Talk:Lift (force)

-- I'm worried here about wading into the debate surrounding the different ways of expressing lift. There are many ways of describing lift, and many of them are right. The Bernoulli/Venturi explanation is not wrong. When you take the integral of the pressure distribution over the surface of the wing, it adds up to the lift. And the velocity does vary with pressure according to Bernoulli's equation. The only error in that common explanation is that there is no reason for the streamlines to meet at the trailing edge. The explanation that's currently there, using the Coanda effect to explain downwash and thus lift by Newton's 3rd, is absolutely right, and a very good way of explaining it, but it being right does not make the Bernoulli explanation wrong. Another popular way of talking about lift is through circulation. This is widely used by practioners of aerodynamics but not particularly clear to the layperson. It is also absolutely correct. In fact, the three methods I just described of explaining lift can all be proven to be mathematically equivalent to the others.

Very interesting. I think the article needs some kind of rewrite or reorganisation that makes all of these points. What I did want to ensure however was that the very common "longer path over the top" explanation was well and truly debunked. Most books aimed at the layman use this explanation and it is definitely wrong. Having eliminated that one, the true explanation is then rather harder to get across to someone without going into some moderately advanced physics, so the Newton's 3rd + Coanda is probably the easiest to understand intuitively. The other effects such as pressure differentials, velocities, etc. are to my mind more easily brought into the picture as resultant phenomena rather than causative ones, though as with many things each person has their own preferences as to how a thing is taught/explained. GRAHAMUK 23:06, 1 Dec 2003 (UTC)

I rewrote it a little, I think it could use some more work. My biggest challenge I think is trying to write it in a way that someone not overly familiar with fluid dynamics and math can understand. Let me know if any of the terms I introduced are unclear. Moink 00:22, 2 Dec 2003 (UTC)

Come help with Wikipedia:WikiProject Fluid dynamics moink 23:19, 27 Dec 2003 (UTC)

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The pressure difference above and below the wing is less important to create lift than one might think. 90% percent of the lift comes from what is called 'downwash'. Simply a deflection of the airflow. Air is accelerated downwards. Lots of air! This requires a force and already Newton now that that creates an eqally but opposing force. If one looks at the force that the pressure difference creates for a small Cessna, it is only about 1500 N (approximately 300 pounds).

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-- The above is correct. In fact, at some level, ALL of the upward force on the wing must be created by accelerating air downward. You can create all the force on the airmass that you want but your plane is gonna fall out of the sky unless you accelate something. The details of fluid dynamics make the process practical. But the primary phenomenon is newton's third law. I've added a paragraph saying so. I think some work still needs to be done tidying up the terms camber, curve and such. I don't have a detailed enough knowledge of airfoils to get that part right. -- Blimpguy - Fri Jan 24 14:39:31 UTC 2003

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For those who will not read the expanded explanation below, I prepend this brief summary:

• Lift can always be accounted for by pressure measurements
• therefore, it is incorrect to say that lift can not be attributed to pressure
• when calculating pressure, terms such as circulation, Bernoulli Effect and Newton's Law (and I will add Navier-Stokes) develop and are all appropriate accountings of Lift, each with its own assumptions about the real world

To explain: I am a NASA engineer and I am always surprised by the bad science on Lift on the internet. Every physical and mathematical description of lift that exists is a model; an approximation of reality (physics itself--down to the superstring--is but an approximation of reality). Some models make more assumptions than others to apply to specific situations and be easier to work with computationally.

Lift does not exist until it is given a definition by people, and it is defined as a force. Right there, assumptions are made about the nature of air and its consistency. As a force, the most accurate accounting of Lift is a measurement. This is done by measuring pressure.

If you will not believe me, perhaps you will believe Dr John D. Anderson, Jr. (Curator of the National Air and Space Museum) and Dr L. Prandtl (Father of Aerodynamics):

In his book, Fundamentals of Aerodynamics, 2nd Ed., Dr John D. Anderson, Jr. states (p218):

"...the true physical sources of the aerodynamic force on a body are the pressure and shear stress distributions exerted on the surface of the body...lift is 'caused' by the net imbalance of the surface pressure distribution..."

(In the above quote, "shear stress" refers to the "aerodynamic force" known as Drag.)

In his book, Applied Hydro- and Aeromechanics, Dr L. Prandtl states:

(p144) "By decomposing the total force into two components, one in the direction of the flow and another perpendicular to the flow, we are led to the conception of lift...In practical aeronautics, we are interested in bodies (airfoils) where the total resulting force is nearly perpendicular to the direction of the flow, so that in this case the lift is great and the drag small."

On the physical origins of this "total force", Prandtl states (p159) "If a body experiences lift, i.e., a force component perpendicular to the flow of the fluid, we can ascribe this phenomenon only to a certain excess pressure on the bottom side of the body and a certain partial vacuum on the top side."

But generally, measuring pressure is not practical. So math is used to calculate it. Along with these math models come useful relationships like "as velocity increases though a constant area, pressure decreases" that apply only to specific and simplified situations.

I will further explain these calculations:

The best models we have are known as the Conservation Equations. These say things such as "any mass going into a system must equal the amount accumulating within the system minus the amount leaving the system because mass is neither created nor destroyed" etc. No one has ever applied these to aeronautics without assumption because the resulting equations are too hard to work with.

The next best model we have are the Navier-Stokes equations. These are derived from the Conservation Equations with some assumptions, such as "the fluid is Newtonian (true for air)", etc. These have a high degree of accuracy but are still difficult to work with, so more assumptions are generally made at the sacrifice of some accuracy. When the assumptions are made intelligently (i.e. apply well to a given situation), not much accuracy is lost. At this level of generalization, one can see Lift's dependance on air temperature (which would still show up in an accurate pressure measurement).

The next best, then, is the circulation equation of Lift (also derived from the Conservation Equations). Assumptions made to arrive at this point include inviscid flow, etc. These equations are good enough for many high-speed situations in which air compressibility is an important factor.

Newton's Law force = mass * acceleration (again, derived from the Conservation Equations) is the next most accurate. Assumptions made to arrive at this point include incompressible flow (good for low speeds only), etc. In this form, the equations can be visualized by the "airfoil pushing air downward" concept. It should be noted here that when aerodynamicists use the term downwash they are referring to the component of the wingtip vortices that is oriented perpendicularly to the freestream velocity, not "the air that is pushed downward by the airfoil"; a definition that certainly developed from pilots or other laymen and could also have been borrowed from helicopter terminology. An important difference between the two is that to aerodynamicists, downwash does not exist for airfoils, which are two-dimensional constructs, but is defined only for a three-dimensional consideration of the entire wing.

The "Bernoulli Effect" is Newton's Law (above) written for fluid flows. In this form, the equations are usually visualized by the "pressure" concept because of the simple relationship between velocity and pressure. This simple relationship only applies accurately to low-speed flight; Lift can not be calculated this way for any other situation because the calculated pressure would not match the measured (thus true) pressure, but nonetheless, Lift is totally accounted for by the true pressure.

As has been said, it is true that as air seperates above and below the wing, the "parcels" of air that were in contact with each other at the leading edge do not come back in contact with each other at the trailing edge. I suppose if a misconception such as that can persist in the public domain with a mighty tenacity, then the current arguments and misconceptions should come as no surprise.

26 Aug 2004 by Lensim

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Hmm. I'm a little confused. I don't think that the article actually says or suggests that "lift can not be attributed to pressure". If so, then it should definitely be fixed. If not, them I'm not sure where the "bad science" is.

It seems to me that there are differences of opinion about how one wishes to think about the phenomenon of lift rather than right vs. wrong. Specifically, aerodynamicists tend to think in terms of pressure differentials and pilots tend to think in terms of Newton's 3rd. My personal opinion is that a description based on Newton is more effective for the untrained reader. From there it seems reasonable to proceed to the other ways of accounting for lift.

So, with that in mind, the use of the term "downwash" in the article is the pilot's meaning rather the aerodynamicist's meaning. I think a description of the two meanings of the term would be an excellent place to start for a "downwash" article. (I would be particularly careful with NPOV here - pilots are not fond of the engineers' habit of lumping them in with "other laymen".)

Blimpguy 15:30, 27 Aug 2004 (UTC)

Sorry for the confusion. When I said "it is incorrect to say that lift can not be attributed to pressure", I wasn't refering to the article, I was actually refering to your comments from 24 Jan 2003 and those above it. If I misinterpreted them, my apologies.

Touche about pilots/engineers...I will mind that. Perhaps I should have said, "so many people love aircraft and aerodynamics, that everyone wants to spread their knowledge, however accurate or inaccurate it may be." Hmm, come to think of it, I probably should avoid explaining the origin of the misconceptions all together.--Mike L (aka Lensim) 22:08, 27 Aug 2004 (UTC)

Lensim - Ah. Yes, that was sloppy/strident of me. Truth be told, I'm not as careful on talk pages as in articles themselves. But, like you, I have my own pet aeronautics crusade. In my case, it is that Newton's 3rd too often gets "lost in the sauce" when discussing airfoils. I know a fair number of folks (both pilots and engineers by the way) who think that pressure gradiants somehow, in and of themselves, "magically" levitate airplanes. In other words, they have never made the connection between the pressure differentials on the airfoil surfaces and the downward acceleration of the airstream. But, just to be completely clear, do you think the Newton's 3rd part of the story is a "misconception" of some sort? Are there other factual errors in the article that should be addressed? Blimpguy 19:10, 28 Aug 2004 (UTC)

If one wants to associate Lift--a measureable force--with a "downward acceleration of the airstream" they are free to do so. However, this is useful only as a theoretical mind experiment that seems to satisfy some people's curiosity as to how aircraft can fly. Dr's Anderson and Prandtl (refer to the quotes in my 26 August 2004 post) would say that this explanation is not a good physical explanation of Lift and furthermore the only good physical explanation is pressure. All others are theoretical and based on models. Note the following points:

• As a measureable quantity, Lift can be completely accounted for by measuring the pressure changes about the entire aircraft.
• As a measurable quantity, it is impracticle to account for lift by measuring the acceleration of the air mass about the entire aircraft, and no one has ever done so, not even for a helicopter.
• When one does want to imagine Lift as a "downward acceleration of the airstream", it is not practical to imagine a stream of air accelerating off the wing, bearing the aircraft's weight. God forbid this be true, lest we be crushed while watching the 747's land at LaGuardia. And if you've been lucky enough to have the Thunderbirds fly low and fast directly above you as they cut perpendicular to the main flightline at an airshow, you know from experience that you don't feel the aircraft's weight even at low altitude. Rather, when imagining this, imagine the aircraft's weight spread over the infinity of air -- thus the theoretical nature of this visualization. But it is not a misconception to think this way; it is just not physically realizable (just as circulation is theoretical and also not physically realizable). Prandlt himself was fond of imagining "an aircraft's weight as it is transfered to the ground".
• The following is a misconception: "Lift develops from pressure differences AND Newton's thrid law" or "one factor in Lift is Newton's third law" etc. It is one or the other, and when it comes to measuring Lift it is always pressure.
• I would consider a statement such as "downward acceleration of the air" to be more of an answer to the question "how does an aircraft fly"--more philosophy than science, than an answer to the question "how is Lift derived".

It is a matter of great curiosity to me when a person decides he or she "knows how an aircraft flies". Truly, to the highest level of resolution of reality possible, we do not know. To the highest level of resolution of reality we can currently muster, all aerodynamic forces are due to electromagnetic interactions (as opposed to the other three fundamental interactions: gravity, strong and weak) amongst air molecules and those of the wing; but even this is but a model of reality, and one that is way too hard for engineers to work with. Most engineers are satisfied when they can produce repeatable and useful measurements, and this they do with pressure considerations. Others are free to imagine as they wish...have fun!

--Lensim 01:40, 29 Aug 2004 (UTC)

I would be careful in saying that circulation is not physically realizable. I'm not sure where you got the idea, really. Consider what the Magnus effect says: that a rotating body in a fluid flow with an axis of rotation perpendicular to the flow will feel a lateral force as a result of its rotation. The key point is that this would work for say, a cylinder--lift would be generated even though a cylindrical tube has no camber and can not have any angle of attack. For the cylinder, the circulation is simply proportional to its rotational speed. In any event, circulation is most definitely "physically realizable," even if our model for it is only an approximation.

--boeman

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Lensim- Would you be so kind as to modify your comment and put it in the article for Lift (force) A section on the general causes and pysics behind lift would be nice. Theon 18:23, Aug 27, 2004 (UTC)

Certainly. So as to not step on any toes, I will familiarize myself more with wikipedia etiquete first though.

Where there is an existing page (as in this case) the best approach is usually to improve/fix the existing text rather than replacing/duplicating. Also working on particular bits rather than large-scale rewrites allows others the opportunity to work with you and minimizes the chances of the dreaded revert war. Nonetheless, don't be shy. Help is always appreciated. Regards. Blimpguy 20:24, 28 Aug 2004 (UTC)

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I need some help here. I seem to have two difficulties with this article, regarding the general disdain towards the "Bernoulli principle". First of all, I do vaguely recall having learned in physics class that viscosity *is* crucial to lift. Are you sure I got it wrong? If not, doesn't viscosity account for the inability to explain lift in terms of the Bernoulli principle? Most webpages that rule out Bernoulli's (like Jeff Ruskin's) assume *laminar flow* - which I find a strange assumption. I do recall reading a couple of sources claiming that without viscosity, you get very poor lift. Also, regarding the "coanda effect" and the use of Newton's third law, I think the article is making the same mistake again. I would have accepted these as alternatives to the use of Bernoullii's principle if they were independant explanations. Trouble is, if not for the Bernoulli force and pressure gradients, what, in heaven's name, could possibly deflect the stream of air *from the top of the wing downwards*??? [User:mousomer|mousomer]] 18:24, 9 Nov 2004 (UTC)

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The Newton's third and Bernoulli explanations are not alternative theories explaining lift. Rather they are different ways of looking at the same phenomenon. Both explanations are valid, but they are useful to different people. Pilots tend to think in terms of the air being shoved downward. Such a view relates in a straightfoward way to the way that an airplane behaves at different speeds and angles of attack. However engineers tend to think in terms of pressure gradiants because they are much easier to model mathamatically.

The "disdain for Bernoulli" is a disagreement over pedagogy, not physics.

I don't know enough about fluid dynamics to comment on the other issues you raise.

Blimpguy 19:11, 9 Nov 2004 (UTC)

Contents

1 Problems with the Boundary Layer discussion 2 Lift underwater 3 Moved from my talk page 3.1 about your (great) artical on lift 3.2 SEVERE Error - D'Alembert's Paradox 3.3 Reinstating the debate, Newton/Bernoulli

Problems with the Boundary Layer discussion

I'm troubled by an error in the paragraph discussing the boundary layer (BL). I'd like to know if any contributors can suggest more accurate language.

The first part of the discussion is limited to definitions, and a concise description of the main differences between laminar and turbulent BL. Unfortunately it goes on to associate the starting vortex with BL separation, and this is incorrect. A starting vortex occurs any time the section lift (circulation) changes. This is required by Helmholtz's laws of vorticity. The state of the BL is not a driving factor in this, and starting vortexes occur - mathematically - in unsteady inviscid flows where there is no BL.

I'm wondering if there's a need to introduce the concept of the starting vortex in a general discussion of aerodynamic lift. It's a pretty advanced concept, and not one I'm used to seeing in overviews for the general public.

What do you think?

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Lift underwater

What about underwater lift? The article is focused on air, not water. I think dynamic lift needs to be merged into this article, but I'm not an expert. Any ideas on how this should be done? KJ 04:30, 2005 Feb 26 (UTC)

Moved from my talk page

The following comment was placed on my talk page. I'm not even sure what he's talking about... isn't there only the one (air density) physical quantity even given a value? Though I do see that he would have issues, since one should be using m/s for velocity and newtons for the force. moink 06:41, 20 Apr 2005 (UTC)

about your (great) artical on lift

you said u wanted to make it easy for anyone to understand u so let me congratulate u on doing just that

the problem is u failed diddent mention some of the mesuaring units i just asumed that since u mesured preasure in kg per m2 that u where using the metric system but my calculations leads me to suspect otherwise

i assumed that vingsurfus where in m2 and velosity in km per hour and lift in kilos

im not really sure how u can get back in touch with me so if u could update the lift-page with the correct units i would be one happy wannabenerd!

thanks a lot Jesper

SEVERE Error - D'Alembert's Paradox

This article incorrectly states that D'Alembert's Paradox says that an inviscid flow around an airfoil will generate no force and therefore no lift. This is entirely incorrect. D'Alembert's Paradox states that an inviscid flow causes no *drag* -- it still predicts lift through Bernoulli's Principle. In fact, the calculation of lift using Bernoulli's Principle is usually done making the inviscid assumption. I will be correcting the article to reflect this.-boeman

addition: Upon further reflection, none of the paragraph in question had anything to do with Bernoulli's Principle at all, so I removed it entirely. The paragraph moved its focus from D'Alembert to the Kutta Condition. The Kutta Condition should go in with circulation if it's going to be included. I may add it in there later.

Reinstating the debate, Newton/Bernoulli

This debate has, somewhat like the article itself, become a little disarrayed. It's hard to know where responses should be inserted, since it looks as if other parts are out of order. I'd like to contribute something, but it's hard to know exactly what stage the debate has reached - as far as I can see little or no progress has been made since about August last year (2004). There appear to be a number of worthwhile points made here, by Lensim and others (Lensim, could you sign ALL your entries please - it would help a lot - just append each one with four tildes (~). Even just knowing the dates is a great aid to following the course of a discussion.).

For me, there is no paradox between the Newton vs. Bernoulli explanations - or Acceleration vs. Pressure if you prefer. I agree with Lensim that the acceleration approach is less practical from an engineering perspective, since it's unmeasurable. However, it MUST be happening - planes stay in the air after all - there is a force acting on them opposing gravity, and that force arises from the reaction of the downward acceleration of the air. I think we can, simply from the most basic principle, accept that. In practice the discussion of pressure is probably much more fruitful however, since that is what can be measured, and is what designers and aerodynamicists work with in real life. Since this is also a valid and equally useful way of looking at the problem, we need not settle on "one or the other", but simply incorporate both ideas into the article. Both exist, both are valid. The argument seems to me to be about which came first - which one is cause and which is effect. Maybe it simply doesn't matter, or cannot be determined.

Lensim states that the acceleration approach is not useful for engineers, and that is probably true. It doesn't mean it is not useful to mention in an encyclopedia aimed at mostly non-engineers, who may find the acceleration explanation easier to comprehend. It also doesn't mean that the pressure explanation is incorrect or should be discounted. However, I do take issue with some of Lensim's arguments against the acceleration approach, since they are erroneous. It does not follow at all from the considerations of acceleration that this force is transferred to the ground, or to anyone standing underneath an aircraft flying overhead. The force is transferred wholly to the aircraft - that's what keeps it in the air! There is no net force "left over" that could be felt underneath - the force is doing work levitating the plane. However, this glaring error apart, I generally agree with Lensim's comments. It's not Newton OR Bernoulli, (or Newton AND Bernoulli if you see it that way), but rather both explanations are equally valid and are each other's consequence. Disentangling them is impossible, so perhaps we would be better off moving this argument forward not by trying to decide WHICH is correct, but by deciding how we can incorporate these ideas into the article in a way that is as informative as possible for the readers of wikipedia, without oversimplifying or dumbing down. As Einstein once said, we need an explanation that is a simple as possible, but no simpler.Graham 00:17, 17 May 2005 (UTC)