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Old Sep 18, 2010, 02:18 PM
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you don't have to seal it, as long as the surface is smooth and you are using epoxy as the resin. the only thing you need to watch out for is resin soak. some of the resin will soak into the pores of the wood, so you'll want to wet it out enough to get a good bond and also, just enough to get a good surface. peel ply is the best way to do this, but not absolutely necessary. if you don't use peel ply, you can simply scrape the cured surface with a razor blade and this will help remove any pitting of the resin. it will also save you a lot of sanding time and also strain on your elbows. LOL. if I were you, I would do a few test pieces first, so you can see what the resin soak looks like and you can adjust accordingly. you just need a few scrap pieces of balsa, some cloth and resin. generally, I would wet it out with a brush and then scrape it till it barely puddles at the edges of the scraper. that's usually enough resin soak for bare balsa. if you've got too much, it'll lift the cloth from the surface, which you don't want :^)

as far as weight is concerned, 4lbs is a little bit on the porky side, but not a complete catastrophe in the making. as long as your wing loading is within spec, there is enough power at .75/.8-1 thrust to weight. this isn't a rocket ship afterall, so it doesn't have to be flown like one. remember that many planes of this type (full size), might only have a thrust to weight of .5-1, or roughly there abouts. I think you'll be ok and you'll know for sure on the maiden. LOL. weight is great for penetration, but presents some problems in the landing/takeoff envelopes. for this, you may need flaps if they were marginal, or not needed before. because it hasn't flown yet, you don't know these variables, so it would require some experimentation. I've always gone with flaps, if I've thought that the weight was a bit on the heavy side. yes, they add extra weight, but the lifting capabilities far outweight the weight that they add. especially when your plane is coming on final like a scalded cat without flap. LOL.
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Old Sep 19, 2010, 06:12 AM
smithy
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Cornwall, UK
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Thanks Rich. Forgive my ignorance but never used glass and resin before, so as i understand it you brush on the resin then lay up the cloth and scrape off the excess? I think trying it on a test piece is sensible. Am i right in saying that you should use the lightest possible cloth?

Regarding the weight, how heavy would an average twin minifan setup with cells for each fan and esc's be roughly? Iv'e got basic flaps on this one so that should help like you say, on the next one iv'e redesigned the wing with more realistic fowler/ extending flaps as per the real one increasing slow speed performance and wing area.

Cheers, Phil
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Old Sep 19, 2010, 07:41 AM
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Don't use a brush on the larger surfaces, it usually adds to much epoxy, if you do use a brush then you'll need to go over it afterwards with a credit card or similar to scrape off excess epoxy. I usually do it like this; cut your fiberglass cloth to size and place on the surface to be covered, then I mix up a batch of epoxy and just pour a small amount onto the surface and spread it out with a credit card or similar tool, I only use a brush around curved sections and around the edges, the goal is just enough epoxy to fill the weave of the cloth, it should not have a shiny surface after it has cured. Only pour on a small amount so it wont take you much time to spread it out, if it sits too long in a pool on the balsa, it will start to soak into the wood too much. Then trim around the edges and do the other side, then when that is also cured lightly sand with 240-320 grit paper, clean and apply a second coat of epoxy, this second batch I like to mix 50/50 with microballoons as this helps a lot with preventing pinholes and also is easier to sand smooth.

I use 25gram cloth, and please get some quality material as it will save you a lot of finish work. I get mine from www.r-g.de, their 25gram cloth is really nice. It is tighlty woven with a smooth finish, some of the cheaper stuff usually has a more uneven texture and surface which means more sanding is required for best results. The best you can do if this your first time glassing is to start with something easy as the tail surfaces and use those to practice on before doing the larger more complicated stuff like wings and fuselage.
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Old Sep 19, 2010, 10:42 AM
smithy
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Thanks Arngeir, very helpfull. I would have dived in and brushed the resin on first if i hadn't read that.

Cheers, Phil
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Old Sep 19, 2010, 12:08 PM
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Also, what epoxy do you have? I usually use Z-poxy finishing resin for these jobs, as it's really nice to sand after curing and cures quickly too, only downside to this is it has a somewhat short working time so do not try to mix up too large batches if you end up using it.
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Old Sep 19, 2010, 01:20 PM
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yeah, you don't want to brush the resin on first. put the cloth on first and wet the resin into the cloth. I've used Arngeir's method of only using a scraper, and it works well on flat surfaces. just be aware of some peculiarities of only using a scraper to apply resin...............if you only use a scraper, the cloth can lift from the wood and/or move around. this is why I use a brush to stipple the resin into the weave first and then use a scraper to spread it evenly, while removing excess resin. this is by far the best way to apply resin. with a scraper only method, you can never be sure that the resin has completely saturated through the weave and soaked into the wood. this will cause your cloth to lift while curing. with experience, I would use Arngeir's method without doubt, but since you are a noob at this, I would stipple in the resin first with a stiff brush, using a vertical motion to push the resin through the cloth, then scrape off the excess till you barely get puddling from the sides of the scraper. this is enough resin to allow complete saturation and soak into the wood underneath.

if at all possible, don't use a 1:1 epoxy resin. your work time will be too short and you'll be forever mixing small batches. if you are stuck with a 1:1 epoxy, the 30 minute variety should be the least amount of working time to buy. if you can buy resin that has a higher mixing ratio (mixing in grams), you'll have an easier time with application and curing. you should still mix fairly small batches, since you don't want it to start curing in the pot. a good scale will be helpful, since you can measure out the resin to the closest gram. I believe my resin is 32 grams hardener to every 100 grams resin. I can check that for sure, but you get the point. I simply zero out the scale with the empty pot on it and start pouring till I hit 132 grams total.
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Old Sep 19, 2010, 01:30 PM
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the other solution of course, is to not use FG at all. since you have a solid wood structure, you may well save yourself some weight by priming and painting the surface instead. just shoot enough primer and paint to get a solid coat of each. this is by far the easiest solution, but you'll have to make sure your surface is primo before you do it. putty and sanding is your best friend here, if you decide to do it that way.

as far as weight of stuff, it would be helpful to know what fans, packs, ESC's you'll be using. if it were me, I'de be weighing all that stuff if you have it and then weight the airframe as it sits now. add it all up and then divide it by your wing area. that will give you your onces per square foot/square inch and you've got your wing loading. then take your total weight and measure it up against your total wattage output. this will tell you what your Watts/pound is for thrust. as long as you are above heading towards the .7:1 area, you wil be fine, or somewhere around 150 Watts/pound. that's the way I would go, rather than ask someone about what will work when they don't have something to relate it to. yours is a rather obscure model to say the least, but a twin is a twin afterall. LOL. if you don't know your total wattage output, just search on here as I'm sure someone has done the same power system as yours at some point in time or another. just list what you have and I'll (or others) help you out
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Old Sep 19, 2010, 06:21 PM
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Quote:
Originally Posted by Arngeir Blakseth View Post
.....
Quote:
Originally Posted by LuvEvolution7 View Post
....
I'm surprised no one said to thin the epoxy with denatured achohol AKA Isopropyl 99%. That's all they add to the finishing stuff from what I was told.

I use actual full scale aircraft Epoxy/Resin from a local composite store. 100g:45g but that can depend on the product.
As discribed cloth first but I even thin the epoxy enough that i can put it in a spray bottle, and brush stipple into the shape of the nooks and crannies as I spray.
My epoxy has a 12 hour pot life so it gives plenty of time to remve access.

Since you guys didn't mention it, is that a bad idea. I've done this with foam and small balsa apps. I started this brush on Denatured alcohol from how Combat models did there early foam covering. The stuff was so thin I just started putting it in spray bottles. The bottle is just one use and the wife is always screeming that all the spray bottle are gone again.
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Old Sep 19, 2010, 07:04 PM
smithy
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Cornwall, UK
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Great advice guys,thanks. Rich as far as the edf setup goes i havn't got it yet, so looking for a hot twin minifan setup. Ive been using an old mk1 minifan that i had for a few years for measurements but i wont be using that one, minifan pro's instead.

Cheers, Phil
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Old Sep 19, 2010, 11:26 PM
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what size are the minifan pro's? my memory sucks, so you are gonna have to refresh me..............is this for 70mm application, or smaller? if it's 70mm, I would look into the WM400MKII from Velocity RC. if they don't have any in stock, you can check with Gary at Efflux RC as he carries them in stock. if it's 60mm, you can look into the WM300 fans. for the money, you can't beat any of the WM series IMHO. 5S should be more than enough for your needs. in fact, 5S might be too much, but you'll have to check into that. I believe on 5S, the WM400 is putting out about 6+ lbs, for a pair of them. that should push this baby around the sky very well. plus the efflux is fairly high and you don't have to worry about an exhaust cone.............it's already done for you. you just have to make a straight pipe from the fan to the exhaust. the other good thing about the WM fans, is that the motor is up front, thereby making it easier to CG, especially on a plane of this design. something to consider at least.

Max

that sounds like a great way to apply the resin, especially on a foamie. I've never tried it that way, so I'll have to give it a whirl some time. you can thin the eposy with denatured alcohol and stipple it in with a brush. it will increase your work time and stretch the resin over a larger area. I've done it this way before and had some good success doing it that way. with balsa, I'm guessing that if it's too thin and sprayed through a bottle, the resin soak would be huge and add a lot of weight to the frame. would be worth experimenting with though.
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Old Sep 20, 2010, 08:29 AM
Pointy end at the front.
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wow!
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Old Sep 20, 2010, 11:00 AM
Some epoxy & it flies again...
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Santa Cruz, California
Joined Sep 2008
507 Posts
Maxthrottle,
nice hint to stretch resin with alcohol. How about adding micro balloons to make it lighter and for easier sanding?
Just for the records I haven't done any glassing yet, just throwings out before trying it.
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Old Sep 20, 2010, 12:05 PM
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Molde, Norway
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Mixing epoxy with microballoons 50/50 works very good for the second coat. Less pinholes and easier sanding. I have tried thinning the epoxy with alcohol and altough some prefer it that way I actually like to use the epoxy as it is. I guess it's a preference, if thinned it's easier to apply epoxy with a wide brush much like painting though, but you can also end up with too much soak into the base material doing it like this so, or if not enough is applied it wont fully saturate the weave. It also affects the curing time and strenght of the epoxy, curing takes longer and strength is lowered.
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Old Sep 21, 2010, 08:16 AM
jean-claude Terrettaz
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Carp, Ontario, Canada
Joined Mar 2000
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Something good to brush a thin coat of aero Gloss70-4 balsa filler first on the balsa surface. This will seal the balsa pores. The balsa is like a sponge and you will have a light plane. I use a bit of alcohol for the epoxy . But you can simply heat the model before applying the cloth and epoxy.
We can tell the winter is approaching!
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Old Oct 26, 2010, 03:01 PM
smithy
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Cornwall, UK
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Well, im back on the build! Concentrating on finishing the wing assembly now, fitting internal controls for flaps and aileron.

Phil
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