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Model Forum / General / Rockets / September 2003broken fin
Well, I finally did it. After 76 flights as a BAR, I broke a fin. It's on my Magnum, and is right above the fillets. It is 3/16" plywood, and I had put a layer of 5 oz glass over it also. Can anyone give me any tips & pointers as to how to repair a broken fin? Unfortunately, I did a REALLY good job of internal fillets on this baby. Also, one side of the remaining piece of fin is fiberglassed / epoxied to one of the outboard 29mm motor mounts (inside the fincan, of course). The flight was a K550 to 4300', and the apogee charge fired but the main charge did not (for reasons unknown). It landed flat on its side. Thanks. David > Well, I finally did it. After 76 flights as a BAR, I broke a fin. It's on > my Magnum, and is right above the fillets. It is 3/16" plywood, and I had > put a layer of 5 oz glass over it also. Can anyone give me any tips & > pointers as to how to repair a broken fin? Unfortunately, I did a REALLY 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! <<< Kaplow Klips & Baffle: http://nira-rocketry.org/LeadingEdge/Phantom4000.pdf www.encompasserve.org/~kaplow_r/ www.nira-rocketry.org www.nar.org Save Model Rocketry from the HSA! http://www.space-rockets.com/congress.html Thanks, Bob, I am able to push the fin back together part-way, but not all the way. Here's a couple of pictures: http://www.nomatech.com/rockets/fin/P1010135.JPG http://www.nomatech.com/rockets/fin/P1010134.JPG 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 > > Well, I finally did it. After 76 flights as a BAR, I broke a fin. It's on > > my Magnum, and is right above the fillets. It is 3/16" plywood, and I had [quoted text clipped - 19 lines] > > Save Model Rocketry from the HSA! http://www.space-rockets.com/congress.html > Thanks, Bob, I am able to push the fin back together part-way, but not all > the way. Here's a couple of pictures: [quoted text clipped - 4 lines] > 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. 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! <<< Kaplow Klips & Baffle: http://nira-rocketry.org/LeadingEdge/Phantom4000.pdf www.encompasserve.org/~kaplow_r/ www.nira-rocketry.org www.nar.org Save Model Rocketry from the HSA! http://www.space-rockets.com/congress.html > 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. Point but it is fine for LMR rings. Jerry  SignatureJerry Irvine, Box 1242, Claremont, California 91711 USA Opinion, the whole thing. <mail to:01rocket@gte.net> Please bring common sense back to rocketry administration. Produce then publish. http://www.usrockets.com My articles valuable? Donate at iKobo.com c/o my email. 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. SignatureMarty Schrader Up and Over... > 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 [quoted text clipped - 6 lines] > 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 SignatureJerry Irvine, Box 1242, Claremont, California 91711 USA Opinion, the whole thing. <mail to:01rocket@gte.net> Please bring common sense back to rocketry administration. Produce then publish. http://www.usrockets.com My articles valuable? Donate at iKobo.com c/o my email. 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. http://www.nomatech.com/rockets/fin/P1010135.JPG -- David > > 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 [quoted text clipped - 17 lines] > > 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 [quoted text clipped - 4 lines] > stayed on. Here's a picture. > http://www.nomatech.com/rockets/fin/P1010135.JPG 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 SignatureJerry Irvine, Box 1242, Claremont, California 91711 USA Opinion, the whole thing. <mail to:01rocket@gte.net> Please bring common sense back to rocketry administration. Produce then publish. http://www.usrockets.com My articles valuable? Donate at iKobo.com c/o my email. > 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 [quoted text clipped - 4 lines] > stayed on. Here's a picture. > http://www.nomatech.com/rockets/fin/P1010135.JPG 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! <<< Kaplow Klips & Baffle: http://nira-rocketry.org/LeadingEdge/Phantom4000.pdf www.encompasserve.org/~kaplow_r/ www.nira-rocketry.org www.nar.org Save Model Rocketry from the HSA! http://www.space-rockets.com/congress.html 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! > > 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 [quoted text clipped - 20 lines] > > Save Model Rocketry from the HSA! http://www.space-rockets.com/congress.html > Bob, > [quoted text clipped - 10 lines] > use wood glue rather than epoxy for the biscuit, and then use epoxy for the > laminate over the new fillets. Yes. > -- 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! Welcome to rmr. And I get criticized for my posts even though most really ARE on-topic if a bit opinionated. Jerry SignatureJerry Irvine, Box 1242, Claremont, California 91711 USA Opinion, the whole thing. <mail to:01rocket@gte.net> Please bring common sense back to rocketry administration. Produce then publish. http://www.usrockets.com My articles valuable? Donate at iKobo.com c/o my email. I think it is time to purchase another kit......heheh Barry > Well, I finally did it. After 76 flights as a BAR, I broke a fin. It's on > my Magnum, and is right above the fillets. It is 3/16" plywood, and I had [quoted text clipped - 10 lines] > > David 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! -- David > I think it is time to purchase another kit......heheh > Barry [quoted text clipped - 15 lines] > > > > David > 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! If only his kits were a bit crappier (flimsier, more prone to damage) like Estes he would be rich! Jerry SignatureJerry Irvine, Box 1242, Claremont, California 91711 USA Opinion, the whole thing. <mail to:01rocket@gte.net> Please bring common sense back to rocketry administration. Produce then publish. http://www.usrockets.com My articles valuable? Donate at iKobo.com c/o my email. > I think it is time to purchase another kit......heheh Buying kits is good for the economy. What barry said. Jerry > Barry > [quoted text clipped - 14 lines] > > > > David SignatureJerry Irvine, Box 1242, Claremont, California 91711 USA Opinion, the whole thing. <mail to:01rocket@gte.net> Please bring common sense back to rocketry administration. Produce then publish. http://www.usrockets.com My articles valuable? Donate at iKobo.com c/o my email. |
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