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Aircraft Ground Effect Part 2  
User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Posted (10 years 3 weeks 18 hours ago) and read 4061 times:

FYI, the first iteration of this post by exact title (aside from the part 2) was way back in October of 2001 by member MTOWBig grin


Question 1: Theoretically, if there was a practical way to elliminate wing tip vortices (or just block the circulation aspect), would the ground effect appear, or be, stronger?

And if I were on the ground and this plane flew (of whatever size and speed at an appropriate height) over me; will the ground effect appear or feel stronger?

Question 2: Does the force of ground effect in general include the volumetric displacement of the plane itself as a whole, or is it just wing lift-induced?


The meaning of life is curiosity; we were put on this planet to explore opportunities.
33 replies: All unread, showing first 25:
 
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 1, posted (10 years 3 weeks 17 hours ago) and read 3978 times:

If you could eliminate the wingtip vortices (infinite wing) there wouldn't be much ground effect at all.

You will always feel the entire weight of the aircraft, if only you are measuring on a large enough area, increasing with increasing altitude of the aircraft. This will not change with ground effect.

Q2: Eh?

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 2, posted (10 years 3 weeks 17 hours ago) and read 3968 times:

Doesn't the airplane take up volume? Like how a ship displaces water? Is that part of the ground effect, like a ship's wake?


The meaning of life is curiosity; we were put on this planet to explore opportunities.
User currently offlineQantasA332 From Australia, joined Dec 2003, 1500 posts, RR: 26
Reply 3, posted (10 years 3 weeks 15 hours ago) and read 3929 times:

As Fred said, no wingtip vortices means no ground effect. This is due to the simple fact that ground affect works by reducing wingtips vortices, and if there were none in the first place the ground would have no affect on the aircraft. That obviously wouldn't matter, though, because you'd be operating at low induced drag anyway.

As for your second question, Lephron, I'm not entirely sure. Supposedly volumetric displacement explanations of how ground affect works are simply "hangar lore," and I do think that this is the case and displacement plays no part, but as I said I'm not 100% sure.

Cheers,
QantasA332


User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 4, posted (10 years 3 weeks 13 hours ago) and read 3868 times:

Does it matter, infinite wing or infinite wing fence, for the purposes of imagining "zero wingtip vortex"? Even forward swept wings have no wingtip flow, cuz they go inwards towards the root and spanwise flow cancels.

************

If there were no wingtip vortices, then it would mean a permanent low pressure exists on top and a permanent higher pressure exists below, I would think that lower energy needs to go somewhere if not allowed to go up and over the tip. I assumed it would go down.

The later version would be if a city bus went by me I'd get tossed back due to the air flowing around the bus, which was why I asked about the effect of volumetric displacement. Displacement flow should go all around. The bus takes up space, and the reaction of air moving away from the front is evident by the wake in the rear. Action-reaction principle.

Aren't the vortices from the wing of a plane basically the wake reaction due to trying to get into the top but can't? You'd have to imagine the wing going downwards like a flat piston, like the bus example. Except the bus experiences a lift vector back, hence drag. In any event, if the flow was prevented from moving over the wingtip, where would it go? The energy has to go somewhere, unless all of it is translated into lift.

Even still, if this perfect plane flew over you, there would be enough airflow going around the plane (i.e. supposid volumetric displacement) such that you briefly loose your balance; like jetblast but cold and less forceful.

For final clairification, would this mysterious side-reacting flow or whatever be part of ground effect only if it comes near the ground?



The meaning of life is curiosity; we were put on this planet to explore opportunities.
User currently offlineVikkyvik From United States of America, joined Jul 2003, 9401 posts, RR: 27
Reply 5, posted (10 years 3 weeks 11 hours ago) and read 3822 times:
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I think you're talking about two different things...

The wake of an aircraft is caused by the drag - the airflow is slowed down in the vicinity of the airplane (assuming the airplane is fixed, and the air is moving past it). This also causes turbulent flow in the air.

The vortices are caused by the low pressure on top/high pressure on the bottom of the wing. This causes the additional induced drag. Ground effect reduces the energy of the vortices, reducing drag.

Ground effect doesn't have to do with the friction drag of the airplane as such. Yes, I guess technically the vortices are part of the wake, but that's separate from the friction wake.

I would assume that given an infinite wing, all of the energy is translated into lift (negating thermal losses). I can't remember, but perhaps this would mean that the wing has an efficiency factor of 1.

~Vik



"Two and a Half Men" was filmed in front of a live ostrich.
User currently offlineMITaero From United States of America, joined Jul 2003, 497 posts, RR: 8
Reply 6, posted (10 years 3 weeks 8 hours ago) and read 3789 times:

Aaaah I am so busy but want to explain this one. Maybe I can get to it in the next few days.

In case I don't get to it later - ground effect is all about how satisfying flow tangency at the ground (this is a physical must) changes downwash on the wing.


User currently offlineQantasA332 From Australia, joined Dec 2003, 1500 posts, RR: 26
Reply 7, posted (10 years 3 weeks 3 hours ago) and read 3723 times:

Hmm, now I'm getting confused; I don't think we're all talking about exactly the same things.

At any rate...

-- I agree with Vikkyvik regarding energy "usage" in the absence of wingtip vortices. There's really nothing else that that energy would go towards, other than lift and a bit of thermal "discharge". Thus, I think you have quite an efficient wing in that case, with all the energy from pressure differential going towards lift, as I said. The energy doesn't really need to "go anywhere", as you suggested, Lehpron.

-- If I'm interpreting the other thing you're asking about correctly, you want to know what role displaced (or "sandwiched") air between the aircraft and the ground plays in ground effect. If this is indeed what you want to know, you're on the wrong track. All ground effect has to do with is the blocking of wingtip vortices and accompanying reduction of induced drag. The reason behind aircraft floating under the effect is simply the reduction of drag and resulting increase in speed and lift, and not sandwiched/displaced air. (There's actually another less major reason which I'll add if I have time, but I'm in a rush so I'll leave it out for now). I'm probably misinterpreting your question completely, so some clarification might clue me in. Otherwise, I'll wait and let MITaero chime in...

Cheers,
QantasA332

Edit: some typos...

[Edited 2004-04-01 10:51:01]

User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 8, posted (10 years 2 weeks 6 days 23 hours ago) and read 3678 times:

Many people seem to fall to the misconception that ground effect is a cushion of air being trapped between the wing and the ground.

It does happen, but while ground effect is noticeable within half a span or so from the ground, the latter effect will only be significant within half a chord or so, IIRC.

Ground effect is one of the things of which I have to admit that my knowledge is seriously in need of some improving though, so I'm waiting for MIT to fill in.

MIT, do you have any good books which cover it in detail? And another question I've been wanting to ask you, have you found any CFD programs in the range betweeen affordable and free? Only 2D strictly necessary. I miss having access to FEMLAB at school...

There's MOUSE, but I never seem to find time to get a Linux system up and running again...

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineKlaus From Germany, joined Jul 2001, 21353 posts, RR: 54
Reply 9, posted (10 years 2 weeks 6 days 21 hours ago) and read 3654 times:

How are the vortices blocked in ground effect? I can´t see how that would work...

The "sandwiched air" explanation doesn´t seem that implausible to me, since when you´re looking at the more extreme cases (very narrow gap) it´s still getting more efficient in producing lift as far as I know and I don´t see how that´s just due to wing tip vortex reduction....


User currently offlineFlightSimFreak From United States of America, joined Oct 2000, 720 posts, RR: 0
Reply 10, posted (10 years 2 weeks 6 days 19 hours ago) and read 3632 times:

The best way to explain ground effect is to first explain induced drag: Lift works perpendicular to the average relative wind. Basically the wind coming straight on, and the wind leaving the wing at a downward angle. The higher the angle of attack, more "downward" directed flow off the wing. So if you are at a slow speed or a high angle of attack, the average relative wind is at an angle to the movement forward, thus some lift is directed backwards. This backwards component of lift acts against thrust, and is what we call induced drag.

Now on to ground effect. When a plane is close to the ground, the flow after it leaves the wing cannot go downward, so it is more parallel to the wing, thus the average relative wind is more parallel to the wing, thus there is less of a backwards component of lift, thus there is less induced drag. It has nothing to do with tip vortices.

I hope I'm understandable, if there are any questions, feel free to email me, and I will send you a diagram.


User currently offlineMITaero From United States of America, joined Jul 2003, 497 posts, RR: 8
Reply 11, posted (10 years 2 weeks 6 days 18 hours ago) and read 3620 times:

FlightSimFreak - that's not all right, but I'm still busy and don't have time to reply properly  Smile You're right that the flow is more parallel to the wing, but it has everything to do with vortices (why do you think there's downwash in the first place?). In fact, a better way to explain induced drag is energy lost to rotation of air behind the aircraft (google 'Trefftz plane' or something). The lift vector rotation argument is also valid.

User currently offlineVikkyvik From United States of America, joined Jul 2003, 9401 posts, RR: 27
Reply 12, posted (10 years 2 weeks 6 days 16 hours ago) and read 3601 times:
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Induced drag has everything to do with tip vortices. As MIT stated, there is energy being put into the vortex flow. And also, regarding the air being turned downwards, tip vortices are what cause the downwash. So either argument you use, the vortices are the defining factor.

Now it is also true that wings are set at some incidence angle relative to the fuse, usually, so it is possible that the lift vector will point slightly backwards. This is not what I learned as induced drag - I thought maybe this was pressure drag? I'm not sure though, corrections welcome.
~Vik



"Two and a Half Men" was filmed in front of a live ostrich.
User currently offlineMITaero From United States of America, joined Jul 2003, 497 posts, RR: 8
Reply 13, posted (10 years 2 weeks 6 days 16 hours ago) and read 3592 times:

FredT - Sorry, I just use the programs we have on campus here, I'm no expert on CFD (yet)  Smile Also, I believe a book by Katz and Plotkin called _Low Speed Aerodynamics_ has a nice explanation of ground effect (mathematical), but most of the things I'm saying come from lecture notes. I'll try to get more info to you later.

Vik - I think the fuselage/wing incidence angle is different from the definition of lift. Aerodynamicists usually talk about lift as if the wing is all that matters (ignore the fuselage), while dynamics people treat the aircraft as a dot and look at the upward force. The lift vector tilting due to downwash change is indeed a way to think about induced drag.


User currently offlineQantasA332 From Australia, joined Dec 2003, 1500 posts, RR: 26
Reply 14, posted (10 years 2 weeks 6 days 15 hours ago) and read 3586 times:

How are the vortices blocked in ground effect? I can´t see how that would work... The "sandwiched air" explanation doesn´t seem that implausible to me, since when you´re looking at the more extreme cases (very narrow gap) it´s still getting more efficient in producing lift as far as I know and I don´t see how that´s just due to wing tip vortex reduction....

Wingtip vortices are blocked simply due to the fact that when the aircraft is near the ground, there's really no room for them (i.e. the vortices) to complete full rotations. They're simply blocked from fully forming, and this in turn reduces induced drag for obvious reasons. And of course, that leads to the principle that ground effect is "stronger" when you're nearer the ground, because there's even less room for those vortices to form...

As for the second part of your question, Klaus, "air cushions" just don't play a part in ground effect, and it's a common misconception that they do, as Fred said. Ground effect is simply a "feature" of low flying, whereby induced drag is reduced. Period. Sure there's a slight "air cushion" at altitudes extremely close to the ground (1/2 chord, according to Fred), but as I said above and before (...and I can't stress this enough!), ground effect is simply the reduction of induced drag at very low altitudes due to the ground blocking wingtips vortices from forming strongly -- or forming at all, in very extreme cases. "Air Cushions" just don't have to do with it (i.e. ground effect) except at very low altitudes, and any "floating" of an aircraft in the effect is due to the reduction of drag and corresponding increase of speed and lift. Sorry to be repeating a lot of this, but it's quite important...

Lift works perpendicular to the average relative wind. Basically the wind coming straight on, and the wind leaving the wing at a downward angle. The higher the angle of attack, more "downward" directed flow off the wing. So if you are at a slow speed or a high angle of attack, the average relative wind is at an angle to the movement forward, thus some lift is directed backwards. This backwards component of lift acts against thrust, and is what we call induced drag.

Induced drag has all to do with wingtip vortices and nothing to do with air "deflection" off the wing, as MITaero and Vikkyvik said; vortices cause downwash, "deflection" doesn't. Induced drag is solely from wingtip vortices. I cannot stress that enough!

Cheers,
QantasA332


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 15, posted (10 years 2 weeks 6 days 2 hours ago) and read 3539 times:

While the explanation of induced lift citing the change in the flow direction due to downwash is mentioned in many respected (in general as well as by myself) books, I still do not like it. Beware, I do not say it is incorrect. I just don’t think it is a great mental model. For starters, it (typically) fails to mention the upwash, which will do the same thing but tilt the lift vector forward. Yes, the contribution is smaller than the contribution from downwash, but... why? Needs to be adressed. Secondly, it tends to get people confused as to where the lift is pointing when talking about the entire aircraft, namely perpendicular to the freestream. Beginning to talk about drag as being in part lift is muddling the waters.

Neglecting shear stress, the local aerodynamic force exerted by the air pressure on a surface panel will always be perpendicular to the panel. It has to be. That will indeed be perpendicular to the local airflow, as the air cannot go into or out of the panel. That the sum of these forces will be canted backwards as the total lift (by the normal definition, i e the aerodynamic force perpendicular to the freestream) increases is not in any way a given when you think about it. It does... but the why is still largely unanswered IMO.

Now, to come up with a better mental model... *grin* It is always easier to complain, isn’t it? But talking about it is likely a good start!

And as I said, I have to do more reading. The stack of books is currently larger than the time available though! Check out www.hfa.se and you’ll know why... Big grin

Cheers,
Fred

P.S. Happy contrails, all!



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineIkarus From United Kingdom, joined Jan 2001, 3524 posts, RR: 2
Reply 16, posted (10 years 2 weeks 5 days 22 hours ago) and read 3506 times:

Q1:
No finite wing could eliminate trailing vorticity.
No wing can produce lift without inducing circulation into the air. A wing acts as vortex.

Where does the ground effect come from? As stated above, it's the flow tangency at ground criterion: Imagine a vortex in the air, above the ground. On one side, it pushes air down, on the other, it sucks it up. Now the air cannot pass through the ground (zero velocity normal to ground plane). Therefore, to mathematically model the flow, you mirror the vortex along the ground plane, so that there is an equal and opposite vortex buried deep inside the ground (imagine the plane not landing on a runway, but on another plane approaching it from underneath:
View Large View Medium
Click here for bigger photo!

Photo © Sam Chui
View Large View Medium
Click here for bigger photo!

Photo © Joan Martorell

.

The result will be tangential airflow on the ground plane. Now the effect of that mirror vortex can be used to calculate the ground effect on the wing (essentially, it ends up being a slight modification of the effective angle of attack, as far as I remember).

Place a vortex near a ground, and you get a "ground effect".

A wing produces lift by acting as vortex. Therefore the wing itself causes most of the ground effect, even without trailing vortices.

Vortices can only end in walls, or go into infinity, or form a circle. Any wing not fixed to the walls of a wind tunnel trails the vortices behind. That can take the shape of very localized wingtip vortices (see the 757), or the shape of a widely spread sheet of trailed vorticity. But on the whole, trailing vortices cannot be eliminated.

Regards

Ikarus



User currently offlineMITaero From United States of America, joined Jul 2003, 497 posts, RR: 8
Reply 17, posted (10 years 2 weeks 5 days 15 hours ago) and read 3484 times:

Ikarus said most of what I wanted to say earlier. This following info requires some knowledge of aerodynamics (vortices, Biot-Savard law, lifting line), I'll try to make it easy to understand.

It's not tip vortices that are mirrored to model ground effect, but instead the vortex that causes lift, or the one used in the Kutta-Joukowski theorem (along the "bound" part of the lifting line vortices).

The ground (mirrored) vortex, with the same distance to the ground plane and the same (but opposite) strength as the wing vortex, ensures flow tangency at the ground plane. If the resulting flow, incorporating both vortices, is analyzed, less downwash is induced on the wing.

Note - the ground vortex also slightly decreases the velocity seen by the wing.


User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 18, posted (10 years 2 weeks 2 days 16 hours ago) and read 3398 times:

okay, i'm going to try to convey my thoughts via pictures, not that any of you folks have got the wrong idea, just for clairification:



the top one is supposed to represent circulation theory.

The purple lines are supposed to represent a body flow away from the fuselage that my be negligible but still happens.

the next two are my guesses at ground effect; which depends, i do not know about the inboard induced flow, does it actually bother to travel all the way to the end or does just the difference in pressure nearest to the tip bother to spill over?

In the mid pix, would that not represent a path of skewed circulation? Now that I look at it more, it looks like it could have a heck of a lot of sidewash, like if you were standing there it would knock you right down.

In the last pix, would that not represent a downwash reflection of the circulation?

In terms of zero vortex, image that the tip flow was cut off, wouldn't that mean that the circulation would simply go straight down by then? Again, if and only if a zero vortex occured there.

Ikarus: >> "Q1: No finite wing could eliminate trailing vorticity. " <<

I could prove you absolutely wrong with a simple phrase: "sonic waverider and compression lift". consider that the shock wave closes in on the leading edge and prevents the induce flow from spilling over -- hence elliminating the trailing vortex -- providing lots of lift that would normally be lost due to vortices.

[Edited 2004-04-05 22:55:22]


The meaning of life is curiosity; we were put on this planet to explore opportunities.
User currently offlineMITaero From United States of America, joined Jul 2003, 497 posts, RR: 8
Reply 19, posted (10 years 2 weeks 2 days 10 hours ago) and read 3363 times:

L, there are a lot of misconceptions in your above post.. no big deal, when I get a break from my $^%@$@# work I'll explain  Smile

User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 20, posted (10 years 2 weeks 2 days 9 hours ago) and read 3360 times:

Indeed, if that were the case, I don't mean to be misconceptual, usually I take the layman's apprach to things and give the impression of dumbness to folks like yourself. Big grin

Think of the layfolks' reaction to center of pressure: "pressure it not central, it is distributed."  Big thumbs up

[Edited 2004-04-06 06:07:28]


The meaning of life is curiosity; we were put on this planet to explore opportunities.
User currently offlineKlaus From Germany, joined Jul 2001, 21353 posts, RR: 54
Reply 21, posted (10 years 2 weeks 1 day 12 hours ago) and read 3281 times:

My problem with the vortex blocking explanation mainly has to do with the vortices being blocked after separation from the wing.

I´m aware that feedback will still be possible as long as we´re dealing with subsonic speeds (especially obvious in near-landing or takeoff situations anyway), I just assumed the blocking of the vortices would mainly result in thermal energy conversion or other effects outside of (behind) the airplane´s reference system without any possibility of recovering the energy lost to the already detached vortices...

An interaction between the high-pressure area below the wing and the approaching ground seemed more plausible because the rigid ground plane would obviously increase the pressure further relative to free and uncompressed air below.

Just trying to get a plausible image, if possible.


User currently offlineMITaero From United States of America, joined Jul 2003, 497 posts, RR: 8
Reply 22, posted (10 years 2 weeks 1 day 11 hours ago) and read 3273 times:

I'm not sure what 'vortex blocking' is. The point is that the ground alters the entire flow solution. Don't think of this as if "after separation from the wing" matters - ground effect is a steady-state phenomenon.

In other words, having the ground there ensures that velocity along the ground can only be tangent to it (no flow through the ground), and this changes the entire flow. (That's why L's images don't really make sense.) It just so happens that the way in which the ground affects the entire flow is to reduce downwash at the wing.

Again, hope to add more later.


User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 23, posted (10 years 2 weeks 1 day 4 hours ago) and read 3248 times:

"That's why L's images don't really make sense.) "

You do know those views were from the front, right?  Wink/being sarcastic

Seriously though, what doesn't make sense? The real situation is either one of those bottom two (IMO, it has to be the third and the flow that goes down, trails) how else can the plane "float" in that effect? Basically it gets low enough to reflect it's own weight into the ground, allowing for less speed during landing and T/O as if it had more wing area. The flow underneath is already slowing and increasing in pressure which super-pronounces the lift coeff. Had the vortices been removed, the reaction of double-pressure (due to effect of the ground and wing) means the plane can literally float on itself with even greater efficiency -- exactly more than double -- provided it had enough thrust to keep it going and not slow down.

***************************************

The fact is that extra force is generated due to moving close to the ground. It may be equal the weight of the plane or the area of the planform (not just the wing) mutiplied by the dynamic force due to motion. I wonder if the pressure due to a plane flying in ground effect over the ground is measureable; if it ends up being soft then it is dynamically dependent more so than mass. Or vice-versa or both, I donno.



The meaning of life is curiosity; we were put on this planet to explore opportunities.
User currently offlineMITaero From United States of America, joined Jul 2003, 497 posts, RR: 8
Reply 24, posted (10 years 2 weeks 21 hours ago) and read 3225 times:

>Seriously though, what doesn't make sense?

You took two 2-d point vortices and drew them on the wingtips, then BS-ed the rest of the flow.  Smile


25 Post contains images Lehpron : They aren't BS-ed, they are guesses and I am not insisting they are correct either. In fact, the point of this whole thread is that I'd like to know w
26 MITaero : First of all, your first pic doesn't represent the actual flow in normal flight. If those are supposed to be streamlines, then you've assumed a single
27 Post contains images Lehpron : >> "Just because you only see tip vortices on aircraft doesn't mean that there are just two vortex filaments trailing from either wingtip"
28 Post contains links MITaero : Hey Lehpron, Downwash is just downward velocity at the wing caused by lift. Here's one site that explains it in some detail, but this is just the firs
29 Post contains links and images Lehpron : Lookie what I found @ popular science.com: http://www.popsci.com/popsci/aviation/article/0,12543,410266-3,00.html I guess my ground-effect guesses wer
30 MITaero : That is incorrect. It's a hand-wavy explanation to give people a general understanding of ground effect. The article is not meant to be scientificiall
31 QantasA332 : As MITaero said, Popular Science is incorrect. Unfortunately, they often water a lot of things down for understandibility, obviously at the expense of
32 AUAE : Is it easier to just think of ground effect as a more efficient means of producing lift? That is all it really amounts to. I must have some better pic
33 Lehpron : Alright, I yield. I had the bias entering that I still thought that ground effect is just wings reflecting their lift by reducing vortex flow. I still
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