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Post by atmosteve on Mar 3, 2008 15:37:25 GMT 10
Hi all, starting a build on a Skua shortly, with the intention of flying it at our larger liftier SE Qld slopes. Now, I have built an RC model or two in my time from all sorts of materials, but have not done the EPP goop thing yet. After having done probably too much reading on the subject I am likely to give this sequence a go, what do you think; 1. Spackle/PU filler treatment 2. 1-2 layers thinned goop 3. Some strapping tape on certain critical areas, not a lot though. 4. Another coating of thin goop, a few days curing time. 5. Pressure pack pretty patterns on the glider 6. Lastly 6-8 coats of thinned goop, up to 1 hour apart and then left to cure in a warm, ventilated environment for at least a week. Before i do the sequence I am contemplating doing an experiment run on a small slab of epp first. As it will be a pull apart, the spars will offer some torsional rigidity and i will compensate for that weight by moderate amounts of bi-di tape and spray paint. It has full length 9mm CF spars. I'm not sure if will be able to get the toulene thinning agent up here, so i suspect i have to source it from further afar. Thanks for reading, atmosteve.
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Post by sean on Mar 3, 2008 16:11:49 GMT 10
Hey Steve, that sounds like a really interesting approach and could give a really nice finish I reckon.
The only thing is there wouldn't be much torsional stiffness with that setup, even with a good spar system. For speed torsional stiffness is a must - if you cover the whole wing with 45 deg bias strapping tape that would do a good job, but a fair bit of weight there I guess.
You can use Xylene to thin the Goop, which is available at most Bunnings stores.
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Post by sf2 on Mar 3, 2008 19:43:43 GMT 10
Having done the EPP thing recently my self, I discovered that putting goop, thinned or otherwise was no longer a recommended technique due weight and the inability to straighten it out after an unpleasent landing. The 45 bias thing seems to be the go, then to remove the edges in the covering material , spray with 3M 77 let dry then spackle and sand. Haven't tried it myself yet but will on my next model. It might prevent the first layer of spackle.Which I've never figured out why you put it on if you cover the wing in tape and do the above, then use your prefered covering(solarfilm, monocote,profilm(my choice)). Hope that helps PS.NCFM have some stuff on it's site.
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Post by thevon on Mar 3, 2008 20:33:13 GMT 10
Steve, If you’re doing PU with the spackle you don’t need to goop over most of it. The Goop will soften up the spackle and make it rubbery, and also add more weight. This is my 2 bobs’ worth for that plane: Personally I like the idea of the curved fiberglass round 2.4mm rods, near the front, with one directly over the other, like I did with the recent Ozprey (see thread). Won’t break, but reasonable stiffness and torsion benefit. But could be hard on a Skua? Dunno. The 9mm spars sound great. Make sure they’re well PU’ed in so they grip to the foam all around. PU/spackle at about 1:6 but work it fast and spatula it very smooth, to avoid needing to sand much. When it starts to gum up, stop and mix some more and keep going. You need to be quick – less than a minute working time. Tape sticks really well to PU/spackle, and if left non-gooped over the main wing area, it adds a surprising amount of stiffness. But if you goop it, you lose that. Thinned Goop around the LE’s and tips only, maybe 1-2cm from the LE’s and 5cm from tips. 2 coats is heaps. No need to do a full coverage, as it adds weight, but as said above, putting a full layer of tape over the spackle helps to keep some stiffness (by the tape protecting the spackle from the Goop). Your choice here. If not fully taping, use an A or X pattern. Skua instructions are good here, but you can put in at closer spacing than the pictures. But make sure for every piece of tape, there’s one directly matching on the other side, to oppose it (put one side on then use pencil to mark the edges at LE and TE, then put the other one on between those marks). Make sure you credit card squeegee the tape, and fine sand every piece of tape immediately after putting it on – rubs off the antistick, and presses it in nicely. You only need 2 layers of thinned Goop over this. More is going to add lots of weight. Just one thin layer over the back of the wing. If you like, put a few more coats over the LE’s and tips. It dries very fast, no need to wait long. Hang it in a breezeway. The only time you’d need to leave it for a day or more is if you were doing the “full goop” treatment, but that’s for a heavy rubbery plane. I’m not sure about the paint. This subject has been extensively discussed in Predator’s “New Stuff” thread on RCG and they’ve had trouble with getting stuff to stick (the paint is a barrier, preventing adhesion). See www.rcgroups.com/forums/showthread.php?t=790302 Matter of fact, half this thread seems to be about colouring. I’d say stick with Profilm (pardon the pun). If you leave the surface as raw Goop it will get really dirty looking no matter what you do. Once you iron the Profilm onto Goop you’ll never be able to get it off again!
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Post by skyboyken on Mar 3, 2008 20:35:31 GMT 10
Hi Steve,
having had an S15 Skua and had heaps of fun flying and crashing it to death I'm sure you'll love yours.
2 suggestions for you:
1. The Skuas love weight. I definitely recommend you put a ballast tube in at the recommended CG. Skuas hold their energy incredibly well and go like stink when ballasted.
2. Don't worry about doing a light build. Make it stiff and durable. I've flown my fully ballasted Skua in conditions where only the light air combat wings could stay up and it goes great provided you fly smoothly (dual rates help). The Skua stays up better in light air than any other fast plank I know of, with the other contenders being the Ozprey and the Moth. I second Sean's recommendation re the strapping tape - that's what mine had.
Whatever you decide, Go Great!
Ken.
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Post by sean on Mar 3, 2008 20:57:57 GMT 10
Thought I might add a further two cents to muddy the waters ... When applying Profilm goop is your best friend. I have found that one coat of well thinned goop over the entire wing adds little weight but adds tremendous toughness and crash resistance to the Profilm - it becomes almost permanently bonded to the wing. The planes I've down this to are insanely crash resistant. If you only use Super 77 the Profilm starts to lift after only a few big hits. NCFM only recommend avoiding a goop-only fuse finish, they say the fuse needs Profilm/Solartex and strapping tape to toughen up the structure, but nothing wrong with goop under the covering.
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Post by atmosteve on Mar 4, 2008 1:01:48 GMT 10
Thanks everyone, plenty to think about in your suggestions. Ok, I will go the profilm, only using two colors and a little vinyl trim. Would like a layer of thinned goop under it for adhesion etc, but dont want to turn the spackle filler under it soft either. It looks like the full goop treatment is an old evolution experiment as you say, so i wont depend on it for rigidity. I gathered that the spackle/pu filler could help rigidity, but if it wont react well to a thin layer of goop over it then I best get liberal with the filament tape, no worries. The spackle/goop relationship seems the weak link. I might follow The Von's idea of pu/spackle, full filament tape covering, acetone cleaned, then a coat of thinned goop before profilming to help it stick as Sean suggested, surface has to be strong, this ones going to have a hard life. It wont be light, but thats ok considering it is likely to be zooming around anyway and having an adventurous life. Ken i have come up with an alternative ballasting idea to the tube system, but may still go that way if it doesnt look like functioning 100% during the build, but its worth a try. Thanks again all for your help.
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Post by thevon on Mar 4, 2008 6:39:36 GMT 10
I have to say I haven't spackled a low density EPP. I've only done it on the denser foams. The Wowings planes are 1.3 density, and I think the foam would "take" a lot more filler than the dense stuff.
Steve I did that method (as you've described) on the Reaper, ie no goop under the tape, and its survival of catastrophic prangs has been excellent! I seem to be able to iron the creases out better than anything else I've had, too.
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Post by skyboyken on Mar 4, 2008 19:59:24 GMT 10
Just a quick correction for your post Andrew, the Skuas are now made from 1.9 Lb EPP. They used to be the lighter 1.3 but no more.
Steve, whatever works for ballast is all good!!
Go Great!
Ken.
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Post by sean on Mar 4, 2008 20:14:39 GMT 10
My 1.2M Skua is made from the light 1.3lb stuff, it is an older Kye era cut so maybe it's different to the current ones? But I've been told the bigger S15 is definitely 1.9lb EPP now.
I spackled my Skua and it worked very nicely indeed, didn't add too much weight so no troubles with 1.3lb EPP and spackle.
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Post by atmosteve on Mar 5, 2008 1:12:45 GMT 10
Speaking of spackle.. was at bunnings today buying xylene, odds and ends and was really intrigued by the range of powder spackle types available- with polymer additives, this and that additives, The selleys polymer flexible exterior one looks ok.
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Post by atmosteve on Mar 5, 2008 11:16:59 GMT 10
Guys, do you still use the kwikset lightweight filler when mixing with the PU, or are there other spackles to be used with the glue. Sorry for the questions, I've searched through some threads here and elsewhere but dont have a definite idea yet, the range of spackle in liquid or powder form is amazing! Some of them might be flexy and strong, but heavy when mixed. Others might be light, but weaker. Not sure at all. Other than that I am set to just do the build pretty much , its all planned out and because its a pull apart, will have some different aspects in its structure. Cheers, Steve.
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Post by thevon on Mar 5, 2008 16:15:58 GMT 10
Steve, the only spackles we've tried are: 1) Bostik Kwikset 2) Selleys Polyfilla rapid lightweight and 3) Red Devil. Sean's used 3) and I've used both 1) and 2). Personally I like 1). I wouldn't try a powder one.I think some of those are like plaster, but the premix ones are like undercoat acrylic with a filler. AND it HAS to be lightweight.
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Post by atmosteve on Mar 6, 2008 1:32:47 GMT 10
Thanks Andrew, I bought some of the Bostik stuff today after reading all the labels on the various containers, yes some of it sure is hard heavy stuff, but the kwikset sure is light. Will do some trial runs with it and PU and goop/coverings etc over the next few days as the kits arrival is delayed, so i've time to muck about with it and get it right before it touches the wings.
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Post by atmosteve on Mar 12, 2008 10:29:36 GMT 10
Started the build yesterday, the foam doesnt appear to be thicker in its properties than any other, it looks like lighter EPP, it could a slightly tougher or concentrated mix of epp that foams up similar to any other density perhaps. Anyway it doesn't feel flimsy.
The CF 9mm extrudud full wing spars are pretty serious! PU'd in they reduce longitudinal flex to a minimum. Working on the pull apart spar system today, which takes a lot of care.
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