Debunking lift theories
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To be or not to be, that is the question
At the end of this very interesting debate it has never been in doubt that we would end up back at the beginning, and so we have with the scientists amongst us concluding that there is no simple way to brief a student PILOT on how their wings/flight control surfaces work. And those that it would appear most knowledgeable on the matter lost for words.
I said at the beginning because, there is nothing wrong to state; " that the thickness of an aerofoil is designed such that the airspeed over the upper cambered surface increases with an associated reduction in pressure, thus the pressure above the aerofoil becomes less than that beneath the wing - where it is not necessary for a change of speed to take place. The force thus generated is called lift and is said to be perpendicular to the surface of the aerofoil. The lift force generated, in order to maintain height, must equal weight (G). The lift force generated will vary with changes in air density, airspeed and Aof A.
I have never heard a pilots briefing done by a knowledgeable instructor different from the above and it needs no further explanation - well, perhaps in the bar.
I said at the beginning because, there is nothing wrong to state; " that the thickness of an aerofoil is designed such that the airspeed over the upper cambered surface increases with an associated reduction in pressure, thus the pressure above the aerofoil becomes less than that beneath the wing - where it is not necessary for a change of speed to take place. The force thus generated is called lift and is said to be perpendicular to the surface of the aerofoil. The lift force generated, in order to maintain height, must equal weight (G). The lift force generated will vary with changes in air density, airspeed and Aof A.
I have never heard a pilots briefing done by a knowledgeable instructor different from the above and it needs no further explanation - well, perhaps in the bar.
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Or there again....
Backpacker,
You could always say to your passenger to stick their hand out of a car window and tilt to to experience different angles of attack. If they do this I’m sure that (like me ) they will FEEL the air below their hand pushing the said hand upwards (rather than feeling a vacuum above their hand sucking it up)
But I still byt the big stick theory
You could always say to your passenger to stick their hand out of a car window and tilt to to experience different angles of attack. If they do this I’m sure that (like me ) they will FEEL the air below their hand pushing the said hand upwards (rather than feeling a vacuum above their hand sucking it up)
But I still byt the big stick theory
The crucial difference, in my view, that made me pick that example, is that beam and ray optics is self-contained. It is not complete, there are limits to its applicability, but within its range of applicability you can use that and only that to obtain a useful result.
Simple aerodynamics, such as inviscid flow approximations, as far as I am aware, is not thus self-contained. For instance, ignoring viscosity you have to pull things like the Kutta criterion out of your hat to explain what the circulation will be; details about the boundary layers such as thickness and where they go turbulent influence the results (especially at high angle of attack) but you can't readily calculate those details from first principles, etc.
Simple aerodynamics, such as inviscid flow approximations, as far as I am aware, is not thus self-contained. For instance, ignoring viscosity you have to pull things like the Kutta criterion out of your hat to explain what the circulation will be; details about the boundary layers such as thickness and where they go turbulent influence the results (especially at high angle of attack) but you can't readily calculate those details from first principles, etc.
When your ray hits a glass surface, you apply Snell's Law to work out by how much the ray refracts. Isn't that "pulled out of your hat" in the same way (unless you happen to remember that the light is a wave, of course, but then you're moving beyond that self-containment)?
According to Newton's Laws I can balance a pin on its point on my desk. Experience says that there's enough vibration around that the unstable equilibrium will never be maintained and the pin will topple. So do Newton's Laws lack this "self-containment" because my pin-balancing experiment fails?
It doesn't take a detailed quantitative understanding of viscosity to conclude that the Kutta condition works, just an empirical understanding of fluids clinging to surfaces. I think singling out aerodynamics as being in some way "incompletely understood" is inappropriate. It is, undoubtedly, harder than maany areas of physics to explain convincingly to the layman in a way that is physically accurate.
The force thus generated is called lift and is said to be perpendicular to the surface of the aerofoil.
To me, it seems intuitively obvious that one cannot get lift unless one is accelerating some air downwards.
Consider a long wind tunnel with a roof and a floor, as well as sides, with an aerofoil section running across it. The air goes in parallel to the tunnel centreline upstream and comes out parallel downstream -- if it doesn't come out parallel, please extend your wind tunnel by another mile and repeat the thought experiment.
The aerofoil will experience lift, but there is no "net" acceleration of the air downwards. There is local acceleration, but all you end up with is an aerofoil being lifted (for the sake of argument) upwards, and an equal and opposite total force on the walls of the tunnel. You might think it an unrealistic situation but it's related to ground effect, and to the realisation that we don't actually fly in an infinite volume of air -- at some point below the aircraft the earth has got to do some pushing up.
The relationship between pressure forces on a notional surface in a fluid and the momentum flux passing into and out of that surface are connected by a fluid mechanics equation usually called the momentum theorem. It's through that equation that the various forces and "accelerations" of the air can be related. How they add up in detail depends on where you draw the notional surface.
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bookworm:
1) Of course, all scientific models are "pulled out of one's hat" at one level or another. That in itself does not make it less self-consistent. It is when you find you have to start adding bits and pieces here and there to solve specific problems that you know you are in trouble; e.g., the Kutta criterion.
2) Newton's laws do not say what you are claiming. Finding a stationary solution does not imply that you will come across such a solution in real life. For that you need to make a stability analysis, which will readily prove that the solution of a pin balancing on its point, while stationary, is unstable. Therefore, with a correct application of Newton's laws, you will conclude that the pin falls.
3) Surely you will agree that the degree of understanding of a physical field should be related to how much the experts understand, and not what one might explain to the layman. If we had a full understanding of aerodynamics, we would have no need for wind tunnels and we would not have a problem with turbulence modelling. But we do.
homeguard: That's my preferred explanation too!
1) Of course, all scientific models are "pulled out of one's hat" at one level or another. That in itself does not make it less self-consistent. It is when you find you have to start adding bits and pieces here and there to solve specific problems that you know you are in trouble; e.g., the Kutta criterion.
2) Newton's laws do not say what you are claiming. Finding a stationary solution does not imply that you will come across such a solution in real life. For that you need to make a stability analysis, which will readily prove that the solution of a pin balancing on its point, while stationary, is unstable. Therefore, with a correct application of Newton's laws, you will conclude that the pin falls.
3) Surely you will agree that the degree of understanding of a physical field should be related to how much the experts understand, and not what one might explain to the layman. If we had a full understanding of aerodynamics, we would have no need for wind tunnels and we would not have a problem with turbulence modelling. But we do.
homeguard: That's my preferred explanation too!
It is when you find you have to start adding bits and pieces here and there to solve specific problems that you know you are in trouble; e.g., the Kutta criterion.
Surely you will agree that the degree of understanding of a physical field should be related to how much the experts understand, and not what one might explain to the layman.
If we had a full understanding of aerodynamics, we would have no need for wind tunnels and we would not have a problem with turbulence modelling. But we do.
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bookworm, I'm rather losing the plot of your argument there... Physicists don't develop models the way you claim that we do, refractive index is certainly not a fudge factor but a material parameter, prototyping is mostly about production matters and not basic design principles... Maybe we'd better give this a rest?
Physicists don't develop models the way you claim that we do,
refractive index is certainly not a fudge factor but a material parameter,
prototyping is mostly about production matters and not basic design principles...
Singling out aerodynamics as mysterious or undiscovered just fuels the dumb conspiracy theories that Gibson was condemning.
One really needs to look to the motivations behind aerodynamic theories before dismissing anything as 'dumb'.
Prandtl was inspired by the success of Lilianthal and the Wrights....
Prandtl was inspired by the success of Lilianthal and the Wrights....
Fancy explaining it to a non-lay person in a way that is physically accurate? I must have been asleep in that lecture.
Circulation isn't really what is happening - it isn't an attempt at an explanation for lift, it's just another tool used in design to calculate values that we wouldn't be able to otherwise.
I understand how to calculate all sort of useful things that would allow me to design a practical working wing, but I still don't get how it works.
Are you being serious?
can you give me some insight as to your understanding of circulation and why you would believe it is 'what is really happening'
So what do you "understand" the workings of? What allows you to say "yes, I get that"?
Explain an aspect of physics of similar complexity in a way that allow you to say that you "understand" it.
Your not stalling are you Bookworm? I understand the lift theories that currently exist, because I understand the mathematics and the motivation. I understand that they don't give an explanation for lift as they were not intended to.
Perhaps the debate on the reality of circulation is separate and simpler? If I were to measure the velocity at closely spaced points on a closed loop around a lifting wing, for the sake of simplicity well away from either root or tip, and use that to construct a line integral of the measured values around the loop, I believe I would come out with a non-zero value, which would, furthermore, be roughly proportional to the lift. Do you dispute that? If not, how can you say that circulation is "not really happening"?
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We'll have to agree to differ on that one.
Ah the "material parameter"! It doesn't get much more empirical than that, does it?
Singling out aerodynamics as mysterious or undiscovered just fuels the dumb conspiracy theories that Gibson was condemning.
I'd wager it's your familiarity that makes you comfortable with assumptions and models in some cases, but not in others (which is the point I was trying to make with bjornhall and the beam-and-ray optics). Subjectively that's fine, but I don't think it's appropriate to generalise your subjective comfort level to an objective assessment of the state of the art.
Do you dispute that? If not, how can you say that circulation is "not really happening"?
Thus calculating a quantative theoretical value for the lift that might be a good approximation to the empirical result...
To say that there is circulation around a lifting wing is just a quantification of the easily observed phenomenon that the air travels faster over the upper than the lower surface (if the wing is lifting upwards). What Joukowski did (and if you believe Anderson then it was Joukowski, not Prandtl) was to relate that to lift per unit span in an inviscid fluid model. You can call the relationship "inexact" or "theoretical" if you like, but that's a far cry from denying the existence of circulation.
Surely you will agree that there is a difference between an empirical model, and a parameter whose numerical value is obtained empirically?
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G-EMMA wrote:
Having missed the last few pages of this thread, I'm reluctant to re-enter the fray at this stage! However, I would just like to say that I believe that statement to be historically incorrect, G-EMMA.
It is Lanchester who is generally credited with an explanation of lift based on circulation and he developed his ideas in the early 1890s, fifteen or more years before Prandtl developed his own wing theory. From a 1930s aerodynamic theory text by Giacomelli and Pistolesi: "With regard to Lanchester's contribution to aerodynamics, there are two great ideas conceived by him: the idea of circulation as the cause of lift, and the idea of tip vortices as the cause of drag known today as induced drag."
Importantly, Lanchester's circulation ideas (as eventually published in his 1907 Aerodynamics) were essentially non quantitative and very much not put forward as a design tool. It was mathematicians Kutta and Joukowski that independently produced the quantitative development of circulation theory that is still used today to calculate the lift generated by wings at airspeeds <Mach0.5.
If you have not read it already, G-EMMA (edited to say I now see you have!), I would urge you to take a look at John D Anderson's seminal A History of Aerodynamics and Its Impact on Flying Machines. As an eminent academic aerodynamicist, historian and one time Special Assistant for Aerodynamics at the Smithsonian, Anderson's book is IMHO the most authoritative and readable account ever written of the story of aerodynamics development. Who knows, it may cause you to question your belief that: "the science of aerodynamics remains in many ways in its infancy, both mysterious and undiscovered", which is a view I struggle to share.
Clearly circulation isn't what happens at all when a wing produces lift, it is just a technique for calculating theoretically the quantative lift that might be expected, Prantdl made it up, to get design results.
It is Lanchester who is generally credited with an explanation of lift based on circulation and he developed his ideas in the early 1890s, fifteen or more years before Prandtl developed his own wing theory. From a 1930s aerodynamic theory text by Giacomelli and Pistolesi: "With regard to Lanchester's contribution to aerodynamics, there are two great ideas conceived by him: the idea of circulation as the cause of lift, and the idea of tip vortices as the cause of drag known today as induced drag."
Importantly, Lanchester's circulation ideas (as eventually published in his 1907 Aerodynamics) were essentially non quantitative and very much not put forward as a design tool. It was mathematicians Kutta and Joukowski that independently produced the quantitative development of circulation theory that is still used today to calculate the lift generated by wings at airspeeds <Mach0.5.
If you have not read it already, G-EMMA (edited to say I now see you have!), I would urge you to take a look at John D Anderson's seminal A History of Aerodynamics and Its Impact on Flying Machines. As an eminent academic aerodynamicist, historian and one time Special Assistant for Aerodynamics at the Smithsonian, Anderson's book is IMHO the most authoritative and readable account ever written of the story of aerodynamics development. Who knows, it may cause you to question your belief that: "the science of aerodynamics remains in many ways in its infancy, both mysterious and undiscovered", which is a view I struggle to share.
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I can obtain the direction of flow at the trailing edge of the aerofoil with a cotton thread, which is probably rather simpler than even measuring a refractive index. Both are measurable quantities whose values can be fed in to useful physical models and are not themselves predictable by those models. If you look to one to prove that the model is not "self-contained", I think you'd be hard pressed to treat the other differently.
Now you're just playing with words... But you're no nearer to the point...
Look: In ray optics, you have a model, that is not merely empirical, in which you have a parameter: refractive index. You know that parameter has to be there, you know why and what it means, you just don't know its value. So you measure that parameter.
In non-viscid fluid dynamics, you have no notion of such a thing as 'viscosity', and you have no clue at all that there is 'something' that makes the zero-circulation solution unphysical, or what the air will do at the trailing edge (or even that there is something especially significant about the trailing edge). So you add a whole new concept to your model; you no longer have non-viscid fluid dynamics, but you now have that + the Kutta criterion.
In the first case you only measure a parameter, and one that you know what it means and why it is there; in the second, you have to 'measure' an entire concept without knowing why you even have to do that.
And it gets worse: If you go outside the most basic theory, you can even calculate refractive index from first principle. But in aerodynamics, especially at high alpha, you have trouble even finding the right fudge factors...