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Old Jul 09, 2003, 06:25 AM
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Fairfield Pa
Joined Jul 2003
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Yippee!
new airfoils, camber control

A couple of years ago, I played with the idea of using a golf ball surface type turbulator on the wing upper surface at close to the upper trip point. The theory being that this might create a more efficient trip since millions of dollars have been spent making golf balls go further a high rate of speed. I hunted around and found a self adhesive perforated tape at home depot, put it on a handlaunch wing and tested using my “subjective” eyes and ears to note performance improvement by thermaling up and then dive bombing myself. Probably looked ridiculous day after day.

The wings – thinned 7080 foil, became almost silent eventually, but was tricky to get right. Did it work? Who knows. I’m bringing it up because I see that laminar flow is being achieved theoretically to around 100% on the lower surface now. Based on noise, I found that at high speed it was hard to keep airflow attached at the tips, is that still the case? Applying tape to the underside at about 70 % on the 7080 foil helped (removed noise and looked faster). Applying zigzag or strip trips reduced performance.

The stock 7080 was not designed for turbulation, don’t know what happened when it was thinned. The theory was that this tape created a more efficient or lower drag turbulent airflow that a smooth surface transition to turbulent.

Before discounting this entirely, consider this. I sent a roll to a friend who teaches at a University in Canada and he had some students test it on a full size LS-4 section in their wind tunnel. They wrote a paper. Conclusion was that “normal” turbulators increased low speed performance and reduced high speed as you would expect relative to smooth, but that the drywall tape increased low AND high speed performance relative to both. In fact I believe I saw a drag reduction decrease of 9% at the highest tested speed and this difference was increasing with speed!

I’m no aerodynamicist but I’m wondering if molding a golf ball surface say around ¾” wide at the upper trip point and somewhere near the trailing edge on the lower surface tip section might still help the latest generation of foils.

Any thoughts? Would be pretty easy to add little glue blobs to mylar in vac bag.
If there is interest I will track down the report. I was surprised when I saw it.
I’m also thinking that muscle wire placed at intervals along the upper surface might allow more efficient camber control than servos. Trade servo weight for bigger batteries if needed. Remember that the new 300ma+ LiPo bats can now dump 20c. Length of wire dictates deflection, thickness affects strength and power required. This wire is being used in some micro models now instead of coil actuators.
Thinking - run Kevlar full chord on the bottom surface and use a flexible muscle wire reinforced upper surface spaced every 6” – 1’ in the flap area on the upper surface.
Maybe flexible dimple tape with muscle wire underneath? I was never able to create curved camber with servos – needed too many.

Am I correct in thinking that the bottom surface can work almost as hard at high speed as the upper surface at low speed as far as attachment goes?

Mike W
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Old Jul 09, 2003, 10:50 AM
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Cambridge, MA USA
Joined May 2001
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Re: new airfoils, camber control

Quote:
Originally posted by Mike Waters
I?m bringing it up because I see that laminar flow is being achieved theoretically to around 100% on the lower surface now.
..snip..
I?m wondering if molding a golf ball surface say around ¾? wide at the upper trip point and somewhere near the trailing edge on the lower surface tip section might still help the latest generation of foils.
In a fast penetration glide, the new DLG airfoils have 100% laminar flow on the bottom, and nearly 100% attached laminar flow on top (the similar Aegea 3m airfoils have roughly 90% laminar flow on top). In either case, a turbulator is pointless because there is no laminar separation to quash.

At min sink, with camber dialed in, the bottoms are still 100% laminar, and the top surfaces are 50-70% laminar. Now there is a separation bubble on top, but it's weak and relatively inconsequential, so turbulators won't help here either. One reason is that in the high-lift condition the airfoils are designed to have a very pronounced LE pressure spike, which tends to promote turbulation farther downstream with none of the downsides of a turbulator strip.

Caveat: For this stuff to work the airfoils must be built accurately and the surface quality must be adequate.
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Old Jul 09, 2003, 11:24 AM
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West Boylston, MA
Joined Feb 2003
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Re: Re: new airfoils, camber control

Quote:
Originally posted by markdrela
Caveat: For this stuff to work the airfoils must be built accurately and the surface quality must be adequate.
Dear Mark,

Re your statement above, one of my gliders has a prototype bagged wing using your SuperGee II/XP-3 airfoils ("t" version, I believe), and there's a groove about 1/2 mm deep and several mm wide along the leading edge, right at the joint between the top and bottom skins. Is it likely I'm losing much performance from that airfoil deviation, and would you recommend sanding it out or leaving well-enough alone?

Thanks much,

Marc
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Old Jul 09, 2003, 07:02 PM
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Cambridge, MA USA
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Re: Re: Re: new airfoils, camber control

Quote:
Originally posted by marctrudeau
one of my gliders has a prototype bagged wing using your SuperGee II/XP-3 airfoils ("t" version, I believe), and there's a groove about 1/2 mm deep and several mm wide along the leading edge, right at the joint between the top and bottom skins. Is it likely I'm losing much performance from that airfoil deviation, and would you recommend sanding it out or leaving well-enough alone?
There's no way to know for sure. You'd have to fill it to see if it makes any difference.
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Old Jul 09, 2003, 07:25 PM
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OK, Mark. Thank you.
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Old Jul 09, 2003, 08:08 PM
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Fairfield Pa
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"In either case, a turbulator is pointless because there is no laminar separation to quash."

So the new wings must make no noise in cruise config at ANY speed, totally quiet, even the tips, thats impressive! how do they handle turbulent air? and a sharp break at the camber hinge line is "inconsequential". Sounds like airfoils are now 100% efficient and can no longer be improved on. Its too bad since the pioneering period of a sport is the most fun and advancement happens quickly. I guess all other classes of model sailplanes have now heard of these advancements and will be dumping all those old fashioned Selig airfoils. Sometimes wind tunnels don't tell the whole story, but I'm sure you may be right.
I find that sometimes its worth trying strange stuff just to keep things interesting. There is an assumption that laminar is laminar and turbulent is just turbulent, but tests results imply many levels of turbulent flow, and an organized trip that is not behaving like a draggy turbulator may have some merit. Also there is still the fuse and tail to think about. Your response rejects hours of research out of hand. Are you sure you are right?
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Old Jul 09, 2003, 08:46 PM
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Nashville, TN USA
Joined Jun 2002
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How new is new?

I was wondering how new is new. The UIUC database does have the thicken airfoils. Could you tell me if there is a set newer than the AG44-47? Is there a write-up on the changes that I could read?
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Old Jul 09, 2003, 09:53 PM
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West Boylston, MA
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LW,

Mark's new SuperGee II DLG (and the XP-3 and Taboo XL, I think) use the AG455ct/AG46ct/AG47ct. The "t" suffix refers to an airfoil with greater thickness at the flaperon hinge line to stiffen the flaps and reduce the liklihood of flutter for the hardest throwers. Best place to look for info would be the files section of the SALglider list at http://groups.yahoo.com

Best,

Marc
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Old Jul 09, 2003, 10:02 PM
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Quote:
Originally posted by Mike Waters
So the new wings must make no noise in cruise config at ANY speed, totally quiet, even the tips
That's about right. Most of the noise seems to be coming from the fuse/wing intersection. My SuperGee is very quiet in the launch or a very fast flyby. My XP3 is noticably noiser, and the main difference between the gliders is that the XP3 has a messier wing/fuse intersection.

I guess all other classes of model sailplanes have now heard of these advancements and will be dumping all those old fashioned Selig airfoils.

There's nothing wrong with the Selig airfoils, or other "old" stuff like MH32, RG15, etc. It's just that these sections are better matched for Reynolds numbers seen on 90oz 3m gliders. They are grossly mismatched to DLGs, and partly suffer even on the new light 3m gliders, especially near the tips. Conversely, the Aegea airfoils would not be a good choice for an XC ship or a ballasted F3B ship. Something like an MH32 or RG15 will be better here.



Sometimes wind tunnels don't tell the whole story, but I'm sure you may be right.

The AG455 was recently tested by Selig & Co, and the AG40 will be tested in a few weeks. We'll see.
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Old Jul 09, 2003, 10:18 PM
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Sounds like airfoils are now 100% efficient and can no longer be improved on. Its too bad since the pioneering period of a sport is the most fun and advancement happens quickly.

The challenge remains in combining the airfoils with a compatible wing layout, structural design, and manufacturing method. Can we get $400-DLG performance and durability for only $200? For $150? That sure is a challenging problem, with far more potential impact on the hobby than squeezing the last 5% out of the airfoils.
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Old Jul 10, 2003, 07:27 AM
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Fairfield Pa
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I think the answer there lies over seas. Eastern block, South America or even better - Asia. Seems that all the skills are there, just not the shapes. Maybe some of our US designers might actually make more by forming andalliance like Sal has done. The market is small of course, but maybe at the lower prices you might sell a lot more. I think we would all prefer to see the best models at half the price. Maybe there is a way where everybody wins. Our top designers, not just in the US really just need an easier avenue to build while still making enough to make the effort worth it.
My own experience was that I loved the development but hated molding fuses and boxing kits after the first 30 or so.
If I had someone with a low labor rate waiting in the wings to produce and give me a cut I would have jumped at it. I ended up basically giving it to the guy that molded my wings. And he did a great job. The cost was $300 to get me $20. Cost of US labor.
$200 world beater should be possible. Does this sound reasonable?
Labor in Costa Rica is $1 for instance, and it's only 2 hrs from Miami.
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Old Jul 10, 2003, 07:51 AM
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My concern about laminar flow truly being 100% is from observations in turbulent air, where the ability to reattach laminar quickly is more the issue. I'm sorry for the rather sarcastic response earlier. The advancement is impressive.
The golf ball surface was interesting to me in that it showed a reduction in drag relative to smooth at high speed, traditionally the opposite is true with turbulators. I was thinking more about reducing drag in the area after the trailing edge "wake" -
Or at the flap hinge area on the upper surface where it might be possible to turbulate without the drag cost of traditional turbulation. I found no degradation in performance even when putting a 1/2" strip well ahead of the laminar transition area. Upper or lower. This shouldn't be true. The observation was subjective, but there was a lot of it.
Same reason to use muscle wire. Allow a smooth curve at the hinge area to reduce min sink. dimple the upper surface around the hinge since traditional hinges are also traditional turbulators. Its very hard to maintain laminar flow in the real world past a hinge line. Especially on the surface opposite the hinge.
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Old Jul 10, 2003, 12:25 PM
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San Francisco
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Over seas manufacturers could do themselves a big favor (and their customers) by emulating the basic dlg platform that has evolved here in the states over the past couple of years. The molded dlgs produced overseas (at least the ones I'm familiar with) are of very high quality but they miss the mark with longer than necessary tail moments for a straight launch (making it necessary to add excessive weight to the nose to acheive proper cg, which in part leads to structural failures in the pod area) , insufficient dihedral and draggy airfoils suited to larger gliders. If they would just tweak the basic platform a bit, these models would perform better. Airfoils better suited to hand launch gliders would just be icing on the cake. It's seems like Mark Drela's airfoils are there for the taking (I could be wrong about this though).


Greg
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Old Jul 10, 2003, 01:36 PM
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Quote:
Originally posted by greg morrison
The molded dlgs produced overseas (at least the ones I'm familiar with) are of very high quality but they miss the mark with longer than necessary tail moments for a straight launch (making it necessary to add excessive weight to the nose to acheive proper cg, which in part leads to structural failures in the pod area) , insufficient dihedral and draggy airfoils suited to larger gliders.
Actually, the new DLG's like the Fireworks III, Aspirin, and a few others, have started to use more appropriate sections. They also seem well proportioned.

But I still don't care for the swept tips with the necessarily truncated ailerons. Some degree of fashion design is OK, but not if it's a performance drawback. I also don't see the benefits of their hollow-molded construction. Solid foam, molded or bagged, makes much more sense in small models.
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Old Jul 10, 2003, 01:45 PM
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Quote:
Originally posted by Mike Waters
Its very hard to maintain laminar flow in the real world past a hinge line. Especially on the surface opposite the hinge.
On my SuperGee and my XP3, the flow is laminar all the way to the TE in the fast reflex condition, on both surfaces. I verified this with an acoustic transition probe in a wind tunnel. This is with roughly a 3/32" hinge gap on top, and no gap seals.

In the high-camber high-CL case, upper surface transition is naturally at or just ahead of the hinge. This is close to the ideal transition location for this operating condition, so tripping due to the more-open hinge gap is not an issue.
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