If you want to bond carbon fiber to balsa surfaces you traditionally had to opt either for cyanocylocrate/hotstuff/flash/CA or epoxy to make a suitable bond.
Recently I've found that you can use hotglue to good effect in conjunction with laminating film. By creating a thin, narrow strip of hotglue where the carbon fiber needs to be bonded and then overlyaing with laminating film and bonding it all together under the pressure of a household iron, a flexible well bonded composite is formed.
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Start by laying two strips of good quality masking tape approximately 12 to 15mm apart along the length of the balsa sheet where you want the carbon fiber strip to be placed.
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Have your hotglue gun warm up for at least 10 minutes so that the hotglue comes out extremely hot, fluid and quite probably steaming.
Apply a portion of hotglue along the area between the two strips of masking tape and immediately scrape it down to a very thin and flat layer of glue which will solidify within a matter of seconds.
Do not attempt to do multiple passes with the squeegee, this will only make things messy.
If you need to fix up the surface then consider using a heat-gun to reflow the hotglue.
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When completed you should be left with a relatively even, thin layer of hotglue which should have spread over most of the area between the masking tape and the tape itself. Allow this layer to cool for a few minutes.
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Once the hotglue has cooled for a couple of minutes, you can commence to peel away the masking tape. If your masking tape is of good quality and the hotglue layer is thin enough you should find that by peeling the tape away at an angle it will cleanly come off leaving a well formed strip of hotglue.
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When all the masking tape has been removed you should be left with a well formed, consistent appearing strip of hotglue. Do not worry too much if there are a few bumps or holes in the strip, so long as it overall is well formed.
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Lay down the strip of carbon fiber that is to be used for the reinforcement spar. You will find that the carbon fiber and hotglue have a natural tendency to bond even though the hotglue is cold.
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Place your sheet of laminating film over the assembly with the glue side down, as if you were doing a normal covering process.
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Iron down and shrink your laminating film covering as per normal.
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Once the covering has been shrunk and applied to the balsa surface, turn the iron up to cotton or linen heat level and apply the iron to the carbon fiber strip using a reasonable amount of pressure. Immediately after removing the iron rub the carbon fiber and hotglue area with a soft cloth. Repeat along the length of the carbon fiber strip.
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The carbon fiber, hotglue and laminating film composite should appear dark, glossy and well saturated (not greyish or blotchy). There should not be a noticable bump or raise where the carbon fiber strip is, if there is you may have to reheat and rub down until the bump is levelled out.
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A job well done. The carbon fiber, hotglue and laminating film composite will provide your airframe with enhanced tensile strength while the hotglue component will ensure that the carbon fiber strip will not delaminate from the balsa, unlike with CA glue.
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A video is also available of this process (along with destruction testing of the resultant composite. The video is available here.
