Nelsonpom

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Explanation of lift

Hello again Nelsonpom. I have been thinking about your search for an intuitive way to explain the phenomenon of lift to high school physics students. You might find the following approach useful. It is advocated by Professor Holger Babinsky of Cambridge University. Go to Iopscience

Babinsky has a video, posted on New Scientist, that uses interrupted smoke streams to show that the top stream arrives at the wing’s trailing edge much sooner than the lower stream. Go to Newscientist

I made some scathing comments about attempts to explain lift using considerations at the molecular level. You might look at the following document to see what is out there. (I think it is pretentious nonsense.)

• http://arxiv.org/PS_cache/nlin/pdf/0507/0507032v3.pdf

Best wishes. Dolphin (t) 23:58, 3 May 2012 (UTC)

Thank you Dolphin. I have seen the Babinski video and in fact had some email contact with him directly a couple of years ago and used screenshots of his video in an article I wrote for a gliding magazine. I'm in a class now so will read the links you gave and write more later. Is there a way I can send you a copy of my article for your comments? In it I used simple inclined plane - air deflection calculations to model the lift of a piper cub at various AoA and airspeeds. Nelsonpom (talk) 00:20, 4 May 2012 (UTC)

I would welcome an opportunity to peruse your article. It is possible to send email to me. Simply go to my Talk page: User talk:Dolphin51. In the left-hand margin there are four groups of options - the third one is Toolbox. One of the options under Toolbox is E-mail this user. Select that option and it should be straightforward after that. (Note that not all Users have the email option enabled. I do, and I see you do too.) Dolphin (t) 01:57, 4 May 2012 (UTC)

Thanks. How do I attach a PDF or Word file? Nelsonpom (talk) 02:54, 4 May 2012 (UTC)

I have never actually needed to send an attachment and I can see there is no obvious facility for doing so. I will send you an email with my email address. Dolphin (t) 03:26, 4 May 2012 (UTC)

Thanks for the email, and the copy of your paper. I have had only a quick look at it, and it looks very professional. I will take a more thorough look in the next day or two.

A couple of days ago I asked for comment at Talk:Flight#Lift-to-drag ratio. Unfortunately no-one has commented yet. Glider pilots would make ideal respondents on the topic so I would appreciate it if you would have a look. Feel free to add your own comments. Regards. Dolphin (t) 05:09, 4 May 2012 (UTC)

Hi again Dolphin - I'd be really interested in any feedback from the article. I'm still caught up with trying to understand the "chemical" properties that relate to the viscosity or air. i.e. it seems there must be either be a tension force that "pulls" air down towards the wing, or that it "expands" down due to the kinetic energy of the particles having a net movement towards a region that lacks a balancing force (i.e. "nature abhoring a vacuum"). Nelsonpom (talk) 05:24, 13 May 2012 (UTC)

Hi Nelsonpom. I cetainly haven't forgotten your article but I have been occupied with a couple of other things during the past week. I will try to send you something this week.

There is one aspect of airfoil aerodynamics that I have always found very helpful - I gave a description of it a few days ago at Talk:Lift (force)#Fundamental physics. I will copy and paste the relevant bit below:

Mr swordfish, Nigelj and I all agree that there is great difficulty in finding an intuitive explanation of what makes the air come down? Unfortunately, it is not sufficient to say the air is merely flowing along the upper surface of the airfoil to avoid a vacuum forming. The truth is that the speed at which the air flows around the leading edge and along the upper surface of an airfoil is much greater than necessary to satisfy continuity and avoid a vacuum forming. Indeed, it is possible to build a streamlined body that is basically a rectangular cylinder but with rounded leading and trailing edges and show that, even with a significant angle of attack, this body with its rounded trailing edge does not generate significant lift. Part of the reason why it doesn't generate lift is that the speed of the fluid around the leading edge and along the upper surface is not fast enough to cause the pressure to be lower than the pressure on the lower surface.

Part of the problem of explaining why airfoils generate lift is to explain why the flow around the leading edge and along the upper surface is so much faster than one would expect - so much faster than in the case of the streamlined body with the rounded trailing edge. The solution to this problem is attributed to Martin Wilhelm Kutta who identified the significance of a sharp trailing edge. His solution is now known as the Kutta condition and it explains why all airfoils have sharp trailing edges, in contrast to the generously rounded leading edges on subsonic airfoils. Any complete explanation of why the fluid flows around the leading edge and along the upper surface so much faster than it flows along the lower surface must point out the necessity of having a sharp trailing edge and acknowledge the work of Kutta in explaining that the flows along both upper and lower surfaces leave the body at the trailing edge. The flow along the lower surface does not flow around the sharp trailing edge. The flow along the upper surface persists all the way to the trailing edge because a vortex of sufficient strength is established in the fluid to ensure the flow does not leave the body until it reaches the trailing edge. It is this vortex, the bound vortex, that quantifies the extraordinary speed of the flow around the leading edge and along the upper surface. Any attempt at a complete explanation of fluid dynamic lift that doesn't incorporate:

1. the necessity for a sharp trailing edge,

2. the bound vortex necessary to achieve the Kutta condition,

is doomed to ultimately fail.

A Rankine body is a good example of a body with an elliptical, sausage-shaped, cross section (rounded trailing edge as well as a rounded leading edge.) Without a sharp trailing edge this body generates little or no lift, despite positioning it with a generous angle of attack. It is a good intellectual exercise to try to explain why the Rankine body generates no lift, but a good airfoil can generate high lift with little drag. The air flows along the upper surface of the Rankine body to avoid a vacuum forming, similar to the way it does along the upper surface of an airfoil. And yet, despite all that, the Rankine body generates no lift. It begs the question "Why lift on an airfoil, but no lift on a Rankine body?" As I have tried to explain above, Martin Kutta is credited with providing the explanation.

In the case of the Rankine body, the air flowing along the lower surface flows around the rounded trailing edge and a short way up the upper surface. There it meets up with the air flowing along the upper surface and then they both separate from the airfoil and flow downstream. In the case of an airfoil with a sharp trailing edge, the air flowing along the lower surface can't flow around the sharp trailing edge. (The sharp trailing edge has a radius of approximately zero. In the case of a free vortex, at radius zero the speed of the flow is infinitely great. It can't happen.)

In the case of the airfoil, the flows along the upper and lower surfaces both reach almost to the sharp trailing edge, and then they flow downstream. (This is significantly different to the situation with the Rankine body.) The mathematical explanation of why the two flows are so different is that embedded in the flow around the airfoil there is a free vortex. The strength of this free vortex is exactly what is required to provide the Kutta condition. This free vortex can be used to explain why the air flows so rapidly around the leading edge of the airfoil and along the upper surface, and why the airfoil generates lift. In the flow around the Rankine body there is no such vortex - the speed of the flow around the leading edge and along the upper surface is significantly slower than in the case of the airfoil. With no vortex there is no circulation around the body and no lift - see Kutta-Joukowski theorem.

So the air flowing along the upper surface of the airfoil is not simply due to the air abhorring a vacuum, or being forced down, or being drawn down by suction. Those things happen around the Rankine body but that body generates no lift. The free vortex around a three-dimensional wing is called the bound vortex (bound to the wing). The bound vortex explains the upwash ahead of the wing, causing the stagnation point to move down around the leading edge to a point on the lower surface of the wing. It also explains the high-speed flow along the upper surface, giving the flow sufficient kinetic energy to reach almost to the sharp trailing edge. (If there were no viscous shear forces, the flow would reach all the way to the trailing edge.)

A vortex cannot simply come to an end in the fluid (the second of Helmholtz's theorems.) At the tips of the wing the bound vortex turns and trails behind the wing, driving a pair of trailing vortices, one from each tip. Vortices contain much angular momentum and they are a very efficient form of fluid motion. The only reason they eventually decay to zero is because of the small amount of viscous shear force contained therein. The trailing vortices from a heavy aircraft persist for many minutes and constitute wake turbulence. Wake turbulence is a serious hazard in busy airspace, particularly for light aircraft straying into the wake of a slow, heavy aircraft taking off. The reason I like to talk about wake turbulence at this point is that the two-dimensional model of lift offers nothing to help explain trailing vortices. (Also, the two-dimensional model of lift offers nothing to help explain downwash and the importance of a glider having a high-aspect-ratio wing.) Pilots, and particularly pilots who will ever share their airspace with heavier aircraft than the ones they fly, must comprehend wake turbulence so they should progress beyond the two-dimensional model of lift. They should become conversant with the three-dimensional model of a vortex system consisting of the bound vortex and the pair of trailing vortices (the horseshoe vortex.) The bound vortex provides a rigorous and simple explanation of why the air flows around the leading edge, along the upper surface and almost all the way to the trailing edge. More later. Regards. Dolphin (t) 11:37, 13 May 2012 (UTC)

I see you've just made some edits here. I still have some queries that I don't have time to write just now... But is this the best place to continue rather than the "Lift force" talk page? I'm new to participating in Wikipedia and don't know even how to get to this page other than using the links in the email! Nelsonpom (talk) 01:51, 14 May 2012 (UTC)

Yes, I have made a couple of corrections, and added a link or two as they occurred to me. This is the best place for a private conversation. Article Talk pages, such as Talk:Lift (force) are intended for discussion about improving the article.

If you look along the top line of the Wikipedia window, when you are logged in, you should see a row of options to the left of Log out - My talk, My preferences, My watchlist, My contributions. This page is your talk page - select My talk and it will take you directly to this page.

To see individual changes we have made, go to the View history tab near the top of the page, to the right of Edit - select View history and you will see a chronology of all edits to this page. To see any individual edit just select prev for that edit. Dolphin (t) 02:05, 14 May 2012 (UTC)

I must say I'm with nigelj in the lift force talk on this one. I've just finished making gliders from 1.5mm flat balsa with a class and they fly reasonably well without very high AoA. I see a continuum from the Rankine body to the classic aerofoil. I am aware of the vortices and have been intrigued to watch little waterspouts in Wellington harbour following landing of the larger jets. I'm a little frustrated feeling I'm nipping at the heels of a superior knowledge base. However I can't help thinking the vortex is a consequence not a cause of the downwash. Surely the vortex is left behind by a normal speed aircraft? And the the language of "induction" smells of force fields that don't exist in fluid physics (I don't believe). Where are the vortices in Holger Babanski's smoke stream videos? As for a vortex explanation being needed to explain the advantage of a glider's high aspect ratio: I see lift as correlated with the total mass of air accelerated down, and the volume moved is primarily linked to the span. A glider can be seen as a high "geared" downwasher of air - not much grunt due to the short cord but a large volume moved. I can see the advantage of a sharp trailing edge: As you say the radius is so tight the air can not rotate easily "up" again. Nelsonpom (talk) 08:46, 14 May 2012 (UTC)

You are right about scale-model gliders made from 1.5 mm balsa. They have the luxury of operating at low Reynolds numbers; they can operate with very poor lift-to-drag ratios; and they have very low wing loadings. Those luxuries aren't available to designers of full-scale working aeroplanes who must go to extraordinary lengths to select the basic airfoil section for their wings, then modify that airfoil section and supplement it with trailing edge flaps, sometimes even leading edge flaps, and so on. Seeing you have your class successfully involved with model gliders it is probably sufficient to say the pressure above the wing is slightly lower than the pressure below the wing, and that is sufficient to support the weight of the glider; and leave it at that. If the purpose of building gliders is to illustrate Bernoulli's principle then obviously you need to explain that the air is flowing faster over the top of the wing etc.

I'm uncertain what you mean by "Surely the vortex is left behind by a normal speed aircraft?"

You are right about the similarity of language used in fluid dynamics and that used in electromagnetism. The word induction pops up regularly in both. The driving force behind a vortex is given the horrible name of vorticity, and it is analogous to the electric current in a conductor inducing a solenoidal magnetic field. The Biot-Savart law is used to calculate the strength of both an electromagnetic field and a vortex field. And so it goes on.

Where are the trailing vortices in Holger Babinski's smoke stream videos? I'm sure his photographs show two-dimensional flow so there are no trailing vortices - the two ends of the bound vortex terminate at the boundary of the fluid (which is presumably the two walls of the wind tunnel.)

The distribution of lift across the span of a wing is not constant - the lift is strongest near mid-span and it falls in an almost-elliptical fashion to zero at the wing tips. That is easily explained using a three-dimensional model of lift but I don't see how it can be explained in a rigorous way using a two-dimensional model. (Hint: The angle of attack is usually not constant along the span, even though the wing may have no structural washout. The angle of attack is reduced by an amount equal to the induced downwash which is strongest in line with the trailing vortices. Uniform downwash is desirable, so R.J. Mitchell gave the Spitfire an almost-elliptical wing planform.)

How do you explain the way the trailing vortices in the wake of an aircraft persist for such a long time, representing a hazard for other aircraft coming behind? Why don't those vortices dissipate almost as soon as the aircraft has passed? (Hint: See the third of Helmholtz's theorems.)

I should say that gaining an understanding of the science of aerodynamics is no different to gaining an understanding in any other branch of science. It doesn't occur quickly, and it is accompanied by a certain amount of fatigue, doubt and scepticism along the way. I'm not critical of anyone grappling to assimilate this stuff - I was once there myself. However, I am willing to help wherever possible.

I will get to your paper from NZ Soaring shortly. I promise! Regards, Dolphin (t) 13:51, 14 May 2012 (UTC)

I thought I'd let you know a bit more about my background reading and motivations (this mode of communication is very anonymous although I understand the advantages of that). I have been an experimental scientist in biology, and "came down" to high school teaching (but not for reasons of inability to continue my University job), and always been an aviation enthusiast. I read "stick and Rudder" which made so much sense to me. I trained as a glider pilot in the 1990s and was unhappy with the path-length lift explanations given by the instructors. Certainly AoA was given full emphasis, but especially after reading Anderson and Eberhardt's article in Sport Aviation and later their book "understanding flight" (as well as Gale Craig's "Stop abusing Bernoulli..." book) I felt that the primacy of AoA (for a pilot, not a designer) and "relative wind" could be diminished if pilots were too focused on airspeed alone. Hence the article for the Soaring magazine. After the article came out one of the senior instructors in NZ invited me to re-try my instructor rating, which I achieved. A few months later, agreeing to do a day's instructing with Air Cadets at a different field I ran out of sky (see images at http://www.flickr.com/photos/99316025@N00/sets/72157622930510797/). I had made the same error previously of trying to fly a full circuit without the necessary height - realised I had a mental block (which to the discredit of my local club they made no attempt to remedy) and I gave up and now fly RC helicopters. (I recently persuaded the helifreak.com form website to add a forum titled "Aerodynamics, physics and engineering" as these things often came up. It was actually in response to a link in a thread there I looked again at the Lift page in Wikipedia. On re-reading I think Anderson and Eberhardt's book pretty well satisfy my earlier questions. On p24 they say "When the air bends around the surface of the wing it tries to separate from the airflow above it (rather anthropomorphic but OK). But since there is a strong resistance to forming voids, the attempt to separate lowers the pressure and bends the adjacent streamlines above. The lowering of the pressure propagates out at the speed of sound, causing a great deal of air to bend around the wing. This is the source of the lowered pressure ... and the production of the down-wash..."

That makes sense to me. i.e what "pulls the air down" is a propagated pulse(?) of decreased pressure. I can see that this would set up the bound vortex too. By the way I'm over trying to explain much to the high school students - I left it at the notion that the angle of attack diverted air down and reduced the pressure on the top of the wing. Regards Nelsonpom (talk) 10:06, 15 May 2012 (UTC)

In my email I promised to send you a link to some material I wrote a few years ago about the books by Langewiesche, and Anderson & Eberhardt. It is now archived at Talk:Bernoulli's principle/Archive 2#A common misconception about wings. Regards. Dolphin (t) 22:32, 15 May 2012 (UTC)