Carbon Fiber/Spruce Spar Cap Construction Questions - RC Groups
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Nov 10, 2003, 08:33 AM
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Carbon Fiber/Spruce Spar Cap Construction Questions

I intend to glue CF and spruce strips to make composite spar caps. The spruce will have a 1/8 x 3/8 inch cross section and the unidirectional CF will be both .007 and .014 x 3/8 inches. Length will be 48 inches.

How would y?all go about gluing and clamping this lamination?

I have had wonderful experience with rubberized CA. If epoxy would be better, I?d like to know which epoxy to use. Would the LHS epoxies do the job. Could I use alaphatic resin? Which of these choices would last longest? What would you do?

Appying reasonable pressure uniformly to a 48? long strip of spruce with a slippery substance between it and the CF sounds like a tough chore. I have a flat bench and a piece of thick glass 48? long.

I can make & glue down straight edges onto the glass that are just tall enough to align the CF & spruce and prevent them from sliding laterally. How would you clamp this assembly?

This is fun

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Nov 10, 2003, 09:53 AM
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The best adhesive for spruce to carbon is laminating epoxy thickened with cabosil (fumed silica). The thickening agent prevents the low viscosity epoxy from running out of the joint. The thin epoxy penetrates the porous wood so you have a combination that is the best of conflicting objectives.

Do two or more spar caps at the same time. Lay down a sheet of polyethylene plastic on the glass. Lay the assembled and freshly glued spar caps parallel, side by side but not touching each other. Lay another sheet of polyethylene over the spar caps and lay a flat board over the second sheet. Weight the board down with bricks, water filled jugs, paint cans, lead pigs, exercise weights or what ever you have handy, while the epoxy cures. The glass and board will be protected by the polyethylene sheets and the epoxy won't stick to polyethylene. If you align the carbon to the spruce and don't disturb the alignment, you won't need any side restraints.

Since the carbon is almost 50 times stronger than spruce, a strip of carbon .007 X 3/8 inch is as strong as a piece of spruce about 5/16 X 3/8 and the carbon is about seven times lighter than the equivalent spruce. Taking these facts into account, why would you use any spruce? Just laminate the number of 0.007 thick carbon together to get the desired strength. At the wing root you need full strength. At half way from the root to the tip, you need less than 25% of the strength at the root. At the tip, the bending strength requirement is zero. In other words, the spar cap crossectional area should taper exponentially from root to tip. If not, the extra material will be along for the ride. Spar caps that do not taper exponentially are at least three times heavier than necessary to carry the bending loads on a cantilevered wing. Shear webs that do not taper linearly from root to tip are at least twice as heavy as necessary.

Most spars are untapered for economy of building effort. Where expensive carbon fiber is concerned tapering saves many dollars and is worth the trouble.
Last edited by Ollie; Nov 10, 2003 at 10:07 AM.

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