Talk:Lift (force)

Lift force: New Theory of Flight

The following information to the reader is being removed by Dolphin51

1. There is no commonly accepted explanation of the generation of large lift at small drag of a wing as expressed as late as 2020 in Scientific American as “No one knows what keeps planes in the air”.

2. Any reference to the peer reviewed published work New Theory of Flight, Journal of Mathematical Fluid Mechanics, 2017, by Hoffman and Johnson, which offers a new explanation, is being removed.

What is the motivation to hiding 1 and 2 from the public? SecretofFlight (talk) 07:00, 31 July 2021 (UTC)

1. The Scientific American article titled “No-one can explain why planes stay in the air” is an article that has been seen here before, and has been analysed in some detail. We weren’t much impressed. See Humility in the face of the unknown.

If you want to initiate a discussion about this Scientific American article you are welcome to do so on this Talk page, but you can see how it has been regarded in the past. Dolphin (t) 07:44, 31 July 2021 (UTC)

2. I reverted the following text: A New Theory of Flight first presented in Computational Turbulent Incompressible Flow as a new explanation of the generation of large lift at small drag of a wing based on computing turbulent solutions of Euler's equations supported by mathematical analysis, has been developed by J. Hoffman, J. Jansson and C. Johnson. See my diff.

This text said almost nothing about the new theory of flight, but it gave prominence to the authors. On Wikipedia, the authors of cited sources are not identified in the article, but they should be identified in an in-line citation.

If you are one of Hoffman, Jansson or Johnson, or you have a close association with them, there may be a Conflict of Interest.

There may be a case for adding this information to the article but it should be added in accordance with encyclopaedic standards. I recommend you have a look at some or all of the following:

• How to edit a page
• Help pages
• Tutorial
• How to write a great article
• Avoiding common mistakes

Regards, Dolphin (t) 08:57, 31 July 2021 (UTC)

The book Understanding Aerodynamics by Doug McLean gives hard evidence that a common agreement on a scientific explanation of the generation of large lift at small drag of an airplane wing is missing, as just one piece of evidence to this very remarkable fact expressed in the Scientific American article. Why should Wikipedia hide this state affairs from the public?

Why is any reference to the New Theory of Flight, which gives the first full scientific explanation backed by solid math and computation, , removed?

Are you open to a section explaining the essence of the New Theory of Flight? Yes I am one of the authors (Johnson). — Preceding unsigned comment added by SecretofFlight (talk • contribs) 08:59, 31 July 2021 (UTC)

You have written about a "common agreement on a scientific explanation of ... lift ..." You have also referred to "this very remarkable fact". In the past decades we have seen many examples of people who believe there can only be one truly correct explanation of aerodynamic lift. These people seem to think "my explanation is correct so all other attempts at explanation must be incorrect." Similarly, there is no reason to expect that scientists will reach agreement on one truly correct way to explain aerodynamic lift. The majority of Users who work regularly on the topic of lift reject as nonsense these suggestions that there is only one truly correct explanation of lift. We also reject as nonsense the suggestion that scientists should reach agreement on one explanation of lift, or that there is something mysterious or sinister in the fact that different scientists display expertise in different ways to explain lift.

The reason your edits have been erased has nothing to do with wishing to hide your theory from public view - it has everything to do with the way it is written and presented in the article. To see how a theory should be written and presented, look closely at the various theories already firmly entrenched in the article - see Sections 2, 4 and 6 in the list of Contents. Also look at the 5 guidance articles I linked in my previous edit. Of course we are open to a section covering a new theory of flight - but it needs to be written in a way that is compatible with the encyclopaedic standards applied across Wikipedia. I recommend you draft the section and present it on this Talk page, or on your own personal sandbox, and then use the Talk page to invite interested Users to peruse it and make their comments.

Please remember to sign your Talk page edits with four tildes. Dolphin (t) 13:06, 31 July 2021 (UTC)

Your latest addition to the article (see the diff) looks like an advertisement for a book or a public lecture, rather than a scholarly entry in an encyclopaedia. It contains no in-line citation of the kind required on Wikipedia. You have written "The new theory reveals the true physics of generation of large lift ..." The true physics - wow, that is a bold claim indeed! It is not an appropriate claim to make on an encyclopaedia, especially when you have correctly revealed that you have a potential conflict of interest in all matters of the New Theory of Lift.

I suggest you look carefully at the existing content of this article and revise your additions so they look consistent with the rest of the content. Dolphin (t) 13:19, 31 July 2021 (UTC)

It also only cites one primary source and a self-published source. According to the manual of style's guidance for reliable sources:

Wikipedia articles should be based on reliable, published secondary sources and, to a lesser extent, on tertiary sources and primary sources. Secondary or tertiary sources are needed to establish the topic's notability and to avoid novel interpretations of primary sources. All analyses and interpretive or synthetic claims about primary sources must be referenced to a secondary or tertiary source, and must not be an original analysis of the primary-source material by Wikipedia editors.

So, the latest edit fails to meet basic inclusion criteria. Mr. Swordfish (talk) 23:06, 31 July 2021 (UTC)

I appreciate that I can have a discussion with Wikipedia on the important scientific question about "what keeps planes on the air" citing Scientific American 2020 reporting that "nobody knows". The fact that this question does not have a proper answer more than 100 years after the take off of powered human flight in 1903 is very remarkable, impossible to understand for the general general public, and kept as a secret kept within the scientific community of fluid dynamics hidden from the public. Yet it is true, and the evidence is massive. There is no convincing theory of flight in the standard scientific literature, and this is clearly evidenced by the Wikipedia article on Lift presenting lots of material but no theory claimed to be correct, because there is none. If there was a correct theory, known to be correct, then Wikipedia would present this theory and all incorrect theories now being presented would serve no role. The New Theory of Flight is a new scientific theory for the generation of lift at small drag of a wing with massive support from computation and mathematical evidence developed by leading academicians and published in leading peer reviewed journals opening a new window in the AIAA HiLift Workshops. Wikipedia can here serve an important role to expose this new theory to the scientific community for scrutiny and the general public for information. Can we agree on this mission?SecretofFlight (talk) 18:39, 31 July 2021 (UTC)

Regarding the Scientific American article, it's a pretty clear case of journalistic malpractice. John D. Anderson said "There is no simple one-liner answer to this...” The author misrepresented his statement as "What Anderson said, however, is that there is actually no agreement on what generates the aerodynamic force known as lift." which is a completely different statement.

This was further compounded by the headline writer (often headlines are written by someone other than the author of the article, so I don't know precisely who to blame here) who turned that into the sensationalist click-bait headline "No One Can Explain Why Planes Stay in the Air" It's utter crap. That said, if the author had turned down the hyperbole there's a decent article there.

Regarding Hoffman et. al. to the best of my knowledge this "theory" has not gained broader acceptance in the aerodynamic community. As such it's WP:FRINGE. Perhaps that will change, but until it does, their work doesn't belong in the article. Maybe Doug can add some perspective here.

While it true that there is no simple, correct, and complete theory of lift, you can say the same thing about any other minimally complex topic, from internal combustion engines to cheese making. There's nothing mysterious going on here - there are very well established models of lift that are quite well understood, at least by practicing aerodynamic professionals. What has happened is that for the better part of the 20th century the most common simple explanation turned out to be just plain wrong, and when that was pointed out, people being human held tightly on to it because nobody ever wants to admit that they were wrong. Much debate and argument followed, with disagreement on how best to take a complex subject and explain it simply. That's very different than "nobody knows" or even "there is no agreement on the (mathematical) theory." Were we to propagate the "nobody knows" shibbolleth we'd be remiss in our duties as wikipedia editors. Mr. Swordfish (talk) 21:46, 31 July 2021 (UTC)

Yes, it is a good idea to call in Doug McLean who has written an excellent book on flight theory with an attempt to come up with something better than the standard theories all know to be incorrect. I have written a post on my blog which I ask you to read and answer the questions posed at the end. Will you do that?SecretofFlight (talk) 11:00, 1 August 2021 (UTC)

Yes, I have read your blog as you requested. I deplore the fact that you have named @Mr swordfish: in your blog in the way you have done, presumably without their consent or prior knowledge. This is immature behavior that is unlikely to find any support in the scientific community.

At the end of your blog you ask several questions. All your questions are rhetorical. I'm sure you don't know what a rhetorical question is, so I will explain. A rhetorical question is one that is asked without any genuine expectation of an answer; usually because no answer exists or because no answer is wanted. For example, "Why do we have to endure this horrible Covid pandemic?" is a rhetorical question. In a genuine scientific or philosophical dialogue people say what they mean; they don't ask rhetorical questions.

I am building a picture of User:SecretofFlight as a somewhat immature and petulant person; someone more interested in advertising his theory than promoting the best quality article on Wikipedia. Please grow up or I will stop communicating with you. Dolphin (t) 12:51, 1 August 2021 (UTC)

You say regarding Hoffman et. al to the best of my knowledge this "theory" has not gained broader acceptance in the aerodynamic community. You are not well informed. The New Theory is now through Jansson an important discussion point at the HiLift Workshops collecting world leading competence.

You say that while it true that there is no simple, correct, and complete theory of lift, you can say the same thing about any other minimally complex topic. This is a misconception about what science is. The main objective of science is to give correct explanations of natural phenomena and it is crucial to distinguish correct theory from false theory. The fact that there is no theory of flight accepted as a correct theory is truly remarkable and efforts to cover up this fact is not science and not in the interest of the public. SecretofFlight (talk) 11:20, 1 August 2021 (UTC)

I did not get any answer on my question posed on my blog so I repeat it here: Why does Wikipedia censor any reference to the well documented New Theory of Flight in a Wikipedia article on Lift (force), which is only an account of old theories all known to be incorrect? I guess the reason is that the Wikipedians exercising the censorship (Dolphin51 and Mr swordfish) do not themselves carry the scientific expertise required to properly evaluate the merits of the New Theory of Flight and so take the simple way out to dismiss it without any scrutiny. But if so, this is not in the interest of the public. If there is a correct theory of flight, it should not be hidden to the people, in particular not to all people relying on safe air transportation. So I add the following question: Which experts are Wikipedia relying on, when dismissing/censoring the New Theory of Flight? SecretofFlight (talk) 18:58, 1 August 2021 (UTC)

The very simple answer as to why the material was removed is that it does not conform to the various wikipedia policies regarding notability, sourcing, and possibly conflict of interest. Dolphin and I have provided links to the help pages that clearly explain the policies and the reasoning behind them. I would suggest you read them, especially WP:ORIGINAL, WP:NPOV, WP:VERIFY, and WP:AGF I'd also suggest you drop the allegations of censorship - they just make your case look weak.

I'm sorry that your theory has apparently not attracted the attention you feel it deserves, but wikipedia is not the place to drum up notoriety. In fact it works exactly the opposite way - first the material must become notable, and only then does it warrant inclusion here. In other words, you need to do your PR work elsewhere first; come back when you have the requisite citations. I'll repeat myself, in case you missed it above:

From reliable sources:

Wikipedia articles should be based on reliable, published secondary sources and, to a lesser extent, on tertiary sources and primary sources. Secondary or tertiary sources are needed to establish the topic's notability and to avoid novel interpretations of primary sources. All analyses and interpretive or synthetic claims about primary sources must be referenced to a secondary or tertiary source, and must not be an original analysis of the primary-source material by Wikipedia editors.

One very clear problem with your edits is that you haven't established notability. Feel free to come back when you can. Mr. Swordfish (talk) 21:30, 1 August 2021 (UTC)

The only reasonable thing to do is to subject New Theory of Flight to scrutiny by some expert such as Doug McLean. My case is strong because I have hard evidence published in leading journals, while the Wikipedia article on Lift (force) is very weak as made very clear in the Talk statement above by Doug. The Wikipedia article starts out with (see also blog post):

"There are several ways to explain how an airfoil generates lift. Some are more complicated or more physically rigorous than others; some have been shown to be incorrect. For example, there are explanations based directly on Newton's laws of motion and explanations based on Bernoulli's principle. Either can be used to explain lift".

This is very serious disinformation Mr Swordfish. Very serious. You apparently agree with the statement above by Anderson: "There is actually no agreement on what generates the aerodynamic force known as lift". You thus know very well that there is no scientific explanation of lift agreed to be correct (only incorrect ones agreed to be incorrect), yet you let Wikipedia inform the people of the World that there is one, or even better that there are many although most (all?) of them are incorrect. You must understand that this against the most basic of all Wikipedia principles your refer to: Wikipedia should not mislead the people. Who is telling you to do that? To cover up what is a fact reported by experts in serious media.

I want to bring this case to highest level at Wikipedia. It is very serious and of great concern to the people. How do I proceed?SecretofFlight (talk) 06:54, 2 August 2021 (UTC)

User:SecretofFlight: To bring this case to the highest level of knowledge of physics at Wikipedia you should take it to the Physics Project team (see Wikipedia:WikiProject Physics). You can do this by posting your case at Wikipedia talk:WikiProject Physics. Dolphin (t) 12:31, 2 August 2021 (UTC)

Another course of action, which will more likely bring it to the attention of the "highest level", is to raise the issue on one of the various noticeboards. There is a process for resolving disputes that cannot be resolved on the talk page, and I think this one qualifies. See https://en.wikipedia.org/wiki/Wikipedia:Noticeboards#List_of_Wikipedia's_noticeboards Mr. Swordfish (talk) 13:47, 2 August 2021 (UTC)

Thanks for this information. I will now prepare material to take the case New Theory of Flight vs Wikipedia Lift (force) to the Physics Project Team and also to Noticeboards.SecretofFlight (talk) 16:02, 2 August 2021 (UTC)

Please post a link here when you have filed your case(s). Thanks. Mr. Swordfish (talk) 19:11, 2 August 2021 (UTC)

@SecretofFlight: When a dispute exists a User will sometimes post their case in two different places on Wikipedia. When this is realised one of the posts gets deleted promptly so Wikipedia’s effort is not divided into two places, potentially producing an ambiguous outcome. I suggest you post first on the Project Physics Talk page and see what happens. If you don’t see a suitable outcome after, say a week, then take it to a Dispute Resolution site. If you post at the Dispute site first it is highly likely that you will be asked to raise the matter first with the subject specialists at Project Physics so they have the opportunity to contribute their views, and their views will be highly valued by others who are trying to arbitrate on any dispute. Dolphin (t) 22:26, 2 August 2021 (UTC)

Seems to me that @SecretofFlight: has larger issues with how wikipedia makes these kinds of editorial decisions than what is within the normal set of issues that Wikipedia:WikiProject Physics deals with. I'm not sure which noticeboard is the best venue to adjudicate this dispute, but my sense is that he will receive a more thorough response at the noticeboards than at Wikipedia:WikiProject Physics. But I'll leave it up to him to choose the venue. Mr. Swordfish (talk) 02:14, 3 August 2021 (UTC)

@Mr. swordfish,Dolphin51: Before I take the case further I pose the following basic questions connecting to e.g the Scientific American article with headline "No One Can Explain Why Planes Stay in the Air. Do recent explanations solve the mysteries of aerodynamic lift?" (i) Is this a correct description of the state of the science of lift according to Wikipedia? If not, what is incorrect? (ii) Is there an accepted scientific theory/explanation of the generation of lift at small drag of an airplane wing? If yes, which is this theory/explanation? (iii) Mr. Swordfish states above "It is true that there is no simple, correct, and complete theory of lift". Does this mean that there is a non-simple, correct and complete theory, if so which, or no such thing? (iv) The Wikipedia article starts out: "There are several ways to explain how an airfoil generates lift. Some are more complicated or more physically rigorous than others; some have been shown to be incorrect. For example, there are explanations based directly on Newton's laws of motion and explanations based on Bernoulli's principle. Either can be used to explain lift". There seems to be a contradiction between (i)+(ii)+(iii) and (iv), that is a contradiction between the statements (a) There is a commonly accepted scientific explanation of lift, and (b) There is no commonly accepted scientific explanation of lift. Which of (a) and (b) is the view of Wikipedia? I want a clear answer, not handwaving that (c) they are both correct since there are many theories carrying different elements, some true some false. It is against this background the New Theory of Flight stands out as the first explanation in both mathematical and physical terms of the generation of lift at small drag of a wing with solid documentation in the scientific literature, which you remove from visibility on Wikipedia. The matter is serious. The role of Wikepedia is to give correct information to the people, not double messages that there both is and is not a scientific explanation of lift. Ok?SecretofFlight (talk) 06:57, 3 August 2021 (UTC)

My views on this matter, and my answers to your questions, are all evident in the posts I have made to this thread. I suggest you take your case further. I will respond there. Dolphin (t) 12:40, 3 August 2021 (UTC)

I agree. We've both already responded to most of this upthread. I fail to see the utility in discussing it further here. Mr. Swordfish (talk) 13:43, 3 August 2021 (UTC)

@Mr. swordfish,Dolphin51: No, you have not answered my questions in your posts! To take the case further it is necessary to make the present standpoint of Wikipedia clear on the matter of scientific explanation of lift. You say you will respond in the next instance. I ask you to do this right away, so that we will not have to start all over again. You have a responsibility to all the readers of Wikipedia and to the scientific community you are representing to answer my questions. What are your answers? SecretofFlight (talk) 13:59, 3 August 2021 (UTC)

@Mr. swordfish,Dolphin51: If you are unable/unwilling to answer the most basic question concerning the article Lift (force) for which you have responsibility, a question of utter scientific importance, then you are not, as I can see, filling the role of a true Wikipedian, which I think will not be appreciated by Wikipedia when made clear in the next instance. Do you see my point? You say that answers are to be found in your posts on this thread. Then point me to them! The world expects clear answers. What are your answers?SecretofFlight (talk) 14:34, 3 August 2021 (UTC)

@Mr. swordfish,Dolphin51: You can choose between two roles as Wikipedians: (i) You can go to history by opening to a much needed scientific discussion on theory of flight with in particular new input from New Theory of Flight, in a situation where there is no commonly accepted correct scientific theory of flight and all current theories basically dating back more than 100 years, are known to both experts and people through popular science press, to be incorrect/incomplete. (ii) You can act as gate keepers with a cover up that for sure there are (many) theories of flight, that science is settled and that New Theory of Flight has no place on Wikipedia. Which role do you prefer? For help to come to a decision I invite you to Secret of Flight with in particular the videos The Secret of Flight and Incorrect Theories of Flight. SecretofFlight (talk) 15:26, 3 August 2021 (UTC)

Here is state of art of standard fluid mechanics as expressed by Doug McLean in his book Understanding Aerodynamics concerning scientific understanding of lift:

"So in one sense, the physics of lift is perfectly understood: Lift happens because the flow obeys the NS equations with a no-slip condition on solid surfaces. On the other hand, physical explanations of lift, without math, pose a more difficult problem. Practically everyone, the nontechnical person included, has heard at least one nonmathematical explanation of how an airfoil produces lift when air flows past it. Such explanations fall into several general categories, with many variations. Unfortunately, most of them are either incomplete or wrong in one way or another. And some give up at one point or another and resort to math. This situation is a consequence of the general difficulty of explaining things physically in fluid mechanics, a problem we’ve touched on several times in the preceding chapters."

We read that generation of lift of a wing is a secret deeply hidden in the Navier-Stokes equations with no slip (but uncomputable because of very thin boundary layer), while scientific understanding in physical terms is a difficult problem, apparently unresolved (as expressed in Proposed revision of simplified explanations of lift below).

The New Theory of Flight reveals the secret of lift hidden in the Euler/Navier-Stokes equations with slip (without boundary layer and thus computable) in a description of slightly viscous incompressible flow around a long wing as potential flow modified by 3d rotational slip separation at the trailing edge into a turbulent wake, with potential flow generating large lift by attaching to the upper surface while gliding with very small friction as expressed by slip combined with 3d rotational slip separation at the trailing edge without the pressure rise of full potential flow destroying lift.

In short: Standard CFD as Navier-Stokes with no-slip is uncomputable and hides the secret of lift, while Euler/Navier-Stokes with slip is computable and opens to reveal the true secret in a New Theory of Flight in the form of potential flow modified by 3d rotational slip separation. It is as simple as that. Details on Secret of FlightSecretofFlight (talk) 07:48, 2 August 2021 (UTC).

@SecretofFlight: @Mr swordfish: @Dolphin51: I read "The Secret of Flight" paper and found the description to be compelling but somewhat hyperbolic in its claims. Although this material is not yet covered in secondary sources, it is not fringe, and it is recent and I think sufficiently strong to be included here in the article on lift. I've included a short description towards the end of the article, in Three Dimensional Flow, where it seems to fit best. Please consider keeping it, making changes, or delete it if you think this is not a valuable addition to the article, as I believe it is. Dilaton (talk) 21:52, 15 August 2021 (UTC)

@Dilaton: Thanks for your thoughts on this one. I concede that this new theory might be regarded as sound in some quarters, and might one day be widely accepted among mathematicians as a theory of flight. At present I see nothing to suggest that it is sufficiently mature to warrant mention in Wikipedia or any other encyclopaedia aimed at a general audience. We have seen two attempts at describing what this new theory of flight looks like, but I am none the wiser. For example, expressions like:

• 3D vortices. There appears to be nothing on Wikipedia to explain 3D vortices so this expression cannot be linked to any existing article to enable the reader to find something about these vortices. (Is this just an alternative to line vortex or vortex filament? Or is it somehow different?)
• potential flow modified by 3D rotational slip separation at the trailing edge into a turbulent wake. This is inaccessible to a general audience. It looks like something from a PhD thesis. Wikipedia is not the place for such a thesis.
• the potential flow generates large lift by attaching to the upper surface while allowing a wing to glide with very small drag from turbulent vortex attachment at the trailing edge. Potential flow attached to the upper surface? Surely every application of potential flow around an airfoil since the time of d’Alembert has assumed the flow is attached to the upper surface, and to the lower surface as well? Sentences like this serve more to confuse than to explain.

If it is to earn a place in this article, it must be described in a way that a general audience might comprehend. Despite your best efforts, your recent addition to the article is unlikely to be comprehended by a specialist audience of fluid-dynamic-literate users, much less by a general audience.

My view is that your recent addition should be removed. I will wait to see what Mr swordfish and other Users think. Dolphin (t) 07:35, 16 August 2021 (UTC)

@Dolphin51: Thanks for considering an addition. "3D vortices" is an attempt to convey that these are a collection of vortex filaments of unequal alternating vorticity, with ends attached to the trailing edge. It is essentially a more accurate refinement of the Kutta condition, in which the sheet of shear leaving the trailing edge is now understood as a sheet of turbulent vortices. The improvement of understanding comes in now seeing that this is where the majority of the drag originates on an airfoil. Perhaps the paragraph I attempted to add could be improved with this or other language? Dilaton (talk) 15:38, 16 August 2021 (UTC)

Wikipedia policy is abundantly clear that there must be secondary sources to include material. So far, there has been none for the "new theory of flight" despite a decades long PR campaign that often spills over into Wikipedia. The academic article itself has been accessed about 720 times and has garnered a total of 6 citations in the literature. Now, it may be that as Dolphin says it "...might one day be widely accepted among mathematicians as a theory of flight." but for now it's not. I've removed it since it clearly does not meet the standards for reliably sourced material. Mr. Swordfish (talk) 14:48, 16 August 2021 (UTC)

@Mr swordfish: I understand your concern and respect your adherence to secondary sources; however, WP:NOR does state that primary sources published in reliable places can be used with care, and I think this published article may thus qualify and be used carefully. Or we can do as you wish and wait for someone else to write about it. I do think that would be a bit of a loss, as the improvement of understanding of drag from attached vortices seems significant. Dilaton (talk) 15:38, 16 August 2021 (UTC)

@Dilaton:While you are correct that primary sources may be used with care, there must be some secondary sources to support notability. At the risk or repeating what I posted upthread, Wikipedia policy on reliable sources says:

Wikipedia articles should be based on reliable, published secondary sources and, to a lesser extent, on tertiary sources and primary sources. Secondary or tertiary sources are needed to establish the topic's notability and to avoid novel interpretations of primary sources. All analyses and interpretive or synthetic claims about primary sources must be referenced to a secondary or tertiary source, and must not be an original analysis of the primary-source material by Wikipedia editors.

Here, we have a paper that was published five years ago and in response the world has shrugged. Now, perhaps it is truly the major scientific breakthrough that the authors claim it to be. Perhaps even you agree that it is and think that the world needs to be told about it. Fine. Go do that. But do it somewhere else. Come back when there are sufficient secondary sources to support the notion that it merits inclusion here. Mr. Swordfish (talk) 23:39, 16 August 2021 (UTC)

I have to say that the sheer volume of debate on these talk pages leads me to believe that the Scientific American article was right after all. --Westwind273 (talk) 05:43, 1 September 2021 (UTC)

The Scientific American article comprises two distinct elements: firstly there is the title “No-one can explain why planes stay in the air.” and secondly there is the body of the article.

My impression of the body of the article is that it contains little to support the title. If you believe the body of the article contains some text addressing what the title says, please let us know what you see - please return to this Talk page and quote the wording you are looking at. Many thanks. Dolphin (t) 05:59, 1 September 2021 (UTC)

Proposed revision of simplified explanations of lift

There is a proposal for a revised treatment of simplified explanations of lift available at

https://en.wikipedia.org/wiki/User:J_Doug_McLean/sandbox

I think in general it is very good. I think it could be improved by addressing the following issues:

1. The current article states "The downward turning of the flow is not produced solely by the lower surface of the airfoil, and the air flow above the airfoil accounts for much of the downward-turning action." This has been removed in the draft. I think it needs to be stated somewhere in the article, otherwise readers may come away with "skipping stone theory"; I'm not seeing a better place than its current location, but I could be persuaded otherwise.
2. it doesn't adequately present the streamtube pinching explanation. Probably most of us reading this are already familiar with this "explanation" which can be found in Anderson and Clancy, but the typical reader will probably have no idea what we're talking about. The current article does present it, and I think if we're going to include this material we should explain it more fully than the draft does.
3. it asserts :

   the "streamtube pinching" explanation also starts by arguing that the flow over the upper surface is faster than the flow over the lower surface

   That's not my understanding of the argument. In the current version of the article (which I believe accurately reflects the reliable sources) the streamtube pinching explanation starts with the fact that theory predicts and experiments confirm that the streamtubes narrow on the top of the wing, and proceeds from there.
4. it lumps streamtube pinching into an "incorrect" subheading, but I'm unconvinced that streamtube pinching is actually incorrect. My view, which I think is born out by the reliable sources, is that it is a correct description of the physical phenomena, but with the logical problem that it begs the question of why the streamtubes change size.
5. it claims that speed/Bernoulli explanations come in two basic versions, but there is a third: the half-venturi tube "explanation". There are probably others. I think this can be easily written around, assuming we don't want to drag half-venturi into the article, by replacing "These explanations come in two basic versions" with "There are two common versions of this explanation"
6. The final subsection "Alternative explanations, misconceptions, and controversies" is reduced to only one explanation, misconception, or controversy after moving previously contained material upwards. It might be appropriate to address half-venturi, skipping stone, "squeeze the soap" and others here. Or just remove this subsection.

There are probably some other minor edits to avoid repetition and improve readability, but I think if the issues above are addressed the revised material will be ready for publication. Thanks for your efforts on this. Mr. Swordfish (talk) 00:42, 1 August 2021 (UTC)

Thanks for the feedback. To respond to the issues raised above:

1. Yes, let's put this back. And let's find a source to cite for it.

2-3. I think the whole paragraph taken together describes the arguments correctly, but I see how it can be confusing if you look at the first sentence by itself. I'll try a rewrite. I don't support retaining the current article's opening statement on what experiments and analyses show. It's a true statement, but no reliable source I know of uses it in the context of a streamtube-pinching explanation of lift, and the current article cites no source for it. My objective is still to stick with the classical sources that propose a reason for the pinching, even if we end up pointing out that the reason doesn't make sense.

4. I agree with your comment on "versions". But I still think streamtube pinching belongs under the "incorrect" heading because its two main steps (streamtube pinching causes higher flow speed, and higher flow speed causes lower pressure) run opposite to actual physical cause-and-effect. In addition to not providing a good reason for the pinching, it has the flaw that conservation of mass isn't a satisfying physical reason why the flow would speed up. Really explaining why something speeds up requires identifying the force that makes it accelerate. I'll add the second "flaw".

5. The upper-surface-as-an-obstacle and the upper-surface-as-a-half-Venturi are really the same argument. Your rewording is OK with me.

6. I'm going to try removing the "Alternative explanations..." subhead and move the "Controversy regarding Coanda effect" sub-subhead up with the flow-deflection explanation, as that's the explanation to which it relates.

I've implemented these changes in my sandbox. Thank you for the suggestions. J Doug McLean (talk) 17:23, 4 August 2021 (UTC)

I have gone through the proposed text and I find it excellent, and an improvement over the current state of the article. I have a few minor language/typographic fixes in mind, which I think will be better carried out once the text is integrated in the article. -- Ariadacapo (talk) 07:09, 5 August 2021 (UTC)

Thanks for your consideration of my suggestions.

1. I have added a citation for the assertion that the upper surface produces "much" of the lift. I'd like to find a better one, but I think this will do for now.

2. My working hypothesis is that the vast majority of our readers will not be familiar with the streamtube pinching explanation. It can be found in Anderson's Introduction to Flight, Eighth Edition, but not in earlier editions (or at least I couldn't find it there), in Clancy's Aerodynamics, and in Eastlake's article for The Physics Teacher. I have been unable to find it elsewhere. A year or so ago I was of the opinion that it was sufficiently obscure that it didn't merit attention in the article, but after acquiring Clancy and seeing it there, my opinion has changed. My best guess is that most people who have taken a college level class in aeronautical engineering have seen it, but it remains mostly unknown to the general population.

Since we can't expect the reader to already be familiar with it, we should provide a more detailed description - the current draft states "When streamtubes become narrower, conservation of mass requires that flow speed must increase." This is certainly true, but a sentence or two along with a picture will help many readers to understant why narrow streamtubes imply faster flow.

3. I think you are correct that the current article's treatment is at variance with the sources. Re-reading Anderson, he starts with "obstruction theory" to explain streamtube pinching, not "Starting with the flow pattern observed in both theory and experiments..." so we should present it his way. Eastlake doesn't explicitly explain why the streamtubes change size, but he alludes to the flow passing "the thickest part of the airfoil" and putting your thumb over the end of a hose, so I'll place him in the "obstruction theory" camp. I don't have my copy of Clancy with me and won't for several weeks, so someone else will need to check that reference.

4. The authors of the current section must have been engaging in an act of charity to re-factor the streamtube pinching explanation so that it is not actually incorrect. Seems that we both agree that the current version is not actually incorrect, but it is different than what is to be found in the cited sources. Since the sources present the explanation as a result of obstruction, we should too. And when we do, I think it is appropriate to lump it under the "Incorrect" heading.

One thing I'd like to see carried over from the current article is

Sometimes a geometrical argument is offered to demonstrate why the streamtubes change size: it is asserted that the top "obstructs" or "constricts" the air more than the bottom, hence narrower streamtubes. For conventional wings that are flat on the bottom and curved on top this makes some intuitive sense. But it does not explain how flat plates, symmetric airfoils, sailboat sails, or conventional airfoils flying upside down can generate lift, and attempts to calculate lift based on the amount of constriction do not predict experimental results.

The material expressed in the first sentence has been carried over, but the rest has not. Since the third sentence above is one of the better (best?) arguments why obstruction theory is lacking I think it makes sense to continue to include it.

5. Seems to have been taken care of. Thanks.

6. Moving the Coanda material up and removing the depleted section on "Alternative theories" makes logical sense. My issue with this version of the draft is that the article now spends more time discussing what is essentially a semantic issue than it does treating the much more central idea of lift as a consequence of conservation of momentum. Moving the Coanda material down in the article would be an acceptable solution, but I'm not sure where to move it. Like the streamtube pinching explanation, I think the "Coanda controversy" is limited to folks who have done some formal study or aerodynamics and not widespread in the general population, so perhaps we don't really need to address it. Or perhaps find a more concise way to present it.

Thanks for considering my suggestions. I think we're making real progress here. Mr. Swordfish (talk) 14:28, 5 August 2021 (UTC)

@Mr Swordfish: If at 3. you are alluding to the citation of Clancy p.76 “This lift force ... ... downward momentum of the air” I can confirm that this is an accurate quotation from Section 5.15 Lift and Downwash (which is on p.76 in my copy.) Dolphin (t) 00:19, 6 August 2021 (UTC)

@Dolphin: My recollection is that Clancy presents the streamtube pinching explanation, but I don't recall whether he starts with "obstruction theory" or proceeds from some other premise (e.g. the "theory & experiment" approach the article uses). We don't cite Clancy in this subsection, so you'll have to look beyond our citations. If you have your copy handy, I'd appreciate if you could take a look at Clancy's approach and report back. Thanks. Mr. Swordfish (talk) 18:18, 6 August 2021 (UTC)

@Mr Swordfish: I have had a quick look through Clancy. He explains lift using the Circulation Theory and the Kutta-Joukowsky theorem. The book appears to contain no linking of lift on an airfoil and stream tube pinching. There are several diagrams that show streamlines of varying spacing around a circular cylinder with circulation, and around an airfoil-shaped cylinder with different amounts of circulation. In the explanatory text adjacent to the diagram of the circular cylinder with circulation Clancy draws attention to the varying spacing of streamlines and links this to pressure variation using Bernoulli (Section 4.5 Circular Cylinder with Circulation on p.38) In the text adjacent to diagrams of airfoils Clancy makes no attempt to draw attention to streamline spacing and its implication for pressure.

In para 4.5(b) Clancy writes “The effect of the circulation is generally to increase the speed over the upper surface of the cylinder and to reduce the speed over the lower surface. This effect is shown by the spacing of the streamlines in Fig 4.4”

In para 4.5(c) he writes “From Bernoulli’s Theorem, therefore, it follows that the pressure is generally reduced on the upper surface and increased on the lower surface. As a result, there is a net force vertically upwards. This is lift.” Dolphin (t) 13:49, 7 August 2021 (UTC)

I stand corrected about streamtube pinching appearing as an explanation of lift in Clancy. I'm now back to wondering if presenting this explanation here is giving it undue weight. Mr. Swordfish (talk) 14:17, 7 August 2021 (UTC)

I've been re-reading the Help article on undue weight, first in the context of the streamtube pinching "explanation", but then in the context of the apparent controversy over the Scientific American article that claims "nobody understands lift". The article on weight states

Neutrality requires that mainspace articles and pages fairly represent all significant viewpoints that have been published by reliable sources, in proportion to the prominence of each viewpoint in the published, reliable sources. Giving due weight and avoiding giving undue weight means articles should not give minority views or aspects as much of or as detailed a description as more widely held views or widely supported aspects.

The current version of the article states succinctly "...there are explanations based directly on Newton's laws of motion and explanations based on Bernoulli's principle. Either can be used to explain lift." The proposed revision does an about face and states "...neither approach, by itself, is a completely satisfactory explanation." (And then there's the SA article, which I'm going to ignore as WP:FRINGE.)

Both of these are valid opinions that are supported by reliable sources. I tend to agree with the latter as my own opinion, but when I put on my editing hat I find it problematical to clearly come down on one side or the other. If we're going to present this controversy, we're supposed to present both sides and "teach the controversy". That said, I don't want to waste our readers' time by rehashing the great Bernoulli v Newton debate that raged back in the late nineties. My preferred solution is to sidestep the issue and avoid sweeping statement about whether both are right, or neither is right, (or whether nobody really knows). The proposed revision clearly explains each approach and its limitations or shortcomings. I think the readers can draw their own conclusions without us having to make sweeping statements like the above examples.

I'll copy the present proposal over to my sandbox and make the proposed changes there so we retain an easy to access "clean" copy of Doug's proposal.

Regarding the streamtube pinching, or "obstruction theory", I'm in agreement that it's essentially the same argument as the "half venturi tube" approach, which seems to be more prevalent in the sources so we should give more prominence to it. I'll take a whack at that, along with an attempt to provide a more concise treatment of the Coanda material. Mr. Swordfish (talk) 13:44, 10 August 2021 (UTC)

My view is that there are multiple explanations of lift, each derived from one or more of the various conservation laws and other laws of physics that are applicable to a solid object immersed in the flow of a fluid. We make use of multiple explanations of lift to serve the needs of the multiple audiences that have an interest in the subject. Even within one audience there are multiple purposes and objectives that cannot be satisfied by just one explanation. For example:

• An explanation of lift that can be presented to 19-year olds will be unsuitable for 13-year olds. An explanation that is both satisfying and satisfactory for student pilots will be unsuitable for students of physics and engineering.
• An explanation that helps explain lift in 2-dimensional flow will not be satisfactory if the objective is to help explain lift-induced drag.

I support the sentiment in the present article: “Either can be used to explain lift.” I don’t support the sentiment that "...neither approach, by itself, is a completely satisfactory explanation." It will be unhelpful, unnecessary and unsound to apologise for certain explanations of lift, or to suggest that no satisfactory explanation exists, or that no-one knows what it is. Wikipedia is able to demonstrate its maturity and soundness by not engaging in a search for a "completely satisfactory explanation". Nor should Wikipedia support a notion that every incomplete explanation must be incorrect.

When we search for the most appropriate explanation of lift for our purposes we are engaging in applied science or applied math or engineering but we aren’t engaging in pure or fundamental science. Bernoulli’s principle and Newton’s laws of motion have universal application and so qualify as fundamental science, but an explanation of lift on an airfoil is simply one of many examples of Bernoulli and Newton in action. There will never be a Committee of eminent scientists whose task is to determine by arbitration the one true explanation of lift.

When we talk about the explanation of lift based on Bernoulli’s principle, it would really be more accurate to say we are using lift as an example of Bernoulli’s principle in action. Similarly, when we talk about the explanation of lift based on Newton’s laws, it would really be more accurate to say we are using lift as an example of Newton’s laws in action. The pure science is always more fundamental than the application of that science to one of a multitude of everyday observations.

I look forward to seeing your latest proposal on your sandbox. Dolphin (t) 12:10, 11 August 2021 (UTC)

Dolphin, Agree that different audiences require different explanations, and it is appropriate for us to present several, starting with the easier to understand and proceeding to the more rigorous. I think we need to be careful about using words like "satisfying" and "satisfactory" because they beg the question of "satisfying to whom?" My hunch is that most people are completely satisfied to know nothing about this topic. Those who bother to read the article may come away satisfied after a section or two, or they may read further until they are "satisfied".

Moving on....

The opinion “Either can be used to explain lift.” is just that - an opinion. So is the opinion "...neither approach, by itself, is a completely satisfactory explanation." If we're going to include either one, we need to present the other, present both as opinion, and provide some context for how widespread each is in the reliable sources. I'd rather not do that, especially early on in the article. Perhaps a later section on the "Bernoulli v Newton Controversy" would be in order, or perhaps a separate article instead. My preference is to just sidestep it as a distraction and present the various approaches, starting from simple and moving to the complex, with some context to address whatever shortcomings or limitations each approach has. And let the material speak for itself without making unnecessary sweeping generalizations.

To that end, I don't think we need the first section "Understanding lift as a physical phenomenon". The article starts with qualitative physical explanations without math and proceeds to the various mathematical models. That is apparent from the table of contents, so I don't think we need to state it explicitly; readers will get it if they bother to read that far. I'm going to remove it from my draft. Comments appreciated.

My view is that no version is 100% complete nor is any version 100% correct. When we do physics, we make abstract models, and in order to make the models tractable we make some simplifying assumptions along the way so the model doesn't exactly describe the actual physical phenomena. That doesn't mean that the models are bad, just that they are always limited, and when criticized for that variance the criticism is often warranted. For instance, 2D potential flow doesn't predict stall, drag, or downwash. But it does a surprisingly good job at predicting lift without making the math impossible. IOW, it's a good but limited model.

Which is to say that every explanation is incomplete to some degree. So, I'm not sure it's "fair" to label the explanation based on flow deflection and Newton's laws that way in the title. I do think it's fair to state that it's incomplete in the body, so I'm removing it from the title but leaving it in the body.

Regarding "Bernoulli-based" explanations, the two we discuss in that section are clearly incorrect. Correct explanations involving Bernoulli (or more properly, explanations that are based on models that have some predictive power) always include many other physical principles to the extent that it's a misnomer to call them "Bernoulli-based". To put a finer point on it, they always include conservation of momentum at some level. Bernoulli's principle is just one piece of the puzzle.

That said, I think a serious shortcoming of this draft as it stands is that readers may come away with the notion that Bernoulli's principle is somehow wrong, or that it is always incorrect to use it when explaining lift. I think we need so say something along the lines of "Although these two simple explanations are incorrect, there is nothing incorrect about Bernoulli's principle, or it's usage in a more complicated explanation of lift." But I'm not sure where to put it or how best to phrase. Suggestions appreciated. Mr. Swordfish (talk) 18:32, 14 August 2021 (UTC)

UPDATE: I've added "Although these two simple explanations are incorrect, there is nothing incorrect about Bernoulli's principle, or it's usage in a more complicated explanation of lift." to the draft. Mr. Swordfish (talk) 18:49, 14 August 2021 (UTC)

I've not gotten much feedback on the draft in my sandbox. I'm not sure if that's because other editors don't like it, or because they think it's fine as is. Assuming the latter, I'll give it a couple of days and if no objections I'll deploy the material in my sandbox. Mr. Swordfish (talk) 22:02, 19 August 2021 (UTC)

I will be happy to give some feedback in the next day or two. Dolphin (t) 22:42, 19 August 2021 (UTC)

Thanks. It's more important that we get it right than that we do it fast. But I want to keep the process moving. Mr. Swordfish (talk) 01:41, 20 August 2021 (UTC)

It looks good to me and I don’t see much to comment on. I have provided my feedback at User talk:Mr swordfish/sandbox. Dolphin (t) 13:31, 20 August 2021 (UTC)

I think the proposed new section "Understanding lift as a physical phenomenon" is important. It clarifies the status of the qualitative explanations relative to the rigorous scientific understanding embodied in the mathematical theories. In so doing, it says a lot more than what a reader could infer from the TOC or what he would be likely to realize even after reading the entire article. I think it makes what follows much easier to understand.

I think we should keep the "Obstruction..." explanation. Anderson is a very prominent author, and this book is a prominent source.

I've put up a new candidate in my sandbox. It avoids the "satisfactory" wording and removes the value judgements from the headings. It also incorporates Swordfish's shortened version of the Coanda section and his separate subheads for "Equal transit time" and "Obstruction...". I added another subhead to separate out the issues common to both explanations that had been swallowed into the "Obstruction..." subsection. I also incorporated his wording on Bernoulli not being incorrect as a principle, with the added qualification that Bernoulli is applicable outside the boundary layer. Comments? J Doug McLean (talk) 19:21, 21 August 2021 (UTC)

I've copied Doug's latest draft over to my sandbox for the purposes of comparison. The diff is here: https://en.wikipedia.org/w/index.php?title=User%3AMr_swordfish%2Fsandbox&type=revision&diff=1040264566&oldid=1039805588 I'll have more to say in a day or so. Mr. Swordfish (talk) 17:29, 23 August 2021 (UTC)

Comments on the latest drafts (24 Aug 2021):

o The first thing I noticed when looking at the diff was that the latest version from Doug is some 10,000 characters shorter. This is mostly refs that didn't make it over. I don't think it will be controversial to restore the refs, although some may be ripe for pruning. I'll restore all of them in my next draft, and if any are deemed to be unnecessary we can remove them on a case-by-case basis.

o Regarding the first section [Understanding lift as a physical phenomena], it reads to me as an opinion rather than a simple statement of fact. The current version of the article also includes the opinion that "Either can be used to explain lift." I prefer the simple factual statement in my previous draft, which I think adequately foreshadows the qualitative vs mathematical dichotomy to come.

There are several ways to explain how an airfoil generates lift. Some are more complicated or more mathematically rigorous than others; some have been shown to be incorrect. Most simplified explanations follow one of two basic approaches, based either on Newton's laws of motion or on Bernoulli's principle.

o I looked into other wikipedia articles that link directly to sub-headings, and only found one that would be affected by the current drafts. I added an anchor tag to that section. We should make sure that it makes it into the final version.

o Agree to keeping the obstruction/constriction/streamtube-pinching explanation. While it's not nearly as widespread as the ETT fallacy, it seems to be common enough for us to reference it here (although I'd be open to an argument that it's not if anyone wants to make it). I think it's worth expending a single sentence on NASA's "Venturi tube" version of it since NASA's site may be the most widely read version.

o I don't think labeling the incorrect explanation as "incorrect" is a value judgement. Seems to be simply a statement of fact, so I'd advocate restoring that in the titles.

o I like the additional subhead to address issues common to both - I wanted to do that myself, but couldn't come up with a good title.

o I'm unconvinced that it's necessary to state that "Bernoulli's principle is applicable to the flow outside the boundary layer." at this point in the article. I think simply stating "Bernoulli's principle can be used correctly as part of a more complicated explanation of lift." is sufficient for the intended audience for this portion of the article. If we're going to address when Bernoull's principle applies and when it doesn't, that should wait until later in the article.

o Regarding "This explanation is correct as far as it goes but is incomplete. " I've come to agree with Dolphin's [opinion] that "as far as it goes" is a colloquial idiomatic expression, that while common in the US may not be understood the way it's meant to by someone unfamiliar with the expression. If we were writing this for a US audience I'd advocate to keep it, but since we're writing for the broader English-speaking world I think the phrase should be excised. We have two candidates for the material at this point:

This explanation is correct but it is incomplete. It doesn't explain how the airfoil can impart downward turning to a much deeper swath of the flow than it actually touches. Furthermore, it doesn't mention that the lift force is exerted by pressure differences, and doesn't explain how those pressure differences are sustained.

and

Flow deflection combined with Newton’s laws is a helpful way to explain some aspects of lift. It leaves some questions unanswered; it doesn't explain how the airfoil imparts downward turning to the flow, and it doesn't mention that the lift force is exerted by pressure differences. It doesn't explain how those pressure differences are sustained.

I prefer the concise "correct but incomplete" phrasing, but could be persuaded otherwise.

I'll merge the the latest draft from Doug with mine, incorporating the ideas above. Comments as always welcome. Mr. Swordfish (talk) 16:32, 24 August 2021 (UTC)

Oversimplification

The current version of that section still refers to Bernoulli's Principle as "there is a relationship between the pressure at a point in a fluid and the speed of the fluid at that point, so if one knows the speed at two points within the fluid and the pressure at one point, one can calculate the pressure at the second point, and vice versa." This sounds great, but it isnt correct, as it is a (fairly significant) oversimplification of his work. In the context of aviation and aerodynamic lift, it is only accurate along a streamline where no heat is being transferred between the wing and the air. Does the cited work include this gross oversimplification? As importantly, does the gross oversimplification make the concept clearer to the reader? PrimalBlueWolf (talk) 08:26, 23 August 2021 (UTC)

@PrimalBlueWolf: Where you have written “but it isn’t correct ...” do you mean Bernoulli’s principle doesn’t correctly represent the reality; or our article doesn’t correctly reflect the principle described by Bernoulli?

It is well known, and always acknowledged in reliable published sources, that Bernoulli’s principle doesn’t take account of viscous forces within the fluid, nor does it apply to a flow field in which heat is being transferred. Despite these assumptions Bernoulli’s principle is a very powerful tool in analysing the subsonic flows around streamlined bodies. I don’t agree with your characterisation that the Wikipedia article represents a “gross oversimplification.” Please explain further. Dolphin (<.,span style="color: blue;">t) 13:57, 23 August 2021 (UTC)

That it doesn't correctly represent the principle as represented in Hydrodynamica. The current version of the article alleges that you can determine velocity and pressure of any other point using Bernoulli's Principle knowing only the velocity and pressure of one point, and the velocity of one other point. That is only valid along a streamline, but the article doesn't acknowledge that. PrimalBlueWolf (talk) 21:25, 23 August 2021 (UTC)

It is often stated that "Bernoulli's principle is only valid along a streamline" but this is a misconception. Within a flow field that exhibits uniform flow as the initial condition, BP applies throughout the flow field. This assumes that the energy is constant, i.e. it assumes no heat loss (as one would find in the example of an airplane wing) or no net work done (as one would find in the example of a sailboat). If one is going to pick nits, BP is not applicable to any real world airfoil due to these energy considerations, however it is commonly used as a approximation or simplification to make mathematical models tractable. Physics is full of these approximations, e.g. assuming sin(x)=x for sufficiently small x. And if we're not going to assume constant energy, BP doesn't apply along a streamline either.

The statement "there is a relationship between the pressure at a point in a fluid and the speed of the fluid at that point, so if one knows the speed at two points within the fluid and the pressure at one point, one can calculate the pressure at the second point, and vice versa." is consistent with how BP is used in practice in mathematical analysis of fluid dynamics. Granted, it's a calculational shortcut that does not precisely model the actual physical world. But it's close enough for engineering work. Note that the section is about "simplified explanations" and is not the proper place for a long technical discussion of exactly when BP applies and when it doesn't. Mr. Swordfish (talk) 03:28, 24 August 2021 (UTC)

@PrimalBlueWolf: As you can see, I have moved your posts and the responses from me and @Mr swordfish: to their own thread under this new heading.

You have written “That is only valid along a streamline, …” That is incorrect in the case of a wing generating lift in the atmosphere. Consider the following:

In Fluid Mechanics by V.L. Streeter (1951 McGraw-Hill), section 3.7 The Bernoulli Equation says:

The constant of integration (called the Bernoulli constant) in general varies from one streamline to another but remains constant along a streamline in steady, frictionless, incompressible flow. These four assumptions are needed and must be kept in mind when applying this equation.

Under special conditions each of the four assumptions underlying Bernoulli's equation may be waived.

1. When all streamlines originate from a reservoir, where the energy content is everywhere the same, the constant of integration does not change from one streamline to another and … [any two points] may be selected arbitrarily, i.e. not necessarily on the same streamline.

In Aerodynamics by L.J. Clancy (1975 Pitman Publishing) section 3.4 Bernoulli's Theorem for Incompressible Flow says:

Further, at some distance upstream of the aircraft, the flow consists of a uniform stream. It follows that on any given streamline in this region the value of p + 1/2 ρ v² is the same as it is on any other streamline.

In Fundamentals of Aerodynamics by John D. Anderson (1984 McGraw-Hill) section 3.2 Bernoulli's Equation says:

For a general, rotational flow, the value of the [Bernoulli constant] will change from one streamline to the next. However, if the flow is irrotational, then Bernoulli's equation holds between any two points in the flow, not necessarily just on the same streamline.

In the language of fluid dynamics we say Bernoulli's principle applies equally at all points on all streamlines in a region of irrotational flow. A wing operates in a stationary atmosphere so there are no viscous forces or vorticity in the air outside the boundary layers. The flow around a wing is irrotational everywhere except in the boundary layers.

You have also written “… only accurate along a streamline where no heat is being transferred between the wing and the air.” I assume you are referring to transonic and supersonic flow. The Wikipedia article presently only refers to lift in subsonic flight. In low-speed flight there is no significant amount of heat being transferred. Dolphin (t) 04:32, 24 August 2021 (UTC)

I'm glad to take the correction and agree with the reasoning. Thanks for the detailed and well sourced explanation. PrimalBlueWolf (talk) 07:19, 24 August 2021 (UTC)

Proposed new version of simplified explanation continued

The last thread had gotten rather long, so starting a new one.

Latest version now available in my sandbox.https://en.wikipedia.org/wiki/User:Mr_swordfish/sandbox

I opted to keep the opening section, at least for now, but as it stands now there is substantial repetition between it and the first paragraph of the next section. Not sure what is the best solution, but I'm out of time for the day. Comments and suggestions appreciated. Mr. Swordfish (talk) 15:33, 26 August 2021 (UTC)

I spent some time today looking at other Wikipedia articles on technical, mathematical, or scientific subjects. I came away with two observations:

1. The articles discuss the topic at hand, rather than discussing the article and how it covers the topic.
2. None of them have language that implies that the topic is difficult to explain or to understand.

With that in mind, the opening section "Understanding lift as a physical phenomena" would be an outlier in terms of Wikipedia style. The more matter-of-fact treatment in the section that follows is in keeping with wider Wikipedia standards.

See Aerodynamics, Wing, Quantum Mechanics, Fluid Mechanics, Fluid Dynamics, Chemistry, Category Theory for a few examples.

On that basis I'm going to remove the section from the draft while repurposing some of the language into the new first section. At this point, I think we have a release candidate. Comments? Mr. Swordfish (talk) 15:31, 27 August 2021 (UTC)

I agree. I encourage you to release the latest version. Dolphin (t) 13:35, 28 August 2021 (UTC)

It's been released. Thanks to everyone who contributed. Mr. Swordfish (talk) 21:06, 28 August 2021 (UTC)

Sorry for not weighing in sooner on the latest changes. I've been away for a few days.

I see that the proposed new section has been removed again and that some of the language has been "repurposed" into the following section. It seems to me that these changes have negatively impacted the article's organizational clarity. The first mention of the mathematical theories now comes under the heading "Simplified explanations.....", and with this placement the mathematical theories are now categorized as one of "several ways to explain how an airfoil generates lift". This isn't an accurate reflection of where the mathematical theories fit in the overall picture. The mathematical theories are the basis of the rigorous scientific understanding of lift. They're not "explanations" of lift.

I think the proposed new section reflected the facts of the matter more clearly. Except for the phrase (referring to the simplified explanations) "and most readers will likely already have been exposed to one or more of them", which I propose we delete, everything that remains is a straightforward statement of fact. Even the one bit of "meta" information ("These issues are discussed in connection...") is a factual statement that more detail on the issues just raised is coming later in the article, not a "discussion" of "how the article covers the topic".

I don't think that providing a bit of factual meta information is out of place in a Wikipedia article. Nor is it out of place to say that a correct qualitative explanation of lift is difficult, given that it's a statement of fact supported by the checkered history of qualitative explanations and by the sources (my TPT paper, at least).

I've tweaked the proposed new section and removed its heading, which makes it part of the "Overview" section, where I think it fits well. I've also taken a crack at removing the resulting duplication from the intro to "Simplified physical explanations..." in my sandbox. My recommendation is to merge these changes into the article in place of the recently released version. J Doug McLean (talk) 19:27, 2 September 2021 (UTC)

Thanks for your continued effort on this page. I've made an attempt to merge your latest version with the current article. It's in my sandbox. https://en.wikipedia.org/wiki/User:Mr_swordfish/sandbox#Overview Comments appreciated. Mr. Swordfish (talk) 20:12, 4 September 2021 (UTC)

Mr Swordfish: I have no objection to the current version in your sandbox being released. Dolphin (t) 12:52, 6 September 2021 (UTC)

Mr Swordfish: Your rendition of the addition to "Background" is more cryptic than my draft, but I'm on board with all of it except the last sentence, which seems to me to be ambiguous. Actually, I think all, not just some, of the simplified explanations we present have the flaw of leaving important things unexplained, even the ones that also have incorrect elements. A possible revision:

There are also many simplified explanations, but all leave significant parts of the phenomenon unexplained, while some also have elements that are simply incorrect.

I think we're almost done and on the verge of completing a significant improvement of the article. J Doug McLean (talk) 00:54, 7 September 2021 (UTC)

I have implemented the suggested change in my sandbox and will deploy that version. However, I failed to start with the latest version from the real article and several changes have been made since I deployed the version from my sandbox so I can't just do a cut and paste or it will override those changes. So, there will be several intermediate versions in my sandbox as I reconcile the two. Mr. Swordfish (talk) 13:46, 8 September 2021 (UTC)

Coandă effect criticism

The following sentence was recently added:

A criticism of the Coandă effect as an explanation for aerodynamic lift is that the Coandă effect itself is not well understood.

With a cite to https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1096&context=etd

The relevant part of that paper says:

The Coanda effect has been widely used in the both aeronautics and medical applications [12], air moving technology, and other fields. Nevertheless, this phenomenon is not completely understood, especially for three-dimensional flow as in the CSM design. The nature of the Coanda effect, with boundary layer separation and entrainment interaction, make for difficulty in solving the flow numerically and analytically.

I'm not seeing where the source criticizes the usage of the Coandă effect to explain lift, so this material appears to be WP:SYNTHESIS. A bigger problem is that saying that "the Coandă effect itself is not well understood" is a very broad statement that would need stronger backing than the carefully worded excerpt from the cited Masters Thesis above. Reading the Coandă effect article I don't see anything supporting the assertion that it is not well understood - were this truly the case I would expect it to be treated in that article.

Of course, that wikipedia article is not dispositive - we're supposed to look at reliable sources, and other wikipedia articles are not reliable sources - but it strikes me that if we're going to publish a broad assertion like that the proper venue for discussing it and presenting the source material would be the talk page for that article, not this one.

I'm removing the material pending the production of better citations. Mr. Swordfish (talk) 20:32, 9 March 2022 (UTC)

I agree with Mr. Swordfish that better citations are necessary. However, as far as I have been able to determine, there are no sources that offer a well thought out explanation for why or how the Coandă effect applies to aerodynamic lift. The popular references quoted in the main article (references 33 and 34) certainly do not offer that explanation. This lack of a source making a detailed argument for applying the Coandă effect to aerodynamic lift is not apparent in the main article. I tried to make this deficit of a source argument, not vey well I must agree, but one that should be made. It is difficult to make this argument since there are no referenceable sources that point out this deficit of a source offering a valid explanation. David Weyburne (talk) 16:51, 10 March 2022 (UTC)

Were I writing this article for myself, I'd include something like:

People often try to explain why the air is deflected on the top of the wing by saying it's because of the Coandă effect, but this doesn't actually explain anything, it just gives it a fancy European name.

But I'm not allowed to just make stuff up on my own and I haven't seen this idea expressed elsewhere so I don't have a source for it. And that means I can't add it to the article. That said, I agree with the sentiment that it's poor pedagogy to explain something via material that the reader doesn't understand either. And I think the article would be improved with a short statement like the one above or something similar to what you added, but unless we can find reliable sources to cite we can't add it. If you find a good source for this I'm all ears. Mr. Swordfish (talk) 23:23, 10 March 2022 (UTC)

Anderson and Eberhardt's "Understanding Flight" (McGraw-Hill, 1st ed. 2001) is the one source I know of that appeals to the Coanda effect in a lift explanation and also tries to explain how Coanda works in physical terms. They attribute the Coanda effect entirely to viscous "shear forces." On p. 23, after explaining no-slip at the surface and the resulting formation of a boundary layer, they say:

"The differences in speed in adjacent layers cause shear forces, which cause the flow of the fluid to want to bend in the direction of the slower layer. This causes the fluid to try to wrap around the object."

This explanation of Coanda is easy to rebut. However, my own book ("Understanding Aerodynamics", Wiley, 2012) is the only citable source I know of that does so explicitly. With reference to using Coanda in lift explanations, I say in sec 7.3.1.7:

"The Coanda effect is erroneously seen as implying that viscosity plays a direct role in the ability of a flow to follow a curved surface. Anderson and Eberhardt assert that viscous forces in the boundary layer tend to make the flow turn toward the surface, specifically, as they put it, that the 'differences in speed in adjacent layers cause shear forces, which cause the flow of the fluid to want to bend in the direction of the slower layer.' Actually, there is no basis in the physics for any direct relationship between shear forces and the tendency of the flow to follow a curved path."

In the paragraphs following the above, I explain in detail my reasons supporting the statement in that last sentence. The gist of it is that the curving of the flow is a result of the interaction between the pressure field and the velocity field, as we explain in the article under "A more comprehensive explanation." It has practically nothing to do with viscous or turbulent shear stresses. As long as the boundary-layer doesn't separate, the curving of the flow to follow the curved surface is an essentially inviscid effect.

Mr. Swordfish has invited us to identify a citable source for his naming-isn't-explaining objection to relying on Coanda. Again, the only one I know of is my own book. In sec 7.3.2 I list things to avoid in an explanation of lift. Item 5 is:

"'Naming' as a substitute for explaining, as, for example, in saying that a jet flow follows a curved surface because of the Coanda effect, where 'Coanda effect' is just a name for the tendency of jet flows to follow curved surfaces."

So we have citable sources for a couple of possible additions to the Coanda subsection that would be of interest to some readers. I'm not enthusiastic about doing it, however, because I think we may already be giving Coanda more prominence than it deserves. On the other hand, I could argue that the article as it stands doesn't present enough of the case against Coanda, and that the additions we're considering here would balance things better and help justify the word "Controversy" in the article's section heading.J Doug McLean (talk) 20:20, 3 April 2022 (UTC)

Thanks very much Doug. Mr swordfish and I will ensure your book is cited as a source where it is appropriate to do so in relation to Coanda effect. Dolphin (t) 23:49, 3 April 2022 (UTC)

Now that we have a cite I've been trying to craft language along these lines, but so far haven't come up with anything that doesn't seem out of place or unencyclopedic. I'll keep trying. Suggestions cheerfully considered. Mr. Swordfish (talk) 23:56, 11 April 2022 (UTC)

Mr swordfish and J Doug McLean I have inserted a paragraph that, hopefully, begins to capture some of Doug's wisdom from above. See my diff. Dolphin (t) 04:41, 27 September 2022 (UTC)

I have also added a sentence on "naming is not explaining". Mr. Swordfish (talk) 18:55, 28 September 2022 (UTC)

A new simplified lift explanation

As if things weren't complicated enough, I have developed a new simplified explanation for aerodynamic lift that I would propose as a add-on to the present version. I am looking for comments and recommendations at this point.

The proposed text is available in my sandbox at https://en.wikipedia.org/wiki/User:David_Weyburne/sandbox

The proposed explanation is based on a graphical interpretation of the mathematical equations governing fluid flow. The key to the approach is the graphical plots of the velocity profiles and the pressure gradient profiles taken at a bunch of locations along the airfoil surface. This permits a one-to-one correspondence between the flow governing equations and the plotted profiles. By invoking the momentum conservation equation in this way, the explanation provides the connection between the velocity and pressure fields that is missing in the other simple explanations. David Weyburne (talk) 13:38, 17 September 2022 (UTC)

Where a Wikipedia User develops a new explanation for something it is called Original Research. Such an explanation is not published in Wikipedia - see WP:NOR.

Your explanation cannot be described as simplified. I find it mystifying. Some of your sentences are statements of the obvious and therefore unnecessary in your description; and others are either incorrect or misleading. If you wish to continue with your work on this subject in order to publish it in an appropriate place, it needs a lot of refinement.

You are relying on four sources but three have been published by yourself. This is usually unwise and I have commented at User talk:David Weyburne/sandbox. Dolphin (t) 23:20, 17 September 2022 (UTC)

Thanks for the feedback. As to original research comment: I do not think any of the explanations presented in the Simplified Explanations section would constitute original research that would be appropriate for a journal article. The explanation may be original but it is not something that can be tested and verified by other research groups. As to the rest of the comment: I am sorry you find it mystifying but I am hoping that is not the case for the majority of readers. You claim there are obvious statements that are unnecessary: I have tried to make the explanation readable for the non-expert and would hope that the expert reader would allow for that. You also claim there are misleading and incorrect statements: It is hard to comment on this claim since you did not bother to outline which statements are false or misleading. David Weyburne (talk) 12:54, 21 September 2022 (UTC)

David Weyburne Thanks David. On 18 September I made some introductory comments about statements I regard as superfluous, and others I regard as misleading. Those comments are on one of your Talk pages - see User talk:David Weyburne/sandbox. Dolphin (t) 23:05, 21 September 2022 (UTC)

Sorry, I initially missed your comments in my sandbox. I appreciate your detailed comments and I have replied to the comments in the Talk section. At this point I will leave the explanation as is and would add that a more detailed explanation is available in the supplied references. David Weyburne (talk) 12:40, 22 September 2022 (UTC)

One further note as to the observation that three of the sources were published by myself and is therefore inappropriate. I would point out that one is a YouTube video, another is an Air Force Technical Report, and the third is an e-book collection of my Air Force Tech Reports. All of them lay out a more detailed version of the condensed simplified explanation provided in my Sandbox. The reason the references are all mine is that I believe that my simplified explanation is original. However, as I stated before, this type of simplified explanation is not something that would be appropriate to be published in a standard journal. It is appropriate for providing a simplified explanation in an encyclopedia-style format. David Weyburne (talk) 12:45, 23 September 2022 (UTC)

As the author of the proposed cited articles you may be subject to Wikipeda's conflict of interest policy. I would suggest familiarizing yourself with that policy. I appreciate the fact that you have disclosed that you are the author of those articles, but that fact remains and is germane and therefore not inappropriate.

That said, the fact that the proposed additional material uses your articles as their source doesn't mean that the material can't be added to the article, or that your articles can't be cited. We've encountered this issue before with a prominent author, who provided some very valuable insights into this topic and helped improve the article. But he made very few edits himself, instead working with the other editors to reach consensus about any proposed revision to the article. I think we are on solid grounds if we follow that model. Mr. Swordfish (talk) 19:38, 23 September 2022 (UTC)

Sorry, been busy. I understand that referencing my own work is problematic. To explain the reason for doing this, I need to give a little background. My simplified explanation for aerodynamic lift is based on showing "graphically" how the conservation of mass, momentum, and energy occurs for a flow around an airfoil. To do this, I start using a simple word-based argument to say that mass diversion results in velocity changes while being diverted around an airfoil. These velocity changes result in a speed up for the flow on the airfoil. How do you graphically show this speed-up? It is possible to use streamline, contour, or vector plots of the velocity but because of the large spatial variations, this approach is not very effective. Hence, most simplified explanations for lift regress to simply stating that "the velocity speeds up". For my simplified explanation I switched to a series of "velocity profile plots" along the airfoil. The profiles show the velocity behavior from a point on the airfoil to a point deep in the free stream above the airfoil. What you see are velocity peaks near the airfoil surface that slowly return to the free stream over distances of ~two chords. These peaks are important in that it gives a visual confirmation of velocity changes and give a one-to-one comparison to the momentum equation du/dy term. The momentum equation says these velocity changes must be conserved which is done, in part, by pressure changes. I then can show a plot of the pressure gradient profiles above and below the wing at the same location as the velocity profiles. The difference in the pressure profile areas, the pressure difference, shows graphically how mass and momentum conservation results in lift.

So what is the problem, why do I only reference my own work? The reason is there is no one doing anything similar using velocity profiles. This velocity profile "peaking" behavior is not discussed or plotted anywhere in the literature that I could find (other than the simple text saying "the velocity speeds up"). Many textbooks show schematics of boundary layer profiles but not ones that show the peaks, the velocity speedup behavior. I observe it my airfoil simulations and in raw mesh data provided by other researchers, but nowhere in the literature. If I had references showing that these velocity and pressure profile peaks exist, I would be less dependent on referencing my own work. For the record, I think for the non-expert, my 15 min. graphics-based YouTube video does a better job of explaining this aerodynamic lift argument than my e-book version.

I would be willing to work with any editor to resolve this issue. David Weyburne (talk) 15:03, 23 March 2023 (UTC)

Recent changes to equal transit time section

The diff is here: https://en.wikipedia.org/w/index.php?title=Lift_%28force%29&diff=1228641725&oldid=1227711027

I don't read the previous version as claiming that equal transit time never happens, only that it cannot be assumed. The "offending" passage is:

This is because the assumption of equal transit time is wrong. There is no physical principle that requires equal transit time and experimental results show that this assumption is false.

By way of analogy, regarding flipping a coin we could write:

This is because the assumption of it always landing heads-up is wrong. There is no physical principle that requires a coin to always land heads-up and experimental results show that this assumption is false.

I don't think that anyone would read that as claiming that coins never land heads-up, only that they don't always land heads-up. Likewise, ETT is not a general physical principle, but that doesn't imply that it never happens. I don't think we need this level of clarification and the recently added/changed language seems to me to make the section more difficult to read. Perhaps we could simply add a sentence to the effect of "ETT does occur in some situations, but when it does there is no lift." But I don't know that it's really necessary. I'll wait for other editors to weigh in before reverting the edit. Mr. Swordfish (talk) 14:24, 12 June 2024 (UTC)

Prior to my recent edit, Wikipedia’s emphasis was that equal transit time (ETT) is wrong, false, incorrect, misleading etc. In fact, the opposite is true. ETT represents the flow past most solid bodies. Airflow past a power line, past each strand of a wire fence, past every flag pole, satisfies the description of ETT. Every rain drop and hail stone that have ever fallen have experienced the 3-dimensional equivalent of ETT. It is only lifting flows that don’t exhibit ETT. Let’s say 99% of flows around solid objects can be described as exhibiting ETT; and only 1% of flows cannot be described in this way. Saying “the assumption of equal transit time is wrong” is a statement that can be soundly challenged unless it is clear that it is confined to lifting flows.

ETT is a very simple 3-word expression. Doug McLean describes it as “an argument that is widespread in explanations aimed at the layman.” (See Understanding Aerodynamics, section 7.3.1.4) A more sophisticated way of saying ETT is “the circulation is equal to zero”. There are many reliable sources that talk about flows where circulation is equal to zero.

Prior to my recent edit, Wikipedia said there is no physical principle that requires equal transit time ... This statement can be soundly challenged unless it is clear that it is confined to lifting flows. The Kutta–Joukowski theorem is a fundamental theorem in the field of aerodynamics and it clearly implies that a non-lifting flow around a body must have a circulation of zero! Similarly it implies that if the circulation is zero, the lift will also be zero. For circulation equal to zero, the layman may read ETT.

Wikipedia needs to say that ETT does not exist around a lifting body or around an airfoil experiencing lift but we need to be careful to avoid versions of this statement that are so universal in their applicability that they can be readily challenged. It can be challenged if Wikipedia implies that ETT is inherently false, or universally inapplicable. ETT is the usual state of affairs, and it is only in the very narrow field of lifting flows that it does not prevail and cannot be assumed. Dolphin (t) 06:15, 13 June 2024 (UTC)

> Airflow past a power line, past each strand of a wire fence, past every flag pole, satisfies the description of ETT. Every rain drop and hail stone that have ever fallen have experienced the 3-dimensional equivalent of ETT.

Is this true? Usually power lines, wires in fences, and flagpoles sway and move in the wind. What force is causing that movement? Do raindrops always fall straight down, or is there sometimes asymmetrical airflow that causes a horizontal force?

Flows with zero circulation are nice simple models so there are lots of textbook examples of that idealized condition. I'm highly skeptical that they occur in nature as the rule rather than the as a first order model in theory; for it to occur, I think you'd need to have the solid object be perfectly symmetrical, not rotating, and the airflow non-turbulent. Perhaps you can provide a reference for your 99% claim? Regardless, this tangent distracts from the main thrust of the section i.e. ETT is not a physical law like conservation of momentum, energy, or mass so it can't be assumed.

The previous version states that "the assumption of ETT is wrong". That's correct. And "There is no physical principle that requires ETT" That is also correct. We should stick by that. Mr. Swordfish (talk) 12:56, 13 June 2024 (UTC)

Interesting article that addresses the history of ETT. It's not peer reviewed so we can't cite it as a reliable source, but worth a read.

https://arxiv.org/pdf/2110.00690

On the Origins and Relevance of the Equal Transit Time Fallacy to Explain Lift

Graham Wild

School of Engineering and Information Technology, UNSW ADFA, Canberra, Australia

G.Wild@ADFA.edu.au

1st of October 2021

Preprint

Not Peer Reviewed

Abstract

Recently, aerodynamics syllabi have changed in high schools, pilot ground training, and even

undergraduate physics. In contrast, there has been no change in the basic theory taught to

aeronautical or aerospace engineers. What has changed is technology, both experimentally and

computationally. The internet and social media have also empowered citizen science such that

the deficiencies in the legacy physics education around flight and lift are well known. The long-

standing equal transit time (ETT) theory to explain lift has been proven false. If incorrect, why

was it ever taught? Through a historical analysis of relevant fluid and aerodynamics literature,

this study attempts to explain why ETT theory is part of our collectively lower-level cognitive

understanding of lift and flight. It was found that in 1744 D’Alembert himself assumed this to

be a feature of moving fluids, and while this initial intuition (ETT 1.0) was incorrect, the

property of ETT (ETT 2.0) was derived in 1752 when applying Newton’s laws of motion to

fluids. This incorrect result was independently confirmed in 1757 by Euler! The conclusion is

that an over simplified treatment of fluids predicts ETT, along with no lift and drag. This then

leads to the open question, can ETT be taught at an appropriately low level as an explanation

for lift? Mr. Swordfish (talk) 12:51, 14 June 2024 (UTC)

I don’t accept your arguments. I explained my arguments in significant detail but you haven’t engaged with that detail or responded to it adequately. For example, I have written about non-lifting flows and you have responded with a little original research suggesting that flows with zero circulation are non-existent or rare.

If you wish, you could make a reasonable defence of the sentence I amended by arguing that the surrounding context makes it clear to all readers that the entire section, and the article, apply exclusively to lifting flows so if Wikipedia says the assumption of ETT is wrong it is not referring to non-lifting flows. I won’t automatically buy that argument but perhaps I will eventually if it is explained persuasively. It is an argument that has much greater potential than the arguments you put forward in your previous edit.

You have written “the assumption of ETT is wrong. That’s correct.” No, it’s not correct in the case of non-lifting flows. I have explained that in detail.

You have written “There is no physical principle that requires ETT. That is also correct.” No, it isn’t correct. The Kutta–Joukowski theorem is a physical principle and it requires ETT in non-lifting flows. I have explained that in detail. Dolphin (t) 13:13, 14 June 2024 (UTC)

I think we both agree that ETT is not a valid assumption for an airfoil with lift. I think we also both agree that there is a body of scholarship that does make the simplifying assumption of ETT in some specific examples. That doesn't imply to me that ETT is the usual state of affairs any more than the assumption of a spherical cow implies anything about the shape or real-world cows.

You assert that "ETT represents the flow past most solid bodies". But you have not provided a citation for that. I'm highly skeptical that this is true since just about everything moves and flutters in the wind. As Norman Smith's paper states:

...the claim that the air must traverse the curved top surface in the same time as it does the flat bottom surface...is fictional. We can quote no physical law that tells us this.

That is, in general there is no physical law that requires ETT. That's not to say it never happens, or that no physical models ever make that simplifying assumption (and when they do, the result is zero lift). Whether "most solid bodies" exhibit ETT is somewhat orthogonal to this section, so perhaps we don't need to settle that here. I do think that the recent additions and changes are a distraction and make the section less readable. I'll take a look at improving the readability while keeping your concerns about overstating the invalidity of ETT. Mr. Swordfish (talk) 16:04, 14 June 2024 (UTC)

You and I both have a thorough understanding of the Kutta–Joukowski theorem. I believe the expression “equal transit time” may be a layman’s way of saying the circulation is equal to zero; I hope we agree on that.

A small part of the problem is that ETT is not a well-defined or rigorously defined expression. To the best of my knowledge this expression is only used by authors who are repudiating this attempt at an explanation of aerodynamic lift. To the best of my knowledge none of the authors and institutions that resort to this naïve explanation of lift actually use the expression “equal transit time“; no-one actually asserts that “ETT” is true or correct. There are only people like us who assert that ETT is not correct (when applied to a body generating lift.)

Your quote from Norman Smith describes a body with “the curved top surface” and “the flat bottom surface.” He is not referring to “most solid bodies” - he is describing an airfoil!

I can supply a quotation from Anderson’s “Fundamentals of Aerodynamics” that will help on this topic. I expect to get access to my copy of Anderson within 7 days. Dolphin (t) 14:30, 15 June 2024 (UTC)

Agree that ETT is not well defined, and that it doesn't appear to be used other than by those repudiating it. Searching for the phrase (or even the word "equal") on my user page collection of works presenting ETT as correct[1] only finds that in the references, not the actual works themselves. Similarly, the obstruction explanation is sometimes derisively referred to as "hump theory" but it's proponents don't use that phrase.

A typical turn of phrase is "The air moving on the top has to travel a greater distance in the same amount of time." or "Air flowing over the top has a greater distance to travel in the same time; that's why it flows faster."

I don't know that the expression “equal transit time” is a layman’s way of saying the circulation is equal to zero, since I would surmize that those advancing the idea probably don't know what circulation is. That said, here's a source basically confirming that ETT and Γ=0 are the same idea.

Regarding whether most flows around solid objects exhibit ETT, if that were true than vortex shedding and Vortex-induced vibration would not pose problems for engineers to overcome.

References

Mr. Swordfish (talk) 16:20, 15 June 2024 (UTC)

Edits finished. Hopefully that addresses the concerns above. Mr. Swordfish (talk) 16:36, 14 June 2024 (UTC)

Your recent edit to the article is an acceptable alternative to my edits. Thank you for making those changes.

The article now avoids giving readers the impression that ETT is inherently false. Hopefully readers can now see that the only falsehood is suggesting ETT exists in the flow around a lifting body. Dolphin (t) 14:46, 15 June 2024 (UTC)

Thanks for the reference to “Flight Physics:Essentials ...” I was not aware of that publication. It looks like it might be essential!

Vortex induced vibrations are an oscillatory phenomenon. They become a problem in structures that have inadequate stiffness or inadequate damping. In our article on lift we are talking about steady flows with zero viscous effects, or only minor viscous effects. We use a reference frame attached to the airfoil or solid body so the consequences of oscillations of a solid body are way beyond the level of analysis we are using in this article, and related articles.

Could it be that after half a lifetime of believing that ETT is false, the work of the devil, it will take a major change of direction to accept that there is nothing false or distasteful about ETT? Could that be why you are finding reasons to deny the inevitability of flows in which circulation is zero, ETT prevails and lift is zero? Dolphin (t) 03:46, 16 June 2024 (UTC)

It's not that I don't believe lift can be zero (and that implies ETT). I just don't think it occurs as often as you seem to think it does i.e. 99% of the time a solid body is immersed in a moving fluid. That's because almost all real world objects are not perfectly symmetrical and that implies an asymmetrical air flow hence non-zero circulation.

Stated another way, ETT is not a valid assumption in general. If you assume ETT, you will get zero lift. I don't have a cite for this and I am willing to consider evidence to the contrary, but real-world airflows around solid objects with zero circulation are the exception rather than the rule. For instance, consider a symmetrical airfoil in a steady flow - it is well established that the lift varies by the angle of attack. For the special case of zero AOA, the lift is zero and ETT occurs (in this simple 2-d model). For all the other values there is lift, circulation is non-zero, and ETT is false. In mathematical terms, the set of values for which ETT holds has measure zero. That's about as rare as you can get without it being never.

Perhaps there is some area of aerodynamic research that assumes ETT or decides that lift is small enough that lift is negligible - many treatments ignore viscosity, or compressibility for example - I'm not aware of any that assume zero lift, but maybe there are. Let me know if you know of any. Mr. Swordfish (talk) 13:00, 16 June 2024 (UTC)

There are several elements of your edit on which I can comment but at present I only have time for one. I will comment on others later.

You write about “real-world solid objects with zero circulation ...” Then you make a sneaky gear change and write about “a symmetrical airfoil ...” The two are very, very different in aerodynamics so your gear change doesn’t go unnoticed. Yes, a well-designed airfoil will produce lift (and lift coefficient and circulation) that varies approximately linearly with angle of attack. The feature of a well-designed airfoil that yields this desirable property is the sharp trailing edge. Clancy’s book Aerodynamics addresses the role of the sharp trailing edge and the way it causes vortex shedding to adjust the strength of the bound vortex to maintain the Kutta condition. I don’t have Clancy with me but I think it is Section 4.5 and/or 4.8 that contains good explanatory diagrams.

In the absence of a sharp trailing edge, any change in orientation of a body is not accompanied by a change in lift (or lift coefficient or circulation.) For example, a cylinder with elliptical cross section, immersed in a flow produces little or no lift; altering the orientation of the cylinder doesn’t produce much change. What lift might be produced is due to asymmetric boundary layers and separated flow, rather than due to the primary flow predicted using an inviscid fluid. If a body doesn’t have a sharp trailing edge, and the orientation of that body is changed, the fluid flow adjusts itself so that circulation remains zero. Circulation greater than zero requires the Kutta condition, and the Kutta condition requires a feature resembling a sharp trailing edge. Airfoils have sharp trailing edges, but real-world solid objects don’t. That is why the only circulation and lift that are observed on bodies without sharp trailing edges is the small amount caused by asymmetric boundary layers on the two sides of the body, separated flow and possibly other minor viscous effects.

Scientists and engineers have to work hard to generate circulation and lift. Typically they use airfoils with thin, sharp trailing edges even though this feature is structurally weak and vulnerable. Flowing fluids are uncooperative - as they flow around bodies their natural state is doing so with zero circulation. Any change in orientation of a real-world solid body causes the fluid to change its flow pattern to avoid circulation developing. If it were not so, aircraft designers would use wings with thick, generously rounded trailing edges so they could get more fuel into the wings, use deeper and lighter spars, and have more room into which to retract the undercarriage. Dolphin (t) 16:18, 16 June 2024 (UTC)

On the matter of the sharp trailing edge there is a very useful quotation by George Batchelor in the short article Trailing edge.

There is also a useful quotation by Richard von Mises at Airfoil, reference number 4. Dolphin (t) 00:38, 17 June 2024 (UTC)

The conventional wisdom is that fluid flow around a real-world solid body experiences zero circulation. Picture the wind blowing around such a body, and then the wind changes direction. Imagine that this change causes a circulation to begin in the flow. This circulation causes a lift force to act on the solid body. Newton’s 3rd law tells us that an identical lift force acts on the flowing air. When a fluid that is free to flow or change shape is subjected to a force or pressure it responds in whatever way will cause that force to diminish. Consequently the lift force on the air flowing around the solid body causes the streamlines, velocities and pressures to change to diminish the circulation that has just begun. This process can be expected to continue until all circulation has been eliminated. Only then has equilibrium been achieved within the flow pattern around the body.

Any residual circulation and lift is not related to the primary flow as would exist in a geometrically similar situation but with an inviscid fluid. It is related to the secondary flow caused by viscous effects such as flow separation. Any residual lift is still accompanied by the original drag force. The lift to drag ratio is so small that this solid body doesn’t qualify as an airfoil. I believe this is an explanation for the operation of oddly shaped lifting bodies which glide without conventional wings. Dolphin (t) 05:22, 17 June 2024 (UTC)

I'm continuing this discussion since I think I may learn something. I'm not trying to be "sneaky", just trying to understand what evidence there is that zero-circulation/zero-lift/ETT is the usual or normal state of affairs rather than a rare exception.

>Circulation greater than zero requires the Kutta condition, and the Kutta condition requires a feature resembling a sharp trailing edge.\

Agree that Kutta condition requires a sharp trailing edge, because without one it's not obvious where the rear stagnation point occurs. And without that it's not clear how much circulation to apply to model the fluid re-joining at the rear stagnation point. But you don't need a sharp trailing edge to have an asymmetrical airflow with non-zero circulation, you just can't apply the Kutta contidion. As Gale Craig states, (paraphrasing) you don't need an airfoil shape to get lift, as anyone who has ever handled a sheet of plywood in the wind knows. Of course, if you want enough lift to fly a plane of propel a sailboat, you'll want something with more lift than a non-arifoil can provide. That doesn't mean only airfoils with sharp trailing edges can generate lift.

I have sailed boats with rudders that have a rounded trailing edge. Performance is sub-optimal, but the rudder most definitely provides enough lift to steer the boat. When I look at leaves on trees or flags on a flagpole in the wind, they never settle down into an equilibrium of zero lift as you describe above. Spinning balls have lift, as any tennis player understands. Here in the US, there's a baseball pitch called the knuckleball where the ball is thrown with a little spin as possible, with the effect that it's impossible to predict which direction the lift will take the ball making it very hard to hit. So, my experience is quite at odds with your assertions.

You say that "The conventional wisdom is that fluid flow around a real-world solid body experiences zero circulation." but don't provide anything to back that up. Along with your 99% figure, I would need some more to go on than your assertion.

Agree that my examples above are anecdotal or original research. Here's an interesting treatment of bluff bodies [2] which seems to be in conflict with your assertion that Circulation greater than zero requires the Kutta condition...

For bluff bodies, the interest is usually in the drag on that body, mainly because experiments have found that drag is the dominant force. This observation, however, does not imply that bluff bodies cannot produce lift because many do. Nevertheless, examining just the drag characteristics of such bodies is convenient in the first instance. Furthermore, bluff bodies may also produce pitching moments, which sometimes need to be known for certain types of engineering work, e.g., to determine torsional loads.

Mr. Swordfish (talk) 13:56, 17 June 2024 (UTC)

Here's an excerpt from another paper BLUFF-BODY AERODYNAMICS

Bluff bodies are obviously also subjected to forces in the across-wind direction and to

moments around the various axes due to non-symmetries of the pressure distribution on their

surface. Therefore, these loads depend fundamentally both of the body shape and on the

orientation of the incoming freestream. Particularly in the two-dimensional case, the force

component in the across-wind direction is often called lift force, in analogy to the

corresponding force acting on an aeronautical wing section (airfoil).

(elision of details about the starting vortex and consequential circulation around an arifoil)

Coming back to bluff bodies, the above described mechanism does not apply in all its

details, particularly because the boundary layer cannot remain attached to their surface even

after the end of the initial transient. However, if the body is sufficiently elongated (like an

ellipse), a starting vortex is shed anyway (even if not as strong as that of an airfoil), and the

asymmetry of the final flow configuration for non-symmetrical wind orientations may be

sufficient for producing significant lateral forces.

Seems to me that if it were the case that almost all bluff bodies experience zero lift the paper would say that at some point. Mr. Swordfish (talk) 14:32, 17 June 2024 (UTC)

One of the frustrating aspects of discussing this subject is the variation in meaning given to the word “airfoil”. On these Talk pages I see the word used with three different meanings:

1. A two-dimensional shape that can be employed in three-dimensional bodies to generate lift. For example, the shape known as NACA 2412 is an airfoil section commonly used for the wings of low-speed aircraft.
2. A three-dimensional body that generates at least a little lift. Some Users point to an irregular body or a sheet of plywood or a sycamore seed and, noting that it experiences a small lift force, say “see, it is an airfoil!”
3. A three-dimensional body that, over a usable range of angle of attack, is capable of generating significantly more lift than drag. With this meaning, airfoils are manmade structures that have the generation of lift as their primary purpose. Airfoils are carefully designed and manufactured structures to ensure the lift-to-drag ratio is high enough to achieve its intended purpose.

Wikipedia’s current definition of airfoil closely matches No 3 above. Airfoil says:

When the wind is obstructed by an object such as a flat plate, a building, or the deck of a bridge, the object will experience drag and also an aerodynamic force perpendicular to the wind. This does not mean the object qualifies as an airfoil. Airfoils are highly-efficient lifting shapes, able to generate more lift than similarly sized flat plates of the same area, and able to generate lift with significantly less drag. Airfoils are used in the design of aircraft, propellers, rotor blades, wind turbines and other applications of aeronautical engineering

The layman imagines that the essential feature of an airfoil (meaning No 3) is its generously rounded leading edge, or its curved surface. In fact it is the trailing edge. That is partly the explanation of why a flat sheet of plywood will experience lift in a flow of air - it has a sharp trailing edge.

Since the days of Joukowski and Kutta, mathematicians and physicists have been able to model the flow of an inviscid fluid around suitable geometric shapes. With a sharp trailing edge it is possible to determine the lift and pitching moment on the shapes. Tests on real models of wings in wind tunnels show there is close agreement between the math and the real world for these shapes with sharp trailing edges. For bodies without a sharp trailing edge, the math shows that an inviscid fluid imparts no lift or pitching moment to the body.

Wind tunnel tests on bodies without sharp trailing edges, and anecdotal evidence, show that these bodies can experience a little lift. This does not mean they qualify as airfoils under meaning No 3 above. Engineering, and most science, have little interest in these bodies. What lift they develop is not due to airfoil action - exploiting the Kutta condition to generate lift. It is due solely to viscous effects such as flow separation. These bodies, at best, have a very low lift-to-drag ratio. Little is written about them in mainstream science or engineering publications. This type of lift has little or no engineering application.

We know that eating a tablespoon of salt a day won’t cure cancer, but it is probably impossible to find a reliable published source that confirms eating a tablespoon of salt a day won’t cure cancer! Similarly it is probably impossible to find a reliable published source that confirms that no bluff body has ever been found that is capable of a high lift-to-drag ratio.

We use the Kutta condition to determine, mathematically, the circulation around a 2-D shape with a sharp trailing edge edge. There is no similar model, theory or equation to determine circulation around a 2-D shape with no sharp trailing edge. I suspect that wind tunnel tests would not show a usable relationship because, being reliant entirely on viscous effects, the results would be strongly influenced by the surface conditions of each model being used - roughness, smoothness, manufacturing imperfections etc.

When I say that bodies without sharp trailing edges do not generate circulation in fluid flows around them, I am speaking as an aerodynamicist applying the model of the inviscid fluid. There is no doubt that my statement is true for inviscid flows, which admittedly are fictitious, but this is usually a good, simple guide to the reality of high Reynolds number flows. When you say that all bodies in a fluid flow experience viscous forces and these forces will provide at least a very small amount of circulation that cannot be eliminated by the flow pattern adjusting itself you are possibly speaking as a scientist focussed on observing the complex realities of the real world. You aren’t able to determine how much circulation there will be, or say exactly how that circulation is sustained. What circulation exists is small and I say it is zero. You possibly describe the same situation by saying circulation is not zero. That might be as close to consensus as we can hope to reach. Dolphin (t) 15:39, 17 June 2024 (UTC)

Agree that I am sometimes a bit loose with the terminology re: airfoil. One other possible avenue of miscommunication here is that when I see the word "lift" in this context I think of the definition used in the first sentence of the article:

When a fluid flows around an object, the fluid exerts a force on the object. Lift is the component of this force that is perpendicular to the oncoming flow direction.

and as a mathematician rather than an aerodynamic engineer lift=0 means actually zero, as opposed to "too small to be useful or significant." One of the arts of engineering is to figure out what things can be ignored, and for most non-airfoil applications the fact that there is some component of the aerodynamic force perpendicular to the airflow is negligible. I'm sure that there are many situations where we would agree that whatever small amount of lift might be present, it's too small to matter so let's assume it is zero. This would imply ETT in that situation.

Other situations I wouldn't agree that it's too small to matter, for instance, a leaf on a tree in a breeze - the leaf repeatedly flutters back and forth in a direction perpendicular to the airflow and this implies to me that there is some force making it move that way and the obvious one is that there is some non-zero component of force transverse to the airflow. I would call that "lift" according to the definition above. But since I doubt either of us will be hired as an engineer to design tree leaves any time soon we can leave it there. Mr. Swordfish (talk) 17:56, 17 June 2024 (UTC)

Thanks. I agree with most, if not all, of what you have written. I now realise that the concepts of streamlines, time slices, circulation and ETT are all concepts that rely on steady flow. When we are talking about a turbulent wake, separated flow, oscillatory flow, the erratic dancing of the leaves and branches of a tree, we can’t claim the protection offered by retreating to steady flow. Debating about streamlines, time slices and ETT in a non-steady flow is deeply flawed.

The dancing of leaves on a tree is definitely caused by the interaction of aerodynamic forces and elastic forces within the highly flexible structures of a tree. This kind of motion could be caused entirely by drag, so I’m not persuaded that the dancing motion of a leaf necessarily shows the presence of lift.

The concepts of lift and drag rely on knowing the direction of the local velocity of the fluid. The air moving through the branches and leaves of a tree is highly disorganised and the velocity at each point is changing rapidly so it is probably true to say that while we can possibly identify aerodynamic forces acting on branches and leaves, the concepts of a drag component and a lift component are not applicable. The distinction between a lift component and a drag component seems to be reliant on steady flow, and flow in which the speed and direction at one point is almost identical to the speed and direction at all nearby points.

The Kutta-Joukowski theorem is remarkably similar to Newton’s 1st and 2nd laws. Scientists and engineers say Newton’s laws are valid. Perhaps a mathematician and philosopher might say Newton’s 1st law is redundant because there is no such thing as a body whose acceleration is truly zero; and no such thing as a body experiencing a net force that is truly zero. Dolphin (t) 00:30, 18 June 2024 (UTC)

In my edit dated 15 June 2024 (14:30) I wrote “I can supply a quotation from Anderson’s Fundamentals of Aerodynamics that will help on this topic.” See the diff. In section 3.16 Anderson writes about the Kutta-Joukowski theorem:

"Although the result given by the equation $L^{\prime }=\rho _{\infty }V_{\infty }\Gamma$ was derived for a circular cylinder, it applies in general to cylindrical bodies of arbitrary cross section."

This confirms that the Kutta-Joukowski theorem is not confined to airfoils. It applies to all cylindrical bodies regardless of their cross sectional shape. If a cylinder of arbitrary cross section causes no circulation in the flow in which it is immersed the cylinder will experience no lift.

It is not too great a leap to say that, just as airfoils are associated with the Kutta condition to explain when they will generate lift, and when they won’t, cylindrical bodies of arbitrary cross section also rely on a feature resembling a sharp edge to obtain a well-defined lift. If these bodies of arbitrary cross section experience lift in the absence of a sharp edge, it is due to viscous effects such as flow separation and asymmetric boundary layers, rather than due to airfoil action.

My mention of a well-defined lift is from "sharp trailing edge to obtain a well-defined lift" as written by Richard von Mises. See citation No. 4 in Airfoil. Dolphin (t) 12:44, 30 June 2024 (UTC)