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Theory on lift

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Theory on lift

Old 20th Oct 2012, 14:01
  #181 (permalink)  
 
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The entire circle, or half of it?

"it can be shown for an elliptic lift distribution that the area of the scoop is equal to a circle with a diameter equal to the wing span."
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Old 20th Oct 2012, 14:28
  #182 (permalink)  
 
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Hi HN39

it can be shown for an elliptic lift distribution that the area of the scoop is equal to a circle with a diameter equal to the wing span.
But what about 2d wings (original point of discussion) or non-elliptic loading (usually the case)? Downwash not uniformly distributed?

Last edited by Owain Glyndwr; 20th Oct 2012 at 14:29.
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Old 20th Oct 2012, 14:51
  #183 (permalink)  
 
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A very large flat plate surface is entering the atmosphere, it dips into the air to a point where it is gliding, and there is no air above its surface. What is holding this flat plate at constant altitude?
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Old 20th Oct 2012, 16:05
  #184 (permalink)  
 
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Lyman,

The entire circle. But of course the downwash velocity is not constant. It reduces with the distance to the wing asymptotically to zero at infinity.
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Old 20th Oct 2012, 16:43
  #185 (permalink)  
 
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Howdy HazelNuts39

The scoop in the paper is oriented above the wing. For the area of the circle to repose above the wing plane, it would be flattened, and perhaps taller than one radius?

What is the descriptor for the area beneath?

The conclusion of the two authors...

"We have also shown that the pressure and velocity of the air over a real wing in flight are not related by Bernoulli’s equation."

Not only does airflow above the wing not create lift, it is detrimental to the work beneath it. It is after all, just additional mass to lift, with the aircraft....

It is the diminution of the airflow pressure above the wing that is a result of increase beneath. If the two are negatively related, how can both be positive?

Last edited by Lyman; 20th Oct 2012 at 17:44.
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Old 20th Oct 2012, 18:45
  #186 (permalink)  
 
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à Italia458, Owain Glyndwr,

We already rejected here these picture (despite coming from NASA).

Try to draw it at infinite...left and right, up and down, and 3-D of course, you have surprises !

The object of that thread is precisely to get a correct theory of lift, not this one !


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Last edited by roulishollandais; 20th Oct 2012 at 18:48.
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Old 20th Oct 2012, 22:10
  #187 (permalink)  
 
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It should have been obvious from my earlier posting that I wasn't trying to use the pictures to explain lift; merely using their entirely correct definition of a streamline.

Or do you have another definition of a streamline?

If you want an entirely correct theory of lift then read Arvin Gentry.

Last edited by Owain Glyndwr; 20th Oct 2012 at 22:19.
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Old 20th Oct 2012, 22:38
  #188 (permalink)  
 
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Hi Owain,
Thank you for your answer. ... I reject the "idea" of streamline, as it is just an idea, and idea do not fly... It was an idea for the time of closed windtunnel! We are in 2012 .We are unable to continue the draw of the streamlines to their limits : beginning, end, the aircraft is flying inside an enormous balloon of same mean density that air around it... The biggest problem is not energy or pressure but information transfer...Dear Holy Shannon Help please ! Or let us find an other model than [email protected]@@@@_______

Last edited by roulishollandais; 23rd Oct 2012 at 18:28. Reason: Holy spelling
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Old 21st Oct 2012, 05:46
  #189 (permalink)  
 
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"Quote:
they both produce lift by compressing air at the leading edge, both above and beneath it"


"Absolutely not. Lift, at its most basic, is created by the turning of the airflow."

And before the airflow can be deflected, it contacts the leading edge, which produces a high pressure "bubble".

Look, I am nowhere near the expert I perceive you to be, and I am profoundly interested in the discussion. Sending people off to the "library" strikes me as dismissive, and tends to smother the discussion, rather than enlarge it. I was scolded at one point by someone for looking at the paper and supporting it, who has now admitted it filled some holes, and he is a convert.

I don't smoke a pipe, nor join groups just to nod and affirm each others' bias.

I think these two guys have some elegant work on display, and honestly, I never liked Bernoulli.

Still air is not a pipe, Air is not a liquid, It is the wing that moves, not its medium, And low pressure is not created by acceleration of the medium.

"It is our hope that teachers will return to the basics and use Newton's laws to describe lift. Then students can explore flight in much more depth than was possible with the popular explanation using Bernoulli."

Last edited by Lyman; 21st Oct 2012 at 05:52.
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Old 21st Oct 2012, 07:35
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I reject the "idea" of streamline, as it is just an idea, and idea do not fly.
Better not tell that to a aerodynamicist. There are so many misconceptions posted on this thread that I've scanned pages from the bible of how lift is created, "Theory of Wing Sections" by Ira Abbott and Albert Von Doenhoff. Some heavy maths, but never the less the text should shed a little light.
He just did

Abbot & von D is heavy on math. I've just located a better article by Arvel Gentry than the one I have been recommending which covers the same ground but without the maths. It talks about lift from sails, but it is all equally applicable to wings. It even describes how you get lift from a flying barndoor, so it should help those who think in those terms!

I can't see how to scan and post via photobucket but you can find it here:
http://www.arvelgentry.com/techs/A%2...l%20Theory.pdf

The bits you want are sections 2 to5

I think you will find it deals pretty clearly with most of the issues that have been raised in this thread. But as Gentry himself said in an interview:

Arvel notes: "Aerodynamics is a difficult subject, and all attempts to simplify it for the average person leads to wrong interpretations. The facts are that lift comes about because air has viscosity, which leads to the starting vortex. This is followed by the formation of a circulation field about the airfoil necessary to meet one of Helmoltz's theorems of vortex motion. Then the Kutta condition is satisfied at the trailing edge, and bingo -- we have lift. These principles, together with knowledge of boundary layer theory, lead to a correct understanding of the interaction between the jib and the mainsail."
Easy for him to say, right?
Gentry puts is as "You can't just sit there and stare at your navel and come up with conclusions."




Last edited by Owain Glyndwr; 21st Oct 2012 at 07:49.
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Old 21st Oct 2012, 07:52
  #191 (permalink)  
 
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You can't just sit there and stare at your navel and come up with conclusions.
From where I'm perched; aft of the jib and abeam and to the lee of a full mainsail, the conclusion is obvious.
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Old 21st Oct 2012, 08:10
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From where I'm perched; aft of the jib and abeam and to the lee of a full mainsail, the conclusion is obvious
Wish I could be with you, but I'd prefer to be further aft and on the windward side with a tiller in my hand
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Old 21st Oct 2012, 12:37
  #193 (permalink)  
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One of the things which continues to throw a spanner in the works each time this subject is broached is the fact that people look at Bernoulli's theorem but only see the equation and forget to look at the complete definition, especially the limits to its applicability.

It applies to a steady flow of an incompressible, inviscid fluid.

If you have compressibility, if you have viscosity, if it's not steady - then Bernoulli does not apply, and will not yield 100% correct results. The error will depend on how much your application deviates from the defined required conditions.

Often, the errors can be ignored for all practical purposes. Outside of the boundary layer (but you have to agree on a definition of the boundary layer - leaving the search for the commonly accepted definition as an exercise for the interested reader) and at low airspeeds where compressibility isn't much of a factor, it'll generally be good enough.

That article seems to want to throw Bernoulli out the window as it doesn't apply in the parts of the flow where viscosity is significant. In my native language, we have a saying about kicking in open doors. That would apply, I think. Noone knowledgeable, especially not mr. Bernoulli himself, has ever claimed that Bernoulli's theorem is without limitations.
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Old 21st Oct 2012, 17:45
  #194 (permalink)  
 
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Hi ft...

"It applies to a steady flow of an incompressible, inviscid fluid."

I think that is the point I attempted to make, inelegantly.

The authors of the paper in question make an elegant case for a new way to teach "Introduction to Lift". The math and the relationships among the variables are clear, and persuasive.

The "sacred" maths and several independent theorems are available on Wikipedia, I checked. I have never understood the apparent need to make lift complex, and pay homage to a Swiss hydrologist.

Any new approach will make "waves" and a vortex or three. Toes will be compressed in its forthcoming popularity...

As to the lack of facts in this post, I will incorporate the paper here, by reference.

Last edited by Lyman; 21st Oct 2012 at 17:58.
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Old 21st Oct 2012, 21:52
  #195 (permalink)  
 
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Lyman...

I have never understood the apparent need to make lift complex, and pay homage to a Swiss hydrologist.
Lift is complex! And Bernoulli wasn't a hydrologist, he was a physicist and mathematician. Daniel Bernoulli versus a hydrologist.

I think the quote that Owain posted needs to be emphasized more. Until you accept this, I see no point in continuing to try to understand the complexities of lift. A scientist or physicist or aerodynamicist all have one thing in common... they all are studying nature and discovering the way it is. If it turns out the way nature is is simple, then that's the way it is. If they find out that nature is complex, then that's the way it is! Lift happens to be one of those complex things.

"Aerodynamics is a difficult subject, and all attempts to simplify it for the average person leads to wrong interpretations." - Arvel

That is fundamentally the point I try to make, the wing compresses air...
When studying lift at low speeds, which is where you always start, it can be said that air is incompressible at speeds below Mach 0.3. That satisfies one of the conditions of Bernoulli. Outside of the boundary layer air can be considered inviscid, another condition for Bernoulli.

Henra...

Yes, expansion means increasing pressure. But increasing can mean increasing back to free stream static pressure. No discrepancy to the 'closed' system here.
Yup... I made an error with my description. What you say makes sense.

Owain Glyndwr...

I disagree. The energy added is used to overcome a side effect of lift generation – drag.
There is abundant evidence from wake survey experiments that total pressure is not constant behind a wing producing lift, so in broad terms they are right, although not because energy is added – rather it is subtracted. However, it is also true that this loss of total pressure is confined to a small area just behind the TE – in the wing wake in fact.
Would it be correct to say that the flow outside the boundary layer does not have energy added or subtracted (and it's incompressible and inviscid) so Bernoulli applies to it? Is the boundary layer then responsible for the induced and parasitic drag?

I'm trying to get an understanding of the energy of the system - how energy is added or subtracted to the air by an airfoil passing through. It makes sense that energy has to be added to overcome drag. Regarding the loss of total pressure behind the TE, where does that energy go?
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Old 21st Oct 2012, 22:50
  #196 (permalink)  
 
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Would it be correct to say that the flow outside the boundary layer does not have energy added or subtracted (and it's incompressible and inviscid) so Bernoulli applies to it?
Yes, Bernouilli's equation is, for all practical purposes, valid outside the boundary layer - but note ft's comment that strictly speaking you have to define the limits of the boundary layer.That can be done in several ways but the fine differences only matter to aerodynamic pedants (I'm not one I hope)

Is the boundary layer then responsible for the induced and parasitic drag?
The boundary layer is not "responsible" for drag in any direct sense of course. It is the viscous forces associated with the velocity shear inside the boundary layer that produce skin friction drag.
If you are looking at a 2D wing then there will be no induced drag in the classic sense since you have an infinite aspect ratio. For sensible finite wings then there will be drag due to lift - for a wing in inviscid flow and with an elliptical loading that will be Prandtl's classic CL^2/(Pi*A.Ratio). For non-elliptic loading and the effect of fuselage Europeans usually put an induced drag factor 'k' in front of that. In the USA it is more common to use the Oswald efficiency e = 1/k. This bit of drag due to lift has nothing to do with the boundary layer.

In real life viscosity and the effect of pressure gradients on the upper surface mean that as AoA (CL) is increased the boundary layer flow will start to separate and the drag will increase. In practical measurements this shows up as an increase in 'k'.

So boundary layer flow is involved in both skin friction drag and part of the induced drag. If there are any separations around at zero lift it can also be involved in the pressure drag.

Regarding the loss of total pressure behind the TE, where does that energy go?
Never really thought about it deeply, but since the energy loss comes from frictional forces why doesn't it (eventually) show up where friction effects always do show up - heat! [That isn't taken into account by Bernouilli either].

Going to be offline next week, but parting thought - if you want a quantitative explanation of lift generation then you have no choice but the circulation explanation L = rho * circulation* airspeed, but if you are happy with a qualitative explanation you can opt for the Newtonian L = mass flow rate * downwash. Neither of those two parameters can be defined numerically (see below) so it doesn't satisfy me, but if you can live with that well "Chacun a son gout" BTW, Bernouilli's equation doesn't figure in either of those explanations


[We don't know the depth of the region affected by the wing or even its shape; downwash is not uniform over this region, in fact it varies with distance below the wing]

Last edited by Owain Glyndwr; 22nd Oct 2012 at 09:33.
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Old 23rd Oct 2012, 17:07
  #197 (permalink)  
 
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Originally Posted by Lyman
That is fundamentally the point I try to make, the wing compresses air,.....
Nope. Wrong again. At the airspeeds encountered in a typical general aviation aircraft, (less than 200 knots) there is no significant compression of the air.

Originally Posted by Lyman
Air is not a liquid
Uhh, yeah, for the purposes of understanding the aerodynamics of a low speed airfoil it is in fact, essentially an incompressible fluid. There is no significant change in the volume of the air as it flows around an airfoil at say 150 knots. A change in pressure, but not in volume.

This gets back to my earlier comment. Most of what you "know" is wrong.

Originally Posted by Lyman
It is the wing that moves, not its medium...
A meaningless distinction. What matters is that there is relative motion. Which is moving and which is not is completely dependent on the frame of reference, arbitrary, and completely irrelevant.

The fact that you beleive that this is meaningful is only an illustration of how poorly you understand physics.
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Old 23rd Oct 2012, 17:16
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Originally Posted by Owain Glydwr
We don't know the depth of the region affected by the wing or even its shape
Well, at subsonic speeds, it is infinite, in all directions. Any boundary is arbitrary.

Last edited by HazelNuts39; 23rd Oct 2012 at 17:18.
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Old 23rd Oct 2012, 18:05
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"Well, at subsonic speeds, it is infinite, in all directions. Any boundary is arbitrary."

AA

"Nope. Wrong again. At the airspeeds encountered in a typical general aviation aircraft, (less than 200 knots) there is no significant compression of the air."

"A meaningless distinction. What matters is that there is relative motion. Which is moving and which is not is completely dependent on the frame of reference, arbitrary, and completely irrelevant."

How is it all of a sudden an energy source is not relevant? It is consistent with your view of Physics as squishy, and dependent on your definitions, definitions that involve a suspension of actual Physical Laws.

Any widely held theory that depends entirely on suspension of fundamentals seems to attract hysterics..

You say...

"A meaningless distinction. What matters is that there is relative motion. Which is moving and which is not is completely dependent on the frame of reference, arbitrary, and completely irrelevant" (my bolding, throughout)


Air is the recipient of added energy, not the source. Watch your landscaper blow debris about with a leafblower, you will perhaps see my point.

The source of added energy is not important? Strange viewpoint from an expert.

Last edited by Lyman; 23rd Oct 2012 at 18:16.
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Old 23rd Oct 2012, 18:09
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Originally Posted by Owain Glydwr
strictly speaking you have to define the limits of the boundary layer.That can be done in several ways but the fine differences only matter to aerodynamic pedants (I'm not one I hope
...If mathematicians are aerodynamic pedants, I accept the mockery at the expense of revenge ..
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