Talk:Lift (force)

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Help with this template Comments: edit – history – watch – refresh This is an attack on the use of the Bernoulli equation as a partial explanation of aerodynamic lift. Bernoulli's equation for the conservation of energy is commonly used to explain aerodynamic lift, however the equation assumes a steady flow, i.e. flow in which particles follow velocity streamlines. A stream line is a line of constant velocity in the flow. When the particles themselves do not follow the streamlines, they are changing velocities, that is, they must cross the streamline bundle. Shear flow is an example of non-steady flow. The flow near the airfoil surface is shear flow because of the existence of the boundary layer, a layer of air in which the velocity changes across the flow; the layers of air shear past one another, faster layers toward the ambient flow and slower layers near the airfoil. This is non-steady flow and Bernoulli's equation is therefore not applicable in this region. (See pg. 9ff. Fluid Mechanics. Landau, L. D. and E. M. Lifshitz. (1959.) Addison-Wesley: Paris.} Here is the beginning of a correct explanation of this part of lift: Upwash (from the bottom of the wing at angle of attack) joins with the main flow at the leading edge to envelop the top of the wing. The combined flow interacts with the boundary layer on the curved part of the top of the wing and, for sufficiently small angles of attack, depletes the boundary layer population there, i.e. the pressure on the top is decreased, by the "blowing" of particles away from the wing's surface as it curves away from the flow. It is the curvature of the wing that is crucial in this component of lift. This decrease in pressure on the top of the wing adds to the lift. It can be seen in the Coanda effect, i.e. a smoke stream near the curved surface is deflected toward the surface by the higher pressure in the flow (the ambient pressure). The difference between the lower pressure at the top surface and the higher pressure under the wing results in lift. —Preceding unsigned comment added by Ccrummer on 27 January 2007. Contents 1 Reply 2 Notes 3 This article is a start, but still has problems 4 New physical description 5 A summary of where I think the physical explanation should go 6 Exact calculation of lift [edit] Reply I have just found these comments published by Ccrummer in January 2007. There is universal agreement with Ccrummer that Bernoulli's principle is not applicable in the boundary layer because BP is stated to be applicable to inviscid flow (or flow that closely resembles inviscid flow) and the flow in the boundary layer is clearly not inviscid flow. However, the great majority of air affected by an airfoil is not part of the boundary layer and its flow closely resembles inviscid flow. Ccrummer’s "correct explation of this part of lift" looks like original research. There is no reference or citation to indicate where the ideas come from, or whether these ideas have ever been published by someone. Wikipedia is not the place for our original research or our personal views on what we believe is true. See Wikipedia:No original research. Ccrummer’s stated views are at odds with statements by most (all?) specialist authors in the fields of fluid dynamics and aerodynamics. For example, in the excellent book Aerodynamics, L.J. Clancy has written “When a stream of air flows past an airfoil, there are local changes in velocity round the airfoil, and consequently changes in static pressure, in accordance with Bernoulli’s Theorem. The distribution of pressure determines the lift, pitching moment and form drag of the airfoil, and the position of its centre of pressure.”[1] Ccrummer has written “steady flow, i.e. flow in which particles follow velocity streamlines. A stream line is a line of constant velocity in the flow.” I have read many books on fluid dynamics, but I haven’t ever read of a “velocity streamline”. Also, when Ccrummer writes “A stream line is a line of constant velocity in the flow” he is at odds with all the authors I have read in this field. The bottom line is - where did this information come from? The threshold for inclusion in Wikipedia is verifiability, not truth. See Wikipedia:Verifiability. See also Wikipedia:Wikipedia is an encyclopedia. In my view, as "an attack on the use of the Bernoulli equation as a partial explanation of aerodynamic lift" these comments have failed completely. Dolphin51 (talk) 12:43, 25 May 2008 (UTC) [edit] Notes ^ Clancy, L.J. (1975), Aerodynamics , Section 5.5, Pitman Publishing Limited, London. ISBN 0 273 01120 0

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[edit] This article is a start, but still has problems

The "physical description" of lift reads like an old argument by another name. There's only one right answer: both. Neither momentum nor pressure explain lift on their own, yet are both tremendously important to it. This should be obvious, since each section talks about the other.

Unfortunately, an accurate physical description of lift is not a trivial task and not even perfectly understood by anyone. A more correct description might talk about viscosity, the starting vortex, pressures, and momentum. Mostly it all comes out of the Navier-Stokes equations (which come from Newton's second law).

The mathematical models section has similar problems. The three “models” are convoluted and not explained very well:

• There is no such thing as the "Lift coefficient model". It's just a nondimensionalization for convenience, allowable thanks to the Buckingham π theorem. I temporarily deleted this, but a link to Lift coefficient would be a good addition somewhere since it comes up frequently.
• Bernoulli's principle is not a model of lift, but it does factor into lift calculations: for example, it's used when deriving the Kutta-Joukowski Lift Theorem. The integrals used here to calculate aerodynamic forces do not follow from Bernoulli's equation and should not be labeled as such, so I retitled this section to “pressure integration”, but there's probably something better. This section seems to confuse a variety of concepts.
• The Kutta-Joukowski Theorem section needs to be cleaned up and better explained.

I'll return to this when I have time. Mbelisle (talk) 05:00, 19 April 2008 (UTC)

[edit] New physical description

I started a more complete physical description of lift production. Until I find a good general reference, it's unreferenced and almost certainly has some problems. It would also benefit from diagrams showing the stages of lift production, which might save some of the verbiage. I acknowledge that it may be overly technical at the moment, which I'll work on once I'm sure it's correct. It think it is, at least, better than NASA or The Straight Dope, and who seem content to conclude "Why the flow moves faster over the upper surface is complicated, don't worry about it."

The "Newton" and "Bernoulli" explanations of lift should be added to the common misconceptions section, perhaps as “Newton vs. Bernoulli”, and/or to a section on ways to measure lift. If you know the details of the flow field (or are making measurements in a wind tunnel), then sure either

• the “Newton” way: a direct measurement of the force (via a force balance) or
• the “Bernoulli” way: integrating the pressures (via pressure taps on the surface of the airfoil)

will give you a reasonably accurate value for lift. But neither answers the real question a physical explanation should address: “Why does a wing generate lift?” Michael Belisle (talk) 00:38, 20 April 2008 (UTC)

We must bear in mind that Wiki is a general encyclopaedia and not a technical treatise, it should be written in language and style that is accessible to all. After all, the formulae are derived from principles not the other way round. The article, in my view, should not primarily be a description of the detailed physics or of the derivation of mathematical models but sections on these can be included. I agree that the physical explanation has to focus on addressing the question "why does a wing generate lift" but it should first do it in layman’s terms.

The value of many Wiki articles are effectively destroyed by us technicians expounding nuances that are absolutely irrelevant to the general user.

Your idea that aspects of the Newton and Bernoulli debate would sit very well in the misconceptions area is an excellent one.

I believe it is relatively straight forward to explain why the flow moves faster over the upper surface to a layman and without once mentioning Bernoulli or conservation of energy. I will give it some thought. Rolo Tamasi (talk) 08:24, 20 April 2008 (UTC)

John D. Anderson says “You cannot get more fundamental than this, conservation of mass and Newton's second law” (JDA, Introduction to Flight, p. 355). I use JD Anderson as a reference because, as Curator of Aerodynamics at the Smithsonian and a standard in Aerodynamics texts, he is quite possibly the most authoritative person on the subject. His explanation cannot be presented on equal footing with eskimo.com.

Bernoulli's principle, as applied, comes from Newton's second law while conservation of mass (in 2-D) is simply ρVL is a constant. In the equal transit-time explanation, people have no trouble accepting that that air moving faster has lower pressure than slower moving air, which is an application of Bernoulli's principle. True, the form of Bernoulli's equation applied is actually “Bernoulli's equation for inviscid, incompressible, steady flow along a streamline”, but in some ways that's more information than is necessary. It's the most common form of the equation and the only way to explain lift without mentioning Bernoulli is to not give the principle a name.

That said, I admit that what I put in there is temporarily not written in laymen's terms. I tried to first to put something in there that is correct and referenced (what was there was understandable but misleading). We can go back and make simplifications as appropriate, especially adding pictures.

After I wrote the section on viscous flow and lift startup (from memory), I searched for references in books by reputable aerodynamicists and I came across JD Anderson's explanation, where he explains lift in steady-state terms with conservation of mass and Newton's second law, neglecting viscosity. Technically, this is correct: if you started up a viscous Navier-Stokes solver and solved only for the steady-state flow field, you would never see the starting vortex. You could then describe the flow field as he has done. So I threw in a sloppy version of his explanation. JD Anderson also explicitly describes downwash (i.e. Newton's third law) as an effect of lift and circulation as a mathematical model of lift. His reasoning there is sound.

I have since located a good explanation of the stages of lift production in K Karamacheti, Principles of Ideal-Fluid Aerodynamics and clear diagram in FM White, Fluid Mechanics. The stages are inclusive of the foregoing. I think a good argument could be made to move the stages to another article. But, if I had looked up lift before I was an Aerospace Engineer, I would have wanted to know about viscosity. Failing to mention the starting vortex or viscosity is hand-waving. “The generation of [lift] depends on entirely on the nature of viscous flow past certain bodies.” (Karamacheti) Michael Belisle (talk) 22:18, 20 April 2008 (UTC)

I think what we need to do here is rewrite the article in a way that it conforms in some way to Wikipedia:Make technical articles accessible, without loss of information. It is unavoidable that, like special relativity, a variety of readers will come here, including aerospace engineers who don't understand how a wing produces lift. (Strange but true: I can guarantee that this article will show up in undergraduate lab reports on airfoils. Back when I was a TA, I mostly solved this problem by taking off points for citing Wikipedia because it was often wrong: for example, see Image:LiftCurve.svg or “At a zero angle of attack, no lift is generated,” which was in this article yesterday. I'd prefer not to have to do that.)

Lift, on its own, isn't significant enough to warrant its own article like Introduction to special relativity, but there is certainly a way to organize it in such a way that laymen don't feel it's overly technical, while aerospace engineers don't feel it's overly simple. Perhaps we need something like an Introduction to flight article, but that should involve a team effort within WP:AVIATION about what to include and how to present it.

The physics community is way ahead of aeronautics in communicating technically correct ideas to both general and technical audiences. I think it's a good model to follow. Michael Belisle (talk) 03:36, 21 April 2008 (UTC)

[edit] A summary of where I think the physical explanation should go

I've done some thinking, examined some more sources, and reviewed a little of the prior Bernoulli v. Newton debate here. Of the debate, I see that pretty much everything has already been said.

My current thinking (which is still evolving) is that the final value for lift comes from an equation where, in physical terms, often acceleration is on the left while the pressure gradient, viscosity, and gravity are on the right: effectively ma=F. Which terms are your favorite? (I like viscosity, pressure, and then acceleration. There's no love for gravity from me.)

We should develop one cogent, understandable explanation that develops the whole picture, not obscure parts under the guise of accessibility. Michael Belisle (talk) 01:43, 22 April 2008 (UTC)

[edit] Exact calculation of lift

In the literature are there any examples, however idealized, where lift can be calculated exactly; i.e., without approximations in the math? I would like to find such an example cited for it would help in understanding lift. Renede (talk) 00:07, 8 May 2008 (UTC)