Glass/Carbon/Glass Boom Lay-up
This type of boom is composed of unidirectional carbon, with glass on the inside and outside to provide hoop stiffness
and torsional stiffness and strength. It is similar in principle to the Al/CF/Al booms, except glass (or Kevlar)
are used instead of the Aluminium.
For small HLG's, 1 layer of 3 oz Uni-web carbon and 0.75 oz glass is sufficient. Full-size HLGs will require at least
2 layers of carbon and perhaps an extra layer of glass or just 1 oz Kevlar instead of the glass.
A non-stick tapered mandrel is required. A 0.4" to 0.5" diameter at the big end is about right. The small end
wants to be about 0.25" to 0.35" diameter.
Lay-up procedure:
Cut the carbon to size. Lightly spray it with 3M-77, and stick it down onto the glass (or Kevlar) stock at 45 deg.
Cut the glass to twice the width of the carbon + 1/8". The 1/8" of glass sticks out past the carbon along one edge,
and the full width sticks out past the carbon along the other edge.
In cross-section:
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Wet out the carbon only (and the glass under it). Remove excess resin by putting down a paper towel on the carbon and rolling
over it with a hard roller. The exposed glass is left dry since there will still be enough excess resin in the carbon
to wet out the glass after rolling. With Kevlar you may want to wet out everything, but blot it as much as possible.
Wipe a thin stripe of highly cabosil'd epoxy onto the mandrel, and stick it down onto the edge of the dry glass. If the edge
of the glass doesn't somewhere, dab on more thickened epoxy in that spot so it does.
Roll the mandrel on the table, wrapping the glass and carbon/glass onto it:
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I spirally wrap the lay-up with a strip of 1/2" wide heat-shrink tape, overlapping 2/3 of the tape width (effectively 3 layers of tape are
applied). You could also wrap it with a polyethylene strip, and then compact it with a vacuum bag or wrapping with a long strip of rubber.
For strength, it's important to get the carbon fibres straight. I leave about 1" of extra carbon on each end extending past the glass.
This allows me to pull hard on it after rolling before spiral wrapping.
- Mark Drela
