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Old Apr 30, 2015, 04:04 PM
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wing molds

Has anyone experimented with simple wing molds? I have in mind to hotwire the forms in styrofoam, line them with FG, then iron on 7 or 10 mil laminating film.

I know this won't produce a highly accurate surface, but I wanted to play around with making a wing skin+structure laid from the inside instead of from the outside.

To keep things simple, the wing will be constant-chord with no twist. That way I'll only have to make up two molds, one top and one bottom. No twist requires a zero to slightly positive pitching moment airfoil, so I'll use the PW-75 section, which has a +.01 Cm/4. The panels will have 9"chord (231mm), 23* sweep. The span will be 87" (2.2m). Aspect ratio comes out to 9.6.

Thoughts?
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Old Apr 30, 2015, 05:41 PM
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If you run off some tests with the lamination stacks and test for the hot wire kerf widths and make your templates to take these things into account I don't see why you can't get both nice finishes AND accurate airfoil shapes.

Instead of glassing right to the foam what about laying in and bonding down aluminium flashing sheet? Wax it well or use a thin mylar release sheet and I think you'll find that the aluminium bridges over any slight waviness in the wire cut surfaces that may occur to give you an even better shape.
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Old Apr 30, 2015, 05:53 PM
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Hold molded... That would be nice!

I know that it's been done before over in the scale sailplane section by yyz. He built a rudder that way. Just as you described.

I would be nice to just lay up some skins, add a spar and have a translucent wing.

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Old Apr 30, 2015, 09:55 PM
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Thanks Bruce: I was thinking about aluminum (aluminium? like platinium? ). It would take some careful forming. Any ideas about how to roll it to shape? It would indeed make a great surface.

Maybe wetted paper would work, like newsprint covering over foam. I'll give it a try.

Kent, a translucent wing would be loverly! Kind of like that Mara being built. So, maybe some transparent Monokote could be the first (outermost) lamination. I was thinking of having the wing be foam cored, but maybe a balsa skin under the outer lamination would work.

I'm going to be awake all night, thinking of the possibilities.

Got to finish the BreakDancer first. Or do I?
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Old May 01, 2015, 04:07 AM
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http://www.roofingflashing.co.uk/lea...uBoaAp2P8P8HAQ

The type of material Bruce mentioned is very flexible,you'd have no problem getting it to conform to shape.
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Old May 01, 2015, 08:05 AM
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Stupot, that material is VERY different from what we call "aluminum flashing". Ours is simply thin (maybe half-millimeter?) sheet aluminum. I found what you're referring to, and I think I used it a few years ago on a greenhouse roof. It might work and would be easier to apply to foam than metal, paper, or FG. I'll have to get to the store and check the surface.

It is fairly thick, so I can't use it around the LE without making new templates. It wouldn't distort the airfoil significantly over the rest of the surface.

Have to think about ease of installation vs. cost... that stuff is fairly spendy.
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Old May 01, 2015, 03:01 PM
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The stuff I get from Home Depot or other lumberyards would certainly require forming around something to get the leading edge radius right. It's pretty springy. But once away from the LE radius it flexes easily enough to just glue it down to the molding beds.

I've done some old control line combat models with foam cores that use newspring and well thinned PVA glue to bond it to the foam Additional layers to create about 6 tapered out layers at the high stress areas such as around the engine mount and the wing's "spar" zone produced a light but very strong wing.

We're not talking about anything vacuum bag smooth through.

If you try this wated PVA and newspaper idea keep both sides equally as wet during the continuous layup process. The glue shrinks like mad and if I didn't keep misting the side done already with a plant mister it started to pull and curl span wise. But when the paper layers are all set to go and sitting in a big shallow tray of the water and glue mix the layup goes fast and misting can either be avoided or minimized. Work from side to side laying first the center cap on each side, then the graduated semi span layers flipping back and forth and only at the end lay on the lower side and wrap around then the upper side and wrap around. brush it all down smooth and tight then hang it from a wing tip or center section piece such that the wing is vertial in some direction spanwise or chordwise and that air can reach both sides equally as well.

The result of all this is a bit rough and ready. Sort of "stand back and squint" quality. But it IS sturdy. It took being buzzsawed in mid air by the other planes to finally put these out of operation. Oh, and one was "bellcranked by another model that hit right on the engine and bellcrank beam mount. NOTHING will stand up to that! ! ! ! Both models were totalled as a result.
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Old May 01, 2015, 03:18 PM
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Ed -- Martin Bamert from Switzerland published his method which basically uses epoxy and sand. It's a bit complex for your rather basic project, but I think it's readily adaptable to what you want to do. His is the lead article in SoarTech 10 which you can access here -- http://soartech-aero.com/SoarTech-Online.htm

Please keep us tuned in -- Herk
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Old May 01, 2015, 04:01 PM
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The article reminds me of concrete finishing. Very attractive.
I wish that I could go into the wayback machine and give Martin a 1080p camera.
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Old May 01, 2015, 04:05 PM
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Quote:
Originally Posted by EdSoars View Post
I have in mind to hotwire the forms in styrofoam, line them with FG, then iron on 7 or 10 mil laminating film.
yyz cut foam "negatives", then laid up glass to them, then vac. bagged on mylars for a nice surface. I guess conventional wisdom would suggest that such a mold would not be durable enough for many lay ups.
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Old May 02, 2015, 08:43 AM
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Herk: those Soartech articles are wonderful reading. Thanks! I was still building Sagitta kits back then. Jeez. It is interesting that the molded, very accurate 78" planes at 15 oz/sq ft were out-climbing the much more lightly loaded built-up sailplanes. Also, 1993 was after the Swiss had developed the nurflugels for F3B, which revealed the performance potential for swept wings. I still believe swept wings have more potential for development than conventional planforms, just because we ignored them for so long. And that is another story.

The process described by Martin Bamert is more time-consuming than is worthwhile for my objective, which is to have a quickly-produced female mold for four or five wings. The yyz process Kent references is just about right for my application.

I want the molds just to prefab the skins: the internal structure won't be laid up in the molds. The skins will be pressed or bagged onto the cores, which will contain the servo pockets, spars (if any) and joiner tubes. That way I can produce several sets of skins that can be applied to cores with different variations in sweep, control surfaces, etc.

I'll post progress as it (sporadically) happens.

ed
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Old May 04, 2015, 09:13 AM
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Another thought: I could produce a smooth surface by using spackle, and it wouldn't even have to be lightweight spackle. It would provide enough bond for the mylar. The mylar could be applied without risk of heat distortion by using a vacuum bag.

But what adhesive to use? Epoxy would be good if it bonds well to the mylar's adhesive coating. Will try a test panel today.
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Old May 04, 2015, 11:53 AM
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Quote:
Originally Posted by EdSoars View Post
Another thought: I could produce a smooth surface by using spackle, and it wouldn't even have to be lightweight spackle. It would provide enough bond for the mylar. The mylar could be applied without risk of heat distortion by using a vacuum bag.

But what adhesive to use? Epoxy would be good if it bonds well to the mylar's adhesive coating. Will try a test panel today.
I'm not sure about how you're using spackle ,but have a google on dry wall adhesive.It comes in a powder,but it's designed not to shrink and is harder than lightweight spackle- probably cheaper as well
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Old May 04, 2015, 12:47 PM
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Nothing can be cheaper than the bucket of premixed dry-wall spackle sitting in my garage!

I just spread it on the foam surface, remove almost all except what's in the pores, (none on blue or pink foam; only on EPP) let dry and carefully sand smooth.

I'll have a look at those adhesives. There may something I can use that will save a lot of time.

thanks!
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Old May 05, 2015, 05:07 PM
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I just cut the mold blanks. The only hitch so far was a glue joint hidden in the foam sheet. I'd made it longer for another project and forgot it was there. The hot wire hung on it for a second or two, but the grooves will go away easily.

Bruce, I may try the aluminum sheet if I can find some 9" wide locally. The PW-75 curvature isn't so bad. Got any recommendations for adhesive?

I spent some time on the zanonia.de website last night looking at some of the high-performance wings there. I think what separates the performers from the rest is very accurate construction. I also spent some time yesterday trying not to drool over a buddy's new Dynamic 80 DS rocket. That was one accurate build all right, but it was also incredibly rigid. Accuracy, rigidity, perfect control sruface movements.

So I'm hoping the limiting factor on this exercise will not be the planform, the airfoil, or the tiplet design!

ed
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