Glassed Foam Core Fins?

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talkin Monkey

Well-Known Member
Jan 17, 2009
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I'm sure I'm not the first to think of it or even try it.

Amongst the pile o' raw scratch build materials stacked up in my garage is some 1/2" high density polystyrene "GreenGuard" foam insulation special ordered as fin stock.

Seems like it would be fairly realistic to cut, airfoil and apply 2 maybe 3 layers of 20oz fiberglass to keep them fairly light but rigid.

I've got plenty of 1/2" Baltic Birch for the fins but I like to build something lighter for the fin/mmt section.

My goal is to scratch build this project with semi-local non-exotic (low $$$) materials AND keep it light enough to fly as an L2 cluster. Basically, it's just an upscale of my L1+L2 PML Endeavour custom builds.

Anybody had any luck just glassing foam up and calling them reliable landing proof fins? If I get them glassed up enough to be flight worthy, will I be saving enough weight to make the effort realistic?

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I have never used foamcore but the honeycomb composite from Giant Leap is great. It weighs almost nothing and all you have to do is apply a wood edge and add 1 layer of 1.4 oz satin weave to cover it. You might have to use too much fiberglass on foamcore to save any weight.
The honeycomb composite fin material from Giant Leap looks like some remarkable stuff, thanks for the tip.

As Giant leap notes, reducing fin weight enables some reduction of nose weight, which in turn enables the use of a wider range of motors.

I was going to make some basswood core fiberglass covered fins for an upcoming project, but will use the honeycomb instead.
Look for information on "vacuum bagging" RC glider wings. I was fiddling around with this technique some years back and commonly vacuum bag fiberglass or carbon fiber onto plywood fin cores to get a ding-resistant surface. I'm sure you could hot-wire cut a fin to airfoil shape and use the same process.
Look into what the home built aircraft guys do, common to wire cut and then glass wings, light strong, cheep.

I have done lots of glass over foam (day job). A few warnings / suggestions,

Polystyrene foam and polyester is a no-no, epoxy only.

Needle the foam before you glass it, I use a 'porcupine roller' typically used for glass mat to needle the foam, significantly increases the peel strength.

Think of a window frame filled with foam, for example make a wood frame for the edges, and put a foam piece in the center. This lets you have something that can be shaped before glassing. Sharp edges in raw foam can be hard to handle.

Why vacuum bag? Use a paint roller to roll out the resin, thin and uniform on a piece of mylar, put the glass on it and squeege or roll a bit more resin (with practice you can do this without excess resin) put the mylar and glass on the core and squeege the mylar until the glass is smooth and void free, after the resin cures peel the mylar. (don't use etched mylar for drafting!!). This technique can give nearly ready to paint finishes.

Add one ply of 120 style glass cloth on the surface, gives a great surface finish and sanding ply.
Thanks for your input and ideas guys.

Lots of food for thought. I'm Still waffling over getting downright mid-evil on these six 1/2" baltic birch fins with a hole saw to create a honeycomb "effect" or just cutting parallel 1" or so longitudinal miters would do the trick. I think the latter would be lighter but not as 2 dimentionally as strong as the former.

Basic idea is to lighten up six big fins o' baltic birch to keep aft end weight/flight costs minimal as possible but still be strong enough to endure a possible "hard landing" or even mach stresses.

The voids will be filled in with 1/2" PolyStyrene-ish foam + a layer of 2oz glass before continuing the rest of assembly.

P.S. Are there generic/local materials thats I can use as "peal ply"?

Thanx again.