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Old Jun 11, 2014, 07:02 AM
Kyle Clayton
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United States, VA, Dinwiddie
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Lets Talk Winglets

So whilst sitting at Miami International Sunday afternoon waiting for my flight back home, it was quite obvious to see that winglets have gone the way of the Snipe with commercial airliners. With all these bull-horns rolling around on the tarmac, and having discussions before with others about how they're mainly cosmetic, I had to do some research to see what the real reasoning behind these were. I'd say 85% of the craft present were sporting these do-dads. Obviously, they must serve some real-world function other than looking cool or being another spot for advertising. As a matter of fact, I didn't see any adverts on any of them. So my research gave me some very interesting reading while waiting for my flight.

My googling for winglets led me to discover that there are many types of these devices and I will discuss a few of them I found. At the moment, the most popular seems to be a style known blended winglets. They are named so because they blend the wing into a winglet in a smooth, up-turning transition. The underlying point of these seems to be to improve efficiency which reduces fuel consumption. Most of these improvements are only in the range of 3-5%, with some of the wilder shapes getting up to a 10%-11% improvement. Granted, 3-5% is not a lot, but with the scale of airline operations, this adds up to millions of gallons of fuel saved and hundreds of millions of dollars they don't have to spend. Of course though, I feel we won't see these savings via incredibly lower fares Now what really intrigued me is the explanation of how they achieve this. First off, and something I already knew, winglets reduce the size of the tip vorticies, which create quite a large amount of drag. These vorticies are created by the higher pressure under the wing flowing down the wing span-wise and rushing to the area of lower pressure on the top of the wing by trying to roll over the tips. Most of you already know this The next goal they achieve that boosts efficiency is that they serve to act as an increase in span and aspect ratio without actually having to lengthen the wing. Longer, higher AR wings are more efficient than short stubby ones. And sexier. Keeping a shorter span is advantageous to airliners because they still need to be able to fit into the gates at already existing airports. The final, and most interesting part I found is that by adjusting the winglet airfoil shape, the "cant" or tilt from the vertical plane of the aircraft, and the "toe" or angle to the forward direction of flight, these can actually create a lifting force that points in the forward direction, propulsion essentially. This is apparently similar to the force created on the keel of a sailboat when it tacks upwind. Granted this propulsion isn't quite like adding another engine, but the best simplified explanation I read of this lift and how it acts on the airplane in flight was to imagine squeezing a watermelon seed between your fingers and shooting it out of your grip.

My searching also revealed all the different types of wing tip devices out there. The most basic is a wing tip fence. At its simplest form, these are basically a delta or triangle that is placed perpendicular to the wing extending above and below on the tips. These are primarily used to reduce tip vorticies by blocking the span-wise flow. They aren't really designed to perform all of the other tasks mentioned before. The next, and most popular shape currently, are the aforementioned blended winglets. Several full-scale sailplanes also currently utilize these. This shape is where all of the other performance enhancements start to happen. From here, the newer and wilder shapes start to evolve. One that is starting to see use on some commercial airlines is the Split-Scimitar winglets, which feature a blended winglet top, with a smaller winglet extending below angling off cant. This shape is supposed to merge the fences with the blended winglets and serve to again reduce vorticies further and also generate more lift in the forward direction. At the experimental-looking end of the spectrum, there are Spiroid winglets and turbine winglets that resemble the tail of one of those old-skool Nerf footballs (yes, skool). These are again supposed to be further refinements in shape that minimize or nearly eliminate vorticies. The final shape I came across wasn't really a winglet at all, but more of a modified tip shape that is supposed to be the optimal shape of a wing and currently being utilized on the 787 Dreamliner, raked wing tips. These are tips that greatly increase in sweep relative to the overall wing and also narrow down to a point at the tip. This shape is supposed to create optimal flow over the tips with the least drag and weight penalty. However, many of these actually increase span, on airliners at least, so there is some talk of making these folding or retractable so that the plane can fit into the terminals easily.

So where am I going with all of this? Well it should be obvious With all of these increases in efficiency that each of these shapes can provide, why don't we utilize any of these devices on our DLG's? Now, before you all jump me, I know there are several quite obvious answers as to why. First off, yes, they would be a royal PITA for you builders between moulding them and also making them strong enough. Also I realize that most of these devices would add weight in the one spot where it is least desirable, on the wing tips. And I know for these to be really designed to the max, some very expensive testing would most likely have to be done, more so than is feasible for a toy airplane . It seems that several designs have tried using these devices before. PCM is about the only I can think of at the moment where they have used tips that served duly as fences and the launch blade and they also used aileron fences on the FW4.2, but these didn't seem to catch on. Currently, I know some larger TD ships employ some wing-tip shaping, such as the Explorers which seem to use a combo shape of a blended winglet with a raked tip. I guess I'm thinking of these being the next direction of wing efficiency improvement without further thinning airfoils, spiking AR's, or playing with venturi ported wings If we are always limited in span, but some of these shapes serve to act as an increase in span and AR without actually having to do that to the wing, wouldn't that be desirable to us? It seems to me that the raked wing tip would be the easiest to implement without any major adjustments to current building processes or adding a bunch of weight out there.

So why haven't we ventured here yet? I know the age of the "experimental builder" has waned quite significantly, but is there anyone even considering these? Will our toys ever sport these hi-tech pieces?
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Old Jun 11, 2014, 07:32 AM
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Hi Kyle

Airliners spend 99% of their flight in one flight regime, which affords for optimization of cruise flight.

Dlgs spend their flights in a broad envelope of speed ranges and lift coefficients.

WInglets allow to minimize induced drag which scales with the square of the lift (IIRC). Hence at zero lift, there is no induced drag to compensate. In that flight regime the winglet just becomes a useless airbrake. CL=0 should not occur for an airliner, but is the condition a DLG spend its zoom climb in. Hence less launch altitude with winglets.

In F3B it was the same deal. In the 90ies they were building wonderfully shaped blended and upcurved wingtips (look for Estrella and Caracho). Now they are use just flat boring elliptical planforms, like in DLG. The consensus seems to be that this is a better compromise to cover all flight conditions in B or K.

With that being said, a sphiroid would be a fun launch blade. A loop of carbon or Kevlar would be virtually unbreakable, in contrast to a normal blade.

Another argument against winglets is that it would be better to just add wingspan to reduce induced drag. Well for airliners you will run out of parking space and DLGs are span limited, so this is no option. In J they are slowly going that route with ever increasing wingspans.

I hope Martin Kopplow can chime in, he built a small DLG with gigantic winglets and flew it successfully at the german opens.

Reto
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Old Jun 11, 2014, 07:38 AM
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In the past AKAModell München has developed a DLG with winglets. I don't no of this was a success.

http://www.akamodell.vo.tum.de/projekte/am10/index.htm
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Old Jun 11, 2014, 08:43 AM
G_T
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Winglets would have shorter chord than the rest of the wing. At DLG Reynolds numbers, the drag coefficient would be disproportionally high. For DLGs in normal flight, the majority of the drag is profile drag not induced drag, by a factor of about 2:1!. Winglets can be useful in span limited cases where the drag is more balanced or the majority of the drag is induced drag. They have no applicability to DLGs.

Additionally, on the launch a DLG experiences a substantial sideways flight component. Winglets would seriously interfere with the flow near the tips.

Gerald
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Old Jun 11, 2014, 10:33 AM
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I did some research mid last year into this subject. I found some great papers on winglet design for full scale racing gliders which, while may not have quite as large of a flight regime as DLGs, has a much broader spectrum then other current applications. I also chatted quite a bit with some aero guys I used to work with who also flew gliders in the Seattle area to get an idea of what tools were needed to do the analysis properly. XFLR can do some of the basic analysis but to really do the finish work that is required to model the transition area between the wing and the winglet, you need some much stronger tools. Supposedly Drela was working on some open source tools that would allow analyzing these properly but I have not heard much more on that front lately.

After reading up, doing a bunch of research, I spend some time in XFLR to see how practical this really would be. Turns out from what I have done, that it is possible to increase the performance over a baseline wing in the glide regime with little penalty at very high speeds for mid AR wings. While I did not look at the low AR area, I did look at high AR (think XXlite levels) and there seems to be much more work involved in just getting the performance with winglets just back to baseline. I could increase L/D in one area but the wing took a big hit at the highspeed end to do it.

So I sent my mid AR design over to some dudes I know who are willing to try things, for science of course, and while they were busy getting their primary planes up and running, they just built a cnc machine for making wings, they did start working on the cad for a small mold. When they get into the work they found that it is pretty much the most difficult mold they have every tried making with its twisting and sweeping parting plane and everything. Our idea was to mount it a wing like the one that they are currently flying and compare it to the wing without winglets. Launch would be off a bungie with possible light discus launches just to test the concept and see if anything is there before it was incorporated into a full mold of a wing and try to solve the structure issue.

Currently it's flying season in Seattle now so probably no progress until next build season in 3-4 months

Now that all said, I fully expect lower launch with a plane with winglets. Just thinking about the added area to the wing to get any benefit is like adding the equivalent of another stab to the wing. That's a sizeable amount of extra drag during launch, which may not be as big of an issue if the glide performance is significantly better. IMO only testing will really tell because while we design all of these winglets in computers which do a pretty good job of modeling, we still test them in real life to establish a true delta in performance over a baseline.
James
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Old Jun 11, 2014, 11:15 AM
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We had a Boeing engineer talk to our soaring club about winglets and what they do. The summary of the whole discussion is that the winglets act like increasing the wing span with out increasing the wing span. They improve the efficiency at the wing tip. For remote control they do nothing but hinder performance due to yaw instability. If your sitting in the plane you can control the yaw much better than from the ground and if the winglets see side slip or a non coordinated turn they will hurt performance. They do look cool.
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Old Jun 11, 2014, 11:51 AM
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If you look at the function of the winglets, the design of the last 1/4 of the wing, and then the approaches some recent designs have put forth, there is, IMHOP, a distinct and easily molded alternative to the winglets that look more like wingpegs. But heck, that wopudl open a can of worms and the free flight guysd would see their secret revealed.

How's that for a tempting flame-on issue. You have to think outside the box for DLGs and the wing types we tend to think are set in stone.

Chris

PS: Why do you think you have to have something that would respond to yaw?
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Old Jun 11, 2014, 11:51 AM
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It took quite a few years of design work to get winglets to be an overall benefit for full size gliders. Now they even use them even on the unlimited span class gliders.

One trick they use on the full-size glider winglets to reduce the yaw sensitivity is to use a lot of sweep back and a scimitar shape to delay stall with yaw.

http://leichtwerk.de/eta/en/technolo...gletsinfo.html

Even a 3% performance increase is huge compared to the other avenues of design changes left at this point in DLG development, like the fractions of a percent from airfoil and planform changes, etc. In a span limited class with gliders that spend a lot of time at high Cl, winglets are attractive. The very low Re, low Cl drag, and yaw issues would be challenging.

Kevin
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Old Jun 11, 2014, 11:58 AM
Kyle Clayton
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United States, VA, Dinwiddie
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Kevin has pretty much hit the nail right on the head. I'm not saying that there aren't further advancements to be made in airfoil and overall wing shapes, but I do feel that we are approaching that limit. I think our main constraint at the moment is building materials that are light yet strong enough to support the crazier hyper-thinned, 20+ AR wing designs that are still sitting in a computer simulation somewhere. In the game of DLG, a 3% improvement in efficiency would be major. I do think it would take a lot of work, a lot, but my thinking is that they can at some point be made to work for these applications. It would definitely be a steep curve to get to that point, but it would be quite interesting to see if they could really be the next big improvement aways down the road.
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Old Jun 11, 2014, 12:34 PM
Kyle Clayton
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Oh and what about raked wing tips? So far most people seem to have skipped right over the part where I said I believe these would be the most applicable for us.
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Old Jun 11, 2014, 12:51 PM
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Hi Kyle

According to the theory behind the Steigeisen model, having the launch peg behind the center of mass was benefitial to launch, as it induced a certain yaw-offset on release.

While the swept back wings of the Steig may have been a reason for its flutter tendencies (increased torsional loads), a raked wingtip could also position the peg a bit further back.

I love the Akamodel Munich design, T-tail and winglets! It is telling that does guys also do manned gliders too, because that thing has inherited some genes from the big ones.

On another note, I remember that Mike Lachowski had a Caracho (an F3B model with 3.1m wingspan and lovely upcurved wingtips). He called it a 2.9m model with two huge airbrakes at the wingtips. Hence he just cut them off and it performed better for him in the speed task (he won one speed round at the 2003 worlds in Germany). I thought that was pretty hardcore to do such surgery on a pristine mouldie at that time, lol.

Also have a look at the C-wing tailless plane: http://www.aerodesign.de/modelle/NF/cwing2.htm

and
http://aero.stanford.edu/reports/non...iguration.html

The C-wing II was flown as a "TD" model with some success! The upper "wings" where actually more elevators, but they allowed to use high lift airfoils (normally unsuited for a tailless wing) to be used on the main wing.

It is a pretty radical way of having more wingspan if you are span limited. Probably unsuited for DLG as the increased inertia would be a nightmare, but fancy looking.

I remember that there was also a diamond wing shape (forward and backwards swept wings meet at the tip) aircraft by Boeing and one by NASA that had an entire tubular wing. Wild stuff!

Reto
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Old Jun 11, 2014, 01:02 PM
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Quote:
Originally Posted by Wave Glider View Post
Oh and what about raked wing tips? So far most people seem to have skipped right over the part where I said I believe these would be the most applicable for us.
I did raked wingtips on my TD ship back in 2008-9 in response to reading this: http://spieltek.com/SunbirdSoaring/S...ormArticle.pdf (remember Kevin???)

My comments above do it without raking and still may retain low drag and not be affected by yaw. As stated by others F3B type ships did it, but then moved away from it. What is needed is an approach that can be molded. But the funny thing is, per my looking, the Snipe may already trend toward it.

Chris
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Old Jun 11, 2014, 01:09 PM
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Originally Posted by RetoF3X View Post
Also have a look at the C-wing tailless plane: http://www.aerodesign.de/modelle/NF/cwing2.htm

and
http://aero.stanford.edu/reports/non...iguration.html

The C-wing was flown as a "TD" model with some success!

It is a pretty radical way of having more wingspan if you are span limited. Probably unsuited for DLG as the increased inertia would be a nightmare, but fancy looking.

Reto

The C-Wing looks a lot like experiments done on flying wing models in the early 70's. The "canted" top surface (winglets) provided more drag than actual lifting contributions. Anytime I see "bi-plane" proposals for our models I shiver.

The real question is function. Why do it when another methods is more optimal and easier to manage? Doing something because it looks good, is really no reason at all.

Chris
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Old Jun 11, 2014, 02:22 PM
RV7guy
Chandler, AZ
Joined May 2004
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They work!!

[IMG][/IMG]I've been flying my Kennedy Composites 2 Meter Sprite with Winglets for years. I flew the plane for over 500 flights before installing the winglets. After installing the plane flew much more like an Open class plane. I've attached a photo of my winglet Sprite and non winglet Sprite. I'll be installing Winglets on the other one soon.

The winglets were designed by Jerry Robertson. They were built and installed to his spec's.

I've often thought they would work great on HL. Although they may add some drag, they effectively add span. The only hitch in the whole deal would be making Winglets large enough to be effective and be able to have a launching peg.

Darwin N. Barrie
Chandler, AZ
Team Futaba
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Old Jun 11, 2014, 03:30 PM
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Quote:
Originally Posted by Thermaln2 View Post
The C-Wing looks a lot like experiments done on flying wing models in the early 70's. The "canted" top surface (winglets) provided more drag than actual lifting contributions. Anytime I see "bi-plane" proposals for our models I shiver.

The real question is function. Why do it when another methods is more optimal and easier to manage? Doing something because it looks good, is really no reason at all.

Chris
Hi Chris,

apparently the C-wing was good enough to be studied by Stanford university, a well recognized institute.

In the case of the model C-wing, it can be debated if it is still a tailless wing or a conventional aircraft (the upper parts being horizontal tails or extensions of the wing), which caused some heated discussions in the german scene. As described, the upper surface was indeed more working like a horizontal tail. Which in turn allowed the main wing to get away with a high lift cambered airfoil, which would not work on a normal flying wing due to the high pitching moment. But it can be debated if the gains in airfoil efficiency and no washout compared to a conventional tailless plane warrants the extra drag of the "upper wings".

If you are only into function, then you should probably not do a tailless plane at all for competition soaring. But some people like tailless planes and hence the C-wing was an experiment to improve some aspects of it (better lift distribution over the wing), which was bought at the price of additional drag (sort of horizontal tails instead of being tail-less).

As stated before, I am not a big fan of winglets or anything put on a wingtip, as it is useless at CL=0. And I like to fly fast.



Best,
Reto
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