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Author Topic: What do you think is stronger - 3/4oz fiberglass cloth with CA or Epoxy  (Read 1703 times)

Offline Dennis Toth

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Going through the final build on a new FOX35 speed ship and am reinforcing a few areas with fiberglass cloth. These are small areas so not much material is needed. Just wanted to know what the group felt was stronger using CA or Expoy with the 3/4oz fiberglass cloth?

Best,   DennisT

Offline Ty Marcucci

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Well, Corvettes aren't built with super glue.  Just saying. D>K
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Offline Dan Berry

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The cloth IS the strength.
All you need to do is keep is attached.  Nitrate dope will attach it.

Offline Dave Hull

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Well..., not exactly. The epoxy resin is the matrix, and it must hold the fibers in all three dimensions. That keeps the fibers from dislocating. buckling, etc. All of which constitute a "composite" failure. So if you expect low static loads and minimal transverse loading to the principal axes of the fibers then most anything will do. But clearly epoxy is superior in achieving the strength that the reinforcement is capable of. And, all epoxies are not equal, nor are epoxies often cured in a way that achieves their own maximum strength. But for a model, just how much is needed? If something has been working for you, then it works. But I have seen a lot of Fox racers with cracks around the nose....

One key is to always follow the load path. Where does the external load come from, and where does it go? Remember that making one area very stiff means that the geometry will likely cause it to carry much if not all of the load. And size accordingly. If you are putting heavy reinforcement over floppy structure, then it needs to form a more continuous shell. Avoid abrupt edges where you have a heavy composite reinforcement and then suddenly just balsa planking.

If you were reinforcing the fillet between a profile fuselage and the wing planking, I would consider two layers of your 3/4 oz. cloth. Cut at least one layer on a 45 degree bias and make it a continuous wrap from the trailing edge all the way around the leading edge and back to the TE. The bias cut should allow it to easily conform to a fillet and having it continuous around the LE might help address the weakest spot in a Fox/Profile setup. Another layer over the top either oriented fore/aft or another 45 bias would be good. Put them on together with one batch of epoxy!

My thoughts.

Dave

Offline Dennis Toth

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Thanks for the thoughts. My ship is pretty solid built like an old log rat racer with inverted engine. I intend to do the fiberglass on the wing. My biggest concern is the resins available kick off very quick and don't allow much time to work the glass in place and smooth. I am thinking I could us clear dope to get it down (kinda like silking), then use a coat of finishing resin to add some strength. Then K&B epoxy color finish.

Is there a better resin (polyester boat resin after glass is applied) that is maybe a little stronger than Bob's 20min Finishing Resin?

Best,   DennisT
« Last Edit: June 13, 2023, 06:35:56 AM by Dennis Toth »

Offline doug coursey

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Thanks for the thoughts. My ship is pretty solid built like an old log rat racer with inverted engine. I intend to do the fiberglass on the wing. My biggest concern is the resins available kick off very quick and don't allow much time to work the glass in place and smooth. I am thinking I could us clear dope to get it down (kinda like silking), then use a coat of finishing resin to add some strength. Then K&B epoxy color finish.

Is there a better resin that is maybe a little stronger than Bob's 20min Finishing Resin?

Best,   DennisT
I USED NITRATE DOPE TO ATTACH FIBERGLASS CLOTH TO THE ENGINE POD ON MY COMBAT PLANE I PUT A COUPLE COATS OF THIN NITRATE DOPE ON AND LET IT DRY,THEN LAYED THE CLOTH 1 OZ ON THEN APPLIED NITRTE DOPE TO STICK IT DOWN...WORKED  GREAT AND IS MUCH EASIER TO WORK WITH THAN EPOXY AND A LOT LESS MESSIER TOO...THEN USED BOB SMITH FINNISHING EPOXY TO COAT THE GLASS....
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Online Ken Culbertson

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When I glass a nose I use slightly thinned 30 min Epoxy over the bare wood. I wear surgical gloves and rub it in, lay down the cloth and brush on a second coat.  Pre doping does the same thing but it is not nearly as strong.

I used slow cure CA once.  It set up while I was still smoothing it. The remains of the gloves didn't leave a mark when I sanded them off. With Epoxy you know when it is setting up and your working time is about up, with CA it just sets. HB~>

Ken
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Offline Dan Berry

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Thanks for the thoughts. My ship is pretty solid built like an old log rat racer with inverted engine. I intend to do the fiberglass on the wing. My biggest concern is the resins available kick off very quick and don't allow much time to work the glass in place and smooth. I am thinking I could us clear dope to get it down (kinda like silking), then use a coat of finishing resin to add some strength. Then K&B epoxy color finish.

Is there a better resin (polyester boat resin after glass is applied) that is maybe a little stronger than Bob's 20min Finishing Resin?

Best,   DennisT

Don't use butyrate dope to stick it down. Use the nitrate.  Use good practices and you'll do fine.
Topcoating it with epoxy or polyester isn't gonna add a lot of strength. It WILL make it more durable.
Polyester resin probably won't cure over any epoxy joints so that's a consideration for using the epoxy finishing resin.

Offline Dave Hull

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Ok, if you are glassing a solid wing for a log-style setup we do that all the time. Everyone probably has a way that uses something they have on hand, but....  Having done a bunch of what are probably similar wings, I have standardized pretty much on the following single pass wet-layup methods:

--Sanded and shaped wing. Can be simply solid balsa, but for racing, we inset a hardwood stub spar for bending strength which also gives a rock solid anchor for the bellcrank.  I generally use a "hardened" leading edge piece. It can be spruce for example. I have made a few with tulipwood which works out really well. (At Home Depot they call it poplar. It is not the same stuff that hobby folks see in their LitePly.) For F2C wings I've used laminated 1/16” basswood strips. All are viable. The front half of the wing can be more dense balsa. The back half can be lighter stock. I sometimes harden the trailing edge by insetting a thin strip of plywood. There are tools that make that quite easy. You can do any combination of the above on a built-up (hollow) wing as well.

--A layer of 0.58 to .75 oz glass cloth. If you want more strength/stiffness in the middle section, use a double-layer in the middle. How you overlap the cloth matters and you can avoid making work for yourself later during finishing.

--Use an epoxy resin that is actually designed for this, not some glue that you try to thin out. I find that the ACP EZ-Lam 30 minute resin is exceptional for this purpose. There is plenty of time to do all the application if you have everything ready. I can't remember a batch ever going off on me before I was done. Why EZ-Lam? You need thinner resin for lighter cloth. And the last thing you want is to try thinning with isopropyl alcohol which traps water in the resin and prevents complete curing and full strength. I suspect that adding acetone does nothing good for the polymerization cycle either, but guys do it and swear it is good. But why settle for "good enough" if you can buy an epoxy that doesn't need a bunch of amateur chemistry added to simply wet out? Finally, the EZ-Lam actually sands quite nicely which is not always true of epoxies (or polyesters!)

--The absolute last thing I would screw around with is polyester resin. There's a reason everyone abandoned using it back in the 1970's. Let's leave it at that....

--Some guys will put a thinned "seal coat" of nitrate on the wing blank before starting to laminate with glass. The logic is to avoid soaking up more epoxy resin and adding weight. Using a decent weight wood for a racer or speed plane (no magical 4 lb stunty stuff), and because nearly the whole wing is long-grain, not soda straw end-grain, I haven't found that the EZ-Lam adds an unreasonable amount of weight. So I don't seal first. And I get rid of a waiting period of multiple days while the volatiles in the dope gas off.
 
--You need a flat table. If your wing is essentially flat-bottomed, you really saved a lot of work.

--The essential truth of a wet layup is that you are doing all of the handling of resin and cloth by adding them to your wing in a planned sequence and using a few techniques. It is a "one-off" fabrication method for us, with no molds...except....

--You can cure in an open layup, a press, or by bagging. We’re not going to talk about resin infusion or glass-in-mold here. I generally use either a press or bagging on solid wings and they aren’t much different in results. On a hollow wing you’re going to want to go with the open layup cure.

--A press is essentially just two urethane foam blocks (think upholstery foam) and two pieces of heavy plywood. The sandwich is:  plywood, foam block, bagged wing, foam block, plywood. If your table is flat, skip the plywood on the bottom. Once all stacked up, you put a lot of weight on top of the plywood and press everything down.

--You need all the MonoKote plastic backing material you can get your hands on--if you want to "press" your wing. This results in pretty much a ready to paint wing. The only sanding might be a very slight scuff with 400 or 600 grit. Unless you got the plastic wrinkled up in the press....

--There is also the vacuum bagging option if you have the equipment. But the press gives excellent results and avoids the risk of a starved layup from too much vacuum over the cure cycle. If you go the vacuum bag route, there are specific techniques and materials needed to make this work. Again, you can end up with a paint-ready wing out of the bag.

--If you want a filled surface that is ready to paint, then using a plastic caul the shape of your wing is really effective. To work well, the wing needs to be single-curvature. I use some .040”(?) poly sheet. The finish on the wing is exactly as smooth as the surface of the caul and/or bag.

--Precut all the glass. Using a green cutting mat and a fabric “pizza cutter” is miraculously better than screwing around with scissors.

--The 99 cent throwaway bristle brushes work great for this. (They might be a buck fifty by now though.) If you paint the EZ-Lam on as thin as it will easily go with one of these brushes, you will find that the layup will be only slightly resin-rich—perfect for the press or bagging and achieving near-minimum weight. If you are doing an open cure (no press or bag) then you may need to blot or squeegee some resin. Doable, but unless you have a hollow wing you are just making work for yourself.

--After scuff sanding, I paint with Klass Kote epoxy. Great stuff!

This isn't a "how-to" description. There are a lot more details that make this work well and all of it saves time over other methods. But if you are interested, ask.

Dave

Offline Lane Puckett

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Dave,  Can you expand on this comment 'I sometimes harden the trailing edge by insetting a thin strip of plywood. There are tools that make that quite easy. '

I'm interested in seeing the tools if they are easy to access.  I'm insetting a thin ply Trailing edge to some light balsa.

Thank you,
Lane

Offline Dan McEntee

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Dave,  Can you expand on this comment 'I sometimes harden the trailing edge by insetting a thin strip of plywood. There are tools that make that quite easy. '

I'm interested in seeing the tools if they are easy to access.  I'm insetting a thin ply Trailing edge to some light balsa.

Thank you,
Lane

    There have been sailplane designs that simply laminated 1/64" ply about 1/4" or 5/16" wide between 1/16" top and bottom trailing edge pieces. When the trailing edge was sanded down to a sharp edge, it was tough as nails. Carbon fiber sheet replaced the plywood when it became available. The Sagitta was one that did that I believe. If trying to do the same in thicker balsa stock, I might try a Master Air screw balsa stripper set to the correct depth, or maybe even make a tool using a knife blade in a fixed position to run down the edge of the balsa.
  Type at you later,
   Dan McEntee
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Offline Dave Hull

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Lane,

This was the method I used on a solid balsa wing. I put it in a separate thread so that it could be found easier, later.

https://stunthanger.com/smf/building-techniques/reinforced-trailing-edge-for-a-solid-wing/msg658788/#msg658788

Dave

Online Ken Culbertson

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The Sagitta was one that did that I believe.
The Sagitta was my weapon of choice in my Sailplane days back in the early 80's.  I don't think the kit version had that.  I built one of Don Chancey's Panteras (actually the 2nd one built) built from his "plans", and we did use that "trick" but I don't think it made it into the kit either.  Don was not very happy with what Larry Jolly did with it.  However, 40 year old memories are funny things so I may be wrong.

Ken
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Offline Dennis Toth

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Dave,
Good information on the glass layup. Just wondering if we could use one of the finishing resins (they are pretty thin and brushable) in place of the ACP EZ-Lam 30 minute resin. Finishing resin is readily available in small quantities I didn't see the ACP EZ in anything less than a quart size (~$50) good if you are doing a large project but a one-off small speed ship only needs about an ounce of resin.

I assume one would do a wrap around the 1/2 the wing at a time with the overlap on the bottom or do you just butt splice the glass on the bottom near the trailing edge? I really need to save some weight in the tail so I can minimize the nose weight needed. Will likely just go with resin on the plywood stab/elevator then one thin coat of some old K&B orange superpoxy paint I have.

Best,    DennisT

Offline Miotch

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"What do you think is stronger - 3/4oz fiberglass cloth with CA or Epoxy"

                            Wouldn't that depend on the particular epoxy ??

Offline Dave Hull

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Dennis,

I've used some West Systems epoxy, but that isn't going to be smaller/cheaper than the ACP EZ-Lam. I suppose next on the list would be the Zap/Pacer Z-Poxy. I can't comment on that because I have not used it. They claim their "finishing" resin is good for laminating and is thin. And it comes in small quantities for the hobby user. All bonuses for your project. I would really try to avoid thinning it. You can read between the lines on their product data sheet that while they say you can thin it some, don't do it much, and test what you are trying to see if it will harden acceptably. They don't even commit on the sheet what to best thin it with. No wonder we have as many opinions on thinning it as we have guys using it.... 

As far as laying in the reinforcement on a wing joint if that is applicable here, I would precut strips the width you think you need--they can be tapered--and avoid overlaps that will leave you with more sanding/filling/pinhole issues. Don't try to "bend" the cloth around the TE. Even lightweight glass on a bias will not stay down.

For glassing an entire wing or stab, the weight of the cloth on a small project is going to be tiny. for example, if you had a 3" x 10" stab+elevator, for an area of 30 in2, and a total finishing area of 60 in2, and you used true 3/4 oz/yd2 fabric the glass weighs less than one gram, if I did the numbers right. That means what you are trying to control is the weight of the matrix, in this case epoxy resin. To do that, you have to make sure the cloth is not "floating" on top of a thicker-than-necessary coating of resin. Pressing eliminates this issue as does vacuum bagging. Even in an open layup you can minimize weight fairly well by applying very limited resin, then the cloth, then sandwich in your MonoKote backing plastic. Now lay the part on the bench and use a credit card to squeegee the excess to the edges and let it cure. If you use the clear plastic you can actually see an starved areas where the white fiberglass has not yet "disappeared," indicating more resin is needed. I work the squeegee to wet everything out before moving the excess resin off the edge of the part. So you want a little extra, but more than that is a waste of material and just more work. For delicate structures (hollow wings, etc.) or complex shapes you can control the initial resin application and then roll off the excess after the glass is down by blotting the shiny spots. A roll of toilet paper can be used for this. Skill matters; practice gets you there.

Remember that the real difference is that without the press or bag, you will need to fill the weave afterwards to get a smooth surface. While you could brush on another coat of epoxy, that usually is heavier and a lot more work than using easy to sand fillers. And then your K&B or KlassKote colors.

Dave


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