Broken fin repair part II

Since I was not skilled enough to drill straight holes in my fins to repair using carbon fiber rods, I am going to repair the fin by making a
splint with fin stock. I made some 1/16" thick fiberglass material following instructions I found at http://www.vatsaas.org/rtv/construction/construction.aspx
Now I need advice on how to proceed.
Here are some pictures of what it looks like
http://deltateamgaming.com/rockets/index.php/Image:Img_0289.jpg
http://deltateamgaming.com/rockets/index.php/Image:IMG_0304.JPG
http://deltateamgaming.com/rockets/index.php/Image:IMG_0307.JPG
There is about from 1/2" to 1/4" of fin above the body tube.
I am trying to decide... Should I cut new slots on either side of the base of the fin and glue new fin stock on either side that goes all the way down to the motor mount? Or, can I get away with just gluing stock down to the body tube?
Gluing all the way down to the motor mount sounds stronger but I do have Aeropoxy on the inside of the body tube against the fin (ie internal fillet) that I feel is giving some strength. If I cut the slots on either side of the fin base I lose this.
I do not have experience with this kind of repair. If I don't go all the way down to the motor mount will the repair be strong enough? I am using good epoxy so I would think so.
The hobby store where I got the fiberglass cloth at, suggested I just use one sheet of cloth and that would be plenty strong enough. Of course he doesn't fly rockets.
What do you all think?
BTW the vertical, burnt, slots you see in the pictures are from my attempt at drilling into the edge. I am going to fill these with epoxy and glass fiber.
Thanks
Mike
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As a new reader Mike, I'm not sure I read Broken Fin Repair Part I
However in my 'net reading' I did come across your site and seem to remember posting the home link.
Much of the construction tips were above my skill however I was particularly aroused at the 'perfect epoxy fins' or such...I'll be using the tape, epoxy and dowel/tongue depressor technique next time around.
tnx...eric
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Since you didn't have the luck/alignment to get internal pins to reinforce the joint, have you considered external ones? You could use some epoxy and embed sewing needles or pins in the layer so that they lay orthogonal to the break. The pins would act as "rebar" and stiffen the fin around the break.
This is all hypothetical for my part with regards to high power rockets. I've used the technique to reinforce broken plastic tabs on more mundane items such as kitchen appliances. That Cuisinart interlock tab held up for a number of years with the epoxy/pin_rebar reattachment. There was a fair amount of sheer force during lid locking, and vibration during use.
ScottE

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First, I'd remove the paint from the broken fin and the body tube on both sides of the fin.
Next, I would put epoxy on both pieces and in the holes you drilled and stick the broken piece on. Use a piece of plywood on each side if the fin and put a spring clamp on the fore and aft end to keep everything straight while drying. Also put some wax paper between the fin and the plywood so the plywood doesn't stick to the fin.
Once that is set, I'd fiberglass from 1/2" below the tip of repaired to the fillet on the adjacent fin. I would do this on both sides of the repaired fin. If you use 9oz cloth it will be very strong.
If the cloth ends at a fillet for the next fin, it is easy to blend in so you cannot see the end of the cloth. For the repaired fin, you can use some Bondo or something similar to blend in the edges of the cloth it. Sand that smooth and once painted, the repair should be unnoticable. It might take a few coats of Bondo & sanding to get it perfect.
It's much easier than it sounds. I see the site you mentioned has instructions for working with fiberglass. You might also want to check www.fiberglast.com
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On Wed, 13 Jul 2005 20:26:04 -0400, Phil Stein wrote:

OK. This is what the guy at the hobby shop recommended. I wasn't sure it would be strong enough.
I don't have 9oz, I only have 6 oz (heavy) cloth. Do you think that will be good enough?
When I am done that one fin it will be thicker than the other 2, is that OK? That is will it fly correctly with one fin thicker than the others? Of course I am learning that one layer is actually very thin.
So to make a splint with G10 fin stock is overkill? Or maybe not even as strong as your method?
Thanks for the info.
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Yes, to make a splint with G10 is way overkill and will not only make that fin 3 times as thick as the others, but will make the CG of the rocket shift off center. It will tend to cone or spin off-center in flight. Making a split with a layer of 6 oz or 9 oz cloth on each side, extending well up the fin and well onto the body tube (basically past the fillet by 1" or more) will be plenty strong, and then you can smooth it out with fill'n'finish (or Bondo if you like really really heavy filler).
A note on Phil's instructions - thin epoxy like West Systems Slow or System 3 will seep through wax paper, so if you use was paper as a barrier, peel it off after the epoxy has set but before it has dried, or else you will have a mess.
By the way - don't feel bad about not getting those pins aligned. 4 pins that are that small would not have been strong enough alone to hold the fin on. Pins work best on a thicker fin such as 3/16 or 1/4 inch plywood.
-- David
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On Thu, 14 Jul 2005 01:46:48 +0000, David wrote:

Excellant. Thanks for the help guys. I am going to finish sanding the area to remove the paint and primer and give it a shot.
Thanks
Mike
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wrote:

Will fill'n'finish stick to G10 & fiberglass? The weight of the amount of Bondo needed for this job is insignificant.

I've never had this happen. Make sure the side of the paper with the wax on it is toward the fin. Saran wrap works too but I think the wax paper works better. Whichever you use, remove it right after the glue has set - allow ~ 1-2 hours with West.

Also, if you use the 6oz cloth as mentioned in a previous post, it should work. If it is that crap they sell at home centers and auto parts stores, I'd either use 2 layers applied at the same time 45 degrees apart.

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ANYTHING glassed is strong enough for HPR (95% of the time :) ).

--
Jerry Irvine, Box 1242, Claremont, California 91711 USA
Opinion, the whole thing. <mail to: snipped-for-privacy@gte.net>
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