Bonding Carbon Fiber to Spruce...

[bronicabill]

I posted this same question to the "Sailplane Talk" forum several days ago and have gotten no response, so maybe that was the wrong place to put it. In case it was, here is my original question...

*** Original Message ***
I've been building and flying R/C for over 20 years now, and at least 17 of those have been with sailplanes in an "on again - off again" mode (mostly "off again"). While I have built a few sailplanes, and have flown many, including an all-composite RnR Synergy 91, I have never constructed a wooden airframe where I've reinforced portions with CF! And that lengthy introduction leads me to my question...

I'm building a Cumic Plus wing and tail set for an existing FG fuselage and would like to reinforce the wing main spars with CF. I have some CF tape that is 1/2" wide and 0.015" thick that I would like to laminate to the outer surface of the top and bottom main spars of the center section to help it withstand zoom launches, but I'm unsure as to the proper method of adhesion such that it will remain bonded for the life of the airframe! I've already glued one strip of CF onto one spar with thick CA, but I'm wondering if this will hold indefinitely since CA is rather brittle compared to epoxy, and I certainly don't want to continue building up the wing until I'm sure this is a suitable method of laminating the CF material.

Any information, comments, complaints, criticism or general observation on this subject would be greatly appreciated! :)

[davidleitch]

I have bonded carbon fibre to wood using both epoxy and ca. I am sure that epoxy gives greater strength in this application at the cost of weight. The strength comes, in my opinion, from the epoxy covering every strand of carbon fibre.

I should add that I can't put two lego blocks together.

[bronicabill]

Thanks! I do believe I'll redo mine using slow cure epoxy... just for grins and giggles, and a bit more peace of mind.

Obviously I just posted the question in the wrong forum last time... DUH! :rolleyes: At least now I know to stick with the thermal guys (well, the Cumic IS a thermal sailplane, isn't it???) when asking questions regarding thermal airplanes... ;)

Cheers,

Bill R.
Madison, AL

[chlee]

Just to echo David's comment, I would highly recommend epoxy over CA. I built a Chrysalis 2M with carbon strips laminated over the wood spars with CA. While moving boxes in my basement, I made a light impact with the wing, bending it in the center. It was a very light impact, and I'd used generous amounts of CA, but it still went *pop* and delaminated.

Epoxy --> good. :)

Chung

[marty o]

Bronicabill,
Maybe you already know this, but after you apply the carbon tape and add the shear webs, you must also wrap the laminated spar with kevlar thread or carbon tow. I've heard some people use unwaxed dental floss for this. The wrap keeps the carbon from buckling and popping loose from the spar. I'm no expert on this but, Ollie Wilson and others have described this process many times in this forum. You may want to do a search for more information. Or go to the Laser Arts Majestic web page and look at the Spar Beef-up Article. The article details this whole process.

[bronicabill]

Thanks, that's sorta what I was thinking might happen under loads. I'll definitely go back and redo the one spar I've already done with epoxy instead of CA, and of course the other 3 spars also.

Thanks again,

Bill R.
Madison, AL

[bronicabill]

Bronicabill,
Maybe you already know this, but after you apply the carbon tape and add the shear webs, you must also wrap the laminated spar with kevlar thread or carbon tow. <<snip>>

No, I didn't know this! Maybe that's why the old Cumic wing I have here (built by somebody else) has CF laminated on the bottom spar only... no buckling tendencies that way. I wouldn't be anywhere near as strong as doing the top AND bottom spars, but it would still be better than stock with no appreciable weight gain.

I think I'll look up those links you mention to see how I wish to proceed here...

Thanks again,

Bill R.
Madison, AL

[markdrela]

I have bonded carbon fibre to wood using both epoxy and ca. I am sure that epoxy gives greater strength in this application at the cost of weight.

Epoxy is not heavier than CA. The two adhesives have similar densities when cured, and you need to fill all gaps with either one. If anything, the CA will be heavier if it wicks more into the wood.

I'm not sure what started the myth that CA is a light adhesive. Indoor FF builders almost never use CA, because in fact it's one of the heaviest adhesives in practice.

[m_acree]

The following link is to a fully documented wing/plane build. The specific info you want is linked at the bottom of the page under building tasks list. On this wing the spar itself is a composite structure built from carbon over end-grain balsa not a carbon capped spruce spar but the technique is similar if you want to cap the top and bottom of the spar structure.

http://www-eng.lbl.gov/~osborn/Alle...e_Introduction/

[m_acree]

This link should work. The following link is to a fully documented wing/plane build. The specific info you want is linked at the bottom of the page under building tasks list. On this wing the spar itself is a composite structure built from carbon over end-grain balsa not a carbon capped spruce spar but the technique is similar if you want to cap the top and bottom of the spar structure.
http://soarheads.com/AllegroLite

You might find a few other useful bulding tips here: http://www.charlesriverrc.org/articles.htm

[davidleitch]

Epoxy is not heavier than CA. The two adhesives have similar densities when cured, and you need to fill all gaps with either one. If anything, the CA will be heavier if it wicks more into the wood.

I'm not sure what started the myth that CA is a light adhesive. Indoor FF builders almost never use CA, because in fact it's one of the heaviest adhesives in practice.

Mark

Thank you for pointing this out. I still labour under the myth that there is a weight saving because of potentially using less CA than epoxy for a given application.

dave