Tandem wing for high efficiency? Case Proteus

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karoliina.t.salminen

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My claim 1: Profile drag increases as a function of decreasing Reynolds number
My claim 2 :profile drag decreases as a function of increasing Reynolds number
My claim 3: Reynolds number below 2 million is problematic if high L/D and very low drag count (in realm of 30 drag counts) is the goal
My claim 4: At really low Reynolds number, laminar flow is rather attempted to be avoided than maximized
My claim 5: At really low Reynolds number, a "turbulent flow airfoil" such as NACA 2412 gives higher L/D than a "laminar flow section" due to boundary layer problems associated with separation bubble, therefore really low Re is better off with e.g. conventional NACA/Clark Y
My claim 6: Modern gliders optimize for practical handling qualities and sacrifice some optimum airfoil performance to do so. Outboard sections are usually turbulent flow sections whereas inboard sections are often chosen as laminar flow sections. Diana 2 is not an exception, check the outboard sections yourself if you don't believe.
My claim 7: Performace of a glider has more to do about the large span and low span loading than the section L/D. L/D has nothing to do with aspect ratio, wing area, only with large wing span. Induced drag (drag due to lift) has only to do with wing span. Nimbus 3 has extremely large span.

I am using XFLR5 which using XFOIL for 2D analysis and MIAREX for 3D panel analysis.

The XFOIL results are off of course when predicting Clmax or at stall region.
However, I what I meant was that section L/D rising as a function of reynolds number is predicted by this software and
also same thing can be seen on numerous papers and publications. Show me a publication which shows that actually the
section L/D is not rising as a function of Reynolds number? Xerox-copied pages even in Bruce Carmichael's booklet also
indicate the very same thing; the higher the Re, the higher the L/D (until a certain Re for the section is reached when it
starts to reduce). Also every book out there also remembers to mention that profile drag will decrease as a function of
the increasing Reynolds number. However, if this conventional wisdom is not correct, please explain to me why because
I fail to understand how this would suddenly be upside down. As a reference about Bruce Carmichael;
Personal Aircraft Drag Reduction, Page 86. Every airfoil has a maximum lift/drag ratio rising as a function of Reynolds number.
Highest L/D is achieved on this list of airfoils NLF(1)0414F with 10 degrees flap at Re 10 million, L/D 240. The lowest L/D for the same
section is pubished at Re 3 million, L/D 170. The same airfoil with 0 degrees flap has L/D 150 at Re 3.0 million and 190 at 10 million.
Wortmann 05H-126 has L/D 120 at 0.7 million and 190 at 2.0 million. Every airfoil listed on the page has increasing section L/D for increasing Re.
Why I am using this as an example? Because Bruce Carmichael has listed conveniently lots of airfoils to same diagrams with the L/D function
visible. Despite the publication is looking very home made, I don't think it is complete rubbish. I am getting similar results from the XFLR5,
on every airfoil, the L/Dmax of airfoil rises as a function of increasing Re. I don't take your word that it would not be a true conclusion.
I don't believe the airfoils is some black magic and some super airfoil will change the world and that some proprietary airfoil would be so superior to anything else than it would be impossible to even think about such thing that only "they can do it".

Also the results from XFOIL simulation or the writings of Bruce Carmichael do not disagree with the recent publications about airfoils I have read.
I have collected a list of documents on my web page which has some publications which are relevant in this topic. From this list I can find no reports which
would support your claim. Unfortunately this list is unsorted and without descriptive publication name, just a link list, but there you have it if you want to check it out:
http://karoliinasalminen.wordpress.com/miscellaneous-links-to-tech-papers-aiaa-sae-final-reports-nasa-etc/

If you can't send me airfoil, but can provide me flight test data that proves that there is airfoils which can achieve 30 drag counts at Re 1 million, I would like to see that.
 
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autoreply

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I don't take your word that it would not be a true conclusion.
You don't have to because I didn't say that. In fact, I concluded exactly the opposite...

Regarding your "claims", 1 and 2 are often correct, within certain boundaries and with lots of ifs and buts. 3 is nonsense since the highest values have been achieved there, 4 is arbitrary, but below 10^5 or so it starts to be very hard to achieve, 5 is again arbitrary, but low Re require completely different airfoils, you can't just throw in any modest turbulent airfoil.
The 2nd part of claim 6 (turbulent outer parts) is nonsense and 7, are you serious? Go read a book about aircraft design or the performance of aircraft....
I don't believe the airfoils is some black magic and some super airfoil will change the world and that some proprietary airfoil would be so superior to anything else than it would be impossible to even think about such thing that only "they can do it".
You are comparing airfoils that are 30 to 60 years old, designed by trial and error and meant for fast turboprops and helicopter rotors to the ones that are very recent and are designed by advanced computer code and have benefitted from recent advances in aerodynamics.

Why do you actually think that modern gliders who's outline and structure is comparable to 25-year old designs perform about 25% better?
 
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karoliina.t.salminen

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Claim 7, I am serious. Induced drag has nothing to do with AR. Many books have got it wrong. They mix downwash and tip vortex together which is an error.
AR only has effect on the steepness of the lift curve slope. Low induced drag comes from low span loading. Weight, wing area are secondary,
it is all about span loading to get low drag due to lift.
 
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karoliina.t.salminen

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Don't shoot the messenger: I am just repeating the words of John Roncz / Oshkosh 2011. My claims were almost direct quotes from his speech. I think John Roncz is not nobody but he has achieved something, hasn't he?

And also I am writing my claims that you could prove me wrong. If you can prove me wrong, then I have learnt something.
If you get out of the discussion, that is not very productive, for neither of us. And so far, if I am not mistaken, you have proven me absolutely nothing except you got offended from my claim that induced drag has a direct relation with wing span and not with aspect ratio. This is by the way the same reason, why it is better to have longer wing span than winglets. Increasing span is better than increasing e.

Downwash is natural phenomenon which occurs from production of lift. It is the opposite force of the lift. To produce lift, the wing must produce acceleration of the air and push it downwards, these two must equal, it is law of pshysics, not black magic. Induced drag has two components, one is tangent of downwash angle and the another is completely parasitic caused by wing tip vortex. Induced drag does not happen because of the wing tip vortex but mainly due to downwash.

There are 2D and 3D induced drags. Example case is wind tunnel: wing tip vortex = zero, but the wing section still has induced drag. That is because of the downwash. On finite wing 3D case, the longer the wing, the more of the flow will produce downwash. The shorter the wing, the more pressure between upper and lower wing surface equalizes over the tip and more strong tip vortex is created. The farther apart the wing tips are from each other, the less pressure differential gets equalized over wing tip and to create wing tip vortex (which is not desired) and more of it becomes productive for producing lift (which is desired).

To get higher L/D you want more downwash to be generated and less pressure to leak over wing tip to create tip vortex. You can do this by a) increasing wing span and b) by decreasing aircraft weight. In case b) what happens, is that you will reduce the weight per your wing span, in other words, you will reduce the span loading. By increasing span, you will again reduce the weight per the finite wing span. So from this standpoint, I think, John Roncz had came into his conclusion that induced drag has nothing to do with aspect ratio, but it has to do with wing span instead.

We discussed about this in Oshkosh a lot, therefore if you have counter arguments, so prove me wrong (anybody) please. I am interested and even thrilled to be proven wrong on this. If you can prove me wrong on this, you will solve lots of problems. Also if you can prove me wrong on claims 1-6, then even more problems gets solved and the aerodynamic design of my concept suddenly becomes super-easy and I can stop wrestling with challenges with the Reynolds number and be happy with low Re.
 
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Rick McWilliams

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Low reynolds number airfoils do not produce exceptionally high L/D. It gets very difficult to design a high L/D airfoil at Re<1M. The L/D gets better and better for laminar flow shapes as Re increases. Laminar flow is easier to obtain with thick airfoil shapes. It is very tough to get high L/D at low Cl. I designed the Lancair IV, and Lancair ES airfoils. I design for low drag laminar flow with the compromise of maximum lift with flaps deployed.

Induced drag is a measure of the energy left in the wake of a lifting system due to lift. I think of induced drag as relating to the mass of air that is deflected by the wing. This is in essence a cylinder of the diameter of the wing span. This is basically the span loading for a well designed aircraft. There is no way to cause downwash beyond the wing tip, the wake will roll up. The span load distribution can have ugly lumps and bumps due to fuselage, canard downwash or tail upwash. These usually get their own drag or trim drag term or interference term. Since they change with lift they can be considered part of the induced drag.

When you run the 2D airfoil codes, plot the surface pressure coefficient. It is most interesting to see how the favorable gradient moves with angle of attack. This provides a very useful airfoil shape "magnifying glass". Thick airfoils have stronger gradients. Big nose airfoils delay the suction peak.
 
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autoreply

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Don't shoot the messenger: I am just repeating the words of John Roncz / Oshkosh 2011. My claims were almost direct quotes from his speech. I think John Roncz is not nobody but he has achieved something, hasn't he?
Ok, maybe I'm a bit blunt or even rude, but I think you've mixed up several things. That's no problem, we've seen it here many times before and things like span loading and effective aspect ratio (winglets for example) can be confusing or even counter intuitive. But if you claim that "many books are wrong" I hope you understand that a I'm not going to "prove" your claims wrong, since it counters so much that's well known and understood since world war I. I've yet to find a well-known book that has these kind of things wrong.

Orion has put up a great list of books. Anderson (introduction to..) gives a pretty good overview of what and why.
Low reynolds number airfoils do not produce exceptionally high L/D. It gets very difficult to design a high L/D airfoil at Re<1M. The L/D gets better and better for laminar flow shapes as Re increases. Laminar flow is easier to obtain with thick airfoil shapes. It is very tough to get high L/D at low Cl. I designed the Lancair IV, and Lancair ES airfoils.
What I tried to say is that while individual airfoils have lower drag when you increase the Re number, it seems that different airfoils, optimized for a certain Re, result in comparble drag numbers, though one would except lower drag airfoils at higher Re. I've also heard that several airfoils seem to "run into a corner" when increasing Re, where drag doesn't really decrease anymore, while they're still far south of Re=10^7. Any thoughts?
 

karoliina.t.salminen

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Orion has put up a great list of books. Anderson (introduction to..) gives a pretty good overview of what and why.
I am sorry but I have read them all. Does not mean though that I have learned 100% or memorized 100% of it, but I have been going through these books since 2004 rather actively because my intent have been since then to design airplane(s). And actually aerodynamics as a separate does not exist. I was originally reading aerodynamics books and not thinking physics, but since then I have started to question what I have read, and almost every book has something to question and by questioning these points, often there is opportunity to understand phenomenon instead of memorizing them and sometimes beyond what the books tell. And in fact, it is all physics. Aerodynamics is a set of rules of thumbs for phenomenons which obey laws of physics. Everything has to agree on thinking on the physics or the aerodynamic "law" can not work.

Apparently it seems that I also I agree with Rick McWilliams 100% (and my claims do not disagree with his view, at least the intent is not to do so).

I have John Roncz presentation on video, but I have not had time to transfer it from my secondary video camera yet. I have only few clips on my DSLR. But in fact, I have the clips taken also with the DSLR which cover the span loading relation to induced drag and I happen to have them on my computer right now. Maybe I need to put them up to youtube soon.
 
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Tom Nalevanko

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I believe that Karollina is correct in her understanding of the relationship between induced drag and span loading. There are slides from various talks of Barnaby Wainfan that show this. And as far as older books go, there are usually a significant level of errors. Don't believe something because it is in a book.

Karollina simply asked for someone to disprove her statements. Disproving something is usually relatively easy when compared to proving something. So she is not asking for the sky...
 

karoliina.t.salminen

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Karollina simply asked for someone to disprove her statements. Disproving something is usually relatively easy when compared to proving something. So she is not asking for the sky...
Exactly. And I would be even happy to be disproven! Because it would solve my (aerodynamic) problems :). Not all of them maybe, but quite many of them.

Actually I started (if someone has followed my blog) with an assumption that I could go by having a rather modest wings but high aspect ratio. However, crunching some numbers, listening presentations, and also thinking with common sense has led me to actually believe that is not the case, but I need physical meters or feet. And that will make the concept a lot larger, larger than I was originally wanting it to be. And larger means more structure, more structure means more weight. More weight means surprise surprise worse span loading. And here we go again, needing more span. I also concluded that why more span is better than winglets. Answer is very simple: More span will cause favorable downwash in addition to reducing pressure on upper and lower side to leak around the tip (and cause the vortex). Adding winglet will reduce the tip vortex, but will not help deflecting more air down. That's why Jonh Roncz is also said "winglets are best when they are laid down along the span rather than up".

Barnaby Wainfan's presentation at Green flight challenge event in Osh 11 was also extremely eye-opening. Great way of thinking and common sense via thinking in terms of physics rather than repeating old mantras from books that some of them were written almost 100 years ago.

I have been reading these old mantras again again again and again, but there has been always something in common sense that why gliders have so long wings, why they are not only skinny, why they are also very long. Then bingo: it is the span loading. Span loading is often ignored by books, they talk about wing loading, and just mention that span loading also exists. But span loading is a very important design parameter if designing for high efficiency. Why does Diamond DA40 climb better than Cirrus SR20 despite the span is the same. Because the SR20 is heavier and thus, it has higher span loading. It also has higher wing loading, but it is because of span loading why it is worse in climbing than the Diamond and also why Diamond can be flown with lower power than the SR20. And then why Dyn'aero MCR01 defeats Cirrus SR22 in climb performance with only 100 hp and similar power loading to Cirrus SR22 and with worse airfoil (with worse section L/D), and lower aspect ratio. Because the span loading is less because the plane is lighter.

So here is the relationship:
- You can get away with more weight by adding span, but adding more span will make the plane even heavier.
- if you reduce span, you need to make the plane lighter to have the same induced drag performance.
- You do not want to add too much extra wetted area by adding more span, hence, the aspect ratio becomes high. Drag has a lot to do with wetted area.

This is why I was questioning the tandem wing. Tandem wing has 1 x span but two wings. Why not bolt these two wings together having double span and double AR. I think I also got the answer for that; practicality, and partly structural considerations. Same thing for conventional design: tail sizing is ruled by practicality in CG range rather than minimum required tail volume coefficient. In the end, many things simplify with the practicality and structural considerations and sacrifices on aerodynamic efficiency happen.

I am quite sure that I have settled back to my original configuration which is:
- conventional (to be able to use efficient flaps)
- large span, low span loading (to reduce induced drag)
- high aspect ratio, relatively high wing loading (to avoid extra wetted area and that way to reduce drag and AR also to have steep lift curve slope (in other words, closer to the 2D airfoil simulations of infinite wings)
 
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Autodidact

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One thing about induced drag that I think I have been mistaken about in the past is that, I believe, it is not just due to tip vortices or the tilting of the lift vector. There is an outward flow component all along the underside of a 3D wing and an inward component over the upper surface also and this results in vortices all along the trailing edge of the wing. The stronger the spanwise flows, the stronger are these vortices and the greater the induced drag locally. The greater the span, the weaker are the inboard vortices (relative to the whole) and so the induced drag goes down. This seems to corroborate Karoliina's assertion but to say that it is all just span loading seems to leave out too many variables. If area and weight is kept the same and only span is increased, then a natural consequence is increased aspect ratio. If chord and weight is the same then area increases and wing loading goes down and hence Cl and the potential energy that's put into the vortex sheet. Same for when AR is kept constant and only span is increased.
 
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BBerson

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Don't shoot the messenger: I am just repeating the words of John Roncz / Oshkosh 2011. My claims were almost direct quotes from his speech. I think John Roncz is not nobody but he has achieved something, hasn't he?
We discussed about this in Oshkosh a lot, therefore if you have counter arguments, so prove me wrong (anybody) please.
Karoliina, I did attend the Roncz forum at Oshkosh 2011 and I am very interested in the topic of induced drag. Actually, I was somewhat shocked when Roncz said induced drag was only determined by span and NOT wing chord or area.
In my view, wing area has a very large effect on the minimum induced drag at low speed.
I have found several different formulas in various books to calculate induced drag. Some formulas use aspect ratio and others do not. I like the formula in my old sailplane Aerodynamics Handbook from SAA. This formula does not use aspect ratio at all, only weight, span, velocity, air density and the efficiency factor.

While wing area is not in this formula, clearly wing area determines the velocity, so wing area or wing chord clearly matters indirectly.

For example, the Gossamer Condor, designed by Paul McCready had a very large wing chord (12 feet) to get low induced drag.

So I think the comment from Roncz was incomplete, just my view.
Hope this helps.
Bill
 

karoliina.t.salminen

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It could be that the Roncz comment is simplification, I have been thinking about it. This is just some common sense, you can't find this speculation from books:
Because for the span wise 3D flow:
- The flow turns towards the tip on the length it travels chordwise and if it succeeds, it equalizes the pressure bottom and top of the wing near the tip and due to the forward motion of the wing, a swirling motion is created, and that's the tip vortex. The faster the wing travels through the air, the less chance the air has to turn towards the tip and less is "leaking" that way. That's why induced drag component decreases when the velocity increases. However, if the wing travels at low speed, and the air stream would like to turn, if the wing chord is conveniently short enough, even if the stream is turning, the wing trailing edge is reached before most of the air stream has been succeeded to turn. And more of the air stream will now cause downwash instead of tip vortex. This common sense would support that AR has something to do with induced drag, but I don't know, that maybe Roncz has noticed that when a long wing with high AR is designed, the AR cancels from the equation and simplifies to span. However, in Daniel Raymer's book on page 347, equation 12.48, K = 1 / (pi * AR * e). In other words aspect ratio, not span. I think opinion of John Roncz might differ. Some pictures of slides of his presentation attached.
 

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autoreply

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An opinion doesn't change the facts, and that's what this is all about. "Opinions" from experts aren't that relevant since none that I know of disagree with the fundamental mathematics (or physics) that rule aerodynamics. Where they might differ in is the explanation, but that doesn't change how it works.

Let's look at those fundamentals. Newton: Every action causes an identical but opposite reaction and force is mass times acceleration. To counter gravity we need an upwards force on our craft. According to Newton this is only possible when we accelerate another mass due to a lack of solid ground.

So to lift our aircraft we need to accelerate air in the opposite direction of where we want to go. We want to go up (to counter gravity), so we accelerate air downwards. We can accelerate a very large mass of air very slowly or a very small mass very fast. As long as their product is the same, we have the same amount of lift.

How much does this "cost"? Well, we know that kinetic energy (that we put in accelerating air downwards) is 0.5*mass*velocity squared.

So the energy that it takes (drag) is proportional to mass*velocity^2, while the lift that it provides is mass*velocity.

Let's assume we have a certain aircraft with a certain "downwash speed". It's not hard to see that if we double the mass of the airflow and we cut the "downwash speed" in half we get the same amount of lift. But the energy that we put into that air is only half the energy it took us originally.

That energy is what we call induced drag.

So let's look what happens in a couple scenarios. We are flying and now accelerate to twice our original speed. We're going twice as fast, so we meet twice the air mass we originally met in the same amount of time. So we only need half as much downwards velocity. So it takes only half the energy we originally need for that same 1 second. But in that same second we've flown twice as far, spreading half the energy out over twice the distance. Thus the drag is only a quarter of what it originally was.

If we increase wingspan and fly at the same speed we get the a comparable result. We double the wingspan and thus double the amount of massflow. This cuts the energy and thus the drag we need/have in half.

So accelerating more mass slower leads to a higher efficiency (lower energy consumption per unit of energy). That's why anything that drives or walks is incredibly efficient, they're accelerating the largest mass we can use (the earth) reducing in ridiculously low "induced drag". That's why jets have such low static thrust compared to turbofans/turboprops. That's why helicopters can generate 10 times more static thrust than the same engine with a 6' prop.

Mind you, nothing of the above is less than 400 years old, it's all plain old Newton.

If you apply the above logic to a wing with a given span, lift and so on, you will probably see that the wing with the lowest energy consumption for that amount of lift has a lift distribution that's flat. Or in other words, the whole wing has a constant downwards velocity of for example 1 m/s.

So we would expect straight wings without any taper wouldn't we?

Now we go past Newton. Airflows can't be binary. You can't just suddenly go from that 1 m/s downwash to the undisturbed air next to the wingtip, so a gradual transition with some kind of transitional effects is going to happen. These are vortices, nothing more than air with a rotation in it and they're behind the entire wing, not just at the tips! With a bit of math, you can prove that a circular/elliptical distribution of a straight wing gives the lowest energy consumption and thus the lowest induced drag.

Mind you, this drag is considerably higher compared to a theoretical wing with a uniform downwash!

Now, the above derivation is only void for straight, horizontal wings. If we allow a third (vertical) dimension, we will see that we can make wings that have a lower energy consumption for a given lift, span and so on. The nice thing about this is that when allowing a wing with a vertical component, we can get to less drag for the same structural weight. I won't go into too much detail, but roughly said can a winglet give a "cut off" elliptical downwash distribution, resulting in almost the same favorable downwash distribution, but without the outer tip downwash who gives lots of bending moment, compared to a winglet. Most winglets are comparable to roughly 2-3 times their height in span increase. Thus, a well designed 1 ft high winglet will give the same induced drag reduction of a wing extension of 2-3 ft, but with only 50 to 33% of the frontal drag. Also bending moments in the root are considerably lower compared to the extended wing.

Theoretically you can prove that in ideal conditions a triple elliptical wing gives the lowest induced drag for a given structural weight or for a given span. The Dreamliner, A350 and many gliders approach that ideal.

As for span loading and so on. They're all different sides of the same dice (if you master the math), but the difference is in their meaning. Span loading is a variable you can use to calculate outcomes, if you know all the other variables, but in itself it's a pretty useless number. But far more usable are dimensionless units. You can apply them directly, to anything. That's why L/D, AR, Cd, Cl, Re and many others are so valuable and uniformly used.

Do the math and try to see the relations between various terms, in physical terms, not in detailed descriptions like vortices, pushing/pulling winglets and so on. Than you'll see that one leads to another, but that the dimensionless ones are far more usable than the other variables, especially since they easily lead to misunderstandings as witnessed all too often.
 
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karoliina.t.salminen

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The question now is that how do you think I would be disagreeing with you? Because what you wrote in the post above I do agree.
So are we disagreeing on something or are we not?
 

autoreply

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The question now is that how do you think I would be disagreeing with you? Because what you wrote in the post above I do agree.
So are we disagreeing on something or are we not?
You've stated numerous things that are simply wrong as noted by me in earlier posts.

As for your last post:
There is no "tip vortex", there's a whole field of vortices and they're a result, not the cause for induced drag. Seems minor, but it's not, even a theoretical wing without vortices will still have induced drag, in fact almost as much as a real wing.
Winglets don't work by "preventing leaking air", they provide a higher downwash at the outer wing panel, without the required longer tip of a planer wing. Downwards winglets are theoretically not any more efficient as upturned ones and this has been supported by virtually every practicable application.

Think fundamentals, not vortices or other details.
 
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Autodidact

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This is an interesting discussion - I had always thought that induced drag WAS a result of the vortices, hmm.

But, on another point, couldn't you rearrange Roncz' "Di" equation to be (L*Cl)/(AR*pi), since lift contains 1/2*p*V² and AR is span²/S? Roncz never said what the wing area was, but he did say what the velocity was and the lift also and it seems like area has to be in there somewhere, doesn't it?
 

autoreply

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This is an interesting discussion - I had always thought that induced drag WAS a result of the vortices, hmm.
Well, that's the problem. The one isn't possible without the other, so the formal math is based on advanced vortices (lifting line theory), but without a comprehensive understanding of those it's pretty hard to make any sense of that. I had been flying for 5 years and only after a year of university I finally fully understood it, instead of all the "laymans" theories. The same for lift production by the way, I guess that about a big majority of the pilots can't give a correct answer to why the wings produce lift, usually followed by "equal time streamlines", all kinds of Bernouilli and so on.

And there lies the problem, half-understood formulas, with "simplified" explanations that are based on advanced math lead to very misleading conclusions and seldom to full understanding without a solid bases in the math and principles behind it.

I much prefer to stick to the real fundamentals. Induced drag is the energy it takes to accelerate the air downwards (hence the "lift induced drag"), nothing more, nothing less.
But, on another point, couldn't you rearrange Roncz' "Di" equation to be L/(AR*pi), since lift contains 1/2*p*V² and AR is span²/S?
Yepz, you can. That's also where Cdi=Cl^2/(PI*A*E) comes from (rewritten to be a drag coefficient instead of a force).

Thus, aircraft drag is Di=0.5*rho*S*v^2*(Cd+CL^2/(PI*A*E)). That describes any non-viscous aircraft that's subsonic...
 
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