Talk:Kutta condition

Why does the Kutta condition exist ?

The flow at the trailing edge of an airfoil separates cleanly because of boundary layer separation effects.

Contrast this situation with potential flow theory which requires the flow to turn around the trailing edge, resulting in zero lift and circulation.

In reality, the flow cannot turn around the trailing edge because of an unstable boundary layer which develops at the trailing edge. Pressure differences in the upper and lower boundary layers generally result in an adverse pressure gradient developing at the trailing edge boundary layer. This instability results in the stagnation and separation of the flow off the trailing edge.

The Kutta condition represents the equilibrium balance in pressure of the upper and lower boundary layers which result in smooth flow off the trailing edge. The equilibrium is always being reset as the flow field changes, to effect the trailing edge separation.

The development of circulation and net lift around the airfoil is a normal consequence of this effect.

Cold-logic 04:52, 9 November 2007 (UTC)

Further observations and misconceptions of the Kutta condition

1. Definition of the Kutta condition

The Kutta condition is NOT DEFINED by zero fluid velocity at the sharp trailing edge. Why .... because all points at the surface of a wing are constrained by viscosity to have zero relative velocity. The Kutta condition, in this event, would therefore satisfy every point on the wing, rendering it a useless criterion for lift analysis.

The Kutta condition is best defined as:- the condition in steady streamlined flow whereby the stagnation streamline leaves the wing at the sharp trailing edge, thereby separating the flows from both sides of the wing.

Note: Only one possible stagnation streamline can be located on the sharp trailing edge of the wing. It is uniquely determined.

2. Starting Vortex

At no time does a starting flow of a wing create a vortex flow around a sharp trailing edge. This is the whole point of the Kutta condition. Vortex flows may be shed FROM the sharp trailing edge, but they do not occur AT and around the sharp trailing edge.

During the starting flow, and subsequently, the fluid is induced to move around the wing so that a stagnation streamline occurs at the sharp trailing edge.

3. Wake Effects

In reality, the wing creates a wake of disturbed fluid so that streamlined flow does not truly occur at the sharp trailing edge as suggested by the Kutta condition. Firstly the wing boundary layer contributes to the thickness of the wake, and so does the actual physical trailing edge thickness of a real wing. The Kutta condition is essentially correct when the wake is thin. However once boundary separation effects become significant, as for stalling, the stagnation (separation) points on the respective wing surfaces diverge significantly, and the Kutta condition is no longer valid.

The separation of the flow from the windward trailing edge during stalling is indicative of boundary separation effects dominating the net wing flows, and not the Kutta condition per se. —Preceding unsigned comment added by 58.110.88.102 (talk) 12:20, 3 February 2008 (UTC)

Where does the asymmetry come from?

I still don't find the explanations for the Kutta condition satisfactory. They introduce some kind of "magic" which renders any explanation of lift useless. How can the existence of the smooth flow from the trailing edge be explained with simple physical arguments? The explanations of viscosity, boundary layer separation and high speed of the air turning around the trailing edge would apply in the same way for the leading edge. Still, the forward stagnation point does not move to the leading edge! What about the argument that the inertia of the airflow following the wing shape downwards acts against the air flowing around the trailing edge? Obviously this inertia effect would not do the same at the leading edge, thus explaining the asymmetry.

Volume Displacement and Kutta Condition.

In the far field, away from the local displacement effects of the airfoil on the fluid, the streamlines fore and aft of the airfoil must align to preserve conservation of mass or volume (if the flow is incompressible). There is no actual fluid circulating around the airfoil, despite the mathematical property of "circulation" in a lifting situation.

Because the flow on the windward side of the airfoil (lower side) cannot go around the trailing edge, as in the potential flow case, that flow must go over the leading edge of the airfoil to preserve continuity.

Therefore, additional flow is induced over the leeward side of the airfoil (upper side) than would be in the case of potential flow over the airfoil.

This induced flow generated by observance of the Kutta Condition on the airfoil, therefore directly causes a net lifting force on the airfoil.

Basically, the Kutta Condition causes more flow to be displaced over one side of the airfoil than the other (asymmetry), which then causes a net lifting force on the airfoil. —Preceding unsigned comment added by 58.106.34.73 (talk) 03:52, 17 April 2008 (UTC)

Is it correct to call Kutta condition as steady state phenomenon?

There are research articles ("Unsteady Kutta condition of pitching aerofoil" IUTAM symposium, France 1981, which clearly states Kutta condition is valid for a range of reduced frequency. After the critical reduced frequency is crossed, the flow no longer is governed by unsteady Kuttta condition. — Preceding unsigned comment added by 14.139.160.4 (talk) 04:39, 28 November 2012 (UTC)

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"An aircraft with a wing with a smoothly rounded trailing edge would generate little or no lift."

This appears to be incorrect. I can take a 1" x 12" x 12" wood plank, hold it out of the window of a car going 60 MPH, and depending how I angle it, get plenty of lift. I doubt that rounding off the sharp edges would change this. Also some seaplanes have reversing propellers to allow them to back out of the slip where they are docked. I an sure that spinning the prop in reverse so that the sharp edge is the leading edge and the rounded edge is the trailing edge isn't very efficient, but it does appear to work. --Guy Macon (talk) 09:55, 10 August 2018 (UTC)

Hello Guy Macon. I’m sorry no-one has responded to your request. I only just noticed it. It looks like I'm the only aerodynamicist on duty these days.

The heading of your new section is a quotation but you haven’t identified whom you are quoting, or where the quote comes from. Perhaps you are attempting to quote the following paragraph which I posted on the Science Reference Desk on 9 August 2018 – see my edit.:

“Designers of all fixed-wing aircraft would love to be able to use a wing with a generously rounded trailing edge. (The wing profile of a Rankine body!) It would provide so much extra volume for a deep rear spar, extra fuel storage space, and extra volume into which the undercarriage could be retracted. Unfortunately a wing with a generously rounded trailing edge will not generate much lift so designers must make use of airfoil sections with what is always called a sharp trailing edge.”

As you can see, I wrote about a wing with a generously rounded trailing edge, not a wing with a smoothly rounded trailing edge. Generously rounded, and smoothly rounded, have very different meanings! A body with a generously rounded trailing edge, and a leading edge of the same radius, and convex surfaces joining the two, is called a Rankine body. Aerospace engineering students perform laboratory sessions with wind-tunnel models in the shape of Rankine bodies, or similar. The students measure the lift and drag on these models. How much lift do you think these Rankine bodies generate? You guessed it – very little! The students are then expected to be able to use the Kutta-Joukowski theorem and the Kutta condition to explain why their Rankine bodies generate so little lift. (The reason they generate any lift at all is because of viscous effects - basic airfoil theory shows that in inviscid flow any body resembling a Rankine body would generate zero lift.)

You wrote about your 1” x 12” x 12” wood plank at around 45° to the wind, held out the window of a car at 60 mph. We all agree that your wood plank experiences an aerodynamic force but at 45° angle of attack it isn't aerodynamic lift. Also, your plank isn’t an airfoil, and nor is it an aircraft. At Airfoil#Overview, Wikipedia explains that flat plates, buildings and the decks of bridges all experience aerodynamic forces when they obstruct the wind, but they aren't airfoils and they aren't experiencing aerodynamic lift.

You wrote: “… some seaplanes have reversing propellers to allow them to back out of the slip where they are docked.” Many propeller-driven airplanes, and especially turbo-propeller airplanes, have propellers that can generate reverse thrust – see Thrust reversal#Propeller-driven aircraft. The primary purpose of reverse thrust is to augment the wheel brakes when stopping, but it is sometimes used to taxi backwards on the runway, or in water in the case of floatplanes and amphibians. It is especially effective on wet runways when the wheel brakes suffer from the reduced coefficient of braking friction. Reverse thrust works by reversing the pitch of the propellers – not by reversing the direction of rotation of the engine and propeller. Even in reverse thrust, the propeller continues to turn such that the leading edge leads, and the trailing edge trails.

The propellers (or screws) of large ships are of fixed pitch. They generate reverse thrust by reversing the direction of rotation of the propeller and its gear-train. Ships' propellers have sharp leading and trailing edges, so even when they are reversed they don't have a generously rounded trailing edge. However, I digress – a ship's screw is very different to an airplane propeller. Dolphin (t) 14:34, 12 September 2018 (UTC)

Already answered at User talk:Guy Macon#Evidence for information supplied at Science Reference Desk. Try to include your fringe theories in this article and you will discover how we deal with pseudoscience on Wikipedia. --Guy Macon (talk) 15:28, 12 September 2018 (UTC)

This is original research, not supported by sources which you claim support it, nor by common experiments which any reader could do on their own with little to no preparation. This is pure crankery and does not belong on this project. ᛗᛁᛟᛚᚾᛁᚱPants Tell me all about it. 15:45, 12 September 2018 (UTC)

Another dubious claim: "Reverse thrust works by reversing the pitch of the propellers – not by reversing the direction of rotation of the engine and propeller." some seaplanes reverse the direction. Sure, it isn't as efficient but -- despite Dolphins claims to the contrary -- it does produce enough reverse thrust to back out of the slip.

Quadcopters are also doing this. "These new quadcopters are using a much simpler method of flying inverted: Spin the motors backwards. Quadcopters control their flight by quickly varying the speed of rotation of each motor. Why not completely reverse the motor then? Today’s brushless outrunner motors have more than enough power to quickly reverse direction.... Propellers will generate reverse thrust if they are spun backwards, however they will not be as efficient as they would when spinning the direction they were designed for." [1] You can go on the quadcopter discussion groups and see that lots of people have flown inverted by reversing the prop rotation (and yes, the props ghas rounded leading edhges and sharp trailing edges) but with noticeably reduced thrust. This is usually followed by a discussion of blade shape that works better when reversed without totally hosing performance when the quadcopter isn't inverted.

This entire article is full of bullshit, First is says something that is true:

"When a smooth symmetric body, such as a cylinder with oval cross-section, moves with zero angle of attack through a fluid it generates no lift."

The is says something that has no basis in reality:

"If the oval cylinder moves with a non-zero angle of attack through the fluid... no lift is generated, despite the positive angle of attack."

--Guy Macon (talk) 16:05, 12 September 2018 (UTC)

Lift theory

Flow between point of separation and trailing edge

Reduced pressure aft of point of separation provides lift. Description of the backfill flow can be taken from the power laws of potential flow:$w=Az^{n}.$ The figure exaggerates the angle at point of separation so power law with n > 10 expected. Entrance of air into backflow zone is facilitated by a sharp trailing edge. The image is taken from flow separation. Reference to experiment suggested. — Rgdboer (talk) 02:11, 9 October 2018 (UTC)

I agree with your description of the physics. This supports the standard scientific view that a sharp trailing edge `works better (more lift and less drag) without implying that the WP:FRINGE view that "An aircraft with a wing with a smoothly rounded trailing edge would generate little or no lift" is correct. The "no lift" theory is clearly wrong, as evidenced by sharp-traling-edge propellers that are still turning backards still working, but not working as well. --Guy Macon (talk) 10:56, 9 October 2018 (UTC)

Thank you Rgdboer. Your ideas are of some interest but not relevant to this article. The Kutta-Joukowski theorem and the Kutta condition are mathematical theorems that assume fluids have no viscosity. These theorems came into existence prior to, or at the same time as, Prandtl’s theory of fluid dynamic drag and its dependence on the viscosity of the fluid.

The assumption that fluids have no viscosity leads to some paradoxes. For example, the K-J theorem and Kutta condition are silent about the drag experienced by a body. The K-J theorem can be used to calculate the lift per unit span experienced by a body with a sharp trailing edge, but it can’t be used to calculate the drag. This is highly unrealistic because we know all bodies moving through a real fluid (with viscosity) experience drag. (However, when we observe drag on a body it doesn’t prove the K-J theorem is incorrect.)

The K-J theorem and Kutta condition assume inviscid fluid so they only inform us about a primary flow past a body. To determine the total flow (with boundary layers, separation point, broad wake, drag etc.) we must find a secondary flow. The drag on a body is not accounted for by the primary flow; it is wholly accounted for by the secondary flow.

Guy Macon has given us some examples of bodies without sharp trailing edges experiencing a little lift. The question in front of us is whether that small amount of lift can be predicted, quantitatively, by the primary flow (K-J theorem and Kutta condition); or whether it must be predicted wholly by the secondary flow. My view is that this small amount of lift is due to circulation provided by the asymmetric boundary layers; and boundary layers are part of the secondary flow.

The Kutta condition tells us that, in a fluid with no viscosity, lift is only experienced by bodies with sharp trailing edges. When real bodies without sharp trailing edges experience lift in viscous fluids it is unrelated to the Kutta condition; it is related to the asymmetric boundary layers that are part of the secondary flow. If asymmetric boundary layers generate lift it is relatively weak - that is why bodies without sharp trailing edges are poor airfoils; Guy Macon has described them very accurately as “piss-poor airfoils”. Dolphin (t) 12:15, 9 October 2018 (UTC)

The Kutta condition tells us that, in a fluid with no viscosity, lift is only experienced by bodies with sharp trailing edges. ᛗᛁᛟᛚᚾᛁᚱPants Tell me all about it. 17:27, 9 October 2018 (UTC)

Note the misquote: "Guy Macon has given us some examples of bodies without sharp trailing edges experiencing a little lift" Totally fabricated. I never said or implied that. I said that bodies without sharp trailing edges have worse lift-to-drag numbers. Go back to the balsawood toy airplane with the rubber-band motor and slabs for wings that Dolphin51 really doesn't want to even think about, much less discuss. Use a sanding block to make the flat trailing edge sharper. The drag will be reduced, but only in Dolphin51's fevered dreams will the lift -- which is purely a function of surface area and angle of attack deflecting air downward -- change by any appreciable amount. --Guy Macon (talk) 19:00, 9 October 2018 (UTC)

I have edited Quadcopter to acknowledge that some quadcopters can fly inverted by reversing the direction of their rotors. (The citation needs some refinement but I will do that when I have access to a real keyboard.) It is reality that some quadcopters can now fly inverted but that is not relevant to this article on the Kutta condition which is a totally theoretical theorem that talks about fluids that have zero viscosity. This theorem is 115 years old! Dolphin (t) 21:37, 9 October 2018 (UTC)

Please don't. You made a claim that was not found in the source (nowhere does it say "normally cambered rotor blades are replaced by blades with a symmetric profile". The source says "Propellers will generate reverse thrust if they are spun backwards, however they will not be as efficient as they would when spinning the direction they were designed for. The quad fliers have found a partial solution to this problem: Remove the curve from the blade. R/C propeller blades are sold by diameter and blade pitch. The pitch is a measure of the angle of attack of the blades. R/C blades also have an airfoil style curve molded into them. Removing this curve (but not changing the pitch) has helped the problem".[2] Plus you didn't cite the actual page, making it harder for other editors to verify your claims. Quadcopters with ordinary propellers work just fine inverted, but the efficiency suffers. That's because airfoils don't work the way you claim they work. --Guy Macon (talk) 02:34, 10 October 2018 (UTC)

Scope of the Kutta condition in the modern world of aeronautics

Guy Macon has written “This entire article is full of bullshit.” I sympathise with Guy and all those who find the article counterintuitive. The root of the problem is that the Kutta-Joukowsky theorem, and the Kutta condition, are based on potential flow theory. This theory takes no account of the viscosity of fluids and makes use of the concept of an ideal fluid. We all know that an ideal, inviscid fluid is remote from the reality. Potential flow theory has its limitations – see Potential flow#Applicability and limitations. For example, potential flow theory is unable to quantify the drag on a body moving relative to a fluid! Potential flow is unable to explain the mechanism by which drag occurs. In fact, potential flow is not even able to acknowledge the existence of fluid dynamic drag! It is indeed a paradox that fluid drag is known to exist but potential flow is unable even to acknowledge its existence – it is known as d'Alembert's paradox.

Potential flow theory also has its critics - Richard Feynman considered potential flow to be so unphysical that the only fluid to obey the assumptions was "dry water". Guy Macon appears to be of the same view – he has used the descriptions “pseudoscience” and “cargo cult science”. Despite the limitations, Wikipedia has many articles on topics based on potential flow theory. For example, see Potential flow around a circular cylinder which shows flow fields that are clearly unrealistic.

The Kutta-Joukowsky theorem ignores the viscosity of fluids but, for many purposes it is the best we have and it appears to be adequate. For analysis of airfoils it relies on the Kutta condition which talks about viscosity but still does not acknowledge any trailing edge other than a sharp edge. The Kutta condition says nothing about precisely what constitutes a sharp trailing edge. Joukowsky and Kutta left it to others to explore lift generated by bodies whose trailing edge was not truly sharp.

Guy Macon has advised that “real airfoils with an oval cross section absolutely generate lift at positive angles of attack.” See his diff. He may be correct. It may be that, like fluid dynamic drag, potential flow theory is unable to predict or explain the lift generated by this class of airfoils. This may be just one more limitation of the Kutta-Joukowsky theorem. The Kutta condition was identified around 115 years ago and it may be that it has outlived its usefulness.

I know of no theory that addresses airfoils without sharp trailing edges. Perhaps we are ready for Kutta Condition Mark II. I look forward to learning more about this class of airfoils with oval cross sections, and seeing what they offer as a replacement for the classical Kutta condition. Dolphin (t) 04:52, 5 October 2018 (UTC)

I know of no theory that addresses airfoils without sharp trailing edges. Then perhaps you should stop making claims about them.

I look forward to learning more about this class of airfoils with oval cross sections Then go fly some balsa wood gliders, paper airplanes and other such toys. ᛗᛁᛟᛚᚾᛁᚱPants Tell me all about it. 14:47, 5 October 2018 (UTC)

In this edit summary[3] Dolphin51 wrote "Citation. The Kutta condition addresses only bodies with sharp trailing edges. It says nothing about the prospect of lift on bodies that have any other shape of trailing edge. The Kutta condition is 115 years old! See Talk page."

I responded by deleting the paragraph with the false information about bodies without sharp trailing edges[4] with the edit summary "If "It says nothing about the prospect of lift on bodies that have any other shape of trailing edge" (see previous edit comment) we shouldn't have a paragraph about "a smooth symmetric body, such as a cylinder with oval cross-section" -- especially one that makes claims that are proven to be untrue (every toy balsa wood glider is a valid counterexample)."

At this point I have completely exhausted my supply of WP:AGF and will from this point instantly delete any material that Dolphin51 adds that states or implies that no lift is generated unless there is a sharp trailing edge.

Likewise for any claims that a propeller spinning in the wrong direction has zero thrust (there is an extensive literature from people flying their quadcopters upside down by reversing the propellers, and some vendors are making special propellers that work better spinning reversed without degrading the normal direction performance too badly), that an aircraft flying upside down doesn't have any lift, or that an aircraft flying backwards has no lift. These are all WP:FRINGE theories and should be treated as such. If Dolphin51 disputes this, I will be happy to post an RfC so that we have an official consensus that balsa wood gliders and paper airplanes do fly, that quadcopters can fly upside down, and that some seaplanes do reverse the propeller direction with transmission gearing instead of using a variable pitch prop to move the seaplane backwards.

In a (most likely fruitless) attempt to short circuit Yet Another Talk Page Full Of Bad Arguments I will add that sharp trailing edges reduce draq and thus save fuel, and that a conventional wing or propeller with the sharp edge on the front and the rounded edge on the back makes a pretty piss-poor airfoil, but nonetheless does generate some lift if the angle of attack is correct. --Guy Macon (talk) 16:11, 5 October 2018 (UTC)

....and of course it didn't short circuit Yet Another Talk Page Full Of Bad Arguments. Instead Dolphin51 now implies that the above means that I agree with him (I don't). --Guy Macon (talk) 17:20, 9 October 2018 (UTC)

Excuse me for being late to the party, but could someone briefly summarize which material in the article is under dispute in the recent threads on this talk page? This forum is for discussing improvements to the article, not for general discussion of the topic. Thank you. Mr. Swordfish (talk) 22:48, 12 October 2018 (UTC)

ᛗᛁᛟᛚᚾᛁᚱPants and I have been removing pseudoscience.[5][6] Dolphin51 has been fighting to retain the pseudoscience. --Guy Macon (talk) 09:11, 13 October 2018 (UTC)

Looking through the diffs, the main issue seems to be that there was an unsupported paragraph regarding oval airfoils. You added a CN tag, Dolphin added a ref in response, and you simply removed the cited paragraph while flinging a bunch of unnecessary and unhelpful invective and accusatory language. I don't have a copy of Clancy readily at hand to check the reference, but my experience with Dolphin is that he's usually very careful with his refs.

That said, I don't think discussion of oval airfoil shapes is crucial to the article so have no problem with it's removal. But please tone down the rhetoric - it's not "psuedoscience" or "cargo-cult". 2-D potential flow is a clearly scientific model, although it has limitations and sometimes the implied results do not match real world airfoils. So, please take the chip off your shoulder and try to work collaboratively. Thank you. Mr. Swordfish (talk) 12:19, 13 October 2018 (UTC)

Request denied. Dolphin51 is the one who refuses to work collaboratively. I tried reasoning with him. He appears to be ineducable.

Dolphin51 isn't just making claims about 2-D potential flow, although he does discuss 2-D potential flow and many other things in his Multiple Walls Of Text. He has made batshit crazy claims about actual, real world wings/propellers -- that they generate no lift/thrust unless they have a sharp trailing edge. That's clearly pseudoscience.

Here in the real world you can mount a propeller and an engine on a barn door and it will generate lift. Here in the real world the claim that wings and propellers rounded leading edges and sharp trailing edges because they don't work otherwise is what we call "wrong". They have rounded leading edges and sharp trailing edges because it improves the lift to drag ratio and has additional benefits related to avoiding stalling.

Our article on Airfoil gets it right:

"The lift on an airfoil is primarily the result of its angle of attack and shape. When oriented at a suitable angle, the airfoil deflects the oncoming air (for fixed-wing aircraft, a downward force), resulting in a force on the airfoil in the direction opposite to the deflection. This force is known as aerodynamic force and can be resolved into two components: lift and drag. Most foil shapes require a positive angle of attack to generate lift, but cambered airfoils can generate lift at zero angle of attack. This "turning" of the air in the vicinity of the airfoil creates curved streamlines, resulting in lower pressure on one side and higher pressure on the other. This pressure difference is accompanied by a velocity difference, via Bernoulli's principle, so the resulting flowfield about the airfoil has a higher average velocity on the upper surface than on the lower surface."

We do not allow WP:FRINGE claims to be presented as facts no matter how much we happen to like the person making the claims. We do allow matereial that is supported by citations to reliable secondary sources no matter how annoying we find the editor making the claim. --Guy Macon (talk) 16:22, 13 October 2018 (UTC)

Thank you Mr swordfish! The root cause of the dispute is that the Kutta-Joukowski theorem and the Kutta condition are talking about the irrotational flow of an inviscid fluid around a two-dimensional cylinder. It is an exercise in mathematics. (A Joukowski airfoil is the result of a conformal transformation.) On this Talk page I have tried to confine my comments to what can be found in reliable published sources on the subject of irrotational flow of inviscid fluids. Some others who have edited here have demonstrated no familiarity with these concepts.

Guy Macon talks about crazy statements that don’t match reality. As you explained so accurately, the potential flow around a body is sometimes radically different from the reality because the reality includes the infinite complexity that comes from viscosity - boundary layers, flow separation, broad wakes, drag etc. Guy Macon is willing to delete statements about potential flow on the grounds that they don’t match reality. For example, he cites the fact that there are websites that talk about quadcopters that can fly inverted by reversing the direction of rotation of their rotors, and implies that this is evidence that the sources on the Kutta condition are somehow wrong. (The Kutta condition mandates a sharp trailing edge if a steady circulation is to exist in the flow of an inviscid fluid around a body.) I don’t deny that quadcopters can fly inverted but that isn’t relevant on this Talk page. For my explanation of why real airfoils in real fluids can perform differently to mathematical airfoils in inviscid fluids see my reply to Rgdboer in the thread above titled “Lift theory”.

Guy Macon has deleted a paragraph from the article on the grounds that it is contrary to his experience. The deleted paragraph referred to “Applying the principles of two-dimensional potential flow ...” and also “inviscid fluid”. As you have acknowledged, potential flow is often different from reality, and at other times it is surprisingly similar. In deleting the paragraph and dismissing it as crazy, Guy has ignored the caveat that it applies to potential flow.

I haven’t challenged Guy’s deletion. Our article on Kutta condition could do with some re-writing so I plan to do that in the coming month or two. I have numerous reliable published sources.

You can assist the present situation simply by reinforcing the notion that articles and text on scientific topics with carefully specified assumptions and limitations, including potential flow of fluids, must not be altered on the grounds that they don’t conform to reality. Dolphin (t) 04:25, 14 October 2018 (UTC)

You have spend months generating page after page of arguments regarding actual airfoils moving though actual air (look at your own sandbox!), arguing again and again that there is no lift without a sharp trailing edge, using real world examples, and now you are claiming that all along you were talking about a mathematical model that doesn't conform to reality? Fine. Edit the page so as to make it clear that your claims have no basis in reality, and nobody will disagree with anything you say about the math. Just don't make any claims about actual airfoils or any claims that imply that they apply to actual airfoils. --Guy Macon (talk) 04:56, 14 October 2018 (UTC)

Thanks Guy. Kutta-Joukowski theorem and Kutta condition are both based on potential flow so, on this Talk page, I have tried to confine my comments to potential flows. My Sandbox is not confined to potential flows so my comments there include many about real airfoils in real fluids, supported as far as possible by citations of reliable published sources that are talking about real airfoils in real fluids. Dolphin (t) 05:34, 14 October 2018 (UTC)

Your sandbox contains multiple statements of the form "Guy Macon says --- but I say" (quoting things that I wrote on this page) without even the slightest hint that those statements are not about the content dispute on this page.

If your sandbox really isn't about the contents of this page, then Wikipedia:User pages#What may I not have in my user pages? may apply, especially the part that says "Generally, you should avoid substantial content on your user page that is unrelated to Wikipedia" and "Unrelated content includes, but is not limited to: Extensive discussion not related to Wikipedia [and] Extensive writings and material on topics having virtually no chance whatsoever of being directly useful to the project, its community, or an encyclopedia article." --Guy Macon (talk) 06:03, 14 October 2018 (UTC)

Guy Macon I have to say that I agree completely with the passage you posted from the Airfoil article. Then again, I wrote it seven years ago in 2011 with assistance from Dolphin, so it's pretty unsurprising that I agree with it. I would assume Dolphin agrees too since he was involved in crafting that re-write. See this diff: https://en.wikipedia.org/w/index.php?title=Airfoil&type=revision&diff=438163347&oldid=437962548

That said, I would like to remind you that assuming good faith, maintaining civility, and generally adhering to Wikipedia etiquette principles and guidelines is not optional. It is not a request that you may decline. So please read and apply the principles. Thank you.

Mr. Swordfish (talk) 20:18, 14 October 2018 (UTC)

The phrase "AGF is not a suicide pact", refers to the Wikipedia "Assume Good Faith" policy, and stems from a statement made by Jimmy Wales in March 2005:

"Our social policies are not a suicide pact. They are in place to help us write the encyclopedia. [...] We need to take due process seriously, but we also need to remember: this is not a democracy, this is not an experiment in anarchy, it's a project to make the world a better place by giving away a free encyclopedia" --Guy Macon (talk) 08:03, 15 October 2018 (UTC)

--Guy Macon (talk) 08:03, 15 October 2018 (UTC)

Mr swordfish: Yes, I agree with the quotation Guy has posted from Airfoil. You will be aware that our excellent article Lift (force) contains two sections dedicated to “physical explanations” of lift; and one section dedicated to “mathematical theories”. Circulation, the Kutta-Joukowski theorem, and the Kutta condition are described explicitly in the section for mathematical theories. All the mathematical theories appear very different to the physical explanations. Guy Macon may be labouring under the misconception that there is “only one true explanation of lift”, and therefore all other explanations must be wrong (pseudoscience.) Guy has made a claim or two about crazy things I have written on the subject of real airfoils and real wings in real fluids. He omitted to supply a diff so we can’t be sure what he is writing about, but I think he may be objecting to information I supplied to him on my User:Dolphin51/Sandbox. (However, this Talk page is not an appropriate place at which to complain about my Sandbox.) Dolphin (t) 23:47, 14 October 2018 (UTC)

"The essential feature of any airfoil is that it has a sharp trailing edge - see Kutta condition. Whether a wing is operating with its “usual side” up, or the usual side down, its trailing edge is sharp so it functions as an airfoil and generates lift. (If the airplane was falling backwards, the wing would not generate lift, for the obvious reason.) --Dolphin51 22:05, 7 August 2018 (UTC)[7] --Guy Macon (talk) 08:03, 15 October 2018 (UTC)

This is, of course, nonsense. Wings with a rectangular cross section are airfoils and do generate lift,[8] even though they lack the alleged "essential feature of any airfoil". As Dolphin51 later admitted, a quadcopter propeller with the usual rounded leading edge and sharp trailing edge spinning backward does keep an inverted quadcopter in the air. The first inverted quadcopters simply reversed the motor direction with no other changes. The main complaint the operators of these early inverted flight quadcopters had was that the battery goes dead too soon (the motors have to work harder because the drag on the propellers is higher) and that it climbs slowly. If indeed the propeller "would not generate lift" (Dolphin51's exact words), these early inverted quadcopters could not hover. --Guy Macon (talk) 08:03, 15 October 2018 (UTC)

Agree that this Talk page is not an appropriate place at which to complain about Dolphin's (or anybody else's) Sandbox. How about we drop the stick? Mr. Swordfish (talk) 12:29, 15 October 2018 (UTC)

I suspect it's be much easier for Guy to drop the stick if Dolphin weren't constantly adding OR to this article and falsely claiming that GUy agrees with it. ᛗᛁᛟᛚᚾᛁᚱPants Tell me all about it. 12:52, 15 October 2018 (UTC)

Dolphin's last edit to this article was ten days ago on Oct 5. So he's hardly "constantly adding OR to this article." If there is some content currently in the article or some proposal for adding, removing, or modifying material to the article, then I'm happy to discuss calmly and rationally. Otherwise, let's move on. Mr. Swordfish (talk) 14:58, 15 October 2018 (UTC)

I think that ignoring the "and falsely claiming that Guy agrees with it" part of my comment makes for quite the juicy cherry to have picked. I also think that Dolphin's proclamation that they intend to "re-write" this article in the coming months is quite worrysome and bears scrutiny, considering the fact that literally every time I have checked their sources against their claims I have found that the source does not support the claim. I do not fault Guy at all for expressing that worry here at talk, considering how germane it is to this subject. ᛗᛁᛟᛚᚾᛁᚱPants Tell me all about it. 15:23, 15 October 2018 (UTC)

It looks like we are now right back where we started! My responses to all Guy Macon’s requests for evidence are still available at User:Dolphin51/Sandbox. All my responses on my Sandbox have been linked to reliable published sources.

I am happy to drop the stick, but if there are further deletions of text from this article I would examine those deletions on their merits. Dolphin (t) 14:44, 15 October 2018 (UTC)

I am not convinced that you are capable of objectively evaluating this subject. ᛗᛁᛟᛚᚾᛁᚱPants Tell me all about it. 15:23, 15 October 2018 (UTC)

I am done. First Dolphin51 says "The Kutta-Joukowski theorem and the Kutta condition are mathematical theorems that assume fluids have no viscosity" and "Kutta-Joukowski theorem and Kutta condition are both based on potential flow so, on this Talk page, I have tried to confine my comments to potential flows. My Sandbox is not confined to potential flows so my comments there include many about real airfoils in real fluids" and then he says "My responses to all Guy Macon’s requests for evidence are still available at User:Dolphin51/Sandbox". So I am supposed to ignore the claims in his sandbox when that is convenient to his argument while at the same time looking at his sandbox for answers to my comments -- and if I disagree Mr. Swordfish (who was called here through WP:CANVASSING to support Dolphin51) complains that "this Talk page is not an appropriate place at which to complain about Dolphin's (or anybody else's) Sandbox".

Meanwhile, Dolphin51 completely ignores the fact that he claimed that "The essential feature of any airfoil is that it has a sharp trailing edge" and "If the airplane was falling backwards, the wing would not generate lift" along with claiming that a normally-shaped propeller spinning backwards doesn't produce any thrust. (There are multiple videos on the internet where owners of RC airplanes accidentally installed the prop backwards. The planes fly, but not as well as they do when they take off the propeller and put it on the right way.)

This is a colossal waste of time, and I believe that I have a clear case for a WP:IDHT topic ban if Dolphin51 attempts to insert his WP:FRINGE theories about how wings and propellers work, as he did at Quadcopter.[9][10][11][12]

I am through discussing this. I will continue watching Dolphin51's contributions, and any further edits that make provably wrong claims about wings or propellers will result in a case at WP:ANI, with the probable result of a topic ban. This will be my last comment. "Never Wrestle with a Pig. You Both Get Dirty and the Pig Likes It".[13] --Guy Macon (talk) 15:48, 15 October 2018 (UTC)

I will work up a refined version for some or all of Kutta condition and put it on my Sandbox. I will then alert all interested Users. We can discuss it on the Sandbox talk page. Dolphin (t) 22:59, 15 October 2018 (UTC)

A warning to editors of this page.

Dolphin51 has been canvassing,[14][15][16] and thus newly-arriving editors are very likely to not have a neutral point of view, and comments by those who were canvassed are likely to not reflect the consensus of the Wikipedia community. --Guy Macon (talk) 05:12, 14 October 2018 (UTC)

Plagiarism by Rathakrishnan

"The flow over the topside is so much faster than the flow along the underside that these two halves never meet again. They do not even re-join in the wake long after the airfoil has passed. This is sometimes known as "cleavage". There is a popular fallacy called the equal transit-time fallacy that claims the two halves rejoin at the trailing edge of the airfoil. This fallacy is in conflict with the phenomenon of cleavage that has been understood since Martin Kutta's discovery" appears on page 140 of Rathakrishnan, Ethirajan (2013). Theoretical Aerodynamics. New York: Wiley. ISBN 9781118479377., but it appeared in Wikipedia in 2008. Congratulations to Dolphin51 who wrote it! cagliost (talk) 09:45, 28 March 2022 (UTC)

An interesting discovery. Thank you for drawing it to our attention! Dolphin (t) 13:21, 28 March 2022 (UTC)