Rear Wing Idea

[variante]

trinidefender, i'm still not convinced about that definition, at all. I've always been taught differently about it, and i suspect that i could confirm my version, for example, making other paradoxical examples. But, instead, i'll leave it there and look for a reliable source giving that definition, since it seems like we're discussing exclusively about a definition (unfortunately. Because there are more interesting and concrete points to discuss, such as those of my simulations).

[trinidefender]

trinidefender, i'm still not convinced about that definition, at all. I've always been taught differently about it, and i suspect that i could confirm my version, for example, making other paradoxical examples. But, instead, i'll leave it there and look for a reliable source giving that definition, since it seems like we're discussing exclusively about a definition (unfortunately. Because there are more interesting and concrete points to discuss, such as those of my simulations).

Well then what have you been taught? Can you provide sources? That is why I have provided about 4 or so different sources that confirm what I know to be induced drag. I don't intend to be malicious or anything, I am just curious as to where you got your definitions.

2nd'ly I still stand behind the statement that the vortex at the end of a wing will always be accompanied by drag. Therefore if you reduce the size and strength of the vortex, you reduce the drag penalty.

Do you agree or disagree with my second statement?

[variante]

Well then what have you been taught?

What i've been saying the whole time, that induced drag is the drag caused by the downwash that occurs near the wingtips, which is in turn caused by wingtip vortices.
What i'm adding to this definition is that some airfoils are "immune" to any drag increase despite being subject to vortices' downwash.

Can you provide sources?

For the main definition i could say "my aerospace professor", but i'd like to find an indisputable source available for anyone.
For what i've added, i am the source as well as my CFD simulations, i believe.

2nd'ly I still stand behind the statement that the vortex at the end of a wing will always be accompanied by drag. Therefore if you reduce the size and strength of the vortex, you reduce the drag penalty.

Do you agree or disagree with my second statement?

Agree if we talk about common airfoils, disagree if we talk about the kind of airfoils of my simulations.

[hollus]

What i'm adding to this definition is that some airfoils are "immune" to any drag increase despite being subject to vortices' downwash.

But if you look at your own numbers, what your zero induced drag wing has done is reduce the lift a lot, and hence the drag a lot. Less work on the air -> less vortex -> less wasted work. But in the finite wing, the vortex is still there, just very small. And the lift to drag ratio still suffers for it (3:1 goes to 2:1) compared to the infinite wing, just like in the curved airfoil case (~85:1 goes to ~25:1).

[Blanchimont]

Some already wrote about your results, but anyway here are my thoughts.

The conventional airfoil seems to be quite good in terms of efficiency, 3010/35 = 86. Your "airfoil", i would rather call it a blunt body, on the other side produces almost 209/35 = 6 times the drag but only 608/3010 = 20,2% of the lift with an efficiency of 2,9. The drag is probably mostly a result of the "trailing edge" of your profile.

If you look at the colour intensity at the upper wing surface for the infinite wings, you'll see the explanation for the differences in lift. The conventional foil shows lower pressure compared to your airfoil, the blue is really intense for the conventional one and barely visible for the other. The streamlines show no deviation in spanwise direction, the only move in a longitudinal vertical plane, as one would expect for an infinite wing where no spanwise flow can occur. The vertical displacement is notable for both wings, but larger for the conventional foil.

If the span is finite as in our second simulations, the difference between the higher pressure at the top and the lower pressure at the lower surface defines both the lift and the magnitude of the now present wing tip vortex. Because the conventional airfoil produces higher pressure differences/more lift, the vortex is bigger. When you look at the streamlines, you'll now also see a displacement in spanwise direction and the pressure at the wing tips is also lower compared with the infinite simulation.

I don't think that your simulations show that your airfoil is immune to induced drag, it is just poor airfoil IMO. A small pressure difference leads to a smaller vortex and for me this is now sign of good quality of airfoil regarding induced drag.


What about adding simple endplates to the finite wings and analyse how much the vortex can be reduced by that and how close to the infinite case the resuts are, if you've got the time for it?

[variante]

The conventional airfoil seems to be quite good in terms of efficiency, 3010/35 = 86. Your "airfoil", i would rather call it a blunt body, on the other side produces almost 209/35 = 6 times the drag but only 608/3010 = 20,2% of the lift with an efficiency of 2,9. The drag is probably mostly a result of the "trailing edge" of your profile.

That's right, the performances of that “trapezoid” are awful. But its only purpose was to show how it behaves with induced drag. Also, I don't exclude that its efficiency can be doubled with minimum revisions.

I don't think that your simulations show that your airfoil is immune to induced drag, it is just poor airfoil IMO. A small pressure difference leads to a smaller vortex and for me this is now sign of good quality of airfoil regarding induced drag.

Yes and, in particular, in the trapezoid case we can observe multiple trailing edge vortices interfering with each other and merging into low energy vortex.
However that doesn't mean that the trapezoid suffers, drag wise, from the presence of that vortex (whatever the energy of that vortex is). In other words, from those last simulations we cannot say with certainty whether that wing suffers from induced drag or not.

In order to reach a definitive conclusion, thus isolate exlusively the phenomenon of induced drag, we should simulate only the effect of induced drag on a wing. How to do that? Simulating the downwash (the cause of induced drag...even though trinidefender doesn't agree about that) on the airfoil. That is what i did in the first simulation i provided.

In case we choose to ignore simulations, we can use logics: the sources of drag of the trapezoid foil are the oblique side and the flat trailing edge. The downwash of the vortex makes the oblique side approach the airflow with lower relative AoA, thus building up less pressure and producing less drag. The very same downwash reduced the wake of the trailing edge with a similar mechanism.

The net result is less drag (and also less lift, unfortunately) when the downwash interacts with the trapezoid.

[DaveW]

Forgive me for butting in, but on the subject of induced drag this might just be of interest.

[trinidefender]

The conventional airfoil seems to be quite good in terms of efficiency, 3010/35 = 86. Your "airfoil", i would rather call it a blunt body, on the other side produces almost 209/35 = 6 times the drag but only 608/3010 = 20,2% of the lift with an efficiency of 2,9. The drag is probably mostly a result of the "trailing edge" of your profile.

That's right, the performances of that “trapezoid” are awful. But its only purpose was to show how it behaves with induced drag. Also, I don't exclude that its efficiency can be doubled with minimum revisions.

I don't think that your simulations show that your airfoil is immune to induced drag, it is just poor airfoil IMO. A small pressure difference leads to a smaller vortex and for me this is now sign of good quality of airfoil regarding induced drag.

Yes and, in particular, in the trapezoid case we can observe multiple trailing edge vortices interfering with each other and merging into low energy vortex.
However that doesn't mean that the trapezoid suffers, drag wise, from the presence of that vortex (whatever the energy of that vortex is). In other words, from those last simulations we cannot say with certainty whether that wing suffers from induced drag or not.

In order to reach a definitive conclusion, thus isolate exlusively the phenomenon of induced drag, we should simulate only the effect of induced drag on a wing. How to do that? Simulating the downwash (the cause of induced drag...even though trinidefender doesn't agree about that) on the airfoil. That is what i did in the first simulation i provided.

In case we choose to ignore simulations, we can use logics: the sources of drag of the trapezoid foil are the oblique side and the flat trailing edge. The downwash of the vortex makes the oblique side approach the airflow with lower relative AoA, thus building up less pressure and producing less drag. The very same downwash reduced the wake of the trailing edge with a similar mechanism.

The net result is less drag (and also less lift, unfortunately) when the downwash interacts with the trapezoid.

Right there is where you misunderstand me. I never said I disagree that downwash contributes to induced drag. You are twisting my words to give new meanings and I don't know if it is intentional but it is highly annoying. What I keep saying is "induced drag is drag created in the production of lift." Downwash contributes to the production of lift, therefore it is a part of induced drag.

However earlier you stated that the vortex at wingtip contributes to the downwash. That is the problem it does not. It actually reduces the local angle of attack of airflow on the outer portion of a wing. Therefore with the local angle of attack reduction that portion of the wing (the part close to the wingtip) produces less downwash and less lift.

That is exactly what I was talking about making assumptions in your CFD simulations. It appears that you are assuming that all the induced drag comes from downwash and at the same time assuming that the vortex at a wingtip INCREASES the downwash when in actuality it DECREASES the downwash produced by a wing.

You say "whatever the energy of that vortex is." While seeming to completely ignore that energy has to be put in to create those vortices. The energy has to come from somewhere and that somewhere is in the creation of drag.

[turbof1]

I readed through this conversation (may I mind to ask to keep it a bit more friendly?). It looks a bit like you guys more or less are basicilly telling the same thing, with a bit confusion over the right terminology (form drag vs. induced drag) and from different perspectives.

I also readed somewhere that the vortices create drag. Although a very accepted statement and used everywhere in the aero world, the vortex is merely a manifestation. The vortex itself isn't causing the drag, but the source that causes it (the collapsing of high pressure flow into low pressure flow at a wingtip). You always will reduce the strength of the vortex when reducing drag at that wingtip, so hence why everybody says that reducing the vortex will reduce the drag.

Hence why I also fail to see how simulating an infinite aerofoil proves anything (sorry variante, the effort is very much appreciated, but I think you got stuck in the wrong mindset). Yes you can try to simulate a vortex above it, but again: it's the collapsing that creates the drag, and not the vortex itself.

[trinidefender]

I readed through this conversation (may I mind to ask to keep it a bit more friendly?). It looks a bit like you guys more or less are basicilly telling the same thing, with a bit confusion over the right terminology (form drag vs. induced drag) and from different perspectives.

I also readed somewhere that the vortices create drag. Although a very accepted statement and used everywhere in the aero world, the vortex is merely a manifestation. The vortex itself isn't causing the drag, but the source that causes it (the collapsing of high pressure flow into low pressure flow at a wingtip). You always will reduce the strength of the vortex when reducing drag at that wingtip, so hence why everybody says that reducing the vortex will reduce the drag.

Hence why I also fail to see how simulating an infinite aerofoil proves anything (sorry variante, the effort is very much appreciated, but I think you got stuck in the wrong mindset). Yes you can try to simulate a vortex above it, but again: it's the collapsing that creates the drag, and not the vortex itself.

While your second paragraph is technically correct it is a little nitpicking. The vortex isn't drag. The energy put in to create the vortex, I.e. creating the spanwise flow around the wingtip and hence the airflow collapsing in on itself, manifests itself as drag which is what I stated previously.

[turbof1]

While your second paragraph is technically correct it is a little nitpicking. The vortex isn't drag. The energy put in to create the vortex, I.e. creating the spanwise flow around the wingtip and hence the airflow collapsing in on itself, manifests itself as drag which is what I stated previously.

It's nitpicking yes, but I feel the conversation is decided by such things.

It's true that an infinite aspect ratio reduced induced drag. Not counting in wingtip induced drag, induced drag gets reduced to zero. But in the end you still need to deal with that wingtip causing extra induced drag.

[trinidefender]

While your second paragraph is technically correct it is a little nitpicking. The vortex isn't drag. The energy put in to create the vortex, I.e. creating the spanwise flow around the wingtip and hence the airflow collapsing in on itself, manifests itself as drag which is what I stated previously.

It's nitpicking yes, but I feel the conversation is decided by such things.

It's true that an infinite aspect ratio reduced induced drag. Not counting in wingtip induced drag, induced drag gets reduced to zero. But in the end you still need to deal with that wingtip causing extra induced drag.

Even with an infinite wingspan how does total induced drag get reduced to zero? The wing is still producing lift is it not? Creating this lift creates extra drag and this drag is still in the form of induced drag, just not induced drag as it is usually talked about here coming from the longitudinal vortex created by the wingtips.

The way you can think about it is induced drag is only zero (in any direction) if the lifting force is also zero. The moment you introduce a lifting force (on an aircraft through the action of increasing angle of attack on the wings) induced drag starts to take effect.

[turbof1]

While your second paragraph is technically correct it is a little nitpicking. The vortex isn't drag. The energy put in to create the vortex, I.e. creating the spanwise flow around the wingtip and hence the airflow collapsing in on itself, manifests itself as drag which is what I stated previously.

It's nitpicking yes, but I feel the conversation is decided by such things.

It's true that an infinite aspect ratio reduced induced drag. Not counting in wingtip induced drag, induced drag gets reduced to zero. But in the end you still need to deal with that wingtip causing extra induced drag.

Even with an infinite wingspan how does total induced drag get reduced to zero? The wing is still producing lift is it not? Creating this lift creates extra drag and this drag is still in the form of induced drag, just not induced drag as it is usually talked about here coming from the longitudinal vortex created by the wingtips.

The way you can think about it is induced drag is only zero (in any direction) if the lifting force is also zero. The moment you introduce a lifting force (on an aircraft through the action of increasing angle of attack on the wings) induced drag starts to take effect.

It's a mathmatical matter:



(not that this is for a planar wing)

I don't fully get it either intuitively. This is one of those things where intuition fails, simply because it isn't appliable in the real world.

[trinidefender]

Even with an infinite wingspan how does total induced drag get reduced to zero? The wing is still producing lift is it not? Creating this lift creates extra drag and this drag is still in the form of induced drag, just not induced drag as it is usually talked about here coming from the longitudinal vortex created by the wingtips.

The way you can think about it is induced drag is only zero (in any direction) if the lifting force is also zero. The moment you introduce a lifting force (on an aircraft through the action of increasing angle of attack on the wings) induced drag starts to take effect.

It's a mathmatical matter:
http://upload.wikimedia.org/math/5/f/0/ ... 29838a.png
http://upload.wikimedia.org/math/8/8/8/ ... 6c5918.png

(not that this is for a planar wing)

I don't fully get it either intuitively. This is one of those things where intuition fails, simply because it isn't appliable in the real world.

You took that straight from Wikipedia, where you must have missed the part where the author gives the statement that proceeds that formula; "For a planar wing with an elliptical lift distribution, induced drag is often calculated as follows. These equations make the induced drag depend on the square of the lift, for a given aspect ratio and surface area (while varying the angle of attack), but as the accompanying graph shows, this is only an approximation and is not valid at high angles of attack (and probably not for very high values of aspect ratio either)."

A wing with infinite wingspan would not really be a planar wing. Therefore you cannot apply that formula. The closest thing possible that exists in real life would be what is known as a closed loop wing. They also made mention that it probably would not work with infinite aspect ratios either.

Yes it is theoretical and isn't as mathematically simple as that formula that you posted. It may confuse people.

[turbof1]

Yeah I was in a hurry. Do we have a good drag formula for conventional aerofoils? From an other website, there's this very simple formula:

Again, very simple and very theoritical (duh, as we are speaking about infinity in the first place). Again, the whole formula is getting devided by consequent A.R. .

(Of course again this isn't relevant to real life wings and relevant to the discussion at hand, doesn't count in induced drag by wing tip.)

There's also this that looks like a very interesting article:
http://www.grc.nasa.gov/WWW/k-12/airplane/induced.html

Again the theory falls short on the fact that the wing tip produces induced drag too. What I think is that moving the A.R. to infinity, wingtip induced drag becomes a constant for a given chord.

I'll ask for Flyboy to come by, he's more used to this and has done cfd work in this area.

EDIT: some more info: http://www.skybrary.aero/index.php/Induced_Drag