Fiberglass covered aircraft plywood fin & Finsim

[LWade]

Hello,

I am designing a L3 7.5″ G12 certification rocket and had questions regarding fin construction and simulation.

I am leaning towards fiberglass covered 3/8″ or 7/16″ aircraft plywood, for weight, availability and cost reasons. Couldn’t locate just 6-8 sq ft of 3/8″ fiber glass covered nomex honeycomb panel, any source recommendations?

Has any one simulated these sandwich panels in FIN SIM to predict flutter susceptibility?

Thanks,

Lwade

[Tim Thomas]

Try ACP Composites. I bought some of their panels. Nice stuff. What are your fins dimensions? I might have some 3/16 or so, panels that might work.

[LWade]

Lawndartman,

ACP has 1/4″ or 1/2″ panels. Did you build up fins using your panel. If so, how did you create the edge’s (leading & trailing)? Did you use carbon fiber or glass over the whole fin, including edges?

[Tim Thomas]

The edges can be done several ways. Dave Way is very good with a wood dowel glued along the edge, then sanded to the profile he wants. Mark @ Stickershock is a lover of either brass or aluminum tubing. I think he did a piece on Rocketry Forum on how he does his. On my L3 Thunderstruck, it had 1/2 in panels. I used a 1″ wide 1/2 in. thick strip of comp wood with a routed 1/16 each side about 3/8 in. up the 1″ side. This left a 3/8 in notch that I glued into the edge of the fins. I finished the edges very nicely. On all of the methods I would Tip to Tip. All of these methods are very easy to do. Getting the panels is the tough part. I will PM you about the panels I have. Tim 8)

[Chad]

I’ve had good luck building up wood fins too before covering them. You can produce a much better fin for about the same thickness as compared to plywood. Look at the construction of wooden airplane or glider wings for ideas. You can use a hardwood spar with the grain running perpendicular and then put lighter wood in front and behind, then finish the edges with a harder wood again (like the dowel suggestion). I’ve also had good luck using a thin sheet of G-10, gluing two spars on either side, and then laying down balsa wood to fill the rest.

Much better strength to weight ratio and I haven’t had problems with warping that I’ve had with plywoods.

[James Russell]

subsonic speeds are not as critical with fins and laminating, less apt to flutter depending on fin semi-span and surface area and thickness. Glassing fins will be more to prevent warping of fins, helps with finishing and will help to prolong life and make it more durable to withstand all the possible damage from storing, transporting, landing and dragging the rocket might see.

If you can find the honeycomb stuff even better. Edge finishing is not hard, as lawndartman has said, wood dowel, tubing or wood with a tongue that will fit into the center of the honeycomb if you cut down the edge as deep as the tongue will be. Glassing over that would be best to keep it from getting damaged and keeping it on.

If you are going to stay under Mach then rounded edges would be fine and take less damage then a sharp edge as well.

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