Carbon Fiber Balsa Laminate

I've been doing the same thing, using the carbon fiber mat to reinforce the sheet balsa in my work. I learned from the article I am using too heavy a carbon mat, the .5 oz weight where the .2 weight would do.

But the application method seems over kill to me.

I've been preparing the balsa sheet stock with two coats of 50/50 Clear/Thinner (Aerogloss) sanded each time to bring a nice smooth base to the balsa surface. Then I iron the carbon fiber to get it real smooth, cut to fit, and then lay it onto the balsa and attach with another thick coat of 50/50 dope/thinner. I find the thinner opens up the mat a little as it seems to be held together by some kind of similar material. The thinner also softens up the underlying dope and when the whole thing dries a couple of days later, I've got a part with a thin carbon fiber shell firmly attached to it.

Going to epoxy to attache the carbon seems to me to be inviting weight and adding fuss and bother to the process.

What am I missing?

Is epoxy really required to properly attach carbon fiber mat to the inside of fuselage sides and the like?

(The best result from my tests on the Nobler turtledeck actually involved silkspan soaked in thinned Titebond, but that's another thread.)

Larry (still fabricating parts) Lowe

What you are doing, for our purposes probably works fine. However, it is a certainty that the dope does not develop the optimum physical properties of the carbon/dope matrix. The critical characteristic is the ability of the material to "wet" and bond to each micro-fiber in the mat. Also, the flexibility and hardness of the binding material (dope or resin) really affects the finished properties. In this regard, the material needs to have a high compressive strength--since loads applied to the finished matrix cause micro compression and tension between fibers. Finally, the mat is normally treated with a chemical coating to enhance the wetting properties and the subsequent fiber bond strength--CF mats and fabrics as well as all laminating glass fiber, are (again, normally) optimized for epoxies and polyesters.

FWIW

It doesn't have to be some exotic, expensive post-cured resin, but it should be epoxy.

JMNSHO.

L.

"I have two very rare photographs. One is a picture of Houdini locking his keys in his car. The other is a rare photograph of Norman Rockwell beating up a child." -- Steven Wright

Me too. I love that column, and Noel is obviously an incredible craftsman, but it seems sometimes that the method and effort way exceeds the benefit.

Also the weights are a little confusing. I am amking single piece molded topblocks for my new planes. I am using 3/32" hand selected 4-6 lb. contest A-grain wood with no laminates or extra glue at all. The one piece top shell weighs 3/4 of an ounce.

Nole said all his fuselage pieces added up to 1.5 oz. That includes fuselage sides, all formers (bunch of formers), and top and bottom shells. All starting at about the high point of the wing.

He used epoxy because that's what contributes a great deal to the strength.

See Ya

Yes but you see what brad is saying. His one 3/32 shell weighs .75 of oz. That is on peice of molded balsa about 42" long. That would equal about the amount Noel would have on his top and bottom peices aft the wing. So all the rear area in his Fuse is only .75oz and that includes all the gluing and stuff. He really does know how to get it light.

DMoon

I'm a believer in epoxy (or...ugh...polyester) resin for carbon or fiberglass. One of the requirements to get the strength potential is to isolate the individual fibers from the others. If not isolated, one fiber cuts the other when flexed. Polyester resin is lighter than epoxy, but shelf-life less and mix more critical, and working time is too short. I mix the epoxy (2 hour), thin with alcohol to the consistency of milk, and brush on. Roll over this with a roll of your favorite TP to blot up the excess. When the TP no longer soaks up epoxy, you're done. Put the FG or Carbon on the outside surfaces, not between two layers of balsa! Steve

I read recipes the same way I read science fiction. I get to the end and I think, "Well, THAT's not going to happen".

The last IntrepidXL I built, I used doubled 1/8th balsa with CF laminate instead of 1/8th and plywood for the fuse front end, and it was plenty rigid all right, but it made the fuse exterior slightly wider than stock. It still finished out at 61 Oz and flew just fine. No vibration at all, even with a ST60 up front. Not sure if I proved anything other than it could be done...since the planes I build with Big Jim construction (motor crutch and cross grain balsa in between) are plenty rigid and don't vibrate either. Add to that, the plywood doubler surface harder and is less likely to be damaged taking engines in and out, so it's more fuel proof in the long run.

I do like to try new stuff as it comes along, but I also try to evaluate it with an open mind... Since I firmly believe new technology used just for it's own sake may have no real benefit. (The king may really have no cloths!)

Now, on my Cavalier, I used the ladder crutch fuse system, and it is very light, and well worth doing if you have urge. Nice and strong, took a little more layout time, but I avoided all those hours hollowing blocks! (and sneezing) Silkspan on the body was the only fuse strength I added and it is good enough to my standards.

Eric V.
Tampa Bay Line Flyers
"If you can't beat'em, join'em then learn all you can and beat'em next time!"

I always thought it was the opposite. You want to get the fibers as close as possible to each other, so you don't have a large layer of resin in between. Resin is heavy, the less of it in the material, the lighter it is, and the stronger it is (as long as there is enough to bind the fibers together.

Hence the vacuum bagging and weights, and pressure molding when you make composite props, and tape wrapping if you're making spiral-wound tubes, and why Glasforms boasts that it's pultruded tubing has very low resin content.

>Polyester resin
>is lighter than epoxy, but
>shelf-life less and mix more
>critical, and working time is
>too short. I mix the
>epoxy (2 hour), thin with
>alcohol to the consistency of
>milk, and brush on. Roll
>over this with a roll
>of your favorite TP to
>blot up the excess. When
>the TP no longer soaks
>up epoxy, you're done. Put
>the FG or Carbon on
>the outside surfaces, not between
>two layers of balsa!

Wouldn't be lighter to then vacuum bag all this? It'll squeeze the resin out of the composite.

But anyhow.. what I meant was that vacuum bagging could be done after you've removed all the excess epoxy by soaking it up with toilet paper.

Howard uses equal parts (by weight) of CF and epoxy,
with no epoxy squeezed out. The vacuum bagging serves
as an especially flexible clamping system.