Hazlenuts

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
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 -

http://i1081.photobucket.com/albums/...Ecomposite.jpg

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.

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.

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.

There I'm afraid, you are just plain wrong - there is ample evidence that two dimensional wings exhibit ground effect characteristics.

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.

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?

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?

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.

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.

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.

Some of us understand it though.

"Upwash only occurs at the leading edge."

Yes, having originated from beneath it. Right?

That's where I got to also, but it is less than helpful isn't it :D

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.

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?

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.

Did Dubois, Robert and Bonin try to understand theory on lift with all the wrong stuff taught in flight schools when they were deepstalling ??? :{

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.

Who is Ms 39? It wasn't me!

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.

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.

But the static pressure on the wing surface does reflect the local velocity at that station outside the boundary layer does it not?

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.

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.

Yes, I would agree with that for that particular picture. As I said, my hope was that people would interpolate for greater heights, although that of course depends to some extent on there being some linearity.

My comments really were based on the data of that NACA TN, but now I come to look at that more closely I see that also was biased towards quite low ground heights. The trend to increased pressures under the wing is still there though even at heights of around 30% span which is reasonable for a modern design close to touchdown. It is MUCH less obvious however and, again looking more closely, I see there is some increase in LE suction peak in this condition.

That may be enough to suggest that for what most would regard as 'normal' the ground effect can be simply regarded as due to increased circulation (arising from the 'mirror image' wing) which adds to the lower surface flow slowing down from the main wing circulation, and must also result in increased circulation around the main wing.

That would satisfy me - now back to Newton :ugh:

This is the established and accepted resultant...


does that fit within that diagrams parameters?

In CFD, would this look the same as a flat bottomed boat with the same side angles as an aircraft wing bottom profile?

Would one assume flap setting affect this calculation?

On final, when wheels are down, would the couple between the cores be influenced?

If the aircraft on final encounters a crosswind, the unbalanced diagram would produce what sort of result?

All questions that need to be answered, but how?

( I would note that images of ac on final can be misleading as the rate of descent of the ac, are typically close to the rollup rate of descent)

http://www.nature.com/nature/journal...5721a-f1.2.jpg

Cheers to evolution!

That is why I'm still convinced that the downwash behind the wing leads to a 'choking' of the mass stream underneath the wing as it displaces the streamlines normally exiting along the lower surface downward toward the ground which themselves are displacing other streamlines.

As the ground puts a fixed barrier to this mutual displacement, the effective freestream area behind the TE is reduced.
Assuming mass flow = free stream speed x effective free stream area this will reduce mass flow and thus velocity underneath the wing, leading to increased static pressure on the lower side.
Sorry for defending Mr. Bernoulli again. :O

Does he need defending? After all, he's not responsible for the misconceptions based on wrong applications of his equation!

Lift=Money

HN39,,
Good form!

Thank you for that drawing from Leonardo da Vinci !
He did not chose to draw streamlines, nor streamtubes, only vortexes :)

The airplane in level steady flight knocks the air into many "pieces", and it is not willingly the means of separation; here is drag. And Friction, and 'resisted', and viscosity, and compression. These nouns describe what is intuitive into the manufacture of what they call 'chaos'. With some forgiveness for the artist, who owns no wind tunnel......

But wait, he is watching the natural tunnel. The boundaries of the tunnel, the visible limits, are described by the shapes so important to the one who is 'visual' in his genius, the Italian.

The artist who cannot see boundaries cannot see shape. Some of us demand precision that is not available, and request of the intuitive that assumptions be made. It is then organized to further 'knowledge'.

The artist is satisfied with space and its inhabitants. The observer demands more, and makes up a personal 'language', a 'jargon'.

In jargon are rules, and code. From chaos, the artist sees order. The inventor makes the order perform, and the circle is complete.

The wing collects the workers, and pushes them away. In pushing, he is raised, and supported.

If I block your exit, will you take another?

Please do not block my exit; I like to keep my aperture open at all times thank you.

ABSOLUTELY!
 

F=Ma

Force = Mass x acelleration


That's the forumla. The speed changes, the pressure changes (all over the wing)-
some higher (leading edge) some lower( aft of that). Cause or effect.
The basic principle is that you must acellerate a mass of air (change its direction and/or velocity. The prop, the wing, the rotor, the jet they all do the same thing. Some do a small mass of air with a big acelleration, some to a big mass of air with a smaller acelleration. Diagrams like the rotating cylinder do nobody any good. That was apparently drawn by somebody who didn't get it himself, and had a hard time explaining it to anyone else.

From the NTSB Report on the G650 flight test takeoff accident at Roswell, NM on April 2, 2011:

Couple of aircraft here illustrating Flaps and Vortex generation...

I didn't see this posted but its very good:

Video: How aeroplanes' wings really work - Telegraph

junebug...interesting video, although the airfoil they used is in stall.

A wind tunnel does not model the compression of the air below the wing due to the weight/force of the aircraft 'planing' through the media.....

Against my better judgement.....

If the air is moving less than .3 mach relative to the airfoil there is no compression.

If there is anything more than an AoA of 0, then there is compression.

versus



http://youtu.be/BY1k1GcZRww

...sorry, I couldn't resist! :)

Technically speaking you are obviously correct. As soon as the static pressure on a medium increases there will be compression (That's true not only for a gaseous medium, even your wood furniture will be compressed if a Battle Tank accidentally runs over it :E). The only question is: Is it big enough that it's relevant. For air this effect is usually neglected up to M0,8 - 0,9.

But
this I don't quite understand.
Why wouldn't the wind tunnel reproduce this effect?

It may have been posted previously (and I missed it) but, just in case it hasn't been ... the article by Arvel Gentry (referred to on p2 - post 33 by Owain Glyndwr) can be found as the final link in Articles - Página web de arvelgentry

Gentry died last year and his original webpage has disappeared.

Thank you John and Owain.
An interesting reference about creation of vortices is the 2016 Nobel Prize of Physic too, despite it matters about topological phase transition and quantic theory.
And don't forget Libchaber vortices .…
What a beautiful science, that theory on lift !:)
roulishollandais

"Scientific Background on the Nobel Prize in Physics 2016
TOPOLOGICAL PHASE TRANSITIONS AND TOPOLOGICAL PHASES OF MATTER
compiled by the Class for Physics of the Royal Swedish Academy of Sciences"