported by citable sources, Wikipedia policy does not say that truth is "irrelevant".

Unapologetic TS is not completely wrong, but it is oversimplified to the point of being wrong much of the time. The problem is that unapologetic TS is not a proper simplification of the correct theory. Ordinarily, a "simplified" physical theory is one that ignores small effects and still gets an answer that is close to correct. The firehose model (the only actual analysis presented in support of TS) isn't based on this benign kind of simplification. The firehose ignores the force exerted on the air by the pressure field. For any subset of the atmosphere (control volume) other than the tall sliver this is not a small effect, and for these other subsets of the atmosphere TS is far from the right answer. Yes, TS "can indisputably be verified", but its limitations are also indisputably clear from the other sources, and we won't be doing right by our readers unless we include clarification nearby, as in my compromise proposal in the previous thread. This is not an "embellishment". It is just what clarity requires. J Doug McLean (talk) 19:17, 19 May 2015 (UTC)

The idea that a near-century of text book writers should have polarised into conflicting elementary and advanced theoretical camps without anyone noticing is frankly untenable. For example Clancy's treatment progresses quite happily and consistently through all of Bernoulli, Newton and Circulation theory. As Burninthruthesky says, there is no verifiable mistake to correct. — Cheers, Steelpillow (Talk) 20:54, 19 May 2015 (UTC)

You're making a straw-man argument here. I never claimed that the aerodynamics community is polarized into "conflicting elementary and advanced theoretical camps". No one since the 1920s has regarded the firehose model as an actual "theory", so the idea that there could be two "theoretical camps" on this topic is absurd. Authors since the 1920s who use the firehose are not proposing it as a "theory"; they are merely using it as a simplified way to illustrate how NII works in a lifting flow. Too simplified, as I have argued and as the more rigorous published analyses clearly show: The firehose model doesn't just neglect small effects; it neglects large ones. The authors of the rigorous control-volume analyses don't bother to explicitly rebut the firehose because none of them would take it seriously as something that needs to be rebutted.

So if you find it implausible that a subset of aerodynamics text-book writers might err on the side of oversimplification in one of their explanations, I think you're being naive.

You don't seem to have read Clancy very carefully. Remember that our topic here is the dp/dt associated with lift. Clancy deals with that topic only in section 5.15, where the only analysis he gives is based on the firehose. He does not deal with the connection between lift and vertical momentum anywhere else in the book, that I can find. He does deal with circulation, but his derivation of Kutta-Joukowski uses only the pressure and velocity at the surface itself and does not use the momentum balance in the field. This is a valid choice, but it means that with regard to dp/dt in the flow he does not "progress" through anything other than the firehose.

But these arguments are beside the point. I know that you and Burninthruthesky hold a different opinion from mine regarding the place of TS in the aerodynamics world. I don't expect you to agree with me, nor am I going to try further to convince you. But I am trying to get you to agree to what seems to me to be a reasonable compromise, i.e. to include my proposed brief caveat in "Limitations of deflection/turning". Look again at what it actually says, separately from the more detailed issues we disagree on. You can't deny that the simple caveat is true, and you can't deny that what it says is supported by the sources on the control-volume analyses. Also note that it doesn't say or even imply that anyone made any "mistake"; it simply points out that there are complications associated with dp/dt that are not mentioned in the simple explanations, and it refers the reader to a later section that deals with the issue. You've objected that this is "unnecessary", but I don't think that's enough. Given our difference of opinion on this, I think the onus is on you to show that my proposal is actually objectionable rather than just unnecessary, and you haven't done that. I'm just asking you to agree to include a simple and quite neutral clarification, even if you think it's unnecessary. So, are you willing to make this small compromise or not? J Doug McLean (talk) 16:06, 22 May 2015 (UTC)

Mentioning there are issues with this is likely the best solution - however, how we word this is likely going to be more important. If we can't agree on the wording proposed previously, are we going to be able to come up with an alternative, or not? Mdann52 (talk) 16:20, 22 May 2015 (UTC)

@Mdann52:, I am wondering whether the encyclopedic ground rules here are clear yet. On what grounds should these issues be mentioned? Until we are clear on the ground rules for inclusion, we cannot possibly be clear on the wording those rules might allow or forbid. I opened this thread by suggesting that WP:POLICY and friends, combined with the lack of appropriate sources, effectively rules out such a mention at this stage of the article. If I have gone wrong in this, can you perhaps answer the question I posed and clarify where and how I have missed the plot? — Cheers, Steelpillow (Talk) 11:56, 24 May 2015 (UTC)

I think I've shown there are ample grounds for mentioning this issue at the point in the article I'm proposing. Steelpillow's move to turn this into a WP:POLICY issue is based on the premise that there is a "lack of appropriate sources" supporting my proposed addition. I think this premise is false. My proposed caveat simply says that the "rate at which downward momentum is imparted to the flow depends on what air is included in the calculation and is thus a more complicated issue than simple explanations of this kind indicate". Here, "simple explanations" refers to the ones in the article section "Flow deflection and Newton's laws", for which several sources are quoted stating that the rate is -L. In the absence of qualification, the implication is that this is the only rate to be found. My caveat refers the reader to the later section "Momentum balance in lifting flows", which describes published analyses from several other sources that find rates of 0, -0.5L, and -L depending on the shape of the region of air included in the calculation. Most of these sources report only one of the analyses and one of the rates each, but one of the sources (Lissaman, 1996) reports all three rates. In any case, saying that the rate depends on what air is included in the calculation is quite a reasonable paraphrase of what these sources say. So I disagree with the premise that my proposal lacks "appropriate sources". On what basis can Steelpillow claim that the sources cited in "Momentum balance in lifting flows" don't count in this regard?

To address Mdann52's question, I'm open to alternate wordings, provided they effectively convey the caveat I'm proposing. J Doug McLean (talk) 23:08, 24 May 2015 (UTC)

My reply to Doug's question has already been given. In essence, analyses based on choosing the wrong air abound and, however well sourced, are irrelevant to the physical principle being introduced here. No sources appropriate to this introductory level have yet included discussions of caveats. The burden is not on me to prove a negative - that there is no suitable source - but is on the claimant to produce and justify such a source. If anybody thinks they have found one, please air it here so we can all see what we are talking about. — Cheers, Steelpillow (Talk) 08:12, 25 May 2015 (UTC)

This discussion was a WP:POLICY issue before Steelpillow and I joined it. I started presenting evidence as such in my second post to it, on 12 August 2014. We can point to policy and sources and ask for them to be respected until we're blue in the face, but we can't uphold policy alone. That's why we are here. Burninthruthesky (talk) 08:18, 25 May 2015 (UTC)

I've argued that the quantitative statement dp/dt = -L isn't appropriate to the "introductory level" of this section of the article in the first place, but I've been overruled. Given that quotes making the unapologetic quantitative statement are to be included in this section, all the other sources that deal quantitatively with dp/dt in a lifting flow become relevant and appropriate. The classical analyses are the authoritative treatments of this question, and your characterization of them as "choosing the wrong air" isn't consistent with the physics or the sources. I believe I've "produced and justified" ample sources; you just don't agree with me.

And I don't expect you to agree with me on this point. Again, what I'm asking is for you to agree to include a simple two-sentence caveat based on analyses that you admit are "well sourced". I'm not even asking for it to be included in the subsection with the explanation itself, but in the subsection listing "Limitations". What the caveat calls attention to is certainly a "limitation" of TS, and what better place is there to include it and to alert the reader that the topic is discussed in greater detail later? So one last time: Are you willing to compromise or not? J Doug McLean (talk) 02:33, 26 May 2015 (UTC)

This is full of logical flaws. First, mention of a principle in an introduction does not mean that "all the other sources that deal quantitatively with dp/dt in a lifting flow become relevant and appropriate" at that same introductory level. Far from it, that's why we have introductory remarks. Then, you misinterpret my remark about "choosing the wrong air". It is wrong only with respect to what this discussion is trying to establish. The choice of which air to analyse is not determined by the physics, it is an arbitrary choice by the engineer. The complex physical analysis can only begin after one has chosen one's control volume to analyse, and I think you will find the sources consistent with that. Again, the section on limitations already mentions some that have been agreed among us all and properly sourced. But it is not sensible to add another that has never been reliably sourced. I tried to find those sources you say that you have produced in the past. I could find nothing that stacked up, nor even identify which ones specifically were supposed to be supporting your position. Yet again we have long essays full of vague pleadings but with no substance. But rather than wait up, as I have done for a year or so, I will offer a counter-example. — Cheers, Steelpillow (Talk) 07:16, 26 May 2015 (UTC)

The outcome of the RFC was to combine the two competing texts suggested. Steelpillow has, very generously, suggested a compromise text that is identical to the one written by J Doug McLean, except for the addition of two words. Under policy, this text, and any other text we write, must of course be cited. The suggestion he is not "willing to compromise" is demonstrably untrue and could be taken as a personal attack.

I have given up challenging every misrepresentation as it arrives, but this one has now been repeated more than once. Burninthruthesky (talk) 09:14, 26 May 2015 (UTC)

Logical flaws are in the eye of the beholder. You see them residing solely in my arguments, and I see them residing solely in yours. My proposed caveat is based on the fact that dp/dt is not equal to -L in the general way implied by TS, but only for a particular choice of control volume. This fact is supported by every source that finds a result different from -L, i.e. Durand, Batchelor, Lissaman, and even Lanchester. I disagree with your assessment that this "has never been reliably sourced" and that it is "without substance".

If TS accurately conveyed the essence of the "basic principle", and my sources were merely adding refinements, your argument might have some force. But these other analyses are part of the essential picture, not refinements. The rotor-disc "counterexample", BTW, doesn't work. See below.

I'm happy with the compromise we reached on TS itself in the article text. The compromise we're debating here is concerned with clarifying what is said in the citation quotes, which is a related but separate question. I'm not suggesting anyone is not "willing to compromise", I'm simply asking if he is willing or not. Simply challenging someone with a question is not a "personal attack". J Doug McLean (talk) 19:42, 26 May 2015 (UTC)

That is a loaded question, implying "do as I say or you're mean". No matter how willing we may be to compromise, we must work within the rules of the encyclopedia. We cannot present opinion as fact.

Whether something is "complicated" is a matter of opinion. I wouldn't describe the fact that dp/dt = 0 for the whole atmosphere as "complicated", neither did Lanchester. It is a separate description of a separate quantity. The only thing it has in common with the text under discussion is that it describes a rate of change of momentum.

As soon as those words are mentioned, reason seems to disappear from this conversation. J Doug McLean previously suggested including the quotation, "Clancy (p. 76): [The] lift force has its reaction in the downward momentum which is imparted to the air as it flows over the wing." Clancy's next sentence does nothing more than to emphasise the application of Newton's second law already made implicitly in his above quote. Both sentences are equally correct; and that is a verifiable fact, not an opinion. Burninthruthesky (talk) 20:17, 26 May 2015 (UTC)

Your reasoning here defies common sense. The picture "dp/dt due to lift depends on the shape of the control volume because of the influence of the pressure field and can range from 0 to -L" is indeed more complicated than the unapologetic "dp/dt = -L for the air". That's just a common-sense meaning of "more complicated", and the facts behind this assessment are well sourced. This is not just an "opinion".

Anyway, I'm tired of trying to rebut such arguments, and I'm leaving Wikipedia. It's been interesting trying to work with you. J Doug McLean (talk) 17:25, 27 May 2015 (UTC)

The rotor disc

It might seem odd to introduce the complexities of a rotating wing into a discussion about the basics, but bear with me. Actuator disc theory treats the whirling rotor as a simple disc which pushes air downwards. For simplicity, we may remove boundary effects by enclosing (or imagining) it in a duct. The disc sucks air in from above and blows it out underneath. This is the bit addressed by the theory of lift. The high-pressure air beneath then spreads out, turns back up and gets sucked in for another cycle. This bit - and this point is key - is not addressed by the theory of lift, it is a side effect.

So, which is "the air" that is deflected downwards? Clearly, it is the flow passing down through the duct. When we do the analysis, we find that half its final momentum is imparted as it is being sucked in under steadily falling pressure. It then passes through the disc, gaining in pressure but not in momentum, before being accelerated downwards by the pressure behind it and having the other half of its momentum imparted in the process. The total momentum imparted turns out to be equal and opposite to the lift: L=−dp/dt.

Now, if we want to we can say, "Oh, it's more complicated than that. I want to take a control volume that is a funny shape and spinning round with one of the rotor blades," and surprise surprise we will get a different amount of momentum passing through our control volume. That is, of course, not because the theory is wrong but because it is a different control volume. From the point of view of deriving the net lift and momentum created by the rotor, it is the wrong choice, although if one wishes to study some particular aspect it may well be the engineer's choice that morning. In particular, if the engineer is interested in the upflow outside the disc then upwards momentum will complicate the picture. But that upflow of course has nothing to do with the source of the lift.

Note too the interplay between momentum and pressure in the functioning of the actuator disc. There is no, "Oh, you have only analysed the one, you haven't taken the other into account." In order to explain either, one finds oneself also accounting for the other. This is a point already made - and reliably sourced - in the article.

Oops, but what about sourcing my own little example? I am not widely-read on helicopters but I can offer the well-known Sikorsky helicopter company's own in-house summary: Montgomery, John R.; Sikorsky Helicopter Flight Theory for Pilots and Mechanics, Sikorsky (1964).

So I hope you can now see why I chose this example: it separates out the specific issues we have been discussing in a clear and physical way that is easy to visualise, and it is verifiable. Any discussion of the lift on a single aerofoil may lose that clarity of visualisation, but it must remain consistent with the result. It is my view that J Doug McLean's arguments do not, and that this is why his sources on this particular point never stack up. But he has not recognised this. So, as Burninthruthesky says above, here we are. — Cheers, Steelpillow (Talk) 07:16, 26 May 2015 (UTC) [copyedited 14:51, 26 May 2015 (UTC)]

Your understanding of the hovering ducted rotor is correct. As long as the control-volume boundaries are far enough from the inlet and outlet faces of the duct, the lift is accounted for entirely by momentum flux, regardless of the shape of the volume. TS would be quite a reasonable statement for this case because it is true for any control volume that is large enough vertically. If TS were being applied only to this case, we wouldn't be having this debate.

But this case doesn't work as a "counterexample" applicable to wing/airfoil lift because the two cases differ fundamentally in the behavior of the pressure in the far field. In the case of the rotor, the pressure force at the outer boundary vanishes with increasing distance regardless of the shape of the volume. For the wing/airfoil, the pressure force at the outer boundary vanishes only for the tall sliver control-volume shape, a qualification that I maintain needs to be stated.

Why are the two cases different? The first difference is that between a powered jet and passive lift. The downdraft below the exit of the rotor duct is a powered jet that remains concentrated. The idealized theory is a reasonable model for the real flow, and in the theory the jet is separated from the quiescent surroundings by a sharp slipstream boundary, or "slip-surface". The jet has a different Bernoulli constant from the surroundings and a large velocity difference, but no pressure difference. For a rotor out of ground effect, the pressure disturbances in the farfield are negligible compared with the momentum flux in the jet.

The second difference is between zero and non-zero free stream velocity, and the implications that has for the pressure differences. In the case of a wing/airfoil in a stream, the pressure disturbances and velocity disturbances in the farfield decay with the same exponent (r^-1 for a 2D airfoil and r^-2 for a 3D wing). In either 2D or 3D, to make the integrated pressure effect vanish relative to the integrated momentum flux (so that TS is true), it is not sufficient simply to make the control volume large; it is also necessary to give it the slender-sliver (or slender vertical slab) shape.

So the momentum-balance issues in the flow around a wing/airfoil are fundamentally different from those in the flow around a hovering rotor. I don't have a source at hand for the comparison as a whole, but the details on the cases I'm comparing are backed by numerous sources. J Doug McLean (talk) 19:48, 26 May 2015 (UTC)

Really? We are being asked to take on trust that sources exist which explain why L = −dp/dt is valid for a wing while it is spinning but not when it is flying straight. No, there are many verifiable differences between the two situations, but that is not one of them. — Cheers, Steelpillow (Talk) 09:56, 27 May 2015 (UTC)

Some of the air

J Doug McLean, if you are still reading, I am sorry to hear of your decision to leave. Your constructive input has been valuable, and will be welcome should you decide to return in future.

The main difference of opinion between us has been whether or not it is reasonable for a non-specialist reader to misinterpret these sources as implying "dp/dt = -L for the air" where "the air" is an arbitrary control volume.

On 18 May, I gave a reason why Clancy's language is consistent with a control volume of infinite height. Our current article says, "some of the air passing the airfoil". To me, this suggests a finite height. For that reason, and to avoid confusing the average reader, I suggested in the RFC the removal of the words "some of". Any comments? Burninthruthesky (talk) 09:19, 28 May 2015 (UTC)

Where are we?

Now that the main critic of the simple Newtonian model has left this discussion, is there any issue left to resolve now? — Cheers, Steelpillow (Talk) 14:18, 29 May 2015 (UTC)

The only issue I have with the current wording of the article is the one I raised above. But it's only a matter of two words. I won't push for it if nobody shares my concern. Burninthruthesky (talk) 14:53, 29 May 2015 (UTC)

I have taken the liberty of implementing the agreed outcome of the RfC. It replaces about one and a half paragraphs with a single, shorter one, including removal of the phrase "some of the air", which nobody was happy with. I consolidated the huge string of cites, then began trimming out some of the surplus, especially those which wandered off the focus of verifying the Newtonian basis for the deflection, got too full of jargon or, like Berriman, are a hundred years old. I am sure there is more trimming to be done there, but I see no reason for this mediation process to oversee it. — Cheers, Steelpillow (Talk) 15:00, 30 May 2015 (UTC)

Thank you, and many thanks for your patience and skill in improving the quality of discussion.

As far as I'm aware, there is now no dispute between active editors. Burninthruthesky (talk) 20:34, 30 May 2015 (UTC)

For lots of reasons I am now taking a long Wikibreak. If this issue turns out not to be wrapped up after all, drop me a PM via Wiki email and I will come back to help do so. — Cheers, Steelpillow (Talk) 09:12, 6 June 2015 (UTC)

@Steelpillow and Burninthruthesky: can I take it that either of you is effectively "withdrawing", and as the main issues are resolved, I am no longer needed and this can be closed? Mdann52 (talk) 19:32, 7 June 2015 (UTC)

In view of his greater experience with this kind of situation, I would be interested to hear Robert McClenon's view on this. Burninthruthesky (talk) 20:01, 7 June 2015 (UTC)

User:Mdann52 is a more experienced mediator than I am. If the editors who are still participating believe that they no longer have an issue, I would agree that this can be closed. Robert McClenon (talk) 03:32, 10 June 2015 (UTC)

@Robert McClenon:I know you have much greater experience in these matters than I do, and I appreciate your guidance through this process.

@Mdann52:As far as I'm concerned, there is nothing else to discuss here, so I've no objection to it being closed. Thank you for your assistance. Burninthruthesky (talk) 06:55, 10 June 2015 (UTC)

Yes, that's fine by me. I'd echo my thanks to all who helped out here. — Cheers, Steelpillow (Talk) 11:56, 10 June 2015 (UTC)

The discussion above is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.