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This is an old revision of this page, as edited by 208.249.136.187 (talk) at 14:24, 2 November 2010 (→‎very basic summary: new section). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

Talk page archive

Archive 4 has been created with a link at above right. It is an exact copy of the talk page as it was before this edit (besides the last four sections which I left here). Archive 5, when needed in the future, should be a new subpage (same as creating an article) titled "Talk:Lift (force)/Archive 5" and the link added to the template on this page's code. For further information on archiving see Wikipedia:How to archive a talk page. See also User:5Q5 for the used archiving procedure. Thank you. -- Crowsnest (talk) 01:23, 15 May 2010 (UTC)[reply]

Request for real world flow diagram (as opposed to idealized flow)

The current animated diagram doesn't show the "big picture" effect of the flows further away from a wing. This avweb article includes an image showing the flow and one animated diagram with a vertical column of dots and a wing passing through, that gives a more realistic picture of how the air is affected by a wing passing through it:

avweb_article.html

Since the flow speed and size of the wing in the current animated diagram (the one with the colored and black dots) isn't known, I don't know how realistic it is, but the upwash effect is exaggerated compared to the flow you'd see with a real airplane. In the real world, starting at a sufficient distance above and in front of an approaching wing, there's virtually no upwash, just an acceleration backwards and downwards towards the low pressure area above the wing which transitions into a mostly downwards path as the wing passes by. I'd like to see an animated diagram showing this "macroscopic" view of the air flow caused by a wing passing through the air.

Jeffareid (talk) 10:00, 22 February 2010 (UTC)[reply]

You can take a look at this video and see the upwash and downwash (as well as the faster flow above the wing; non-equal transit time) in a "real flow". -- Crowsnest (talk) 19:45, 12 May 2010 (UTC)[reply]

What interactions cause the increase of energy of the air when producing lift (and drag)?

Using the air as a frame of reference, after a wing passes through a volume of air, the affected air ends up with an increase in energy, a downwards (lift related) and somewhat forwards (drag related) non-zero "exit" velocity where the affected flow's pressure returns to ambient.

Using a 1500 lb glider with 60:1 glide ratio at 60 mph as an example, (a Nimbus 4T with an 80 foot wingspan does this at 68 mph), in a steady 1 mph descent, the gravitational power applied to the glider is 1500lb x 1 mph = 4 horsepower. The glider in turn applies this power to the air, adding 2200 lb ft of energy to the air every second.

Most of the energy is in the form of an impluse wave that transmits the 1500 lb weight of the glider through the air and eventually onto the earth's surface.

Clearly Bernoulli is violated by this increase of energy of air (work is peformed). What are the interactions between a wing and air that are responsible for this increase in energy? In real world situation, such as assuming a finite wingspan of 100 feet or less, is there a lower limit to the minimal amount of energy that must be added to the air in order to produce lift for a 1500 lb glider at some range of speed, perhaps 35 mph to 70 mph?

Jeffareid (talk) 20:18, 23 February 2010 (UTC)[reply]

Viscosity? When anything moves through the air you get molecules hitting the surface and they inevitably bounce off faster, they then hit each other and similar things happen there. The net upshot is that the thermal energy of the air around the object increases. If you think about it, drag has to involve heating because energy is force times distance, in any second the object has moved a distance against the drag force, and that energy has to go somewhere.- Wolfkeeper 01:56, 24 February 2010 (UTC)[reply]
In this case of a glider, the temperature increase of the affected air is small, and the amount of change in velocity of the air in the direction of flight, related to drag, is also small. Most of the change in velocity (from zero relative to the air, to the "exit velocity" speed) is in the direction of lift, and most of the increase in total mechanical energy is related to the increase of "exit velocity" in the direction of lift. Since the wing is at an effective angle of attack, it could be possible to estimate work done in the direction of lift as the average force of the wing times the average component of distance the wing surfaces move perpendicular to the direction of flight, as a wing moves through a cross-sectional plane perpendicular to the direction of flight, but I don't know how to take the pressure differential effects into account. Jeffareid (talk) 03:31, 24 February 2010 (UTC)[reply]
There's a handy formula for relating fluid velocity to pressure - it's called Bernoulli's principle :-) If the temperature increase of the affected air is small, it will provide good answers. But, there's no work done in the directin of lift for straight and level flight. (No distance travelled in the dirction of the force = no work.) The propulsion system does the work in overcoming drag.69.1.23.134 (talk) 02:54, 14 March 2010 (UTC)[reply]
I agree that no work is done by the lift vector, but it isn't only in straight and level flight. Providing the frame of reference is the one in which the atmosphere has zero velocity, lift is the component of aerodynamic force that is perpendicular to the vector representing the velocity of the airfoil (or wing or aircraft) and so zero work is done, regardless of whether the aircraft is in straight flight or turning, level flight, climbing or descending. Thrust, drag and weight are all capable of doing work on the airfoil (or wing or aircraft) but lift does no work when using this particular reference frame. Dolphin51 (talk) 11:32, 14 March 2010 (UTC)[reply]
True for an airfoil, but not for a wing or aircraft. Turning, climbing and descending all involve work in the direction of lift on the wing/body resulting in energy storage/release by the wing/body. 214.4.238.180 (talk) 18:11, 12 May 2010 (UTC)[reply]
True for an airfoil, but not for a wing ...? What is a wing if it isn't an airfoil? Climbing and descending involve work, but it is work done by the weight and thrust. Turning flight doesn't involve work. If we choose the reference frame attached to the atmosphere, lift never does work because lift is defined to be the component of aerodynamic force that is perpendicular to the relative velocity, and any force that acts perpendicular to the displacement or velocity vector does zero work. See the definition of Work (physics). Dolphin (t) 21:56, 27 October 2010 (UTC)[reply]
You asked what interactions cause the increase in energy involved in lift and drag. I don't know if this really answers your question, or just leads to another question, but here is a stab.
(Hope we are indirectly supporting the article here, since Wikipedia policy forbids discussions for other purposes.)
Each tiny patch of wing collides with air molecules. Each collision imparts a change in momentum of the particle (and the glider). For some of those patches, for example at the forward stagnation point, the collisions increase the magnitude of the momentum of the particles, increasing their kinetic energy (by one half the square of the increase, divided by the mass). For others, they decrease it, as on the top surface aft. A patch there is receding from the particle, which dulls the rebounds like a snowball hitting a moving car from behind. The power transfer from a given patch is the sum of those energy changes per unit time, which smooths out to a constant value if all else is steady. For some patches that power is positive, for others negative, But the sum of the powers from all the patches is positive. These are the interactions that cause the increase in energy, I think.
Mark.camp (talk) 16:05, 27 October 2010 (UTC)[reply]

Weltner & Ingelman quote

I removed the Weltner & Ingelman-Sundberg quote:

The cause of the aerodynamic lifting force is the downward acceleration of air by the airfoil

— Klaus Weltner and Martin Ingelman-Sundberg

since it does not seem to be written by someone who has published much on the subject of lift in renowned journals in the field, or is often cited. Nor does it seem to have been published in a peer-reviewed article, or being a reliable secondary source. So, why should this particular quote go into the article? If such a quote is needed, are there no better phrased alternatives? Also note that the wording is quite opposite to the common wording of Newton's 2nd law: this quote says that force is caused by acceleration, while the common description is that net force causes acceleration. So, instead of an explanation, this quote may confuse. (Apart from wordings, as in the original phrasing by Newton, of the 2nd law as a balance between net force and momentum change rate without reference to cause and effect). -- Crowsnest (talk) 23:42, 23 April 2010 (UTC)[reply]

I have no objection to Lift (force) supplying a number of different explanations for the phenomenon of lift, and I have no objection to Lift and the deflection of the flow being one of those explanations. It has a legitimate place in this article.
I see no benefit in having one or more of the different explanations supported by a quotation, regardless of the significance of the author of that quotation. If one of the different explanations is to be supported by a quotation, they should all be supported by such a quotation.
The quotation from Weltner and Ingelman-Sundberg is probably technically correct when surrounded by the complete text by these authors. However, this quotation, isolated from its surrounding text, is without merit. Essentially, all it says is the cause of the ... force is the ... acceleration of air ...
There is nothing in physics, and certainly nothing in Newton's 2nd or 3rd Law of Motion, that says the cause of a force is an acceleration. So taking these words out of their intended context does Weltner and Ingelman-Sundberg a great disservice. Without the support of the surrounding text provided by these two authors, this quotation is technically incorrect.
If each of these different explanations for lift is to be supported by a quotation that quotation must be eloquent and scrupulously correct. The quotation by Weltner and Ingelman-Sundberg is neither of these, so I am not in favour of it being used in Lift (force). In fact, I think it is sufficient to state each of these different explanations and provide suitable in-line citations. Quotations are not warranted. Dolphin (t) 05:21, 24 April 2010 (UTC)[reply]
Dolphin,
I do not think that the quote is taking Weltner out of context. Here is the entirety of the Abstract:
Abstract. The conventional or standard explanation of aerodynamic lift states the higher streaming velocity at the upper side of the airfoil as cause of the lower pressure, due to Bernoulli’s law. But a higher streaming velocity is the effect of a lower pressure and never its cause. The cause of the aerodynamic lifting force is the downward acceleration of air by the airfoil - which depends on the angle of attack and its velocity.
In relation to the airfoil the normal acceleration of the air in case of curved streamlines must be regarded which results in pressure gradients perpendicular to the streamlines and reaction forces acting perpendicular on the deflecting surfaces.
And here is what he says in the summary, in it's entirety:
The conventional explanation of aerodynamical lift based on Bernoulli’s law and velocity differences mixes up cause and effect. The faster flow at the upper side of the wing is the consequence of low pressure and not its cause.
The generation of lift by an airfoil can be explained correctly and simply taking the downward acceleration of air into consideration. This approach allows to derive the dependency of lift from angle of attack, flow velocity and the air’s density in a streightforward and coherent way.
A detailed explanation of the generation of pressure differences is possible if the normal acceleration of streaming air is taken into consideration.
I don't think it could be much clearer what he is saying: deflection of the air causes lift. See the quotation from the American Journal of Physics in my reply to Crowsnest below for further corroboration.
As to the question of whether it merits a quotation as opposed to an inline citation, that's a question of style. I don't claim to be an expert on Wiki style, so I'll defer to those who are.
My actual preference would be to dispense with the cut-and-paste style of a quote from Weltner followed by a quote from Anderson followed by a quote by Langewiesche and replace it with plain prose (with citations, of course). Stitched-together quotes are less readable than plain statements and the article would be improved by simply describing the theory. Unfortunately, the subject is enough of hot-potato that we (the various editors) are ultra-careful about the language and have resorted to quotations in order to protect ourselves from criticism by other editors. The result is that the article evolves to be written for the editors rather than for the general public. Unfortunately, I don't know that it's possible under the current circumstances to come to agreement on language.
Mr swordfish (talk) 20:33, 26 April 2010 (UTC)[reply]
Hello Mr swordfish. Thanks for your comprehensive and thoughtful response.
I fully agree with your final paragraph. (I particularly agree with your comment that pasting of quotations in this section of Lift (force) is related to the contentious nature of the subject. I think this Talk page is the place for the quotations so that all interested editors can see that in-line citations accurately support the text in the article.) Let’s move the quotations from the article to this Talk page, and rely on plain prose that will best serve the general public.
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. The quotation belongs on this Talk page so we can all see exactly what the in-line citation is pointing to. Regards. Dolphin (t) 23:18, 26 April 2010 (UTC)[reply]
Crowsnest,
Google Scholar, while a useful tool, is not comprehensive. In particular, your search missed Weltner's seminal paper in the American Journal of Physics 1987 vol 55 No. 1 A comparison of explanations of the aerodynamic lifting force. This paper is in a peer reviewed journal, but unfortunately is behind a paywall. It is similar to the online paper cited by the wiki article, and contains the following quote:
"Textbooks stating that the higher streaming velocity is the reason for the low pressure are wrong. It is the other way around. The low pressure is the reason for the higher velocity of the streaming air." (emphasis in the original)
Although the paper cited is self-published, Weltner certainly qualifies as a reliable secondary source. The online paper says basically the same thing as the AJP paper; I could have quoted the AJP paper instead, but the online article said it more succinctly, and it seems valuable to have an easily accessible reference rather than one that's behind a paywall.
Newton's 2nd law is usually presented as F=ma. Whether force causes the acceleration or the other way around is open to interpretation. For example, when you are sitting in the passenger seat on the bus and the driver presses the accelerator you feel a force from the back of the seat. The force you feel is caused by the acceleration of bus; it would be absurd say it was the other way around, i.e. to insist that you are making the bus speed up by pressing on the back of your seat. So, in some situations, it makes complete sense to say that acceleration causes a force.
As for the quote "confusing" the reader, there is nothing confusing about the quote at all: it is the simplest, most direct and straightforward statement of the deflection theory. That's why it was chosen. I understand that you disagree with it. That's your prerogative. Mr swordfish (talk) 19:45, 26 April 2010 (UTC)[reply]
The quote is from a self-published source, so in general not acceptable. A possible reason to still include it in the article would be, when Weltner and Ingelman-Sundberg are renowned experts in the field. And the Google search was just intended to get an indication about that (how much have they published on the subject), as well as an indication about the amount of citations to their work.
Nor is it logical to take a quote from a self-published source because it is "basically the same thing" as what was said in another, peer-reviewed paper by the same author. Wikipedia is full of refences to articles in peer-reviewed papers which are in general not freely accessible for everyone, and references to for many difficult to obtain books. There is no policy stating that quotes should be from easily accessible references.
The example given about the bus is off-topic, while the representation F=ma is not a suitable depiction of Newton's 2nd law for use in fluid-dynamical flow modeling. Nor do I believe that in fluid dynamics, in general, flow forces cause acceleration or the other way around. On the contrary, in fluid dynamics most flows are balances between pressure (and viscous stresses), and momentum changes; in complex interactions.
People like John D. Anderson (note 9 at this moment in the article), and Landau & Lifshitz (note 26) are very careful in their wording of the relationship between lift force and momentum changes, preventing to end up in endless discussions about cause and effect (which may be useful for the philosophy of science).
Your opinion or my opinion on what is "confusing" or "the simplest, most direct and straightforward statement of the deflection theory" is of limited importance. The point is that a stand-alone quote in that location in the article has to represent scientific consensus (or the consensus within the notable scientific faction it stands for). The cause-and-effect formulation of Weltner & Ingelman-Sundberg gives an opinion, not a scientific fact that is widely accepted (notable in the WP sense). -- Crowsnest (talk) 07:45, 27 April 2010 (UTC)[reply]
Mr. Swordfish, you wrote "it would be absurd say it was the other way around, i.e. to insist that you are making the bus speed up by pressing on the back of your seat."
Did you mean to say, "it would be absurd say it was the other way around, i.e. to insist that you are creating a force on your back by accelerating." ?
I think so. You were making the (good) point that simply reversing F and a in the cause and effect statement seems intuitively absurd in this case. But the F in statement 1 is the forward force exerted on the back, not the backward force by the back, and the a is the acceleration of the person, not of the bus.
If we make the reversal one of switching equal and opposite forces carefully, we come up with "The backward force of the back on the bus causes the acceleration of the bus to be lower than it otherwise would be" which is different than the way you wrote it. In this case there is no intuitive absurdity at all. Both cause and effect statements are equally intuitive.
Crowsfoot is right that F=ma, or its fluid dynamics equivalent, doesn't by itself indicate an asymmetric cause and effect relationship. If causes must precede effects, then we could even argue that F=ma forbids a either causal interpretation, since it refers to instantaneous values of both F and a (and m).
But its helpful to the reader, I think, to go with the intuitively appealing bias that net forces (pressure differences) cause accelerations (of bodies or particles, or for us, of differential volumes of fluid). The real difficulty comes when the reader wants to know if the pressure field is caused by the flow field, or vice versa. We must get him/r to abandon this truly meaningless question, I believe, and understand that Newton's differential equation, and the continuity and mass conservation constraints all have to hold at the same time. The infinite set of 3d velocity vectors and the infinite set of local pressures have to be consistent with each other, and there is no intuitively meaningful way to say that one causes the other.
91.103.41.50 (talk) 21:48, 27 October 2010 (UTC)[reply]


Re-casting Deflection Theory section in plain prose

Per the discussion above, I've taken a stab at recasting the deflection theory in plain language, removing the quotes from the article and placing them in footnotes. I think this approach is more readable to the general public than the 'cut and paste' set of quotations that is there now. The proposed new language is in my sandbox: http://en.wikipedia.org/wiki/User:Mr_swordfish/Lift#Deflection Please take a look and offer comments or edits.

One thing that would help the article is a nice picture or diagram for this section, but I haven't found a public domain one yet. Suggestions cheerfully appreciated. Mr swordfish (talk) 21:20, 3 May 2010 (UTC)[reply]

Thanks for the work you have done here, and for the invitation to comment. I have made my comments at User talk:Mr swordfish/Lift#May 2010. Dolphin (t) 00:09, 4 May 2010 (UTC)[reply]

As of tomorrow it will be ten days since I posted the edits in my sandbox. Dolphin, Crowsnest and I have made some minor tweaks in that time, and while it is still a work in progress (and probably always will be) I think it's time to merge the changes into the main article. Pending any objections, I'll make the changes live tomorrow. Mr swordfish (talk) 17:25, 13 May 2010 (UTC)[reply]

Bernoulli's Principle

Today I reverted Sliceofmiami's good faith edit to the "equal transit time" section of the article. If we want to present a description of lift based on Bernoulli's principle (and perhaps we should) it deserves it's own section, not a rear-guard action tacked on to the end of equal-transit-time.

The difficulty in presenting a simple explanation based on Bernoulli's principle is explaining why the air changes velocity as it flows past the airfoil. I don't necessarily disagree with Sliceofmiami's statement "lift is still generated by Bernoulli's principle", but without some supporting arguments explaining why the velocity changes the assertion is out of context.

The next section (a more rigorous physical description) explains it pretty well, but is not exactly simple. My take is that it's not possible to explain lift simply via Bernoulli's principle, but I'm all ears if someone wants to take a crack at it.

BTW, to answer his question "Bernoulli's principle is discussed negatively in this section, why?", I don't think Bernoulli's principle is discussed negatively, only the mis-application of it via equal transit time. Mr swordfish (talk) 13:37, 14 June 2010 (UTC)[reply]

I agree that Bernoulli's principle on its own is not sufficient to explain lift because it doesn't explain why the air travels faster past the upper surface of the airfoil than the lower surface. In the past I have grappled with this predicament by incorporating the Kutta condition. When a reader is comfortable with the Kutta condition, the extra information provided by Bernoulli's principle should complete the picture. In addition, the vortex necessary to meet the Kutta condition completes the equation at Kutta-Joukowski theorem so that the amount of lift can be considered to be quantified. Dolphin (t) 04:08, 15 June 2010 (UTC)[reply]
Bernoulli's principle gives the pressure (dynamics) provided the velocity field (kinematics) is known (in incompressible flow). As already said by Dolphin51, the problem is in the determination of the flow kinematics. The equal transit time assumption is known to be false. Also the descriptions purely based on blockage and mass conservation (Lift (force)#In terms of a difference in areas) are inadequate. As pointed out by Dolphin51, the Kutta condition in combination with irrotational and incompressible flow are sufficient to provide the flow kinematics (as a potential flow). However, this is not a simple explanation: the Kutta condition has to be justified through Prandtl's description of boundary layers, and the flow fields from potential flow theory require quite some math (but have -- according to Lord Kelvin -- minimum kinetic energy).
However, we could rephrase the last sentence in the section from a double negation ("...does not imply that Bernoulli's principle is incorrect...") to a more positive one, e.g.: "Note that while this theory depends on Bernoulli's principle, the fact that this theory has been discredited is independent from the validity of Bernoulli's principle". -- Crowsnest (talk) 09:00, 15 June 2010 (UTC)[reply]

The animation contradicts the text?

The animation that is supposed to show the flow on top is faster than the flow on the bottom appears to show the top flow slower than the bottom, visually it doesn't look faster, and after the air leaves the airfoil the top part is behind the bottom part; is it an optical ilusion or is the animation really showing the top flow being slower? --TiagoTiago (talk) 00:48, 27 August 2010 (UTC)[reply]

It is neither. You are not looking at the animation carefully enough. The flow on top is faster, particularly between the leading edge of the airfoil and about the quarter-chord position. Above the airfoil the spacing between the vertical black dotted lines is about twice the spacing of the lines below the airfoil. That shows the average speed above the airfoil is about twice the speed below the airfoil.
The vertical black dotted lines in the fluid are all one color, but if they were alternating colors (say alternating red and blue) you would see clearly that upstream of the airfoil each vertical line was all one color but downstream one color above the wake would almost line up with the other color below the wake. It is a brilliant piece of animation and there is no error or optical illusion. Dolphin (t) 03:02, 27 August 2010 (UTC)[reply]

very basic summary

So, as a very, very basic summary that glosses over it, would it be fair to say that the airfoil's angle of attack or camber "deflects" the upper stream towards other layers, compressing it and causing it to speed up, while it allows more room for the lower stream, expanding it and causing it to slow down?208.249.136.187 (talk) 14:24, 2 November 2010 (UTC)[reply]

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