Cherokee-M Construction - Fin lamination
July 6-13, 2003:
Reinforcing the fins tip-to-tip with carbon fiber cloth. These fins have a pretty big span, so fin flutter could be a problem. Finsim from Apogee components doesn't have material data for the carbon-nomex honeycomb I'm using, but according to finsim, plain 3/8" G10 with this big a span would be marginal at best past mach. And fins this big out of 3/8" G10 would be way to heavy anyway.
But tip-to-tip reinforcement essentially gives a cantilever affect, like a suspension bridge. Each fin has got cloth on two sides providing opposing force, and since carbon doesn't really stretch it's really hard for the fins to bend without breaking the carbon or delaminating. Thanks to Sue McMurray and others at Aeropac for this explanation of why tip-to-tip lamination provides such great strength.
To support the fins during the lamination process, I'll be using a tip-to-tip fin lamination jig borrowed from John Rockdale, based on the original jig designed by John Coker, both members of Aeropac. The problem here is that the fin can is too big for convenient home vacuum bagging. So the idea is to apply release film, breather, and outer film as if you are vacuum bagging, but then just pile some convenient ballast on top of the fins to squeeze out excess resin. Not as good as real vacuum bagging, but better than nothing.
As usual, my epoxy of choice is Aeropoxy (R2032 Resin and PH3660 hardener). For lamination, straight out of the can with no thickeners.
Tip-to-tip fin lamination jig:
Measuring how much cloth I'll need and creating a template out of newspaper:
Masking off the aft bulkhead and motor retainer:
Cutting out the carbon cloth:
Carbon cloth ready for lamination:
Applying the carbon cloth:
Release film:
Polyester breather cloth:
Ballast:
Results:
You can see some bubbles where air got trapped under the release cloth and excess epoxy oozed into the voids. This was the first side. On the other two sides I was more careful not to get air bubbles under the release cloth.
There are also a few air bubbles under the cloth where the cloth bunched up. I'll have to deal with those, either fill them with epoxy or tack the cloth down with epoxy before I paint, or else the paint will crack any time you push on one of those hollow spots.
I'm not completely satisfied with the results. Maybe some sort of air bladder instead of ballast would give a more even surface without the voids (and maybe even squeeze out a little more resin). But I think the problems are mostly cosmetic and I think the results are plenty strong enough to fly with.
