Debunking lift theories
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When a person writes a letter (as opposed to an article) to a periodical or newspaper for publication, who owns the copyright, the author or the publisher - or both?
The author owns the copyright, unless he/she has assigned it to someone else.
However, there is a second copyright in the typographical layout of the published version which belongs to the publisher.
If you re-type the letter, you infringe only the first.
If you scan it and post an image, you infringe both.
And if it had been a very short letter, would we even be having this discussion?
All this can be complicated by concepts like the public domain and implied licences, which I will save for another day.
Leaving the legal details aside, I'd say a pretty good test is to ask yourself how you would feel if you'd written the letter. If you wouldn't mind it being reproduced here, then probably the author won't complain. Common sense, and common courtesy, is often a good guide.
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True, but without viscosity, you can't have any circulation and therefore lift is zero.
Really?
Really?
True, but without viscosity, you can't have any circulation and therefore lift is zero.
It's a little like standing a pin on its point and saying "the acceleration due to gravity determines which way it falls". It's true, but it doesn't really matter if g is 10 m/s2 or 10 m/yr2, the pin still falls the same way. It's the phenomenon, not the value, that is important.
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Surely one can get "lift" through simply redirecting some material that is flowing along. The "material" could be air, or a load of bricks. I don't think bricks will have a whole lot of viscosity.
Bookworm's post is intriguing and I am sure he is right (he is more or less always right). But the viscosity required must be negligible.
Bookworm's post is intriguing and I am sure he is right (he is more or less always right). But the viscosity required must be negligible.
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IIRC the amount of 'lift' one gets from 'deflecting' the 'air bricks' that are directly intersected by the wing's forward movement is negligible. It is because of viscosity that this initial movement of a relatively small airmass generates the circulation of quite a large airmass which results in an extensive downward deflection of air (and us flying 
) .
I seem to remember, in something like a TB-20, all of the air within about 15 feet above the wing is involved in this deflection. It is also why minor interruptions in the flow along the top surface (ie. a bit of rough frost) can so catastrophically kill the lift the wing generates.
) . I seem to remember, in something like a TB-20, all of the air within about 15 feet above the wing is involved in this deflection. It is also why minor interruptions in the flow along the top surface (ie. a bit of rough frost) can so catastrophically kill the lift the wing generates.
Last edited by mm_flynn; 10th Oct 2007 at 09:45. Reason: changed 30 to 15
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Islander2, is this what you disagree with?
http://www.aviation-history.com/theory/lift.htm
If so, why?
http://www.aviation-history.com/theory/lift.htm
If so, why?
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Regarding the link tigerbatics posted, in my view:
- They are throwing out the baby with the bath water when they say Bernoulli is wrong. What they are right in is that you can't use the "equal time" argument to determine the velocity field around the wing. But if you can determine the velocity field by some other means, then you can determine the pressure field almost trivially (and quite correctly!) using Bernoulli. The problem is that it is very tricky to obtain the velocity field, and hand waving approaches like the "equal time" concept fail miserably. But that is hardly Bernoulli's fault!
- People criticizing descriptions like those in the link, usually have gotten it all backwards: They believe the "Newtonian" approach amounts to claiming that the air is deflected down by "bouncing off the bottom of the wing", as if the air behaved like particles. That would be nonsense, and it is not what the authors here claim.
- In blaming the deflection of the air (and thus the generation of lift) not on Bernoulli but on the Coanda effect, they are replacing one "easily understood" but totally wrong description with another equally false one. The Coanda effect deals with jets, not with bulk fluids. The Coanda effect comes into play if you use some rare aviation gizmos like blown flaps, but it has nothing to do with why most aircraft fly.
Last edited by bjornhall; 10th Oct 2007 at 18:49.
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for why the Coanda effect is not the explanation for lift
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Islander2, is this what you disagree with?
http://www.aviation-history.com/theory/lift.htm
If so, why?
http://www.aviation-history.com/theory/lift.htm
If so, why?
Their paper was submitted in 2001 to the American Journal of Physics, but not accepted for publication. I fancy the peer review would make for interesting reading. Here's one damning critique by the av8n on-line aeronautics book author:
http://www.av8n.com/fly/lift.htm
Anderson and Eberhardt subsequently produced an expanded version of their erroneous ideas in a book entitled "Understanding Flight". Serious students of aerodynamics may prefer to re-title this: "Misunderstanding Flight"!
However, for an excellent and authoritative overview of the Bernoulli vs Newton debate, I commend to you the John Gibson letter (and no, I'm not going to reproduce it here!)
Good to see av8n.com given a bit of an airing again and nice to see the site's still working. Prof Denker seems to know his stuff and I've recommended his website to many over the years.
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Many thanks for the explanation and links Islander2.
Very interesting in an academic way and well worth thinking about; but for safe flying it seems a pilot can get by with one of several approximations to the truth provided he/she does not really try to understand it!
Very interesting in an academic way and well worth thinking about; but for safe flying it seems a pilot can get by with one of several approximations to the truth provided he/she does not really try to understand it!
Spoon PPRuNerist & Mad Inistrator
FBW,
I have followed this thread in its "interesting" directions, and I thought it would be a useful idea to contact Today's Pilot to actually ask their permission to reproduce the letter.
I received permission to do so from Dave Unwin, Editor of Today's Pilot,
this afternoon, so if anyone has access to the letter (and a scanner)
, then please go ahead and put up a copy.
I won't post the email correspondence here (to protect identities), but I have sent BRL a copy to prove the bona-fides!
I hope that someone will oblige, and the discussion can then become more "uplifting". Sorry, just came to me
SD
I have followed this thread in its "interesting" directions, and I thought it would be a useful idea to contact Today's Pilot to actually ask their permission to reproduce the letter.
I received permission to do so from Dave Unwin, Editor of Today's Pilot,
this afternoon, so if anyone has access to the letter (and a scanner)
, then please go ahead and put up a copy.I won't post the email correspondence here (to protect identities), but I have sent BRL a copy to prove the bona-fides!
I hope that someone will oblige, and the discussion can then become more "uplifting". Sorry, just came to me
SD
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As a trainee pilot it is just as useful to me to think of lift as 'downwash'
I'd agree that, for pilots, a deep understanding of the cause of lift is unnecessary; what we need to concentrate on is the effects of lift.
The downwash explanation is fine for students of flight. As you're probably aware, though, serious aerodynamics texts tend to put it into the category of effect rather than cause.
It certainly produces some interesting tangles for those attempting to build a lift theory around downwash when they come to explain two-dimensional lift. A wing with infinite length (which can readily be simulated in a wind tunnel by having the wings protrude through the tunnel walls) generates strong lift with zero net downwash. In other words, a molecule of air that is initially displaced upwards as the wing approaches (upwash) is then displaced downwards (downwash) back to its original position.
This 'inconvenient fact' leads Anderson and Eberhardt to offer the ludicrous proposition that there is a net downwash in two-dimensional lift with the wing diverting "an infinite amount of air at zero velocity"!
Even Denker struggles with this one, leading him to assert that: "both the incoming upwash and the outgoing downwash make positive contributions to lift". Whilst that statement is true enough, it's hardly supportive of downwash as a lifting theory based on Newton's third law!
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Part of the reason I enjoyed studying it was that I realised there is some 'magic' going on we can't explain yet.
Fly Conventional Gear
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There isn't much mystery left here.
Some may remember the "Downwind turn" thread, which while is not directly related to lift theory demonstrated that there is still sufficient inconstistency in some parts of theory of flight to enable a lot of myths to be peddled.
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John Gibson letter
I bought TP on recommendation from the folks here just yesterday, to read said letter. I was disappointed to see that:
a. It seems to be a rebuttal to a letter responding to an original article that John Gibson wrote earlier, probably in August. I do not have the original article, nor the response letter.
b. It only talks about the theory names, without going into detail on what each theory claims and why that claim is unfounded
c. I did not understand half of it.
(Fortunately there are a few other articles in this TP worth reading...)
Nevertheless, since Saab Dastard has obtained permission and nobody else seems to have gotten around putting it up, here goes:
a. It seems to be a rebuttal to a letter responding to an original article that John Gibson wrote earlier, probably in August. I do not have the original article, nor the response letter.
b. It only talks about the theory names, without going into detail on what each theory claims and why that claim is unfounded
c. I did not understand half of it.
(Fortunately there are a few other articles in this TP worth reading...)
Nevertheless, since Saab Dastard has obtained permission and nobody else seems to have gotten around putting it up, here goes:
NEWTON, BERNOULLI AND LIFT THEORY
Rob Davies (Letters, September issue) is upset by what he calls my opinions on Newton and lift theory. They are rather more than that. I prefer to call them historical facts plus knowledge gained in 55 years in employment and consultation at Warton, mostly within the aerodynamics department. It has been claimed recently that aerodynamicists have always been wrong (if you believe that, you can believe anything), because they have used the "equal transit time" theory. That claim is nonsense. The theory, rightly described as incorrect in that NASA website, is a fairy tale that is sometimes taught by pilots to other pilots. It was never part of lift theory or presented in serious textbooks, and NASA does not say it was.
I did not say that Newton's laws are invalid or nonsense. He was first to prove theoretically that fluid forces on a body are proportional to the fluid density, the body dimensions and the velocity squared. He formulated the theoretical structure of frictional shear stress in viscous fluids. His second law plays a part within the general three-dimensional fluid flow motion equations of Euler and Navier-Stokes of the 18th and 19th centuries, but these were insoluble until high power digital computing arrived long after two-dimensional aerofoil lift theory was well established and proven. He also proposed a fluid force model assuming a stream of individual particles colliding with an object in motion through it, giving up the component of their momentum normal to the surface and then moving off tangentially to it. When applied later to flat plates at small angles of attack, this led to the notorious sine squared law with very low lift and high drag, making flight impossible if true. Cayley knew about two French experiments proving the law was false. Nobody took much notice and he turned for a while to balloons and airships. Later events also disproved it, up to Langley's 1890 finding of 20 times more lift on a flat plate at five degrees AoA than predicted by the law. Up to the time of the Wrights' first flights, the few people believing in powered flight were widely dismissed as cranks. The fact that Newton had expressed uncertainty about this model was ignored, or more likely forgotten. Lanchester conceived the circulation theory of lift in 1893 from the principles of fluid vortex and streamline behaviour derived after Newton's time. Kutta and Joukowski established its mathematical structure soon after, from which the classic low speed aerofoil shape of a moderate thickness, well-rounded nose and sharp trailing edge was first obtained. Its essence is that aerofoils generate a circulating flow which is combined with the free stream velocity, increasing the local flow velocity above and decreasing it below the aerofoil. The integration around the aerofoil of these velocity variations gives the circulation strength. The lift per unit span can be found from the basic equation "lift equals circulation times velocity times density", without the direct use of either Bernoulli or Newton, as can be proved by measurement. In two-dimensional flow (no tip effects) there is no net downwards air movement. At negative AoA, negative lift is provided by the reversal of the circulation direction.
As noted in the NASA website, applying the Bernoulli equation to the local velocities obtains the pressure distribution around the aerofoil, and hence the total lift. 'Bernoulli' is not a lift theory, nor did Bernoulli write it. It was derived by Euler from a subset of his fluid motion equations, and gives the relationship between velocity and pressure along streamlines. Together they show that lift is generated largely or entirely by reduced pressure above the aerofoil, first proved by measurement in Eiffel's 1911 wind tunnel. Though it is essentially a low speed, or more specifically a low Mach number theory, it is still used today in greatly developed forms. It cannot be said that the current enthusiasts for the sole application of Newton's laws to explain lift have succeeded in rubbishing circulation theory, because they appear not to know it exists. Their ideas are assertions that have utterly failed to provide any quantitative measures of lift, or to explain the physical basis of lift generation, or to suggest how an aerofil might be desgined. It is these ideas that I referred to as nonsense.
John Gibson
(For the record, this letter appeared on page 56 of Today's Pilot, November 2007, and is copied here with permission obtained by Saab Dastard. All typos are my fault.)
Rob Davies (Letters, September issue) is upset by what he calls my opinions on Newton and lift theory. They are rather more than that. I prefer to call them historical facts plus knowledge gained in 55 years in employment and consultation at Warton, mostly within the aerodynamics department. It has been claimed recently that aerodynamicists have always been wrong (if you believe that, you can believe anything), because they have used the "equal transit time" theory. That claim is nonsense. The theory, rightly described as incorrect in that NASA website, is a fairy tale that is sometimes taught by pilots to other pilots. It was never part of lift theory or presented in serious textbooks, and NASA does not say it was.
I did not say that Newton's laws are invalid or nonsense. He was first to prove theoretically that fluid forces on a body are proportional to the fluid density, the body dimensions and the velocity squared. He formulated the theoretical structure of frictional shear stress in viscous fluids. His second law plays a part within the general three-dimensional fluid flow motion equations of Euler and Navier-Stokes of the 18th and 19th centuries, but these were insoluble until high power digital computing arrived long after two-dimensional aerofoil lift theory was well established and proven. He also proposed a fluid force model assuming a stream of individual particles colliding with an object in motion through it, giving up the component of their momentum normal to the surface and then moving off tangentially to it. When applied later to flat plates at small angles of attack, this led to the notorious sine squared law with very low lift and high drag, making flight impossible if true. Cayley knew about two French experiments proving the law was false. Nobody took much notice and he turned for a while to balloons and airships. Later events also disproved it, up to Langley's 1890 finding of 20 times more lift on a flat plate at five degrees AoA than predicted by the law. Up to the time of the Wrights' first flights, the few people believing in powered flight were widely dismissed as cranks. The fact that Newton had expressed uncertainty about this model was ignored, or more likely forgotten. Lanchester conceived the circulation theory of lift in 1893 from the principles of fluid vortex and streamline behaviour derived after Newton's time. Kutta and Joukowski established its mathematical structure soon after, from which the classic low speed aerofoil shape of a moderate thickness, well-rounded nose and sharp trailing edge was first obtained. Its essence is that aerofoils generate a circulating flow which is combined with the free stream velocity, increasing the local flow velocity above and decreasing it below the aerofoil. The integration around the aerofoil of these velocity variations gives the circulation strength. The lift per unit span can be found from the basic equation "lift equals circulation times velocity times density", without the direct use of either Bernoulli or Newton, as can be proved by measurement. In two-dimensional flow (no tip effects) there is no net downwards air movement. At negative AoA, negative lift is provided by the reversal of the circulation direction.
As noted in the NASA website, applying the Bernoulli equation to the local velocities obtains the pressure distribution around the aerofoil, and hence the total lift. 'Bernoulli' is not a lift theory, nor did Bernoulli write it. It was derived by Euler from a subset of his fluid motion equations, and gives the relationship between velocity and pressure along streamlines. Together they show that lift is generated largely or entirely by reduced pressure above the aerofoil, first proved by measurement in Eiffel's 1911 wind tunnel. Though it is essentially a low speed, or more specifically a low Mach number theory, it is still used today in greatly developed forms. It cannot be said that the current enthusiasts for the sole application of Newton's laws to explain lift have succeeded in rubbishing circulation theory, because they appear not to know it exists. Their ideas are assertions that have utterly failed to provide any quantitative measures of lift, or to explain the physical basis of lift generation, or to suggest how an aerofil might be desgined. It is these ideas that I referred to as nonsense.
John Gibson
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none of the theories we currently have explain lift in a way ..... that stands up to much scrutiny, each falls apart in its own way given the right set of circumstances.
In my experience, the theories that have existed for a century or more explain lift very well, stand up to intense scrutiny, and for incompressible flow (say <Mach0.5) quantify lift with a very close match to experimental observation.
In all the instrumented wind tunnel test results I've seen (and I spent a number of years in aircraft design with Hawker's in my formative engineering days), the measured lift force equates extremely well with the integrated pressure differential around the wing, and the pressure at any position equates equally well via Bernoulli's prediction to the streamline flow velocity at that position.
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Originally Posted by contacttower
I don't think that's completely true, scientific theories come and go
One example often cited is the replacement of Newtonian physics by Einsteinian, but that's not really true. We're still taught Newtonian physics in college. Einstein's equations reduce to Newton's when velocities are low.
Some may remember the "Downwind turn" thread, which while is not directly related to lift theory demonstrated that there is still sufficient inconstistency in some parts of theory of flight to enable a lot of myths to be peddled.