User talk:Mr swordfish/Lift

Hi Mr swordfish. Thanks for inviting us to comment on your re-work of Lift (force). I think the re-work is definitely an improvement. I approve of the way you have handled the equal transit time model.

There is a problem with Note No. 18. It appears as a red link. There is a heading Other Alternative explanations for the generation of lift. The word Alternative should not begin with a capital letter. Keep up the good work. Dolphin51 (talk) 23:18, 7 July 2009 (UTC)

Thanks. These issues have been corrected.

Mr swordfish (talk) 14:26, 8 July 2009 (UTC)

Dummy section break

I have tested the link to FIXED WING AIRCRAFT FACTS AND HOW AIRCRAFT FLY. It leads to the home page for AviationExplorer.com, but no further. If there is something about equal transit time that links to the home page it is not immediately obvious. It may be necessary to provide further explanation as to how to get from the home page to the page about ETT. Dolphin51 (talk) 22:54, 8 July 2009 (UTC)

The link is to http://www.aviationexplorer.com/fixed_wing_aircraft.htm If you scroll down about 60% you'll see the cited quote. I'll add some other citations. —Preceding unsigned comment added by Mr swordfish (talk • contribs) 04:28, 9 July 2009 (UTC)

Thanks for that. I found it.

A number of your proposed headings are spelt with the first letter of each word capitalized. In Wikipedia, only the first word is capitalized unless it is a proper noun such as the name of a person or city. Dolphin51 (talk) 04:43, 9 July 2009 (UTC)

Capitalization corrected. (BTW, spelt is a hexaploid species of wheat, the word you want is "spelled". \pedantic (c: ) Mr swordfish (talk) 13:52, 9 July 2009 (UTC)

Making this page live. Mr swordfish (talk) 19:57, 10 July 2009 (UTC)

May 2010

Mr swordfish, thanks very much for the invitation to comment on your proposed edits to Lift (force). Generally, your changes appear to make an improvement to the article. There are two areas that cause me problems.

1. The following sentence has been added: Note that for a conventional airfoil, both the upper and lower sides contribute to the deflection of the air and therefore lift is generated on both sides of the foil. This sentence is supported by citing the Glenn Research Center’s Incorrect Lift Theory #2.

Firstly, the cited source does not say anything about both sides of an airfoil contributing to deflection or generating lift.

Secondly, the sentence implies that there is some sort of unconventional airfoil in which only one side contributes to deflection of the air, and only one side contributes to generation of lift. I believe that to be incorrect so I challenge you to identify an airfoil that generates lift on only one side.

2. The following sentence has been added: This downward deflection of the air is known as downwash. This is misleading.

The downwash in the region between the trailing vortices is associated with induced drag, not lift.

• Think of a wing mounted in a wind tunnel so that the tips of the wing touch the walls of the tunnel. The wing can generate lift but there is no downwash.
• In two-dimensional flow about an airfoil there is no downwash but lift can be generated.
• If two wings of very different aspect ratio are generating the same amount of lift, the downwash behind the two wings will be very different – downwash will be greater behind the wing of lower aspect ratio. Similarly, induced drag will be greater on the wing of lower aspect ratio.

Lift is not associated with the trailing vortices. Lift is associated with the bound vortex. (The bound vortex can be imagined as running spanwise across the wing, joining the two trailing vortices. See Horseshoe vortex.) The bound vortex causes the air a short distance upstream of the wing to deflect upwards, and the air a short distance downstream of the wing to deflect downwards. The lift on a wing is proportional to the circulation driven by the bound vortex. (See Kutta-Joukowski theorem.) The downward speed of the air a great distance downstream of the wing is associated with the trailing vortices.

This problem can be avoided simply by eliminating the sentence about downward deflection being known as downwash. Dolphin (t) 00:06, 4 May 2010 (UTC)

Thank you for your constructive comments and for helping to make the article better. To address your concerns:

1a The cited source actually does discuss lift occurring on both sides of the foil. The incorrectness that it debunks is the idea that only the bottom of the wing generates lift: This theory is concerned with only the interaction of the lower surface of the moving object and the air. It assumes that all of the flow turning (and therefore all the lift) is produced by the lower surface. But as we have seen in our experiment, the upper surface also turns the flow. In fact, when one considers the downwash produced by a lifting airfoil, the upper surface contributes more flow turning than the lower surface. This theory does not predict or explain this effect.

I inserted this as a succinct way to address a common misconception. Perhaps the article would be better to give the "skipping stone" theory it's own section. My feeling is that it doesn't deserve it's own section, but that we should address it somewhere, albeit briefly. I'm open to suggestions of how to better handle this issue.

1b I inserted the word conventional was a weasel-word because I'm not sure that there isn't a counterexample somewhere, i.e. a foil that in certain circumstances generates lift on only one side. For instance, any body with flow on only one side will generate lift on only one side (e.g. a surfboard). Of course, that's not really an airfoil and if we restrict ourselves to airfoils the statement is probably correct without having to qualify it as "conventional" . I have no problem with removing this caveat.

2 Downwash has several meanings. I was using it in the layman's sense (definition 1 in downwash). Since this is a loaded term subject to different interpretations that may generate controversy, and since it is not central to the objectives of the section I have no problem with simply deleting that sentence. Mr swordfish (talk) 13:53, 4 May 2010 (UTC)

Thanks for your prompt reply.

I agree that the cited Glenn Research Center (GRC) website does in fact say the upper surface also turns the flow. I overlooked the significance of that paragraph in relation to your words about both sides of an airfoil contributing to lift and deflection of air. I apologize for that.

In Lift (force) and in your latest proposal there is a quotation by John D Anderson (from Introduction to Flight) saying the debate centers on which explanation is the most fundamental. Anderson goes on to conclude that the most fundamental explanation of lift is that there is a difference in pressure on the two surfaces of the airfoil. It is not the pressure on either surface that is sufficient to explain lift, but the difference in pressure between the two surfaces. For me, this underscores the foolishness of announcing that both surfaces of an airfoil generate lift because that sounds like saying the lift is the vector sum of the lift on the upper surface and on the lower surface.

There is always a risk in using elementary sources as citations for encyclopedias. (The GRC websites unashamedly describe themselves as beginners’ guides.) In relation to the influence of the upper surface the GRC website says the upper surface contributes more flow turning than the lower surface. While this might be a neat way of convincing newcomers that the skipping-stone theory is incorrect, it is scientifically unsound to say that the upper surface of an airfoil contributes more than the lower surface.

I concede I have a problem with any attempted explanation in mechanics that focuses on which surface of a body is generating a force. As you know, mechanics, and especially Newton’s Laws of Motion, focus on bodies exerting forces on other bodies. It is the bodies that are exerting and experiencing forces, not one surface or another on a body. So a statement that both surfaces of a body are generating a force seems a bit like observing someone clapping hands and announcing that both hands are contributing to the noise! Dolphin (t) 00:15, 5 May 2010 (UTC)

For me, this underscores the foolishness of announcing that both surfaces of an airfoil generate lift because that sounds like saying the lift is the vector sum of the lift on the upper surface and on the lower surface.

Isn't this exactly what we're doing when we do pressure integration?

$L=\oint p\mathbf {n} \cdot \mathbf {k} \;\mathrm {d} A,$

i.e. sum the pressure at each point on the surface to arrive at a total force? Mr swordfish (talk) 16:38, 5 May 2010 (UTC)

Your point is a good one. I hope I can explain my point just as well.

We can integrate the pressure forces over the whole wetted area of the wing to determine the lift and the pressure drag. The pressure on the underside of the wing causes an upward force but the pressure on the top surface of the wing causes a downward force. (Fortunately, the force on the underside exceeds the force on the top surface, yielding a resultant upward force.) However, if we pursue that line of reasoning we would conclude that the top surface of the wing is not contributing to lift, it is opposing it; and so we would conclude that it is only the underside of the wing that causes lift. That would be an inappropriate conclusion because we are trying to avoid the skipping-stone model and write about the flow-turning model of lift.

Our pressure integration is $\oint$ and, as you know, this symbol means an integration over the entire surface, not just a part of it. In the case of an airfoil it means the top surface, the underside and the nose of the airfoil section. If we were to write we do the pressure integration over both surfaces of the airfoil, not just one of them it would be stating the obvious because that is what an integral $\oint$ is all about.

The primary contributor to lift is the high-speed airflow adjacent to the top surface of an airfoil so I am not challenging the view that the top surface participates in the generation of lift (even though the force on the top surface is opposed to the resultant lift.) I am in favour of the view that it is the whole airfoil that diverts the airflow and causes the forces between the airfoil and the air. I am challenging any implication that the two surfaces are somehow acting independently of each other.

The skipping-stone theory incorrectly says it is only the underside that causes lift. Some people (erroneously) say it is the top surface that causes lift. Some people say (with a little justification) both surfaces cause lift. On the subject of the flow-turning model of lift I am advocating that Wikipedia avoid all reference to the two surfaces and say it is the airfoil, the 3-D body, that turns the flow, causing a force to act continuously on the passing air and on the airfoil. Dolphin (t) 23:32, 5 May 2010 (UTC)

Hi. I made a small adjustment to the text. Still worrying me is the suggested differences between "Deflection" ("One way...") and "Flow turning" ("Another way..."), which are presented as different views on the generation of lift, but in fact is a continuation and an expansion on the above line of reasoning ("Deflection"). -- Crowsnest (talk) 13:50, 8 May 2010 (UTC)

Agree with your concern about presenting deflection and flow turning as different "ways to think about it" - they are really just different ways of describing the same thing. I've edited the text to reflect this. As a result, it didn't make sense to have the treatment of pressure under flow turning, so I broke it out into it's own sub-heading and re-cast the language to apply to both turning and deflection. I'm not sure if the article is better with three sub-headings, or whether it would be better to go with just one.

I also made a number of other small tweaks based on the discussion above. See history for details. Mr swordfish (talk) 16:05, 10 May 2010 (UTC)

Looks good. I made some tweaks to the criticism on the deflection/turning explanation. Feel free to amend. -- Crowsnest (talk) 18:53, 12 May 2010 (UTC)

Thanks for pitching in on this effort. I made a small edit to your tweak to make the section more readable.

That said, I'm skeptical that phrases like Constituitive equations and fluid deformations are going to be easily understood by the lay readership. The article begins by stating that it will present the more easily understood explanations first, so my take is that it's premature to start discussing differential equations at this early stage. I do think it would be appropriate to mention that the more complex models are explained elsewhere in the article under "more rigorous physical description". Mr swordfish (talk) 21:52, 12 May 2010 (UTC)

In-line citation No. 12 is intended to be the NASA site leading to the NACA Report The determination of downwash. The URL didn't work for me - it led to a report by Lieut Diehl on the performance of the F-5-L BOAT SEAPLANE. However, I was able to locate the intended report so I have corrected the URL. Others should check that the URL which I have inserted at in-line citation No. 12 also works for them.

The NACA Report The determination of downwash is Report No. 42. The in-line citation appears to call it Volume No. 124. Is this a typo? Dolphin (t) 00:10, 13 May 2010 (UTC)

Thanks, Dolphin. I corrected the report no. and tried to add a stable link. I further rephrased the bit on the additional modeling needed for a full description of lift (a model for the relation stress-deformation rate) -- Crowsnest (talk) 09:00, 13 May 2010 (UTC)

Good. This is even more readable. One quibble is the use of the word "need" in Fluid stresses – including pressure – need to be related to the fluid motion As Dolphin said in the main article discussion re Weltner:

The first sentence at Lift (force)#Description of lift on an airfoil is There are several ways to explain how an airfoil generates lift. I fully agree with this statement and I am glad it is the opening sentence in this potentially-contentious section. There are too many people who seem to be saying My favourite explanation of lift is correct and therefore all other explanations must be incorrect. The quotation by Weltner and Ingelman-Sundberg is dogmatic in that it asserts a single cause of aerodynamic lift. It fails to acknowledge that this is one explanation of lift, but not the only one. This quotation is incompatible with the first sentence which establishes Wikipedia’s position that there are several ways to explain lift.

I think the use of the word "need" here may be similarly dogmatic - i.e. it may imply that the only way to explain lift (or to do engineering) is to relate fluid stresses to the fluid motion. I'd like to replace the word need with something more in keeping with the "several ways" overall theme. Thoughts? Mr swordfish (talk) 16:01, 13 May 2010 (UTC)

Well, more is needed besides Newton's 2nd and 3rd law, in order to obtain a full description of the flow and the resulting lift. And this "more" can be accomplished in many ways, ranging from assuming incompressibility and irrotationality at low Mach numbers (resulting in potential flow) to e.g. the compressible Navier-Stokes equations combined with a set of thermodynamic equations to determine pressure and density. -- Crowsnest (talk) 19:40, 13 May 2010 (UTC)

Agree that more is needed than Newton's 2nd & 3rd, but as you say "more" can be any of several approaches. To say you "need" to relate stresses to flow implies that it's necessary, that it's the only way to do it. Since there are other approaches, I'd like to stay away from absolute words like "need".

That said, I'm probably going to publish this as-is tomorrow, unless someone comes up with better verbiage in the meantime. Mr swordfish (talk) 21:26, 13 May 2010 (UTC)

Replacing the article text is fine with me. Regarding the wording of the "more" needed to obtain a full description of lift: the only "exception" I know of is potential flow (eventually with vortex sheets). The final physical justification of the underlying assumptions was provided by Prandtl, showing that viscous effects are limited to a thin boundary layer, and the outer flow stays irrotatational (due to Kelvin's circulation theorem). -- Crowsnest (talk) 07:13, 14 May 2010 (UTC)

I have no objection to Mr swordfish's version going live immediately. In the discussion above, I challenged two sentences - the one ascribing the name downwash to the downstream flow induced by the bound vortex; and the one stating that both surfaces of an airfoil contribute to the lift on the airfoil. Mr swordfish replied that he would have no problem if these two sentences were deleted. Seeing Mr swordfish's version is about to go live I have deleted these two sentences. I also refined one of the remaining sentences to confirm that both an upward force and a downward force are at work simultaneously. I tried to avoid the impression that the downward force comes first and the upward force follows, merely as the equal and opposite force. (Of course, both forces appear and disappear simultaneously. Neither is more or less significant or important than the other.) I am happy for my deletions and my altered sentence to be discussed and dissected in detail. That can happen before or after Mr swordfish's version goes live. Dolphin (t) 11:14, 14 May 2010 (UTC)

Hi Dolphin51. Just a comment: also the airfoil in 2D flow (infinite span) has downwash (= upwash), but for a finite span the downwash is (strongly) dominated by the trailing vortices. See e.g.: R.H. Barnard, D.R. Philpott (2004) "Aircraft flight: a description of the physical principles of aircraft flight", 3rd ed., pp. 15 & 44-45. -- Crowsnest (talk) 01:07, 15 May 2010 (UTC)

Hello Crowsnest. Different authors approach this matter in either of two ways. Some say On a wing of infinite span (2D flow) there is no downwash. Others say The bound vorticity causes downwash and upwash, and if the wing has finite span the downwash exceeds the upwash. If the article is to take the latter approach it needs to refer to both downwash and upwash. It needs to say that when a wing is generating lift there is downwash of the downstream flow and upwash of the upstream flow. The text proposed by Mr swordfish made no reference to upwash.

As you know, in the deflection model of lift the angle of deflection is not the angle of the downwash measured relative to the freestream velocity; it is the angle of the downwash measured relative to the direction of the upwash ahead of the wing. The lift on the wing is proportional to the vector difference between the momentum of the air in the upwash as it approaches the wing minus the momentum of the air in the downwash as it leaves the wing. Dolphin (t) 11:59, 15 May 2010 (UTC)

Hello Dolphin. Thank you, this is very interesting. The differences between these two definitions of downwash, could perhaps have more emphasis in the downwash article. While bound vortex does not have an article yet (starting vortex has). I will be more careful about using the terms downwash (and upwash), knowing this. Downwash is still mentioned twice in the article (regarding the momentum approach to determine lift, as also discussed by you). From Landau & Lifshitz, vol. 6, pp. 68-69, the lift is related to the vertical momentum changes before and after the wing:

$L=\iint _{x=x_{1}}\rho U_{\infty }v\;dy\;dz-\iint _{x=x_{2}}\rho U_{\infty }v\;dy\;dz,$

with x=x₁ being a cross section behind the wing, and likewise x=x₂ before the wing. Further v is the vertical velocity component. The integration is over the whole cross section, for y and z ranging from minus to plus infinity. In case x₂ and x₁ are chosen just before and after the wing, your above description is obtained: the lift equals the difference between the vertical momentum fluxes just behind (downwash) and before (upwash) the wing. Otherwise, if x₂ is chosen far before the wing and x₁ far behind, the contribution from the plane x=x₂ far in front of the plane can be neglected. But in the wake the momentum changes at x=x₁ persist, and determine the lift (as stated in the article). I think I now have the picture. -- Crowsnest (talk) 16:35, 15 May 2010 (UTC)

Hello Crowsnest. Thank you for your prompt and detailed reply. I agree there is much scope for Downwash to be improved. Dolphin (t) 12:07, 16 May 2010 (UTC)

June 2012

In the section Limitations of explanations based on Bernoulli’s principle there is a couple of anomalous quotations:

Hoffren: starting from the shape streamtubes above and below an airfoil. There appears to be a word or two missing between "streamtubes" and "above"

Anderson and Eberhardt: we are missing a vital piece when we apply Bernoulli’s principle Missing a vital piece of what? There appears to be a word or two missing. Dolphin (t) 07:58, 30 June 2012 (UTC)

Thanks. Hoffren's quote was indeed missing a word. I've expanded the A & E quote for context. Mr. Swordfish (talk) 12:22, 30 June 2012 (UTC)

You have introduced a new section called Conservation of mass. In my experience, a minority of books on fluid dynamics use the expression conservation of mass, or mass conservation. The majority use the term continuity. Wikipedia has a sub-article devoted to the concept, using the term continuity. See Continuity equation#Fluid dynamics. Dolphin (t) 08:20, 30 June 2012 (UTC)

I have not done a survey to determine whether "conservation of mass" or "mass continuity" is in the majority. They are essentially the same concept, in that the continuity equation is directly derived from conservation of mass. I think either could be used, but since the article was already using "conservation of mass" I opted to continue to use that phraseology in the interest of consistent style. There is certainly enough usage of "conservation of mass" in the literature to justify is usage:

• In Introduction to Aeronautics Brandt et al: "Equation 3.2 is known as the continuity equation. It is a statement of conservation of mass for fluid flows." pg 62
• Eastlake says: "Analysis of fluid flow is typically presented to engineering students in terms of three fundamental principles: conservation of mass, conservation of momentum, and conservation of energy."
• In Introduction to Flight John D Anderson introduces Chapter 4.1 titled Continuity Equation thusly: "The laws of aerodynamics are formulated by applying to a flowing gas several basic principles form physics. For example, Physical principle: Mass can neither be created nor destroyed."
• Skip Smith says in Illistrated Guide to Aerodynamics "This principle is known as the 'Law of Continuity' or simply the "continuity equation.' This law is really the Law of Conservation of Matter applied to a moving fluid.
• As Hoffren puts it: "From the mass conservation law, the scaler type continuity equation is obtained."

My take is that "conservation of mass" is the underlying physical principle and the continuity equation is a formula directly derived from it. Since this section is aimed a lay persons, it's more appropriate to state the underlying principles instead of equations derived from them. I'm not dogmatic about this and sometimes an equation can be illuminating (and if I were writing an engineering textbook I would derive the formula and use it thereafter), but I'm not sure how I'd recast the explanation in terms of continuity or the continuity equation. Mr. Swordfish (talk) 13:04, 30 June 2012 (UTC)

I see the problem. I can only offer a couple of suggestions. Firstly, the cited source is John D. Anderson's Introduction to Flight so there is value in using whatever terminology he uses. Secondly, regardless of what expression is used, at its first appearance it should be linked to one of Wikipedia's articles on the subject. Dolphin (t) 03:13, 1 July 2012 (UTC)

Sorry about taking so long to get back to you. There were live articles commanding my attention, so this sandbox article was put on the back burner.

Both Anderson's 'Introduction to Flight and 'Introduction to Aerodynamics introduce conservation of mass before introducing the continuity equation and base the derivation of the continuity equation on conservation of mass. Using conservation of mass is well supported. It's also consistent with the Fluid_dynamics article.

Personally, I prefer using 'conservation of mass' to 'continuity equation' because the latter can be confused with the similar sounding continuum assumption which is a related but different concept.

Regarding linking the first appearance of a term, we're not supposed to put links in section titles and this is where "conservation of mass" first appears. I considered moving the words "conservation of mass" up a paragraph so that I could link it on first appearance, but the result was not an improvement. Putting the link in the first sentence of the section would seem to meet the requirement. I would think most readers would be able to find the link without much effort.

At this point, I think we have a release candidate. I'll give folks about a week to comment and then move it over to the live article. Mr. Swordfish (talk) 21:40, 17 July 2012 (UTC)

July 2014

Well done guys! So far I have had only a quick look at the release candidate.

The intro contains the sentence: Lift is the component of this force that is perpendicular to the oncoming flow direction. This sentence implies, incorrectly, that all bodies experience a component of force perpendicular to the oncoming flow direction. The article should be limited to saying all bodies experience a drag force, but only airfoils and other bodies capable of behaving a bit like airfoils experience a lift force.

There is a diagram with a caption ending: ... when the airfoil deflects the air downwards the foil is deflected upwards. I suggest this be changed to “... the foil experiences an upwards force.” The word deflection implies movement and it is doubtful whether an airfoil moves upwards. If the airfoil moved upwards in response to an upwards force, the lift force would do positive mechanical work on the aircraft, causing its kinetic energy to increase. In the reference frame attached to the atmosphere, lift is perpendicular to the velocity vector and incapable of doing work (same as a centripetal force). In a wind tunnel the airfoil doesn’t move anywhere, and certainly not upwards. Dolphin (t) 06:34, 26 July 2014 (UTC)

This sentence implies, incorrectly, that all bodies experience a component of force perpendicular to the oncoming flow direction. From a strict mathematical perspective there is always a lift component, but it may be zero. I suppose one could infer from the sentence that the component is always non-zero, but that would be a logical leap.

As for the text on the picture, I agree that it makes a bit of a shortcut that is perhaps not as rigorous as we might like. In the absence of gravity or other forces it's correct that both the foil and the air deflect, and so for a simple explanation it's applicable if one understands that once you add gravity to the model things change. The diagram including the text was lifted from NASA's site, and if it's good enough for them it's good enough for me. Perhaps we could change out the text using a picture editor, but at that point are we misrepresenting NASA's presentation? Mr. Swordfish (talk) 21:02, 31 July 2014 (UTC)

The article is not written from a strict mathematical perspective but I guess I could live with the current implication that all bodies moving relative to a fluid experience lift. (I acknowledge that this implication exists in the article at present and therefore the release candidate is not the first time we have seen it.) As an aside, what do you think of the following re-arrangement of sentences?

A fluid flowing past the surface of a body exerts a force on it. If the fluid is air, the force is called an aerodynamic force. In water, it is called a hydrodynamic force. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the surface force parallel to the flow direction.

You have written "if it's good enough for them it's good enough for me". (This is the argument from authority!) It is tempting to see the mention of NASA on this website and imagine that it has been written by all those good scientists, engineers and astronauts who used to send men to the moon and now do clever things with satellites and space stations. I have seen enough amateurish comments on these so-called NASA websites to make me sceptical about who can write their material, and who can edit it. These articles might have come from a bunch of retired NASA employees, or the Friends of NASA, or similar, but I don't imagine there is a bunch of Federally funded scientists, engineers and astronauts whose duties actually include writing popular articles about basic aviation topics, and maintaining them on a website. I question whether these popular articles on the NASA website can be regarded as reliable, especially where their content conflicts with the content in high-end scientific and engineering literature. For this reason, I say not only can we amend NASA's caption on their diagram, we have a duty to do so. Dolphin (t) 07:19, 1 August 2014 (UTC)

PS Some of these NASA websites, perhaps all of them, are qualified with the statement that they are intended for college, high school, or middle school students. Therefore, the authors of these articles can defend themselves against any allegation of lack of rigor by pointing to this qualification. The authors don't intend their articles to be high-end scientific and engineering literature and so we shouldn't treat them as such. Dolphin (t) 07:36, 1 August 2014 (UTC)

Point taken about the argument from authority. That said, the diagram is correct in the simplified situation where gravity not included in the model. I've edited the caption to reflect this assumption. The words included in the gif image are not so easy to change.

For context, the diagram appears on this page: http://www.grc.nasa.gov/WWW/K-12/airplane/newton3.html That presentation is intended for middle school through college students so I believe it aligns with the intended audience for the introductory sections of this article.

Regarding the proposed language, I'm not opposed to it, but I don't see it as an improvement. It places the distinction between aerodynamic and hydrodyamic before the definition of Lift; since this article is about lift I think that should come first. But I'm not wedded to that and would have no real problem if we adopt your proposed intro. Mr. Swordfish (talk) 14:06, 1 August 2014 (UTC)

I have looked at the NASA website you quoted above. It contains the statement: "In this problem, the air is deflected downward by the action of the airfoil, and in reaction the wing is pushed upward." This statement is correct if we interpret the phrase pushed upward to mean the wing experiences an upward push, an upward force. What appears to have happened next is that someone other than the author has prepared the diagram in question. This someone has interpreted the phrase pushed upward to mean displaced upward or deflected upward. The diagram shows the air deflected downward and the airfoil deflected upward. This implies that Newton's third law of motion says something about every deflection being accompanied by an equal deflection in the opposite direction. As you know, none of Newton's laws says anything about deflections; nor is there a law by anyone else that equates deflections.

The diagram comes close to illustrating Newton's second law as it applies to the flow of fluids. If a fluid flows into a control volume with momentum p₁ every second, and flows out with a momentum p₂ every second, a force F must be acting on the fluid to change its momentum. An equal and opposite force -F acts on the part of the conduit within the control volume. The force F is given by Newton's second law F = Δp/Δt. If the directions of the momentum vectors change as the fluid passes through the control volume, the fluid is clearly being deflected but there is no reason to assume the conduit is also being deflected. This is the situation with fluid flowing through a stationary pipe with a bend - the fluid is being deflected as it passes through the bend but the pipe is not being deflected.

So in a page devoted to Newton's third law, the good folks at NASA have inserted a diagram that looks like a mixture of Newton's second law, Newton's third law and some non-existent law saying "for every deflection there is an equal but opposite deflection."

I see that we can amend the caption accompanying the diagram but we can't amend the words embedded in the diagram. Until a more satisfactory diagram can be found, it might be sufficient to refine the caption. In my view, changing the caption to "in the absence (of) any other forces" doesn't solve the problem; we require a diagram that universally represents the generation of lift on an airfoil; restricting it only to those situations in which there is no other force departs from any suggestion of universality. Besides, restricting it to a situation where there is no other force makes the diagram look like that for a windmilling propeller - as the propeller begins to rotate the angle of attack reduces; eventually the windmilling propeller reaches the speed at which the angle of attack is zero and consequently no lift is generated.

I can suggest the offending sentence in the caption be replaced by When an airfoil deflects the air downwards, the air exerts an upward force on the airfoil. Dolphin (t) 12:17, 3 August 2014 (UTC)

I noticed the strange wording as well. I have been able to change the text in the diagram. I hope you agree it's an improvement. Burninthruthesky (talk) 09:09, 14 August 2014 (UTC)

Yes. Thank you. I've edited the caption to reflect the discussion above. Mr. Swordfish (talk) 11:43, 14 August 2014 (UTC)