On my 98mm mongoose kit, my fins are very very rough. This came as a surprise to me because my 38mm mongoose had very smooth, finished fins can anyone help me undertand why that is and what the best process to smooth them out would be? Thanks
I would imagine that the fins are left rough to allow for doing a composite lay-up. You want the fins to have some tooth for the epoxy to grab. You should seriously score and rough up the surface of the a/f tube for the fillets and the lay-up, too. I'd suggest chopped Kevlar in the fillets to give them structure, followed by tip-to-tip carbon and a thin glass veil, with peel-ply and batting used, vacuum applied if possible. That will leave a smooth enough surface for priming, and keep the fins from fluttering in the breeze. Shear forces on fins are hardly worth considering... It's the side forces that start them to fluttering that will kill an airframe dead.
You might want to rethink the kevlar-carbon hybrid for the outer layer of a fin. The outer layers give you the most stiffness in bending, kevlar is significantly less stiff that carbon and will actually make the fine more likely to flutter. Kevlar needs to be dry before laminating and can be more difficult to wet out.
Couple other reasons why Kevlar isn't a great idea for outer layers,
Sanding - if you need to sand the surface smooth, Kevlar int the right answer it fuzzes up and can be difficult to smooth out, primers don't stick to it as well as a good carbon/epoxy laminate either.
Strength- Kevlar will decrease the strength of your fins not increase it. Here is how to think of it, think rubber bands attached to window glass (ok im reaching here but work with me), the carbon composite (window glass) is about three times stiffer than the Kevlar (rubber band), when it starts to bend, the carbon takes up much more of the load than the Kevlar, the Kevlar won't actually take up much load until the carbon has failed. Some sporting goods applications use a combination so after the carbon fails the Kevlar holds the pieces together, but this is after a failure.
oh one side note that isn't talked about much, double cup!
Weigh out and mix resin in a single cup, I use uncoated paper buckets (popcorn buckets) for mixing, mix the resin fully, then pour it all into a SECOND cup, scrape as much as you can into the second cup and then with a NEW mixer (I use tongue depressors mcmaster 54845T85 with one end cut straight) re mix it. Good shop practice..
While I'll agree that Kevlar and Kevlar hybrids are a lot tougher to wet out properly, and you dang sure can't sand 'em, I must disagree with your analysis of their use on fins.
Kevlar doesn't stretch. At all. That's its advantage in a lay-up.
Look at a G10 fin from the front. Measure one side, from the root to the tip. It's a fixed measurement. Now, bend that fin into an arc. Unless that fin is infinitely thin, the radius of the outside arc is greater than the radius of the inside arc. You've stretched the outside of the fin, and compressed the inside. Now, do a composite lay-up on that fin, and try to stretch that side. You're putting the lay-up under tension. But the carbon, Kevlar, or hybrid material doesn't stretch. Do the lay-up on both sides, and the fin won't bend.
IMHO, the only real drawbacks to using Kevlar or a hybrid is getting it wet out, and trying to sand it.
James
While I'll agree that Kevlar and Kevlar hybrids are a lot tougher to wet out properly, and you dang sure can't sand 'em, I must disagree with your analysis of their use on fins.
Kevlar doesn't stretch. At all. That's its advantage in a lay-up.
Look at a G10 fin from the front. Measure one side, from the root to the tip. It's a fixed measurement. Now, bend that fin into an arc. Unless that fin is infinitely thin, the radius of the outside arc is greater than the radius of the inside arc. You've stretched the outside of the fin, and compressed the inside. Now, do a composite lay-up on that fin, and try to stretch that side. You're putting the lay-up under tension. But the carbon, Kevlar, or hybrid material doesn't stretch. Do the lay-up on both sides, and the fin won't bend.
IMHO, the only real drawbacks to using Kevlar or a hybrid is getting it wet out, and trying to sand it.
James
Everything stretches..
how stiffness the fin is is a function of the stiffness of the materials (modulus E), the stiffness of the composite in shear (Shear Modulus - G), and the geometry (goes into Moment Of Inertia - I).
Stiffer materials stiffer fin,
higher Moment Of inertia - stiffer fin
When you bend the fin, the outer fibers do stretch a little and the inner fibers do stretch a little, plus the layers shear a little between them.
How likely a fin is to flutter is dependent on the same factors, E,I, and G.
For the same thickness fin, a graphite fin will be stiffer and have a higher natural frequency than a Kevlar fin or fiberglass fin.
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