Talk:Lift (force)

Increased flow speed and Bernoulli's principle

There is a cycle: "For any airfoil generating lift, there must be a pressure imbalance" → "an increase in speed" → "a lower pressure". It needs a correction. Probably without "False explanation based on equal transit-time" this explanation lost sense (not Bernoulli's principle itself, of course). --109.174.113.181 (talk) 18:29, 12 June 2021 (UTC)Serg

I'm trying to understand the nature of the confusion here, but I can't make sense of the above comment. Perhaps others can explain. I'll give it a few days and then remove the confusion tag if no further comments. Mr. Swordfish (talk) 15:30, 3 July 2021 (UTC)

Now, having read the comment several times and also the article it appears that the comment author 'is' confused. They seem to think that the article states that "an increase in speed" → "a lower pressure". But it doesn't say that. However, given the prevalence of "explanations" that say the pressure differences are caused by speed differences I can see how one might think our article says that. I propose the following language, which might help clear things up:

Bernoulli's principle states that 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. For any airfoil generating lift, there must be a pressure imbalance, (i.e., lower average air pressure on one side than on the other). Bernoulli's principle implies that these changes in the air pressure must be accompanied by changes in the air speed. In the case of an airplane wing, air flows faster and at a lower pressure over the top of the wing than under it.

Mr. Swordfish (talk) 13:12, 6 July 2021 (UTC)

If we have "a pressure imbalance", then we have the lift. So if it states "there must be a pressure imbalance", we don't need speeds and Bernoulli's principle to explain the lift. On the other hand, the actual contribution of Bernoulli’s principle to the lift theory is not required for a simple explanation. 109.174.113.183 (talk) 20:04, 8 July 2021 (UTC)Serg

I disagree with the comment above from 109.174.113.183. The occasion when we must make use of Bernoulli's principle is not when we begin "there is a pressure imbalance"; it is when we begin "the air flows faster past one side of the airfoil than past the other side." That demands an explanation as to why a difference in flow speed leads to a pressure imbalance - and the answer is "Bernoulli's principle!"

I agree that it is possible to point to the existence of lift in airfoil action and accurately state that there is a pressure imbalance whenever the airfoil meets the oncoming air at an angle of attack greater than zero; Bernoulli's principle is not required because it is a simple but accurate description of airfoil action. This is an explanation that is reasonable and sufficient for many people, but for others it is too simple and completely fails to relate lift to the known kinematics of the flow field - why does the air flow faster past one side than the other etc.? For this latter group of people a sufficient explanation is available when they begin to take into account the Kutta condition and the Kutta-Joukowski theorem, and these two concepts ultimately require that use is made of Bernoulli's principle. Dolphin (t) 07:41, 9 July 2021 (UTC)

In the text, it starts from "For any airfoil generating lift, there must be a pressure imbalance". After that we don't need "air flows faster ..." since we have "a pressure imbalance". Moreover, this explanation doesn't describe a source. It can be based on cause-and-effect misunderstanding only.

I don't think this explanation is simple. Probably it is habitual (rather than clear) for people who heard about "equal transit time" theory. As an example, in comparison to the flow turning explanation:

• it doesn't explain a source (this leads to misunderstandings).
• it's impractical and unsafe. For example:
  • the stall warning is based on low pressure sensor. So we have low pressure over the wing, but the airplane falls.
  • it doesn't explain why it is dangerous to fly behind an airliner.
• it doesn't explain wingtip vortices (and this can be observed in contrast to speed difference of the airflows).
• this doesn't help to understand wing configuration (e.g. aspect ratio) because it is based on wing area.
• it is harder to explain Bernoulli's principle rather than Newton's third law.

Bernoulli's principle can be useful in advanced practices (e.g. KTAS vs. KIAS calculation).109.174.113.178 (talk) 14:39, 11 July 2021 (UTC)Serg

I agree with you that the flow turning explanation is both simpler and more useful than the Bernoulli explanation, especially for an audience that is not mathematically inclined. I also agree that you don't need to know that the air is going faster in order to have a basic concept of lift. However, the changes in speed are a very real phenomenon and just about every reliable source includes a treatment of Bernoulli's principle, so we would be remiss as wikipedia editors if we didn't include something about it. So, I'm not sure what you are asking for here. Do you want us to remove all references to Bernoulli's principle in the article? Something else? Do you think the material is incorrect or misrepresents the reliable sources it's based upon? Mr. Swordfish (talk) 17:32, 11 July 2021 (UTC)

Yes, there are a lot of incorrect explanations based on misinterpretation of Bernoulli's principle. For example, FAA PHAK describes lift via venturi tube and refers to NASA at the same time (I guess this is not a mistake because NASA published description about Venturi nozzle in 2000). So I propose to mark the current section "Increased flow speed and Bernoulli's principle" as false explanation in addition to "False explanation based on equal transit-time" (this should be clear enough, since we have already references to McLean about correct usage of Bernoulli's principle). For example, the seminar from University of Michigan Engineering has a good structure for a simplified explanation. It even explains the pressure imbalance (same as Holger Babinsky) [however, 13:45 - "you don't need to go into pressure in order to explain lift"]. According to this seminar or NASA we have three false explanations wrong1 "Equal transit time", wrong2 "Particle kinetics" based on Newton's third law, wrong3 "Venturi tube", and correct one.

In regards to Bernoulli's principle description in a comprehensive explanation, I think the current picture of the pressure field around an airfoil could mislead. Color gradients (like this ) could help to understand that the lowest pressure is not at the trailing edge. The current picture shows the same (almost), but it requires more attention to understand.109.174.113.177 (talk) 15:53, 12 July 2021 (UTC)Serg

Serg, you propose to mark the current section "Increased flow speed and Bernoulli's principle" as false explanation. To do so, you will need to find a reliable source that declares it to be false, and also convince the other editors that the reliable sources that the article is based on are wrong. I sincerely doubt that you will be successful.

There is nothing incorrect about the current section. I'm willing to entertain the notion that it may be confusing, or that it could be written in a more easily understood manner. But it's not wrong and marking it as a false explanation would itself be false. Both Bernoulli's principle and Newton's laws can be used to correctly explain lift. Both have also been used to incorrectly explain lift - that doesn't mean that every explanation using one or the other is wrong. If you read past the small section under discussion, I think you will see that most of your concerns are addressed later in the article. Mr. Swordfish (talk) 20:06, 12 July 2021 (UTC)

I see three discussion points about the section:

1. I agree, the section properly describes Bernoulli's principle. No issues here.
2. The explanation is false because it is based on cause-and-effect misunderstanding.
3. Probably it is not easy to explain this misunderstanding simply. Thus in regards to your "we would be remiss as wikipedia editors if we didn't include something about it", I propose to add popular false explanation about Venturi tube (instead of the current section). It mentions Bernoulli's principle, and no problems with refs.

A comprehensive explanation could require both Newton's laws and Bernoulli's principle (and maybe mention of adverse pressure gradient). A simplified explanation don't need Bernoulli's principle or pressure imbalance.109.174.113.177 (talk) 11:25, 13 July 2021 (UTC)Serg

1. Good. We have agreement that the section correctly describes Bernoulli's principle.
2. Where does this section say anything about cause-and-effect? If you think it does, then either you are reading something into it that is not there, or I'm missing something. Could you be more specific?
3. Agree that it's not easy to explain. I have no issues with adding a subsection on the venturi tube false explanation, as long as we point out that it is false. However, I don't think removing the four sentence introduction that simply states Bernoulli's principle and how it applies to an airfoil is the right approach. If we're going to discuss BP then we have to say what it means first. I'll propose a redraft.

As for whether BP should be part of a simplified explanation, I'm inclined to agree with you. If we hadn't had a century of the false equal-transit-time explanation being the more or less standard explanation then my preference would be to move the BP material further down in the article as part of the mathematical treatment and only give the flow-turning Newton's 3nd law explanation in the "simplified" section. But, the reliable source material treats BP prominently (often incorrectly, although the more reliable sources treat it correctly) so I think as editors we are bound to reflect that in the article. That said, if there is consensus to pull BP out of the simplified section I will readily agree. I doubt there will be. Mr. Swordfish (talk) 14:00, 13 July 2021 (UTC)

2 and 3 are addressed below.

By "misrepresents the reliable sources it's based upon". There are two references #12 and #13 that could be refined. The 13th doesn't seems to be reliable source. It mixes statements e.g. it refers to FAA PHAK and Jeppesen’s PPM (that use Venturi tube to describe lift) as to textbooks with a valid Bernoulli-based explanations and manifest Venturi tube as a false base at the same time. The 12th states the following:

• "Newton’s laws may be a simpler description as long as one does not need to evaluate the details of the flow field."
• "conservation of mass and conservation of energy, Bernoulli’s law" and "conservation-of-linear-momentum principle, Newton’s laws" are "mathematical models that correctly calculate the force".

I have two concerns here:

1. I think it could be helpful to point that BP is not enough for classic Bernoulli-based explanations (disregarding unexplained reasons mentioned in the limitations section). In pilot schools BP is manifested as a complete explanation. In fact it requires conservation of mass and conservation of energy including BP.
2. Regarding mathematical models we have already "This explanation is largely mathematical, and its general progression is based on logical inference, not physical cause-and-effect." with ref to McLean at Lift (force)#Circulation and the Kutta–Joukowski theorem section. Probably this source is not enough to proof equivalence of simplified physical explanations.

109.174.113.170 (talk) 06:28, 15 July 2021 (UTC)Serg

I agree that it is possible to explain aerodynamic lift without including a discussion of Bernoulli's principle. However, a description of the physics would be incomplete without saying that as the air flows from a region of ambient pressure to a region lower pressure it must speed up. More pressure behind than in front accelerates the air. That's Bernoulli's principle, and at least one of our sources claims that the change in pressure causes the speed increase, rather than the other way around. It's the [speed increase] -> [pressure drop] causality so common in the incorrect explanations that causes the confusion. We've tried to avoid that in the article. Mr. Swordfish (talk) 16:03, 9 July 2021 (UTC)

Airflow acceleration is a part of a comprehensive explanation. So, the airflow turning is a simple explanation , and as an extension we have the acceleration and conservation of momentum.109.174.113.178 (talk) 14:39, 11 July 2021 (UTC)Serg

I don't find the paragraph confusing, but I concede it does little to clarify the relationship between Bernoulli's principle and the phenomenon of aerodynamic lift, and therefore could be confusing to many readers. I will give some thought to some words that might be an improvement. Dolphin (t) 13:24, 6 July 2021 (UTC)

The way I explain it to my sailing students is:

The air flows around the sail and changes direction. That means that the sail has exerted a force on the air, and according to Newton's 3rd the air must exert a force on the sail that is equal in magnitude and opposite in direction. How does the air do this? The only way it can: through air pressure. (I elide over shear stress to keep it simple). So there is a region of reduced pressure on the outside surface of the sail and a higher pressure on the inside. As the air flows along the outside surface it moves from a region of ambient pressure to a region of lower pressure. Since there is more pressure behind than in front, the air speeds up and goes faster as it travels along the outside surface of the sail. Is this important? Not really; I can't think of a single practical implication that is useful for trimming your sails. But it has to happen. You can't have the force without the pressure difference, and you can't have the pressure difference without the speed change. Sometimes this algebraic relationship between speed and pressure is used in the mathematical analysis of how sails work, but it doesn't do much for you out on the water.

Putting this into the article would probably be categorized as synthesis, but perhaps we could use it as a starting point for a rewrite to better "...clarify the relationship between Bernoulli's principle and the phenomenon of aerodynamic lift." Mr. Swordfish (talk) 12:49, 10 July 2021 (UTC)

The discussion here has highlighted some problems in the existing presentation of this sub-topic. Rather than trying to patch up the existing presentation I want to propose a complete re-write. Please see my proposed re-write in my sandbox. All comments will be welcome. Dolphin (t) 07:28, 12 July 2021 (UTC)

I do not think introducing the concept of circulation and the Kutta condition at this point in the article will reduce confusion. Just the opposite IMHO. The main section is titled "Simplified physical explanations of lift on an airfoil" That's should not include circulation and the Kutta condition.

My take is that the problem with the current sub-section is that we studiously avoid making any concrete statements regarding cause and effect. While the bulk of the incorrect Bernoulli-based explanations say that the air speeds up for some reason and this causes the pressure to be lower, our presentation is much more passive - "an increase in speed must accompany any reduction in pressure". My sense is that most lay readers are looking for some cause-and-effect language and when they don't get it don't feel like the article has answered their question. I think the section taken in it's totality eventually clears this up, but not until somewhat later. I'm thinking that perhaps re-ordering the material might help, but thus far I haven't come up with a coherent approach. My thought is that he conservation of mass explanation could wait until later since it's not so "simple" (and really is not much of an explanation either). But as it's discussed in the following section, we'd need to move that too and I'm not seeing a simple way to do that. I'll give it some more thought. Mr. Swordfish (talk) 20:30, 12 July 2021 (UTC)

I agree, it seems complex for an introduction. I also agree, cause-and-effect language will work better for a simplified explanation. The article has a good start: propeller, helicopter rotor, wing share the same physical principles and work in the same way. Thus fans, propellers, helicopter rotors and wings generate lift and produce airflow in the same manner. Airflow is evident for fans, propellers and helicopter rotors; it is also observable as wingtip vortices for wings. The following section about flow turning describes how it happens.109.174.113.177 (talk) 11:25, 13 July 2021 (UTC)Serg

@ Mr swordfish We are talking about the section titled "Simplified physical explanations of lift on an airfoil". On close examination, the explanation under scrutiny uses Bernoulli's principle to explain why the air flows faster over one side of the airfoil than over the other. It isn't using Bernoulli to provide an explanation of lift on an airfoil, but rather the other way around. Perhaps this is why the originator of the thread (Serg) found it confusing. I take your point that my first proposal isn't a simplified explanation and is therefore inappropriate. I have revised my proposal to simplify it. Please peruse it at my sandbox.

As has been discussed many times on these Talk pages, no-one has ever claimed that Bernoulli's principle can be applied to an airfoil section at a particular angle of attack in order to determine the lift. Bernoulli's principle can only be applied once the kinematics of the flow field are known. Bernoulli's principle reveals nothing about the kinematics but critics of Bernoulli seem invariably to dismiss it on the grounds that it fails to provide the complete solution of the lift problem from A to Z. First, the kinematics of the flow field must be determined and that is typically done by such methods as the Kutta condition and the Joukowski airfoil or similar.

I suspect a rigorous "cause-and-effect" explanation will prove elusive. We have seen debates about which occurs first - the increase in speed or the reduction in pressure. As happens in so many areas of physics, two or more quantities are observed to change simultaneously, and it is naive to imagine that one changes first and drives the change in the second. (In the cases of a venturi or an airfoil, it is the shape of the solid body and its orientation to the flow that causes flow speed and pressure, and sometimes also density and temperature, to change simultaneously.) Dolphin (t) 13:19, 13 July 2021 (UTC)

I don't think this revision is an improvement. The first sentence ("When an airfoil is inclined to the oncoming flow at a non-zero angle of attack, the flow on one side of the airfoil moves at a faster speed than the flow on the other side.") is crying out for a tag. While it is technically true, some readers may think it implies the pressure differences are caused by the speed differences. A decade or so ago, after the equal transit time fallacy (ETTF) had been well publicized, many lay-person explanations changed their materials to say "the wing is designed to make the air go faster over the top..." to avoid ETTF. While this is a technically true statement, it's misleading and doesn't begin to explain how one would go about designing such a wing.

Let me give it another try, providing context and building on the previous subsection:

As stated in the previous subsection, the air exerts an upward force on the wing when the air is deflected downward. This upward force manifests itself as air pressure with a lower air pressure on the top of the wing than on the bottom. Bernoulli's principle states that there is a relationship between the pressure in a fluid and the speed of the fluid, with lower pressure implying higher speed and vice versa. For any airfoil generating lift, there must be a pressure imbalance, (i.e., lower average air pressure on one side than on the other). Bernoulli's principle implies that these changes in the air pressure must be accompanied by changes in the air speed. In the case of an airplane wing, air flows faster and at a lower pressure over the top of the wing than under it.

I think this revision avoids the pitfalls of cause-and-effect or "which happens first" while providing some logical reasoning as to why the pressure is lower. The current article "buries" the reason there are pressure differences in the second sentence, which may be the source of the confusion. Mr. Swordfish (talk) 14:34, 13 July 2021 (UTC)

Mr swordfish, by our conversation about cuase-and-effect misunderstanding above, I agree with you that the current text can be interpreted differently. For example, by the version in the sandbox one might think it manifests [speeds]→[pressure imbalance]→[lift]. The current text of the subsection ends with "a lower pressure over the top of the wing than under it" that implies lift. Note, this subsection is intended to explain the lift force. I guess you read it as explanation of speeds only. In that case it is correct. I think that "As happens in so many areas of physics, two or more quantities are observed to change simultaneously" from Dolphin51 sounds better than "accompanied". Since the pressure and speed are changed at the same time and this change doesn't produce energy, it cannot explain the force. But we need BP to construct a wing (or to calculate the airspeed by the pressure using pitot tube).109.174.113.177 (talk) 06:15, 14 July 2021 (UTC)Serg

Serg, yes I read this little four sentence subsection as explaining why the air speeds up, not why there is lift. Conceptually, it's [lift]->[pressure differences]->[speed differences]. I'm not really seeing how to read it differently. And I don't think it was intended to explain the lift force - explanations based on pressure differences follow the subsection, along with another subsection discussing their limitations. This subsection is merely an introduction to what follows. Mr. Swordfish (talk) 15:39, 14 July 2021 (UTC)

I think it will better to end with "[speed differences]" e.g.:

For any airfoil generating lift, there must be a pressure imbalance, i.e., a lower-than-ambient pressure accompanied by higher airflow speed over the top of the wing and a higher-than-ambient pressure accompanied by lower airflow speed under it.

, and rename the subsection to "Conservation of mass and Bernoulli's principle" (and one more link to McLean to the limitations)109.174.113.170 (talk) 04:48, 15 July 2021 (UTC)Serg

@Serg - it is incorrect to imagine that the air under the wing is at a pressure significantly higher than ambient pressure. Underneath the wing the pressure is close to ambient and the airspeed is close to that in the free-stream. At the stagnation point the pressure coefficient is 1, and in the free-stream it is 0. Underneath the wing the pressure coefficient is between 0 and 1, but much closer to 0; at the top surface of the wing the pressure coefficient is less than 0 for example -1, -2, -3, even -4 etc. Dolphin (t) 06:17, 15 July 2021 (UTC)

Agree. I used this picture as ref.109.174.113.170 (talk) 06:48, 15 July 2021 (UTC)Serg

@Mr Swordfish: Your proposal begins “… the air exerts an upward force on the wing when the air is deflected downward.” I see two objections to this proposal:

Firstly, “an upward force … when the air is deflected downward.” This is the explanation of lift based on Newton’s 3rd law! If we were to use this approach it would imply that the role of Bernoulli’s principle in explaining lift is only as an appendage to the explanation based on Newton’s 3rd law. Overall, it would imply that the explanation based on Newton’s 3rd law is complete without Bernoulli, but the explanation based on Bernoulli is not complete without Newton. That would seriously devalue the role of Bernoulli and would be unacceptable.

Secondly, we appear to agree that Bernoulli alone cannot explain the lift on an airfoil – it works in conjunction with some other information. (Reference 30 is from Resnick & Halliday and states "There is no way to predict, from Bernoulli's equation alone, what the pattern of streamlines will be for a particular wing.") Some extra information is required; and in the interest of simplicity that extra information cannot be explained – too confusing. So that extra information must be presented without explanation – “stated without proof”. In the alternative in my sandbox I state without proof that “the flow on one side of the airfoil moves at a faster speed than the flow on the other side.” You have pointed out that this will be challenged because it “is crying out for a WHY tag.” In your proposal you state without explanation or proof that “the air is deflected downward.” This can also be challenged – why is the air deflected downward? Who said it is? Etc.

It appears that the key issue to be decided at this point is what extra information should be presented, without proof, to support the simplified Bernoulli explanation of lift? (Should it be some information about the kinematics of the flow field; or some information about Newton’s 3rd law?)

I have adjusted my proposed alternative wording in my sandbox. Dolphin (t) 09:28, 14 July 2021 (UTC)

Dolphin, I think there are basically three approaches to explaining lift via Bernoulli's Principle (BP):

1. A mathematical approach involving partial differential equations, vector fields, line integrals, boundary conditions, etc.
2. Starting with Newton's 3rd law and using the obvious fact that the air is deflected downward to establish a force, then noting that force implies a pressure difference, and finally using BP to conclude that the air goes faster on the top of the wing.
3. Magical thinking involving non-physical reasons for why the air speeds up e.g. equal transit time.

The first option is not simple. The third is not correct. That seems to leave option 2, assuming we want to explain things rather than just make assertions. e.g. "the wing is designed to make the air go faster over the top". And I think that it is obvious why a wing at an angle of attack will deflect the air downward, so I don't think saying so will make readers wonder "why" in the same manner that asserting "air goes faster on top" will.

As for implying that the Bernoulli explanation is merely an appendage to Newton's 3rd law, well, it is. Look at the derivation of BP as originally published - it assumes pressure changes, uses the pressure change to calculate the net force, then uses F=ma to calculate the acceleration, and finally integrates the acceleration expression to obtain speed. BP is a direct consequence of Newton's laws. (Granted, BP can be derived by applying conservation of energy, but conservation of energy is itself derived from Newton's laws.)

The subsection in its current form basically uses option 2 when it says "For any airfoil generating lift, there must be a pressure imbalance..." I suggested stating that idea first to make the section more readable; I don't think it really changes the content (and I'm not sure it actually makes it more readable).

Serg is correct to point out above that one way to read this little four sentence sub-section is that it does not explain how lift comes about. It merely explains that lift requires a pressure imbalance and this pressure imbalance implies changes in airspeed. i.e. it explains why the air speeds up, not why there is lift. The "explanations" that follow are flawed, as we state clearly in the "Limitations" subsection. I'm not sure that we can do better without dragging a bunch of math into it, and I don't think that's appropriate for a "Simplified" explanation. Mr. Swordfish (talk) 15:22, 14 July 2021 (UTC)

@Mr Swordfish: I see that the simplified Bernoulli explanation I am looking for is actually included in the sub-section titled Conservation of mass. (It includes “Starting with the flow pattern observed in both theory and experiments, the increased flow speed over the upper surface can be explained in terms of streamtube pinching and conservation of mass.” It ends with “From Bernoulli's principle, the pressure on the upper surface where the flow is moving faster is lower than the pressure on the lower surface where it is moving slower.”) This is satisfactory as a simplified explanation using Bernoulli.

The sub-section that we are discussing (titled Increased flow speed and Bernoulli's principle) is actually a simplified explanation of the flow speed being faster beside the top surface and slower beside the lower surface. As Serg has suggested, a change in the titles of these sections might be warranted. Dolphin (t) 13:33, 17 July 2021 (UTC)

We all agree that the explanation under discussion doesn't satisfactorily explain lift. It avoids saying anything that's incorrect, but also avoids identifying any actual cause-and-effect relationships. It also falls short on sourcing. It cites reliable sources only for separate pieces, but cites none for the overall explanation. And I don't know of any source that supports the overall approach. So in addition to not explaining lift, it's also guilty of synthesis.

So far in this discussion we seem to be trying to figure out how to repair our version, so as to make it coherent in a cause-and-effect sense. I happen to think that this isn't logically possible. But whether it's possible or not isn't the issue, in my opinion. Our job isn't to fix an inherently faulty explanation of lift; it's to report what the sources actually say. And the classic Bernoulli-based explanations actually purport to explain lift, not just to explain why the upper flow is faster.

True, the traditional Bernoulli-based explanations in the old sources are wrong. But I also agree with Swordfish that they deserve a place in the article because they're prominent historically and likely familiar to many readers. So it seems to me that our reason to include a Bernoulli-based explanation at all is primarily historical. It thus needs to be rewritten so as to faithfully present what the classical sources say, right or wrong. It should present the three major purported reasons why the flow speeds up (equal transit time, half-Venturi, and streamtube pinching) and then present all the reliably reported objections. I don't think it needs a separate subhead for "Conservation of mass".

Giving "False explanation based on equal transit time" separate billing in its own subsection implies that equal transit time is the only thing false about the Bernoulli explanations. The material in that subsection should be moved up and placed with the other objections to the Bernoulli explanations.

And I think we should rethink the "Limitations..." subsection. For one thing, the "Limitations" heading is polite to a fault. It sounds like we're talking about limits on the range of applicability of a mathematical theory, while what we're really talking about are shortcomings or outright errors in all the qualitative explanations, including the one based on flow deflection. In what I propose below, I would eliminate the separate "Limitations..." subsection and fold the discussion of shortcomings and errors into the headings and explanations themselves.

After reading about all the shortcomings and errors, an attentive reader will have noticed that everything under "Simplified explanations..." is pretty weak and likely wonder why it's all featured so prominently.

To head off such questions, I think we should do more up front to put the simplified explanations in perspective relative to our understanding overall. I'd suggest adding a brief new section just ahead of the simplified explanations and changing some of the headings within. Something like this:

"Understanding lift as a physical phenomenon (New section)

The flow around a lifting wing is a complex fluid-mechanics phenomenon that can be understood on essentially two levels:

1) The level of the mathematical theories (link to that section), which are based on established laws of physics and represent the flow accurately, but which require solving partial differential equations, and

2) The level of qualitative physical explanations without math. Correctly explaining lift is difficult because the cause-and-effect relationships involved are subtle. A comprehensive explanation (link to that section) that captures all of the essential aspects is rather long. There are also many simplified explanations, and most readers will likely already have been exposed to one or more of them. But simplifying the explanation of lift is inherently problematic, and no simplified explanation is completely satisfactory. Each simplified explanation presented below is therefore accompanied by a discussion of its shortcomings or errors.

Simplified physical explanations of lift (Existing heading)

Over the last hundred years or so, many different simplified explanations have been proposed. Most follow either of two basic approaches, being based either on Newton's laws of motion or on Bernoulli's principle. Both approaches have positive aspects, but neither approach, by itself, is a completely satisfactory explanation.

Incomplete explanation based on flow deflection and Newton's laws (revised heading)

(Revised subsection combines material from "Flow deflection and Newton's laws" and "Limitations...")

Incorrect explanations based on an increase in flow speed and Bernoulli's principle (revised heading)

(Revised subsection combines a major rewrite of "Increased flow speed and Bernoulli's principle" and "Conservation of mass" with material from "Limitations...")"

Altogether this approach may seem harsh, but I think it more accurately reflects the status of the simplified explanations. I haven't filled them in, but there are plenty of sources to cite. And we don't have to cite all the sources we currently cite.

If the group thinks this is a promising approach, I could be persuaded to draft it in in my sandbox.

The changes I've listed above would improve things, I think. But there may be more we should do. Serg has raised the issue of Weltner's (also Babinsky's) streamline-curvature explanation, which has definite virtues, but which isn't currently treated anywhere in the article. I'd support adding it to the "Simplified explanations...". I think it qualifies as "simplified" because it explains lift based only on the cross-stream components of the pressure gradient the fluid acceleration. If we decide to add it, we should also describe its shortcomings, for which I know of some citable sources.

J Doug McLean (talk) 22:26, 27 July 2021 (UTC)

Thanks Doug, and welcome back! I think it is a promising approach and I encourage you to draft it in your sandbox, Dolphin (t) 02:34, 28 July 2021 (UTC)

I also agree that this is a good approach. Presenting each "simple" explanation and immediately following it with clear statements of the "limitations" is preferable to lumping the limitations into it's own section that the reader may not even get to. I also agree that it is overly polite to describe explanations that are simply wrong as "limited". "Incomplete" and "incorrect" are more accurate labels, and I think the sources support this.

Doug suggests adding material about the half-venturi non-explanation, which is currently not treated by the article. I have no objection to that; we might want to consider including the skipping-stone explanation as well.

As for the streamline curvature theorem approach, we already include that in the article - see https://en.wikipedia.org/wiki/Lift_(force)#Basic_attributes_of_lift. I'm not convinced that moving it up to the simplified section would be an improvement - for some audiences saying "it's really simple - just take a look at this differential equation" is appropriate, but I don't think most of our audience would think an explanation based on a differential equation is simple. But I could be persuaded otherwise if we can explain it with a careful choice of language.

I look forward to seeing Doug's draft; I may take a stab at it myself. Mr. Swordfish (talk) 17:04, 28 July 2021 (UTC)

I have a draft almost done, and I'll post it shortly.

I've run into a bit of a surprise with sourcing for equal transit time. Above, I was advocating citing original sources that seriously presented equal transit time as being correct. Now I find I don't have any. All the relevant sources in the current article are "second-hand", describing equal transit time only to debunk it. The second-hand sources I've looked at so far don't cite any original source, but offer only vague references to "text books" and such. So I currently have no original source. Of course the second-hand sources are correct and reliable, and we'll definitely cite them for the objections to equal transit time, but for the explanation itself I was hoping for something closer to "the horse's mouth". Does anyone here know of a source that presents equal transit time as true?

Our treatment of streamline curvature under "Basic attributes of lift" deals only with the equation relating streamline curvature to pressure gradient. The Weltner paper and Babinsky paper I was thinking of present a coherent explanation of lift based on the equation. Still, I'm okay with not adding it to the "Simplified explanations...". J Doug McLean (talk) 23:58, 29 July 2021 (UTC)

See https://en.wikipedia.org/wiki/User:Mr_swordfish/List_of_works_with_the_equal_transit-time_fallacy Cheers! Mr. Swordfish (talk) 01:32, 30 July 2021 (UTC)

Thank you, Mr. Swordfish. This is an impressive list indeed. I wasn't aware that the fallacy was as common as this, even into recent years. Judging by the numbers through 2009, I'm guessing that the only reason there are no later entries is that you stopped gathering them.

For my draft of my proposed revisions I tentatively chose three of Mr. Swordfish's references spanning the decades and limited to authors involved in the aeronautical community. I'm open to other suggestions. My draft is now posted in my sandbox. J Doug McLean (talk) 00:03, 31 July 2021 (UTC)

I can't take credit for the list; it was a collaborative effort among wikipedia editors when it was a full-on page. At some point, around 2009 it was nominated for afd (articles for deletions) and consensus was to remove. I requested that the page be "userfied" so it still exists on my user page. Glad to see that it is of some use.

My sense is that although the list stopped being updated in 2009, there are actually fewer works that present ETT as truth. It really slowed down when Glenn Research Center started calling it "Incorrect Theory of Flight #1". Perhaps this article has helped too.

Looking forward to reading the draft. I'll post whatever minor detail suggestions there and reserve or this space whatever bigger picture concerns I may have. Mr. Swordfish (talk) 21:19, 31 July 2021 (UTC)

Pressure gradient coexistent with curved flow explained in addition to Bernoulli related factors

I'm wondering why this hasn't been brought up yet. From a wings frame of reference the airflow is curved (diverted), coexistent with a pressure gradient where pressure decreases perpendicular to the flow towards the center of curvature. In a typical case, above a wing, the pressure gradient is from ambient to lower pressure, and below the wing, from higher pressure to ambient. At the same time, there is a coexistent Bernoulli relationship between pressure and speed of the flow, corresponding to the pressure related to the pressure gradient along the streamlines. The article can go into details about how a wing curves the flow (such as why flow tends to remain mostly attached to convex surfaces within reason (without significant separation of flow)), but it seems enough to just note that a wing curves (diverts) the air flow. Rcgldr (talk) 16:00, 13 July 2021 (UTC)

You are describing the explanation of lift presented so well by Holger Babinsky. Reference No 61 is a citation of Babinsky’s work. See under Lift (force)#Pressure differences. Dolphin (t) 22:24, 13 July 2021 (UTC)

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)

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)

Section headings using the word lift

I tried [1] to remove the word "lift" from the section headings, as it certainly seems unnecessary for an article that is already named that. It is also in the Wikipedia manual of style WP:HEADINGS. @Mr swordfish: which sections title do you wish to keep?--ReyHahn (talk) 14:13, 17 August 2021 (UTC)

"Simplified physical explanations on an airfoil" didn't make much sense to me. The others are fine without the word lift. We should probably keep the lift in "Lift Coefficient". Other than that I think the section headers read fine without the word lift. Mr. Swordfish (talk) 22:27, 17 August 2021 (UTC)

One issue with re-titling sections is that it may break links that point to that subsection. There's a workaround, which I'm looking into for the proposed changes to the "simplified explanation" section. We should make sure we do the same for these changes. Mr. Swordfish (talk) 20:47, 20 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)

References

References