Theory on lift

[awblain]

Circulation theory?

I suggest that you could also call `circulation theory' a simple piece of legislation by Mr Newton.

The flowing air lifts up the wing, and the wing pushes down the flowing air.

Behind the aircraft, air is now descending, to a good first order, to the tune of `lift = rate of change of vertical momentum in the airflow', while behind the aircraft the air is also made to follow along, to a first order, to the tune of `drag = rate of change of horizontal momentum in the airflow'.

[roulishollandais]

@Owain Glyndwr,

Thank you for the excellent Arvel E. Gentry document. That can be taught to begin. The most of the false theories are well explained. In his document "streamlines" may be considered as a good pedagogic tool.

My question about "streamline" is still a little deeper : where do you put the turbulence in the unhomogeneous "streamline" ? David Ruelle and Floris Taken (Institut des Hautes Etudes Scientifiques, Bures sur Yvette (France)) showed (1) that turbulence in his phases space has an strange attractor which is of length infinite in the area, and then it is a fractale.
Jerry P. Gollub and Harry L. Swinney showed by experimenting on the Couette-Taylor (2) flux that Lev D. Landau's theory of turbulence was wrong, and they discovered only very few transitions in increasing the speed, and chaos appeared very quickly.

roulishollandais

(1) "On the nature of turbulence" Communications in Mathematical Physics, n°23, 1971,pp.155-183
(2) "Onset of turbulence in a Rotating Fluid" Physical Review Letters n°35, 1975, p.927

[Owain Glyndwr]

@roulishollandais

I'm not sure I know how to reply to your question roulis. Could you expand a little what you mean by

"put the turbulence in the unhomogeneous "streamline"

please.

If I simply said that I see the flow inside the boundary layer as essentially wholly turbulent (since transition on practical wings is never aft of the slat TE) and that inside this turbulence the flow is chaotic (strange attractor/fractal or whatever description) so that the concept of streamlines is irrelevant, would that meet your needs? Streamline flow then is limited to that region outside the boundary layer (however you define that)

[roulishollandais]

@Owain Glyndwr,

Originally Posted by Owain Glyndwr

outside the boundary layer

The problem with streamlines exists inside and outside of the boundary layer.
You have chaos everywhere turbulence exists...

Site Web pour cette image

Turbulence is chaotic by definition, ...

princeton.edu

[FlightPathOBN]

When the wing vortex (technically called the bound vortex) arrives at the wingtip the changed flow conditions cause it to change direction and continue rearwards as a trailing (aka wingtip) vortex. Since for any vortex the velocity increases and the static pressure drops as you move towards the core if the conditions are right any water vapour condenses out and we “see” the vortex as a contrail. These vortices are real!

The only part of that statement that is correct is "these vortices are real"

The illustrations that are provided with the entire post are oversimplified and misapplied to wake turbulence...


This is likely the result of a reliance on CFD models, and other nonsense that has prevailed for many years, but is now thankfully being addressed. These illustrations show a wing section in a wind tunnel, with simulated winds. Smoke or other methods are used to detect the airflow around the wing section, or even a wave tank with colored water.

Notice that the airflow in the section is not deflected? How is lift generated of airflow is not deflected?

Rollup of the vortices occurs at the location of the wing where the laminar flow over the top of the wing disconnects from the top of the wing surface, NOT at the wingtips. (notice on your illustration that the condensation trail originates from the outward edge of the flap)
Here is a better example...757 (note how wing/outboard flap config affects vortex gen? just wait until you see the vortex behind a 787....


The rollup...



What has been oversimplified is the illustrations noted. There is no accounting for the weight and varied surfaces of the aircraft/wing, and in general, the overall issue that air can be compressed, while water cannot.

It is relatively easy to see the illustrations that have used CFD, such as those you have shown, but as we are finding out...cannot explain wake vortex creation.

Just look at winglets touted as reducing wake as a great example.

[Lyman]

Hi FPOBN....

Do you have a permission slip to inject compression and viscosity?

Cheers

[FlightPathOBN]

plausible deniability...

[Owain Glyndwr]

roulishollandais
Nice pictures!

But they are theoretical evaluations of the flow in a channel and the flow is subject to three (open channel) or four (closed rectangular channel) boundary layers. With fully developed boundary layers there is no real freestream conditions.
I would be surprised if the results shown resulted from anything more complex than application of a single initial condition. But we know that the output from strange attractors is extremely sensitive to initial conditions (butterfly wing flap to tornado). In real life the boundary layer will be subjected to an infinitely variable and random set of initial conditions as it passes over rivet heads, steps, gaps etc. So I suspect that the chaos in the boundary layer flow will be a lot more complicated than that shown in your photos.
]If we do consider freestream of course then there will always be some level of turbulence, but we must consider how the scale of the turbulence relates to the scale of the object passing through. [Scale here means the magnitude of a characteristic physical dimensional quantity such as wavelength]
Nobody would expect to see a neat set of streamlines surrounding an aircraft flying through a patch of turbulence, and the characteristic length used for load prediction is 2500 ft. I don’t know what the characteristic length would be for a hand launched model glider on a calm day, but I bet it isn’t as high as 762 meters!
If the air is calm (small scale turbulence) then although you would be correct in saying that there must be some turbulence the effect of this at distances greater than the boundary layer height away from the solid surface will be small and for all practical purposes we may assume streamline flow.
The picture below (taken by Prandtl) shows what I mean



@FlightPathOBN

Ah well, PJ2 told me I would get into difficulties with tip vortices ...
When the wing vortex (technically called the bound vortex) arrives at the wingtip the changed flow conditions cause it to change direction and continue rearwards as a trailing (aka wingtip) vortex. Since for any vortex the velocity increases and the static pressure drops as you move towards the core if the conditions are right any water vapour condenses out and we “see” the vortex as a contrail. These vortices are real! The only part of that statement that is correct is "these vortices are real" Let me take that bit by bit....

When the wing vortex (technically called the bound vortex) arrives at the wingtip the changed flow conditions cause it to change direction and continue rearwards as a trailing (aka wingtip) vortex.

To use someone else’s words (taken from Wikipedia, my emphasis)

Three-dimensional lift and the occurrence of wingtip vortices can be approached with the concept of horseshoe vortex and described accurately with the Lanchester–Prandtl theory. In this view, the trailing vortex is a continuation of the wing-bound vortex inherent to the lift generation

I stick with my statement

Since for any vortex the velocity increases and the static pressure drops as you move towards the core if the conditions are right any water vapour condenses out and we “see” the vortex as a contrail

Sorry, but you are just plain wrong when you say this isn’t true – again taking words from Wikpedia as I can’t be bothered to look up more erudite references

Depending on ambient atmospheric humidity as well as the geometry and wing loading of aircraft, water may condense or freeze in the core of the vortices, rendering them visible.
The core of a vortex in air is sometimes visible because of a plume of water vapor caused by condensation in the low pressure and low temperature of the core; the spout of a tornado is a classic example

[quote]The illustrations that are provided with the entire post are oversimplified and misapplied to wake turbulence...Well I prefaced my explanation with :

With apologies to both those who think the following is dumbed down and those who think it too complicated, this is an attempt to put the math into plain words.

And I never once mentioned wake turbulence – I was attempting to explain the circulation theory of lift generation on the wing.

This is likely the result of a reliance on CFD models, and other nonsense that has prevailed for many years, but is now thankfully being addressed. These illustrations show a wing section in a wind tunnel, with simulated winds. Smoke or other methods are used to detect the airflow around the wing section, or even a wave tank with colored water.

Let me say that I am an old fashioned aerodynamicist – I have never in my life used or calculated using CFD. Similarly, the illustrations I used predated extensive use of CFD by many years – Shevell (1983), Gentry (1981) and even Prandtl (1908 I think). So I think your argument that the explanation results from over reliance on CFD is misplaced – and incidentally I don’t think you would find many modern aerodynamicists who would agree with you that CFD is “nonsense”!

Notice that the airflow in the section is not deflected? How is lift generated of airflow is not deflected?

Good spot! But the reason the airflow is not deflected is that this was a one of a sequence of photographs (taken by Prandtl) and chosen by me to illustrate the starting vortex formation. At that point in time the starting vortex had barely left the wing TE and the opposing wing circulation had not been generated. No circulation/no lift and no air deflection (downwash).
But a very short time later in the sequence the starting vortex has moved away, there is circulation around the wing and there is downwash, as in the picture above.

Rollup of the vortices occurs at the location of the wing where the laminar flow over the top of the wing disconnects from the top of the wing surface, NOT at the wingtips. (notice on your illustration that the condensation trail originates from the outward edge of the flap)

I think you must have a completely different conception of laminar flow to that held by most people working in the field. There is no laminar flow over pretty well the whole of the wing top surface (or over the whole aircraft for that matter. The boundary layers are turbulent as shown by roulishollandais photos above. I think what you mean is simple attached flow?

What has been oversimplified is the illustrations noted. There is no accounting for the weight and varied surfaces of the aircraft/wing, and in general, the overall issue that air can be compressed, while water cannot.

As I have said, I was not trying to do anything other than explain how lift is generated. Weight, control surfaces, compressibility are all extraneous factors with no relevance to a simple explanation of lift although they can obviously affect wake turbulence which I understand to be a preoccupation.

It is relatively easy to see the illustrations that have used CFD, such as those you have shown, but as we are finding out...cannot explain wake vortex creation.

As I said, I tried to keep it simple, but since you have brought it up, the explanation for the strong vortices off the flap tip is that the lift/unit span is given by rho*airspeed*Circulation. Over the part of the wing covered by flaps the lift/unit span is much greater than that on the unflapped bit. Since rho and airspeed are the same that means that the circulation is much bigger over the flapped part so that when the flap stops there is an abrupt change in circulation - and circulation is simply the strength of the vortex. Vorticity cannot just start or stop in midair and since the outer part of the wing cannot support it this high level of flap vorticity becomes another trailing vortex. Rather like a wing within a wing perhaps?
The 757 picture you posted is very interesting as it clearly shows the vortex ‘bursting’ some way behind the wing. As AoA increases this bursting point gets closer and closer to the TE.

[Mr Optimistic]

This all started with a 23yr old OP who stated 'Pardon me, this isn't a very technical ques.'

Think he got his monies worth but is he any the wiser ?

[Lyman]

I have never understood the apparent need of some people to instantly complicate what it is they do, or believe in. It is beyond me, and is probably a topic for a social science website.

I take a look at the wake of the C-17, and note an immense trough of disturbed air beneath and behind the aircraft. It is notable in its lack of visible moisture, evidently a high pressure zone, perhaps even heated.

The metallic and composite mass that is maintaining its altitude at the expense of an intense disturbance in the airmass remains aloft how? Kutta? Bernoulli? Glyndwr?

I make it as the result of a massive deflection of mass, period.

Can we discuss this without instantly devolving into a willy waving contest to impress? Attack? Defend? Promulgate? Dismiss? Promote?

edit... "Ah well, PJ2 told me I would get into difficulties with tip vortices ..."

So, Owain, maybe not so soon with the wing tip? Maybe the wing itself, and the paper on Newton? Simple concepts for simple people?

Luddite pilot......

[Owain Glyndwr]

As I said at the beginning:

If you are happy with the Newtonian explanation far be it from me to seek to change your mind, better switch threads or have a drink and watch a ballgame on TV,

[roulishollandais]

@Owain Glyndwr, FlightPathOBN,

I don't have much time now, but immediately Thank you for your contribution.

I liked that pictures only to show turbulence, and to help to forget the post #1 shematic... But yes it is just in a wind tunnel, and Iself often am saying it is a dangerous limitation to explain lift, how aircrafts fly, aso.

Originally Posted by @Mr Optimistic

This all started with a 23yr old OP who stated 'Pardon me, this isn't a very technical ques.'

Sorry, Lyman !

[Mr Optimistic]

From the op's perspective it is perhaps worth stepping back to observe that lift as conjectured is that net force which acts in a direction opposite to that of gravity and that arises very simply from the net force over the wing surface. The net force is the sum/integral of the pressure over the surface. When a mass of air is deflected it takes force to do that and that force has to be transmitted as a pressure distribution in the flow. Aerodynamics attempts to reconcile the equations which arbitrate between the fluid and solid elements but newton physics is all you need.

[FlightPathOBN]

on a lighter note...Newton rules and Bernoulli drools...



just let that sink in....

I make it as the result of a massive deflection of mass, period.

agreed, if the deflection of the combined compressive air mass did not equal the weight of the aircraft...you would have?
for every force the is an equally reactive force...

after all, it is called an air PLANE...not an air LIFT

[Owain Glyndwr]

In the conclusions of the Anderson/Eberhardt paper that Lyman cites so often the authors say:

Although circulation theory can be used for accurate calculations of lift, it does not give a simple, intuitive description of the lift on the wing.

I admit I thought (hoped) that professional pilots might be interested in an accurate description of how lift is generated and smart enough to understand it if presented in non-mathematical language. Seems like both expectations were optimistic.

So Brian Abrahams was right - it is a waste of time and I will follow his lead by leaving you to wallow happily.

But allow me a parting question. Would some advocate of the Newtonian explanation tell us how it explains the presence of those strong vortices at the outer ends of flaps or wings? Looks to me as if there is as much air moving up in those rotational flows as there is moving down. But Newton says all the air is moving downwards doesn't it? [BTW, the explanation should not involve drooling Bernouilli or pressures above the wingtips being lower than the pressure on the lower surface]

Eppur si muove

[FlexibleResponse]

A simple intuitive description of lift?

When an air vehicle slides through the air it displaces downwards a mass of air that can be seen as downwash.

When the mass of air being depressed equals the mass of the air vehicle level flight is possible.

When the mass of air being depressed is less than the mass of the vehicle, then the vehicle descends and when the mass of air exceeds the mass of the vehicle, the opposite occurs.

All the rest of the observables are symptomatic of the process of lift production and are for the Aerodynamicists to measure and worry over so that they may optimize design efficiencies for particular applications (and to confuse pilots!).

[Lyman]

That is a good description. The unfortunate and unnecessary "battle" is joined when any two blokes start to 'defend'.

The paper Owain references is a case in point. Two highly educated and experienced scientists are proposing a better way to TEACH lift. They suggest a return to an emphasis on Newton.

And why not, they list several demonstrably false premises in the history of teaching Bernoulli.....

I won't pretend to understand why Bernoulli is preferred by a group of highly skilled and smart people, except to say that Bernoulli is well understood and along with circulation theory, satisfies a desire to unpack the complexities....

If Owain cannot explain a vortex, I won't even try. But I would start with Newton, Mass, energy...... And viscosity....

[FlightPathOBN]

Would some advocate of the Newtonian explanation tell us how it explains the presence of those strong vortices at the outer ends of flaps or wings?

The aircraft is planing through the air, the weight of the aircraft, with angle of attack of the wings, compresses the airmass to provide level flight. (notice as the aircraft fuel load lightens, the angle of attack to fly level is reduced.)
The wing section is constantly changing, with the leading edge sweep angle, thickness, and width. The combination of angle upward of the bottom of the wing, and the component airflow direction over the top, causes the rollup, while the air beyond the wing tip will rollover the compressed section of air created by the aircraft passing through....



A surface vessel wake does not compress the water, and the vessel moving through, does not create a wake that sinks into the water

The aircraft wake is compressed, and with the combination of the 2 counter-rotating vortices, will sink to a certain altitude until crow instability occurs.

[Lyman]

From the paper

Power and lift
In aerodynamics, the subject of power requirements is seldom considered. Some introductory textbooks do not even have power listed in the index. In aeronautics this discussion would be about drag, which is a retarding force, the effect of which is proportional to the speed of the airplane. Unfortunately, drag is difficult to derive and is usually presented without derivation.

Which is a shame, but joins some other problems which make the standard model problematic.

One if which, imo, is the immediate suggestion that it is tha air that moves, rather than the wing, which although sustainable logically (albeit 'visually'), adds confusion to the model when presented as introduction...

Because that would suggest the energy is bound in the airmass, rather than in the propulsors. It is reasonable to assume, if the airmass is presented as the energetic mass, that "flow" is present. It is not. Not initially, and that is what the authors isolate as damning of the model. They pointedly state "the acceleration is unexplained".

The air is compressed, and then accelerated downward aft of the trailing edge of the wing. Vortices and chaotic turbulence are artifacts of the wings development of "lift". Which brings up perhaps my most serious criticism, that somehow the aircraft is "pulled up by suction". That to me is ludicrous, as low pressure cannot be created in this instance without compressing (pressurizing) airmass somewhere close by. In this case, it is critical to look to the work as "push" NOT "pull".

It is not incorrect to say that "lift" is developed not by flow, but by preventing it.
Preventing flow into low pressure zones is what wings do, and what makes them useful.

[HazelNuts39]

Which brings up perhaps my most serious criticism, that somehow the aircraft is "pulled up by suction".

The wind blows from high pressure to low pressure. Viewed from the high pressure area, it looks like being pushed outwards. In the center of a hurricane, it looks more like suction. It all depends on your point of view.