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Old Sep 05, 2011, 07:45 PM
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modifying the gentle lady airfoil

I built a GL about ten years ago and have a lot of fond memories about calm days and long flights, BUT it's ability to penetrate in wind left a lot to be desired. I want to build another but this time change the ribs to a different airfoil - S3021. Seems that this should be easy considering that the spars are in the center of the ribs.

Has anyone else done this?
I realize that this will require the wing saddle to change to allow correct incedence with the new reduced camber - trailing edge will have to be raised or the leading edge lowered.

Anyone see any other problems?

Does anyone know where to get the airfoil coordinates for the stock gentle lady airfoil.
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Old Sep 05, 2011, 08:23 PM
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look up the "miller mod" that skip miller did to an Aquila. I did this on a gentle lady eons ago and it did seem to help with wind penetration . could also have been the increased wing loading. the stock airfoil is a clark y I believe. If you are going to build another plane maybe you might look at the Chrysalis 2m from DJ Aerotech a nice build, flies great and is several orders of magnitude above the GL.
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Old Sep 05, 2011, 09:03 PM
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What's the fun in building a stock plane?
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Old Sep 05, 2011, 09:05 PM
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WHat I liked about the GL was even if the airfoil was a clark Y, it was thinned out which ment with some added weight (and reinforcement) it could penetrate well. Of course the added weight will probably hurt your thermalling ability. The two gliders I played with over the years was the heavier Electra but with extended wings to 90" which gave great combination of penetration and thermal ability. I also built a 60" sloper with the same airfoil that was a great fast flyer.

By changing the airfoil you are really changing the plane. One option I often thought of was to use the last spar as a hinge point so you can elevate the trailing edge for penetration and still have it fly like the GL you enjoy the rest of the time.
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Old Sep 05, 2011, 09:54 PM
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I've recently been playing with a hand-me-down Electra. Repowered it with a brushless speed 480 and 3s Lipo. I know it's heresy, especially for a hand launch guy, having that whirly thing on the front. Flies at about 37 oz. Quite a bit heavier than my GL. I've been amazed at how well it moves around the sky in windy conditions, but still thermals well. Moves much faster than my Oly II without the fluttering.

I'm thinking that the design is maybe a bit underated. I've been having loads of fun with it regardless.

I just finished adding spoilers and re-covering another one of my Electra wings. Kind of fun, I took a plaster mold of the top of the wing, and vac bagged some carbon spoiler blades that follow the covering sag. I'll take some pics and post later, but now I can finally get the bird down on the ground
I've yet to speck it out from a javelin toss though.
Dave
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Old Sep 05, 2011, 09:55 PM
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The GL is NOT a Clark Y. It's more like a size 9 Nike. Seriously, airfoils with totally flat bottoms are NOT Clark Y. Take a look at this picture:
http://www.ae.illinois.edu/m-selig/a...ots/clarky.gif
Note what's going on in the forward 15% or 20% of the airfoil. This curve makes a big difference and if you leave it off, don't call it a Clark Y. The real Clark Y actually has a decent speed range.

For a trainer, speed range isn't necessarily a good thing to have! Easier to break the glider up in the air if you spend a bit too much time pushing down when you shouldn't.

I had a GL fuse with an E205 wing on it, which was considerably faster than the original GL, but still could slow down reasonably well. I would be tempted to use a Drela airfoil if I was doing it. The aspect ratio isn't terribly high, so the slow end of the speed range will have a lot of induced drag if you use a lifty airfoil.

If I had Profili up and running at the moment, I would check out a thinned Clark Y, as I think some people have had success with such a foil. But I suspect an appropriate Drela airfoil would be somewhat better. Lots of info about them in the articles at charlesriverrc.org.

If you're not going to do a d-tube, then your airfoil will be pretty hard to analyze because of the sag of the covering.

I'm part way through (and stalled) on building a Chrysalis 2 meter. I expect it will have a pretty good speed range, just as the Chrysalis hand launch I had did. But if you want a really high performance two meter RES, bite the bullet, get the short kit, and build an Allegro Lite. At least if you have really good eyes and you can build light. That wing is skinny and so is the fuselage. Building light is really just being stubborn and insisting on weighing all the wood, using the appropriate weight in the right place, and being careful not to use too much of anything.

I suspect the "Miller Mod" might be a lot easier than most of the above suggestions and still get you a lot more speed range.
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Old Sep 05, 2011, 10:52 PM
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Quote:
Originally Posted by lincoln View Post
The GL is NOT a Clark Y. It's more like a size 9 Nike. Seriously, airfoils with totally flat bottoms are NOT Clark Y. Take a look at this picture:
http://www.ae.illinois.edu/m-selig/a...ots/clarky.gif
Note what's going on in the forward 15% or 20% of the airfoil. This curve makes a big difference and if you leave it off, don't call it a Clark Y. The real Clark Y actually has a decent speed range.

For a trainer, speed range isn't necessarily a good thing to have! Easier to break the glider up in the air if you spend a bit too much time pushing down when you shouldn't.

I had a GL fuse with an E205 wing on it, which was considerably faster than the original GL, but still could slow down reasonably well. I would be tempted to use a Drela airfoil if I was doing it. The aspect ratio isn't terribly high, so the slow end of the speed range will have a lot of induced drag if you use a lifty airfoil.

If I had Profili up and running at the moment, I would check out a thinned Clark Y, as I think some people have had success with such a foil. But I suspect an appropriate Drela airfoil would be somewhat better. Lots of info about them in the articles at charlesriverrc.org.

If you're not going to do a d-tube, then your airfoil will be pretty hard to analyze because of the sag of the covering.

I'm part way through (and stalled) on building a Chrysalis 2 meter. I expect it will have a pretty good speed range, just as the Chrysalis hand launch I had did. But if you want a really high performance two meter RES, bite the bullet, get the short kit, and build an Allegro Lite. At least if you have really good eyes and you can build light. That wing is skinny and so is the fuselage. Building light is really just being stubborn and insisting on weighing all the wood, using the appropriate weight in the right place, and being careful not to use too much of anything.

I suspect the "Miller Mod" might be a lot easier than most of the above suggestions and still get you a lot more speed range.
Thanks to all who responded.
Lincoln,
I don't plan do a D tube, in fact weight is more important for this ALES project. So Drela airfoils are out.
I've dreamed of building the allegrolite but funds are tight right now so this first ALES will be on the cheap.
I figured with the gentle lady I could just replace the ribs and still wind up wiht a light weight for the two meter class. Someone else built a 26 oz electric GL and mine will probably be close to his setup but with a better airfoil.
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Old Sep 06, 2011, 12:48 AM
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Oops, didn't notice the budget issue when I wrote the paragraphs that come after this one. Since you can accept a fairly stock GL, it seems you wouldn't need NEARLY the strength that the Allegro Lite has. The electric version's wing is 50% derated from the glider only version, but I've seen that glider only version take a full power winch launch from a pretty powerful winch. It doesn't look right, but it works. You could get by, I bet, with just .014" by .5" carbon top and bottom on the inner half of the wing, or perhaps just maple or fir caps. The carbon is only about $10 and maybe a few more dollars shipping, and if you get far enough with this project I might be persuaded to send you some for free. The short kit is $22 and the boom is $33. Or you could find an appropriately sized carbon fish pole. (I may have one I could send you!) So we're looking at a total of $65, plus maybe $10 more shipping than you'd expect with a GL kit, and let's assume an extra $10 for wood you can't scrounge from your own supplies. That's $85. The GL kit is $60, and for $25 more you can build something with the performance of an Allegro Lite, which needs a slightly smaller motor and battery anyway. Or less if you can scrounge, or if you want to cut your own ribs.
--------
So what you're saying is you can't build anywhere near as light (or you don't want to do the work) as Mark Drela. The Allegro Lite is LIGHT. The electric version, with a heavy, ordinary Speed 400 brushed ferrite motor and 7 nicads, weighed 23 oz. That's a d-tube sheeted design.
----
Another way to look at it:
I figure, these days, you could save an ounce or more on the motor, another ounce or more on the battery pack, maybe half an ounce on the receiver, and another half an ounce on the speed control. Probably your servos would weigh about the same. So Mark's airframe, minus components, was about 13 oz. You could afford to build it at 16 oz. and still have this very low weight.
and another:
Of course, I know we're talking about a GL here, which has maybe 25 oz. weight. Add a couple of ounces for a brushless motor, another ounce for prop, spinner, and ESC, assume a lipo motor pack weighs what the old 4 cell rx pack did, subtract at least two ounces compared to the old, large servos the GL was designed with, subtract another ounce because of the old, large receiver that we replace, and you're at the same place. GL wing area is listed at 663 square inches. It looks like a third of the top sheeting is already there (i.e. 1/6 of total required). Assume the d-tube should cover 1/3 or so of the wing area. 368 (221 on top and bottom, times 5/6) square inches of 1/16" balsa, 6 lb density weighs a little over an ounce, but you can probably lighten up the spar if you make it two caps and a shear web, especially if appropriately tapered. So significantly better aerodynamic performance only costs you an ounce or so in weight, if you pick the wood carefully. Obviously it's more work, however.
-----
If the work is an issue, here's another approach which I predict Don Stackhouse won't like. The Chrysalis has the same span as the GL and only about 9 percent more wing area, and I think it's a safe bet that the pitching moment of it's airfoil is a bit less, as it seems like it has a bit less camber. So you could probably just adapt the wing to a GL fuse. Or perhaps you'd have to enlarge the tail slightly or stretch the fuse just a little. No one would mistake it for a standard kit airplane, either!

The easiest way would just be to build a Chrysalis, or perhaps some other glider with a faster airfoil. There used to be gobs of these, but now not so many. You can still get plans for many of them. For instance, Carstens has plans for a very pretty 2 meter glider called the Sapphire which could be adapted.
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Old Sep 06, 2011, 09:57 AM
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About ten years ago, I had one of the original Allegrolite kits and it cost $160 without a fuse pod. I sold it a few years ago for $180 (still kicking myself over this).
I have to believe that it would cost almost as much to replace the stuff in that kit.
I actually have an allegrolite boom that I was saving for a future DLG, but I'm getting too old for 1.5 meter DLG.
Also have some .007 X 1" carbon strips for the spar reinforcement, but I'd have to order lightweight balsa. A couple of years ago, I stripped the only local hobby store of all their light balsa and they haven't restocked - it's more an arts and crafts store now.
Think your weight estimate is a bit low without a kit - probably more like 19 ounces scratch built but still light.
The Allegrolite probably handles much better than even a GL with a better airfoil.


On the other hand:
I built my first GL (also sold) to 22.5 ounces, so for this one 25 or 26 ounces powered would not be unreasonable.
My older eyes would probably see the GL at altitude much easier.
I'd scratch build the GL.
I could build the GL with heavier balsa without feeling that I'm insulting the design.
I still think it would be cheaper.

I will think on it. I have to finish a couple of other builds before I get to this one anyway.

Thanks.
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Old Sep 06, 2011, 08:15 PM
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Depends on what you mean by better handling. I remember that the performance was good (or perhaps great) and the control response was good too, but the stall was sharper than some. It's been a few years since I flew one, and then not for very long, so I don't have any more detail.
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Old Sep 07, 2011, 04:58 AM
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I'd build the SL...all the things you want better than the GL.
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Old Sep 07, 2011, 12:53 PM
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Lincoln beat me to it. GL does not use a clark Y airfoil. The GL airfoil is actually a rather nice design. It's relatively thin and moves the max thickness aft to reduce drag and still maintain good low speed capability. It was used in a number of Goldberg power planes, like Eagle and Eaglet and they certainly weren't slow. One of the best thing you can do to speed up you GL, is one of the easiest. Remove the wash out out of the wing! It's there as a crutch to help beginners and delay tip stalling, but adds tremendously to the overall drag of the wing. If you're an experienced flyer, you won't be needing it and wont' even miss it.

sean
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Old Sep 07, 2011, 01:26 PM
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Originally Posted by flystoolow View Post
I'd build the SL...all the things you want better than the GL.
What's an SL?
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Old Sep 07, 2011, 04:38 PM
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What's an SL?
Slippery Lady?
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Old Sep 07, 2011, 04:57 PM
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Originally Posted by John Gallagher View Post
What's an SL?
Sophisticated Lady
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