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Old Jan 08, 2010, 06:12 AM
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Glassing Center Sections

I'm about to glass the center section of my .60 sized Super Kaos. It's been awhile since I did this, most the planes I've built recently have used internal structure to to reinforce the wing joint.

I have used polyester resin, epoxy and CA as the materials used to bond the glass to the wing. Although they worked, none of these methods were a way I was overly enthusiastic about! Do you have a method that you feel is fool proof or at least works well? If so, please explain.
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Old Jan 08, 2010, 07:33 AM
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Joe'n Kody's Avatar
Yeppoon Queensland
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Polyester resin is ok for joining the wings if you are hard up for anything else. I never use it on my planes as it is rather brittle and will crack from extended vibration. This cracking can be stopped by using more glass and resin but the plane will be too heavy to fly.
Never use CA for joining wings together by spreading it over the glass tape or strips being used. CA is too dangerous to your health in such huge areas and it is a pain to sand. It is also rather brittle but the expense is crippling to your wallet.
The best way to glass the wing join is to use "laminating epoxy resin" and two layers of 2oz glass cloth that has been cut on the bias. The first strip is about 2 1/2" wide and the second strip is at least 3 1/2" wide. By overlapping the glass in this manner you dont get a thick build up at the edges. Before you start to fit the glass cloth, paint/roll the laminating epoxy on the wing to a distance of at least 1 1/2" past the edge of the widest strip that you will use. When this coat has cured, gently sand the area smooth where the resin runs into the sheet covering. This is to give you a hard surface to sand down to as you feather the edge of the glass.
Roll the two layers into a dense layer and squeeze out as much resin as you can. The strength is in the glass, not the resin. All the resin does is fill the weave and bond the cloth to the balsa. Obviously there is a lot of strength applied by the resin but you don't want to add unnecessary weight. If you have a big plane you can add a third layer of glass if really needed and use wider strips of glass. Cutting the cloth on the bias is essential and this is where all the strength is obtained. Many builders don't understand the importance of cutting the glass on the bias.
If you don't understand this vital aspect for glassing, ask for info.
Bias cut glass is used for reinforcing the firewall, covering compound curves, covering the balsa sides and internal areas and much more. I believe you can buy this form of glass off a roll but I have never seen it.
If you haven't used bias cut cloth for joining the wings, do yourself a big favour, toss the wing away and start again.
When you are satisfied with the join of the wing, you can then cover the glassed area with 3/4oz cloth for a fabulous smooth finish.

Joe
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Old Jan 08, 2010, 07:45 AM
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You can stick the fiberglas down with Tite-bond glue and it will be less expensive and easier to work than epoxy or polyester, and you can sand it too, but I wouldn't sand it personally. Another thing I have done is to use very lightweight cloth and then stick it on with Balsa-cote, which is a heat-set adhesive you paint on to adhere non-adhesive-backed shrink covering. Then you mono-cote or whatever right over it. Again not so many steps as polyester or epoxy.

I would paint a wet layer onto the wing, then lay on the glass, then paint some more whatever over in either of the above cases.

Jake
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Old Jan 08, 2010, 08:40 AM
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Per Joe'nKody: Never use CA for joining wings together by spreading it over the glass tape or strips being used. CA is too dangerous to your health in such huge areas and it is a pain to sand. It is also rather brittle but the expense is crippling to your wallet.

I have used the above method and it works and isn't any more a danger to your health (then the other methods) if you use precautions such as good ventilation and a fan to move the fumes away from you and don't stick your face right down on top of the work where the fumes are coming from. I'll agree, there is significant cost in this method.

If you feel the bias is so important to the process, why haven't you explained what Bias cut FG cloth is?
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Old Jan 08, 2010, 09:24 AM
Will fly for food
Maryland
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If you put down epoxy and let it cure, then add another layer, the only bond will be a mechanical one between the sanded surface of the cured epoxy.

Better to put the epoxy onto the wood and glass cloth, that way the epoxy soaks into the wood and the glass and makes in a single matrix.

A trick for reducing the amount of resin in the final layup is to gently heat the layup with your Monokote gun, then press with paper towels to remove the excess resin. A proper layup is 50% glass, 50% resin. This method gets close to that, not as good as vacuum bagging, but pretty good for a lot less work.
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Old Jan 09, 2010, 09:16 AM
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Although I believe Bias cut glass is stronger, it is fairly wasteful of the cloth. I have never used Bias cut wing tapes and I've never had a failure. I guess I'll do it like I've always done. I was hopeful there were some new/better ways of doing this.
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Old Jan 09, 2010, 12:52 PM
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I also think this bias cut application would be very strong. I just might try it sometime. Just as in the grain of balsa/wood/ply applications, grain/weave is all very relevant to the overall outcome.. in printing.. presswork.. paper-grain is ever on a press operators mind..

I've used this method in a few apps before and didn't know there was a term for it. I've not done any wing-center sections with it.. all has been straight-grain apps.. just basically/precisely bonding with resin/glass directly applied at same time. I've also used woven carbon-fiber.. which is really cool stuff.
I've never had anything remotely let loose, or come apart in any way.. I have on the other hand, had total disasters(crashes, I'm fortunate for just a few in the last several years).. whereby the wing center-section was the ONLY thing to have survived... so done conventionally its already passed my tests.


I basically just mix some 30min apply a thin film within my glass area, lay the glass down, then mix some more, enough to cover the whole area.. I basically use a credit-card to apply that mix just like thin putty, pressing it into the fabric and smoothing it, with about 1/2to3/4" over on each side... sand and cover.. this works for me for many years.. as I'm sure there's alot of us that do it like this. My 3cents here.. but thanks all for the tips.. for sure.. its that outside the box thinking that makes it interesting.
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Old Jan 09, 2010, 01:01 PM
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Quote:
Originally Posted by Mode One View Post
Although I believe Bias cut glass is stronger, it is fairly wasteful of the cloth. I have never used Bias cut wing tapes and I've never had a failure. I guess I'll do it like I've always done. I was hopeful there were some new/better ways of doing this.
you should get about twice the ultimate strength from bias cut, as ALL the strands are working rather than just half of them. However glass is SO much stronger than balsa, that unless its really serious stress, it doesn't actually matter.
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Old Jan 09, 2010, 01:10 PM
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Just had another thought for that "bias-cut" cloth.. as I was just in the shop looking at one of my planes.. a bias cut would work excellent if you wanted to glass your tail(hor/vert. stabs)on your plane.. or to wrap any areas of the fuse that might be subject to flex/stress. The weave would lay nicely into/onto compound curves/joints.. hmm ..

Anyone done tail sections/area on anything?? Like a "SuperKaos"?? Might be an idea for ya Mode.. don't know.. but if you might need some torsional reinforcement would that work decently? .. I've not owned a SuperKaos.. well .. I did for a few hours..thats another story though, but I didn't build it... but how do those long tails hold out?? Probably for years.. but I've heard of pattern ships just giving out down there.. albeit this is a wooden ship here..

I dont know... I'm just throwin thoughts out now.. you'll do fine.
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Old Jan 09, 2010, 07:36 PM
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The last time I ever used glass cloth and epoxy laminating resin for a center section I tried something new that is sort of 1/2 leave it open and 1/2 vacuum bagging. I laid the cloth and resin down and then covered the area with Saran Wrap or similar. Then I clamped big pads of soft foam rubber on until it crushed quite well and left it all to cure. The results were a nice smooth job with lovely tapered edges. No raised edge fibers that needed to be laboriously sanded off and no need for a second coat of resin to fill the weave pattern. Just a light scuffing to lose the shine and promote adhesion of the covering and painting. It worked so well I'll definetly use this method again if I feel the need for a similar center section treatment.
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Old Jan 10, 2010, 04:23 AM
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That's basically how I glass, too. Cover with thick plastic bag, then modelling clay, then weight.

vacuum is the king of techniques, but its not the only one.
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Old Jan 10, 2010, 07:54 AM
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Bias explanation

For those that are wondering what bias cut fiberglass is, It's merely bidirectional fiberglass (equal amount of threads in both directions) that has been cut to orient the fibers at a 45 degree orientation to the surface that you're applying it to as opposed to 0 and 90 degrees.
Bias ply layups are for increasing torsional strength. The 0 and 90 degree orientation is for bending strength.

Burt Rutan's Scaled composite booth at Oshkosh used to demonstrate this every year with two little 5 in X 5 in layups on 1/4 inch foam. One layup would be one ply of Bidirectional fabric on each side oriented at 0 and 90 degrees and the other piece would be with the one ply on each side of fiberglass at 45 degrees . The piece that had the layups at 0 and 90 degrees was very difficult to bend but could easily be twisted. The one with the 45 degree (bias) layup was easy to bend but difficult to twist.
Make two samples yourself and try it.

What I would do and have done when joining similar wings is one ply 1 1/2 long oriented at 0 and 90 degrees. Another ply oriented the same that's 2 1/2 long (for bending loads) and then the third ply 3 1/2 long that's cut and oriented at 45 degrees for torsional loads. (known as a bias layup)

It is true that the fibers going in the 90 degree or chordwise direction are doing nothing to add to the tension and compression loads in the wing, but the ones that are aligned correctly with the loads imposed on them are much stronger than the bias cut ones. This is what unidirectional fabric is for. (fabric with most of it's threads oriented in one direction)

There's another finishing option too called peel-ply. It's available at most places that sell fiberglass and is simply a polyester fabric that the epoxy won't hang on to. You lay it up as you would the fiberglass as the top ply over all the other plies. Once the epoxy is cured you peel it off and leave a nice finish. It isn't as nice as the plastic wrapped ideas mentioned above but it's quick, easy, helps to remove the excess epoxy which makes it light and does give you the good transition between plies.

Mike
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Old Jan 10, 2010, 08:41 AM
Will fly for food
Maryland
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The problem with just laying plastic on the layup is it leaves a lot of HEAVY resin on the plane. And resin has virtually no strength by itself.

If you want to do a semi vacuum bag, do the same thing with a layer of peel ply (dacron fabric) between the plastic and the layup (even better if you used my previously mentioned trick of light warming with a heat gun and blot the excess resin off with a heavy duty paper towel). This will remove some of the excess resin, keep all the strands down. But it won't give you a perfectly smooth surface, but it requires no sanding to prep, just add some lightweight filler.
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Old Jan 10, 2010, 08:44 AM
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Quote:
Originally Posted by ThermalSeeker View Post
What I would do and have done when joining similar wings is one ply 1 1/2 long oriented at 0 and 90 degrees. Another ply oriented the same that's 2 1/2 long (for bending loads) and then the third ply 3 1/2 long that's cut and oriented at 45 degrees for torsional loads. (known as a bias layup)
Didn't I say that already?

But I only mentioned two layers, one bias, one standard orientation.

And as you said, Unidirection is great for single axis bending. To make a very strong wing, run a layer of two of uni from tip to tip, then cover the whole wing with layers of bias and standard.

Hmm, sounds like a Varieze wing construction layup.
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Old Jan 31, 2010, 04:04 PM
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Using peel ply is great allows the excess resin to be squeeged away and prevents the weave of the cloth from distorting under the pushing of the squeegie and helps remove trapped air under the cloth.
The best way to handle bias cut CF or FG cloth is cut it on the bias after its wetted out between plastic sheets. It can be cut to the exact shape you want with both sides covered in plastic, remove the plastic on the side your putting down on the wood, squeegie the excess resin and air out to the edges of the plastic wipe away and let it cure under the second piece of plastic. That will peal off when the resin has cured and leaves a nice filled surface or you can remove the second piece of plastic as soon as it's applied when its wet this lets the weave show more better for gluing something to it later.
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