broken fin

Depends on how the break looks. Splintery is OK, in fact prefered, as long as you can push it back together and have it fit pretty well. Mix up some thin epoxy like System3 and cover all the splinters well. Then push it back together, cover each side with something non stick (wax paper), and clamp between a couple flat plates like 1/4" plywood sheets while curing. Then cover with a fiberglass strip overlapping the break and again sandwich while curing. Repeat with a wider strip if necessary.

If the break won't go together, then you've either got to splice new material into the fin, perhaps cutting dovetail like notches into both sides, or route out all of the old fin and replace the whole thing.

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

You might consider chopping the remaining fin -- fillet and all -- off at the airframe. Then, drill a set of 3/16" holes (same diameter as the width of the wood) into the portion of the fin between the airframe and the engine mount tube. Insert some 3/16" hardwood dowels. Trim the dowels off at some reasonable length. Notch out the new fin to match the dowels sticking out of the airframe. Glue everything back up the way you did it originally.

I have used this technique for small and medium power, but not on the scale you need. It should work, though.

Or you might sand it flat AT the fillet, make a proper size and shape replacement part, drill 4 holes in the fillet and fin for steel pins and insert allthread with epoxy.

Then glass the outside surface both sides again, and if it is a bit thicker, fine. Put the launch lug and railguide on the opposite sides.

Make all the edges and surfaces airfoiled well. Sand as needed.

Jerry

Interesting idea. What would you think of using a 3/32" aluminum plate, rather than the pins, extending 3/4" or so into both pieces? I could drill holes in the plate to ensure the glue keeps it in. I wonder if that would be stiffer?

The break is not even. One of the fillets came off with the fin; the other stayed on. Here's a picture.

Thanks, Bob, I am able to push the fin back together part-way, but not all the way. Here's a couple of pictures:

I wonder how much material I can cut/sand away in order to get the fin pushed back together, before I lose the strength of the wood.

-- David

I thought of plate firse but considered the difficulty of routing the groove right. But you could try it. I would still use steel. It is fully enclosed.

The photo seems more like surface delamination not breaking off "at the top of the fillet" as described.

I would use the pins as described, glue it and CLAMP IT in a position to ASSURE ALIGNMENT, and be done with it. Reglass the fillet.

Jerry

Now I think I see your problem. You didn't use that cheap "luan" plywood subfloor material, did you? It's great for subfloors, but it's CRAP for fins. You can break it in half with your bare hands. 2 thin verniers of wood onthe surface, with something no better than cardboard in between. Very much like the Lite-Ply.

If you used this stuff, hack off ALL the fins and replace them. You can do it now, or you can do it later...

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

Actually that looks close enough to try to mend. But read my other post. This looks like it might be Luan subfloor plywood, which is CRAP for fins.

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

Bob,

I noticed the same thing. The inner two plys seem to be a bit softer / spongier than the others. I don't think it is luan though. I've seen that, and it sucks.

I didn't make these fins myself. They are the stock LOC fins that came with the Magnum kit. They've held up under several K motors, and several other sub-optimal recovery scenarios, but this one fell from 4300' with no chute at all and landed right on that one fin.

I think I am going to try a biscuit-style repair. I assume that I should use wood glue rather than epoxy for the biscuit, and then use epoxy for the laminate over the new fillets.

-- David

PS - Bob and Jerry, thanks for your help. For the rest of you, you should be ashamed that this tech question got 1/10 the responses that a SPAM post got!

Point but it is fine for LMR rings.

Jerry

I think it is time to purchase another kit......heheh Barry

I hear ya Barry! But don't worry, I have my sights on a Bruiser EXP... :-)

I just want to say that I have 16 flights on this Magnum. I've had an EX J motor CATO, an Aerotech J350 CATO, and 2 other landings with little to no parachute. So far I've been able to fix everything. Nice kit!

Buying kits is good for the economy. What barry said.

Jerry

Yes.

Welcome to rmr. And I get criticized for my posts even though most really ARE on-topic if a bit opinionated.

Jerry

If only his kits were a bit crappier (flimsier, more prone to damage) like Estes he would be rich!

Jerry