Broken fin repair part II

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

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

Mike wrote in news:pan.2005.07.13.23.27.42.695012 @kordik.net:

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

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.

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.

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

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

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.