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Theory on lift

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Theory on lift

Old 3rd Dec 2012, 13:18
  #321 (permalink)  
 
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Hazlenuts

Perhaps the 'effective' aspect ratio is increased?
I think perhaps we are talking at cross purposes, as you are offering a reason why drag is reduced, and I am seeking a Newtonian explanation of why lift is increased.
As a straight response to your suggestion I would only remark that there is nothing in Newton's equations that relates to AR and that infinite AR (2D) wings also show ground effect.
In the case of drag reduction I think it might be very simple, and lift and drag changes are two sides of the same coin:
When we talk of a lift increase in ground effect we usually mean an increase at constant alpha. When we talk of a drag reduction we usually mean a reduction at constant lift coefficient. Invert that first statement and we have 'a given lift coefficient is obtained at a lower alpha when in ground effect' That means that at constant lift the alpha is reduced in GE, so that the resultant force vector is inclined further forward leading to a reduction in drag.

Lyman
Hence the compression, increased pressure, and lift without benefit of incidence increase?
Newton's equations as applied to an explanation of lift generation contain mass, velocity, momentum and their derivatives. There is no mention of compression or pressure - indeed such things are foreign to Newton's work.
Consequently I think that you cannot invoke such concepts in a valid Newtonian explanation of how ground effect works.

For myself, I can't get my head around anything further than:

The Newtonian explanation of lift in free air is that starting with a block of air at rest (zero momentum), passage of a wing leaves some of that air with a downward velocity and momentum. to impart that change there must have been some force whose magnitude is given by the rate of change of momentum. There must also have been an equal and opposite force acting on the wing, which we call lift.

Now it seems to me that when the wing is placed near the ground the underwing mass available for deflection is reduced - vanishing to zero if the TE actually touches the ground. In addition I would think that the average downward velocity of the mass of air going under the wing would also be lower than in free air because of the constriction imposed by the ground - in effect being half the downwash actually at the TE.
Both of those seem to me to lead to a reduction in lift rather than an increase. If anyone can point out my mistake or suggest some other Newtonian mechanism that would explain the lift increase one finds in ground effect I will be very interested!

Finally, I found these piccies that might be interesting in this discussion -


Last edited by Owain Glyndwr; 3rd Dec 2012 at 13:22.
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Old 3rd Dec 2012, 13:57
  #322 (permalink)  
 
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Those are unhelpful, and incorrect diagrams. They give the impression that ground effect is something to do with the height of the wing from the ground being related to the chord, which it isn't. (Just because it's published in a book doesn't make it correct. Doubly so when it comes to explaining aerodynamics to pilots.)

"Ground Effect" is noticeable when the height of the wing from the ground is less than the wingspan.

You can't show ground effect in a two-dimensional diagram, because it's inherently an effect that occurs only in three dimensions, like wingtip vortices.

Last edited by photofly; 3rd Dec 2012 at 13:58.
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Old 3rd Dec 2012, 14:28
  #323 (permalink)  
 
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Those are unhelpful, and incorrect diagrams. They give the impression that ground effect is something to do with the height of the wing from the ground being related to the chord, which it isn't.
Well I'm sorry you find them unhelpful, but incorrect they certainly are not. The pressure distributions are consistent with measurements on a wing of AR 5.0, so what basis have you for saying that is incorrect please? The streamline picture is meant to be consistent with the pressure diagram which, incidentally, relates to a point near midspan. Obviously it relates to a height far less than one would see in actual flight operations, but that does not make it wrong - it is an extreme case to demonstrate effects.

"Ground Effect" is noticeable when the height of the wing from the ground is less than the wingspan.
Sure, when you are flying airplanes that is when it starts to show up. But I certainly didn't say that ground effect depends on how many chords above the ground the wing is - that was your interpretation of the diagrams.

You can't show ground effect in a two-dimensional diagram, because it's inherently an effect that occurs only in three dimensions, like wingtip vortices.
There I'm afraid, you are just plain wrong - there is ample evidence that two dimensional wings exhibit ground effect characteristics.
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Old 3rd Dec 2012, 14:48
  #324 (permalink)  
 
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Maybe we should consider that the downwash produced by the lifting wing doesn't change in ground proximity, but is reflected by the ground as an upwash. The wing benefits from the upwash in a manner that is somewhat similar to an airplane (or glider) flying in an updraft, or the migrating birds flying in a V-formation. Replace the ground surface by the virtual mirror wing (what Serebrisky&Biachuev call the "method of images") and the upwash it 'induces'.

P.S.
It is perhaps of interest to note that S&B's test setup uses a 'mirror' wing opposite to the test wing to simulate a ground plane.

Last edited by HazelNuts39; 3rd Dec 2012 at 15:36. Reason: P.S.
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Old 3rd Dec 2012, 14:56
  #325 (permalink)  
 
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Hi Owain

I understand the problem related to compression (pressure) and Newton. That is why I always use mass when discussing the theory.

Any (local) 'modification' of a dynamic system involving a gas involves transient exchanges of energy, and is far more complicated than pushing an object up an inclined plane.

There are seductive traps which require patience. Newton #3 works fine for thrust; there is no reason to eliminate the other two from GE discussion, imho.

Thanks for all your time here, I am learning.

HazelNuts39

I like the upwash imagery. In fact, I like your approach in general, involving as it does visualisation. So far, I am looking at the enhanced products of GE as a result of Newton's Second Law. The 'shape' of the area under the a/c describes a volume of increased pressure, due to the stream tube resisting the descent of the planform onto it. I have used the image 'piston' in the past.

The stream tube is being pinched as it migrates aft toward the convergence of the underwing and ground streamlines?

Last edited by Lyman; 3rd Dec 2012 at 15:05.
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Old 3rd Dec 2012, 15:06
  #326 (permalink)  
 
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there is ample evidence that two dimensional wings exhibit ground effect characteristics.
Well, of course. Two dimensional wings are always in ground effect, because they have the flow pattern of a three dimensional wing with infinite wingspan. Not co-incidentally, they also generate lift with zero drag. If you want to see what ground effect does on a real wing, looking at two dimensional diagrams isn't helpful.

Those diagrams may be mid-span flow patterns but without an idea of the scale of the wingspan they're not much use. And since they're unrealistically low for aircraft flight, what use are they for indicating ground effect in relation to aircraft?
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Old 3rd Dec 2012, 15:33
  #327 (permalink)  
 
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Ground effect & downwash?

If you have less "downwash", I suggest you would have less lift, not more, from Newton's second law - less change of downward momentum to the air, less upward force on the wing.

How's this for size: the (above-ground) flow pattern when in ground effect can be described by replacing the ground by a mirror image of the wing and airflow. This flowing air at zero height speeds the wake air backwards, as compared with the flow pattern when far above the ground, thus reducing the slowing parallel to the ground/drag when close to the ground.

For lift: the speeding of the air behind the wing backwards, when close to the ground, as compared with at height, causes an increases in the flow rate over the wing, greater downwards mass flux, so a greater change of momentum and more lift. Also consistent with less slowing in the flow downstream, also consistent with less drag.
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Old 3rd Dec 2012, 15:42
  #328 (permalink)  
 
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Two dimensional wings are always in ground effect, because they have the flow pattern of a three dimensional wing with infinite wingspan.
No, they are not always in ground effect. In free air they have the flow pattern of a three dimensional wing with infinite wingspan in free air. Two dimensional wings placed near a ground board have different characteristics than when in free air.

No one is suggesting that studying the behaviour of 2D wings is going to help understanding of a 'real' wing, but looking at the flow on inboard sections of an Aspect Ratio 5 wing doesn't come into that category in my book.

The points to be made are (for a real AR 5 wing)
1. The upper surface flow is not much affected by ground proximity
2. The underwing flow develops steadily into a low velocity/ high pressure zone over most of the lower surface, from which comes the extra lift.

Wing tip vortices don't come into it - bound vorticity might be used to explain things, but I have learned that it doesn't pay to try to describe the flow to pilots in terms of bound vorticity!

Again I say they are meant to illustrate the sort of flow changes that occur near the ground. I assumed (and assume) that viewers would be able to interpolate for greater heights for themselves - it was anyway the only set of illustrations I could find at short notice.

In fairness I should also say that the illustrations were taken from a report discussing wings operating really close to the ground but, and I would emphasise this, the pressure distribution diagram and the general flow behaviour correspond closely to that observed on the AR 5 wing, which was tested at more realistic aircraft heights.

Last edited by Owain Glyndwr; 3rd Dec 2012 at 15:54.
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Old 3rd Dec 2012, 15:43
  #329 (permalink)  
 
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Let's keep it simple.

When downwash angle decreases due to ground effect, the total aerodynamic force vector is tilted forward.

Upwash only occurs at the leading edge.

but I have learned that it doesn't pay to try to describe the flow to pilots in
terms of bound vorticity!
Some of us understand it though.

Last edited by Lightning Mate; 3rd Dec 2012 at 15:44.
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Old 3rd Dec 2012, 15:59
  #330 (permalink)  
 
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"Upwash only occurs at the leading edge."

Yes, having originated from beneath it. Right?
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Old 3rd Dec 2012, 16:34
  #331 (permalink)  
 
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If you have less "downwash", I suggest you would have less lift, not more, from Newton's second law - less change of downward momentum to the air, less upward force on the wing.
That's where I got to also, but it is less than helpful isn't it

For lift: the speeding of the air behind the wing backwards, when close to the ground, as compared with at height, causes an increases in the flow rate over the wing, greater downwards mass flux, so a greater change of momentum and more lift.
I flirted with that idea, but the problem is that the measured data shows that neither the upper nor lower TE static pressures varies with height above ground at a given AoA. That means (using Bernouilli) that the velocities at the TE are not varying with height either, which sort of puts the mockers on it.
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Old 3rd Dec 2012, 16:41
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The reason that I included wake, and wake calculations was relevant. If the resultant cannot be accurately measured, it leads one to doubt the rest of the calculations, methodologies used in determination and validation of wing design, and understanding of principles.

Of the two major manufacturers, ones models the entire aircraft, while the other models wings and fuselage separately. Would one expect the same results?
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Old 3rd Dec 2012, 17:18
  #333 (permalink)  
 
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If the resultant cannot be accurately measured, it leads one to doubt the rest of the calculations, methodologies used in determination and validation of wing design, and understanding of principles.

Of the two major manufacturers, ones models the entire aircraft, while the other models wings and fuselage separately. Would one expect the same results?
With respect, I beg to differ. There is an enormous difference between measuring, let alone calculating, the flow well downstream of the wing with all the attendant uncertainties and random influences and designing a wing. I find it difficult to believe that the major manufacturers should be lacking in understanding of how their wings operate.

So far as the differences in their techniques are concerned, that is really a matter for them. At the end of the day it is their money that will be paid to the customer airlines if they don't meet their guarantees. There is in any case a lot more to the design of a wing than making theoretical calculations of its characteristics.
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Old 3rd Dec 2012, 17:38
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Snoop

Did Dubois, Robert and Bonin try to understand theory on lift with all the wrong stuff taught in flight schools when they were deepstalling ???
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Old 3rd Dec 2012, 17:59
  #335 (permalink)  
 
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Downwash/Flirting

Owain,

To the first quote: I'd reply that Ms 39's explanation that there's `more upwash' isn't right, as there is in fact more downwash, or else there would not be more lift.

To the second quote: I'd reply that when you flirted with the idea that an increased downwards mass flux, and a greater flow speed over the wing, I think that's absolutely fine: the pressure on the wing is not just static. As ground effect works for very slow gliders, any worries about thermal equilibrium can't be essential either. The boundary layer details must change, since Sir Isaac demands his pound of flesh for the increased rate of change of momentum, and the only way it can be delivered is via changing the integrated pressure normal to the surface over the wing section: the flow changes, the pressure changes. This can only happen if the flow above the wing speeds up by more than the flow below when the underground image wing comes close enough to modify the flow field.
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Old 3rd Dec 2012, 18:18
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Who is Ms 39? It wasn't me!
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Old 3rd Dec 2012, 18:23
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Upwash increases due the increasing pressure beneath, which is caused by restricted flow due to the 'funnel' created by the proximity of the solid ground.

It is a 'channel', and resists the entry of air that would otherwise flow beneath.

The same thing happens in freestream, but the below wing pressure is less there because the escape route is not bound by a second underwing slipstream, the "ground".

It is all about Rate of flow / velocity of wing. And flow restriction, which is ultimately the creator of all lift.

Altering the rate of flow creates the additional lift. But all these 'causes' are concurrent. At altitude, upwash is produced in the same fashion. Ground Effect demonstrates the complete theory, and Bernoulli does not explain lift, as said prior, he explains conservation of energy. Newton describes the mechanics of moving mass around to create a "plane".

Awblain, how can more upwash result in less downwash?

Owain.

The freebie from Ground Effect is the answer to the dilemma. Without increasing AoA, we get more lift at a constant velocity. Why? Because there is an increase of pressure underneath the wing.

The whole idea is to do just that, by deflecting and compressing it.

Last edited by Lyman; 3rd Dec 2012 at 19:07.
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Old 3rd Dec 2012, 18:40
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Lyman,

Lift is equal to the net rate of change in momentum of the air in the downwards direction as it flows by the wing.

On entering ground effect, `upwash' may increase in some places, presumably ahead of the wing, but `downwash' has to increase BY MORE in others, or else there would not be an increase in lift.

Newton deals with changes in momentum, Bernouilli with changes in energy. They are not redundant. Both must be satisfied.
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Old 3rd Dec 2012, 19:02
  #339 (permalink)  
 
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To the second quote: I'd reply that when you flirted with the idea that an increased downwards mass flux, and a greater flow speed over the wing, I think that's absolutely fine: the pressure on the wing is not just static.
But the static pressure on the wing surface does reflect the local velocity at that station outside the boundary layer does it not?

Sir Isaac demands his pound of flesh for the increased rate of change of momentum, and the only way it can be delivered is via changing the integrated pressure normal to the surface over the wing section: the flow changes, the pressure changes. This can only happen if the flow above the wing speeds up by more than the flow below when the underground image wing comes close enough to modify the flow field.
No problem with Sir Isaac and changes to the integrated pressures normal to the surface, but the measurements (NACA TN 1095 again) show that the pressure changes are such that the pressures on the upper surface are barely affected by the presence of ground whereas the pressures on the lower surface increase substantially in a manner that indicates that the flow over the lower surface is retarded. This is just the opposite of what you are suggesting here.
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Old 3rd Dec 2012, 19:37
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Owain,

I think your baulked flow beneath the wing picture, where the wing is within a fraction of a chord distance of the ground (Fig 4 of your diagram) is effectively describing a sort of `ram-air hovercraft', where there is an enhanced static pressure beneath the vehicle - plus a lot of extra drag, as you're slowing more mass down in the horizontal direction than you would a long way above the ground.

I would suggest that enhanced lift and reduced drag in what a glider would recognize as ground effect is a more subtle dynamic process where you slow air less horizontally, which allows you to move more air vertically, thus simultaneously increasing lift and reducing drag.

I agree completely that the surface pressure on the wing is related to the speed just outside the boundary layer, and any compression that's occurred. I'm just not sure the picture in the diagram is right to describe what I'm assuming about the flow around a wing in ground effect.
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