How to fix a broken fin

Mike wrote in news: snipped-for-privacy@kordik.net:

Is the fin thick enough to fit reinforcing *within* the fin?

ie: the same way one might use dowel to reinforce two pieces of timber being joined edge to edge. (top of this page):

Unfortunately, no. I would guess that the fin is only 0.040" - 0.060" thick.

Thanks for the idea though.

I've had to repair several fins of several different types. (Note that all the fins were broken on landing; none during flight).

On plastic aerotech fins: I found some epoxy at Ace Hardware that is supposed to stick to plastic. I sanded off the paint; epoxied a piece of carbon fiber cloth to one side; smoothed it out with Fill'n'Finish, and repainted.

On plywood fins: I used an internal joint. On one, I made a biscuit out of aluminum. On another, I used 3 1/8" stainless steel pins. I also sanded the paint and external fillets off, re-applied the fillets, and added a small fiberglass patch about 1" up the fin and 1" onto the booster.

On fiberglass fins like the PML kits: I sanded the paint away about 1" on either side of the break, and applied a small fiberglass patch 1" up the fin and 1" onto the booster; smoothed it back out with fill'n'finish, and re-painted.

All of the repairs have held, but the fiberglass fin patches have in some cases left the fin slightly warped (the fin didn't break all the way off; it tended to tear 1" or 2" up from the aft end, right above the fillet. I couldn't get the fin to stay straight no matter what I tried). This causes a bit of rotation during flight, but nothing more than that.

I have had to do this twice now.

I dremel the fin away just above the fillet along the breakline, following the original crack. Clean it with the dremel. Not too much, just enough to get rif of any real shredded timber.

Then using a tiny dremel bit (depending on the fin thickness), I drill into the fin stock and root at intervals (matching of course), then use Carbon fibre rod pins to pin it together. Using milled fibre reinforced epoxy to glue it in place and fill.

If u do it carefully you will never know, and it is very very strong.

If u do need, a layer of fibre glass of CF tissuue over each side of the joint will give a good finish.

I havce pics of the two I've done. Never had a problem with them, on up to a Pro38 4grain .I';m sure theyd take a lot more.

Kev UKRA #1383

I believe that the PML Ariel uses fiberglass fins; do you have a source for carbon fiber rods that will fit inside fiberglass sheet?

It's G10 and probably 1/8 or 3/16" thick and if you are drilling holes to put rods in it, you had better have a real small drill and a precision machine or a REALLY steady hand.

This should be in the anti-FAQ.

Yes, I know. They look to be about 1/8". I found some carbon fiber rods online that are 0.020" thick.

Let us know how it works out. I'd be interested in all the details.

I would guess that .020" rods would snap easier than a toothpick, and would be all but impossible to align - but I could be wrong - let us know how it works out!

I received the carbon fiber rods today. I ordered 3 different sizes.

The fins are about 1/8". I figured I would drill 1/16" with a drill press. Since the fin is so thin I figured that the bit is likely go through the one or both walls of the fin. It probably doesn't matter that much does it?

You used fiber reinforced epoxy, I do not have that but I do use Aeropoxy which is extremely strong. Is that enough?

What are the intervals that you used?

How deep did you go? I figured I wouldn't have to go very deep.

Thanks for the help. I won't get to it this week but I will report back (with pictures) when I am done.

Thanks Mike

You can make your own fiber-reinforced epoxy by cutting some 1/8" to 1/4" long fiberglass fibers from the edge of a sheet of fiberglass, and mixing into your epoxy.

That might be to course for what he's doing. Some of the stuff you get from West and others is more like a powder and IMO would work better in this case. I've done this and it worked well for me.

The carbon fiber rods did not work out for me. I think it is a good idea but I couldn't get the holes lined up properly between the two fin halves.

I am going to cut slots on either side of the fin base and glue some fin stock on either side. This will make a new fin slot (or splint) for the fin.

Mike

That's a shame, I've used this technique twice with good results, once to reattach all three fins on a rocket! While it's a good technique, the drilling of the holes is hard as the alignment is absolutely critical. One fin I reattached ended up having oversized holes drilled to allow some "slop" when reattaching the pinned fin.