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Looking for info on controlling airfoil shape in moldless composite wing

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Royal

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Yep, it's tedious but not hard. When I contoured a very badly wavy wing I made the mistake of working on small areas. We felt the wing and marked low spots with pencil then mixed small (Dixie cup size) batches of dry micro. We had a lot of troughs to fill and it took a long time. Should have mixed the micro in a salad bowl and slathered it on the whole wing in one pass.
This is exactly how i have done all my restoration. Doing a door even if it looks straight...fill the entire door and block with a 4' bock using 80 grit and working it all the way down to 500 grit BEFORE primer. Then if you use wax and grease remover and spray it on your panel you can check your low spots usually from over sanding or pressing too hard deforming your base material.
 

Royal

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You guys should try out guide coat. Spray on. Start blocking. It wont clog your sand paper. Its very thin and wipes off with alcohol.
 

TFF

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Sanding micro is like sanding bodywork. Two things of note. It’s harder but less dense than plastic filler. If it’s on your plane, your plane is heavier because of it. Surface perfection can be a performance boost and performance detractor at the same time. Walking the line at Oshkosh, you can tell the builders who cared more about looks and the ones who picked performance.
 

Royal

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Sanding micro is like sanding bodywork. Two things of note. It’s harder but less dense than plastic filler. If it’s on your plane, your plane is heavier because of it. Surface perfection can be a performance boost and performance detractor at the same time. Walking the line at Oshkosh, you can tell the builders who cared more about looks and the ones who picked performance.
I care about both
 

TFF

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And there lies the dilemma. Pretty porker or lean and mean. Same with welding, you don’t grind a weld in aviation. What you lay is what you got.
 

enderw88

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It's important to understand that professionals working at first-rate competition sailplane manufacturers traditionally couldn't make accurate wings either, but made great sailplanes and won lots of contests regardless. Do template checks on any LS6 or LS8 and you'll come away wondering if the left and right wings were made in the same factory. And yet these are fine sailplanes that win Nationals level comps and fly straight as arrows.
I guess I have unrealistic expectations.

Here's the thing: Unless you are working with very aggressively laminar airfoils like the middle Wortmans or the current IF1 front runners, once you get aft of about 10% chord, absolute fidelity to contour takes a distant second priority to freedom from waviness. If your airfoil is in the ballpark and you get the waviness below 0.004" per the Johnson gauge (lower is better, of course), it'll be fine.
I thought they were much more sensitive back to mid chord. I figured "in the ballpark" meant +/-0.010".

I've seen regionals and even nationals won with some incredibly sketchy-looking wings. Their secret was getting the waviness down to a dull roar, getting good sealing and ventilation exhaust, tuning their instruments well, and then concentrating on strategy and tactics. And I've also seen as backmarkers the guys (and, yes, it seems to be a guy thing) who put hundreds of hours into profiling to laser-cut templates. If you want to fly, fly. If you want to sand, well, via con Dios, amigo.
There is a certain Zen to sanding...
 

Royal

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I guess I just don't like moldless construction. Just seems like going through making a plug over and over.
 

Victor Bravo

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Some really really smart innovative guy (that's you, BoKu) should come up with a way to run a strong laser or a 400 grit jitterbug wet sander on a CNC gantry mechanism, with the airfoil coordinates, twist, and all that programmed in. If you invested the time/money/sweat into making such a machine, you could probably make a good profit, letting it run on somebody's old Libelle in the background while you're laying up HP-24 fuselages in the foreground.

All composite gliders, Lancairs, Glasairs, the 1000+ Rutan canards, all those European LSA's, Pipistrels.... then you can play with re-contouring a layer of Bondo on racing Mustangs like they did on Dago Red, work with Jon and Steve on the NXT's , on and on and on.

 

stanislavz

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should come up with a way to run a strong laser or a 400 grit jitterbug wet sander on a CNC gantry mechanism, with the airfoil coordinates, twist, and all that programmed in.
It does not matter. It will make perfect shape, but not lightest one. Problem is - irregular thickness of fiberglass layer and sponginess of foam.

And just adding one extra layout of glass or carbon fiber to expandable then sanding - is again not worth - zero rigidity due to sanded fibers and heavy.
 

Vigilant1

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But - i was lucky, to find, that foil on hot wire cnc cutted foam in female mold shape make as good as possible mold with no effort.
Two minor questions:
1) What is the foil: Are you using Mylar (toughened PET), or very thin polyethylene, or "stretchalon", or something else? I'm trying to get an idea of how thick/stiff the stuff is. Are you using it to bridge/smooth the slight surface bumpiness in the foam for a better finish, or is it there strictly to keep the layup from sticking to the foam?
2) While the skin is still in the mold and everything is still supported by the mold, would/are you bonding in the spar and ribs to lock in the shape, or do/would you place the skin onto spars and ribs built separately. Doing it all in the mold sounds easier and less prone to errors.
 
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stanislavz

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Two minor questions:
They are not minor.

1. Is more on foam, than on foil. For rigid foams (xps/ pu / modeling) you may use as thin as possible foil. And use some vacuum for it to cover all small details. But - rigid foam have some dravback in it. I am cutting styrofoam for 10+ years. And xps styrfoam is not easily available in blocks needed for full fuselage. And it have some internal stress. You are forced to glue it to rigid board before cutting to get good result, but it will still bend when removed from cut block. Cutting some preglued blocks leads to mixed and unpredictable results. Do not ask me how i did know that :)

Going to eps foam (white beads, same as used in packing) - you:
a. Can buy it in 4 x 1.2 x 1 metre blocks.
b. Any required density. Eps-200 is as hard as it can get - you stand on it, no sign left after
c. Easier to cut than xps.
d. No internal stress - cutted straight - left straight
e. no need for external support - put it on flat surface - and you have nice and rigid molds.
F. You have 4 sides to cut mold into. As i did shown before - 2 sides goes for tail part of fuselage, 1-2 for straight/tapered wing.

But - it is not sand-able, and it have some imperfection - different density around beds leds to slight waviness (or dimpleness ?) which is sanded easily. So - just use thicker foil. I did tried with mylar / ordinary PE /brown packing tape / waxed paper / paper with aluminium foil all glued using spray glue - all went well. No stress on mold. If you can glue this foil on sunny day without any wrinkles - it is ok. If it is to rigid to cover some sharp bends - go to thinner one.
2. 1 one. Not tried it on big part yet, but would cut mold for nose part of the wing an top rear in one with required twist. Add/mold in place all left parts - nose ribs, main spar web, spar, ribs. Or one wWww shear web. And close it with rear bottom part of wing skin. Or make it from fabric later. This left only one exposed join - just below main spar.
 

Heliano

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Interesting discussion. I agree with BoKu: for "normal" airfoils the first 10-15% are critical, and a lot less critical behind that. The wing of my plane is moldless glass/foam, but the idea of dozens and dozens of hours of sanding terrifies me. So we´ve developed a method - you can get the idea from the attached sketch - that simply eliminates the need of sanding and allows a pretty good, waveless surface. We are calling it it "método caipira" (hillbilly method) because it is a no brainer and does not require any hi-tech tool.
 

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Royal

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Interesting discussion. I agree with BoKu: for "normal" airfoils the first 10-15% are critical, and a lot less critical behind that. The wing of my plane is moldless glass/foam, but the idea of dozens and dozens of hours of sanding terrifies me. So we´ve developed a method - you can get the idea from the attached sketch - that simply eliminates the need of sanding and allows a pretty good, waveless surface. We are calling it it "método caipira" (hillbilly method) because it is a no brainer and does not require any hi-tech tool.
Are you still doing a wet layup with fiberglass on top? Or is that going to be your "mold"?
 

Jon Matcho

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So we´ve developed a method - you can get the idea from the attached sketch - that simply eliminates the need of sanding and allows a pretty good, waveless surface. We are calling it it "método caipira" (hillbilly method) because it is a no brainer and does not require any hi-tech tool.
This sounds quite promising, but I'm struggling to understand the approach from the attached sketch. Do you have any pictures to help me understand?
 

sming

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From what i understand:
1) you glass the top first
2) with only the top you can still conform the foam to the rib. Create a corner on the rib to support it.
3) then glass the bottom and glue it to the ribs corner.
Got it right?
I dont understand how you do it for the other side of the wing ;)
 

BJC

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Interesting discussion. I agree with BoKu: for "normal" airfoils the first 10-15% are critical, and a lot less critical behind that. The wing of my plane is moldless glass/foam, but the idea of dozens and dozens of hours of sanding terrifies me. So we´ve developed a method - you can get the idea from the attached sketch - that simply eliminates the need of sanding and allows a pretty good, waveless surface. We are calling it it "método caipira" (hillbilly method) because it is a no brainer and does not require any hi-tech tool.
That sounds like the process used in the Vision.

Very labor intensive.


BJC
 

Heliano

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The critical step is using the strip as template to laminate the 90-degree flange (strip has the inner surface covered with two layers of fiberglass - same as on the inner surface of the skin plates); laminate two layers of fiberglass on the skin plate's inner surface (plate remains flexible as long as only one surface has fiberglass); glue the skin to the flanges using flox (epoxi with flocked cotton fiber, or milled glass fiber)- the outer surface will be flush with the rib edges. All glued surfaces must be sanded with 40-grit paper. Yes, it requires a certain amount of labor, but certainly it is much less labor than sculpting the foam with sandpaper. There is another aspect too: foam plates are higher density near the surface and lower density in its core. The method I described preserves the higher density outer layer. Now I do not recommend using this process in aircraft designed and tested using another construction method, unless one is willing to do static testing and aeroelastic evaluation from scratch.
 
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