Repair of through the wall fin

I have a polecat aerospace 5.5 Phoenix with a broken lower fin, at the body tube. Any ideas on how to get the old fin root outta there and replace it?

M60

m60a3tts said:

I have a polecat aerospace 5.5 Phoenix with a broken lower fin, at the body tube. Any ideas on how to get the old fin root outta there and replace it?

M60

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That doesn't sound fun at all.

First thoughts begin with the tedious task of dremeling/routing/barrel sanding/chipping/gouging it all out and starting over.

If that didn't work and I'm beyond my threshold for patience then I would go with the tried and true method of placing a small shaped C4 charge along each of the fillets and then moving back for the detonation.

Can you post pictures of the area that includes the broken fin and a shot of the aft centering ring/motor mount area. It would be easier to maybe figure something out if we could see it.

Good Luck and may the patience of Job be with you.

Andrew

I have to agree with everything Andrew said. There is no real miracle fix except to rout out the remaining portion of fin. I have a HP bird that also broke a fin and of course being a HP rocket, you're left with no choice but to make it "good as new".
After I routed out the wood all the way to the MMT, the rest was actually quite simple. I applied JB Weld quickset to the MMT as well as the root edge of the new fin, and let cure.
I then did the outer fillets and let them cure. I drilled a 3/16" hole in the BT on both sides of the fin (for internal fillets) and poured a 2 part clear (West Systems) in each hole and rolled it back and forth to fully cover the fin and MMT inside.
Routing out the remaining piece is where the work is at. Take your time and you'll be fine.
Hope that helps......Good luck
_____________________________
-Chris

I have the pics.









M60

If I were you I'd be sorely tempted to cut it flush with the AF and make the replacement a surface mount. Here you might want to use some glass for reinforcement--say ay least 1/2 way up the span and well past where the fillet ends on the the body tube. It will stay put. Likely lot less work than the surgery required otherwise. Another option if you're really determined to do TTW mount, would be to leave the remnant of the broken fin in place, and open up the tubing enough to gratft some glass that can be run from the new to the old. Generous fillets afterwards.

Agree there ,glue it back to what remains ,the joint will be stronger than the wood.

m60a3tts said:

I have the pics.

hmmm...my pics uploaded...but not here.

M60

Click to expand...

Once they upload, you have to add the attachment.

I would take my time and definitely repair the area as it was before. With a dremel tool this is a bit tedious, and will take a little time, but the repair will be stronger. Drilling the holes on each side of the fin is something that will help once you get the fin back in there. If you make small semi-circles with a dremel on each side of the fin you can fill them in with a little glass cloth "stuffing", saturate them with CA and grind back smooth before filleting over them. There are lots of tools ina dremel kit that will help with this kind of thing, and before you get done, you'll have 5 or 10 of them out. hehe

I too have a busted fin on my 5.5" Phoenix, so this thread is really helpful. Good insights too!

denverdoc said:

If I were you I'd be sorely tempted to cut it flush with the AF and make the replacement a surface mount. Here you might want to use some glass for reinforcement--say ay least 1/2 way up the span and well past where the fillet ends on the the body tube. It will stay put. Likely lot less work than the surgery required otherwise. Another option if you're really determined to do TTW mount, would be to leave the remnant of the broken fin in place, and open up the tubing enough to gratft some glass that can be run from the new to the old. Generous fillets afterwards.

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I have to agree with the idea of surface mounting the replacement. It will be more than strong enough, and far easier to do.

SpartaChris said:

I too have a busted fin on my 5.5" Phoenix, so this thread is really helpful. Good insights too!

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Any photos of the disaster? We can weep together.:cry:

M60

Doesn't this fin also in a way "draft" behind the front fin so it doesn't take as much of a beating as the front one?

Bigander said:

Doesn't this fin also in a way "draft" behind the front fin so it doesn't take as much of a beating as the front one?

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The beating it takes is upon landing. The rocket came down like a brick at saturdays launch. Even with a 70" chute.

M60

Next time try coming in ballistic to take advantage of the draft...

Can you cut the bottom centering ring about 1'' on both sides of the fin and get to it that way .Replace the fin and foam the fin can.

I replaced a fin on my EZI-65 a couple of weekends ago. It broke off right at the body tube. I used my Dremel to carve out the old fin. Eventually, I ground it down past the body tube enough to leave a gap. Then I was able to stick a screwdriver in and break off the old fin tab where it met the motor mount. I used the screw driver to strip away as much of the epoxy fillet as possible. I enlarged the fin slot a bit. Then I cut a new fin from plywood and glued it it. I used the gap in the fin slot to add some extra glue then created a fillet along the body tube.

-- Roger

m60a3tts said:

I have a polecat aerospace 5.5 Phoenix with a broken lower fin, at the body tube. Any ideas on how to get the old fin root outta there and replace it?

M60

Click to expand...

I have had to do this before. It's a pain in the *##.

Here is my approach.

I use a RotoZip with the bit set at the proper depth to remove all traces of the fin from the motor mount. The depth has to be right on, but it's not that hard to do.

Next start cutting. It will get a bit sloppy and cut some airframe, but if you are careful it will work very well.

Once all traces of the old fin are gone, simply insert the new fin and repair whatever airframe you damaged.

Worked for me.

I have a LOC Aura that had a hard landing on a streamer in Orangeburg this weekend. The fin is still hanging by a "thread" and has not come off completely. There is about 3/8ths of an inch of fin sticking out of the body then the cracks begin and then it's whole again. Is there a way to fix this without digging the thing out? I'd like to salvage the existing fin and find a way to reinforce it in place.

I have no fiberglassing experience.

I agree with using the Dremel tool and cutting out the fin. I like to use these diamond cut-offs from Harbour Freight. If you watch the add, they sometime drop the price to CHEAP. https://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=31501
I have also used a heat gun to remove fins that were epoxied in, but then I was trying to recover just the fins since the rest of the rocket was trash from a ballistic recovery. This rocket looks nice and the heat gun would cook your great paint job.
Harry Keyes
TRA10375
POTROCS

I agree with Pyrovette. I'd go at it from the bottom. I'd cut a slot big enough through the bottom ring to get a razor saw in there to cut away from the body tube and the motor mount. You may be able to grind away much of the epoxy fillets and slide the remaining piece of fin out through the bottom. After replacing the fin and doing the internal fillets I'd epoxy a whole new ring on the bottom over the existing one, looks like you have the room to do that. I don't know any other way to get an epoxy fillet on the fin and the internal body tube? Uncrichie.

Cut broken fit off flush to the body tube.

Drill 3 3/32" holes along the length into the stub.

Make new fin an drill 3 matching 3/32 holes into the root.

Go to your hobby store and buy some 3/32 music wire.

Cut three pins from the wire. Insert into the fin. Epoxy and insert into stub.

Fillet and done.

If you can foam the fin can you won't even need a new bottom certering ring.

Alright - I know this is too late but I was given some great advice from a friend that has a 10 Phoenix - he uses a coffee can attached to the bottom to absorb the landing. Those square fins tend to dig in and he too was afraid it would be ripped off.

Here is a shot of the can attached to my 6" DRM...these big fins dig in on landing and I would get small surface cracks. I used the coffee can and it takes all the energy on the landing and helps keep it off the fins. The can crushed obviously - but it is cheaper and easier to make a new one than fix the fin.

Hope this helps...

View attachment may7 -1.JPG

uncle_vanya said:

I have no fiberglassing experience.

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Hey Brad, yeah I saw that, what a sickening feeling hugh. Anywho, glassing isn't really that difficult, just messy (if you're like me that is.) A pair of gloves, a shallow pan, epoxy mix, and a medium to fine mesh cloth, plus of course your filler for finishing.
To the others with broke fins, here is another approach for you (providing you have access to a table saw) 1. trim remaining fin down to the BT 2. set saw depth not to exceed your EM or your CR's if you don't wish to repair those also 3. set up a pinch fence (2 fences so the rocket doesn't roll) 4. power up n cut. Now this may seem drastic to some, but, I've done it this way, and using various other tools and methods suggested here also. Personally, I won't hesitate using this method down to the EM and having to repair the aft CR also, as I have found this to be efficient, simple, and quick, leaving more time for repairs and less aggrivation in the removal process. Although, that roto-tool would work well for removal, there is that chance of it getting away from you and OOPS, now a large BT mishap and possibable another fin to repair. But that's just my opinion, and we all know that opinions are like butts...everybody has one, and most of 'em stink. GOOD LUCK! Hope this helped.

oops, i didn't look at the pics before posting that last suggestion. now i see you have fins inline and in front of, but, the table saw method can still work simply by using a 2x4 cut to or layed at the depth of your forward fins then cutting a slot in it for your fins to "ride", although you will probably need a table saw larger than a standard 10" since you need more blade heigth then i originally assumed...

I do not honestly know much about high power but I have had this happen on a few lpr rockers and mpr more recently. I like to use a small diamond dreamel bit and cut off the remainder of the fin. then carefully drill out the old fin ever 1/8 or 1/4 inch. Then taking a small cold chisel and fine nose break off the tabs and bull them out slowly. then clean off the epoxy with the chisel. and sand lightly.

cut the fin flush and glue on a new one.. you are essentially gluing the tab back on the fin this way... there is no need to dig the old tab out.

the glue joint will be stronger than the wood..

exactly, why people even bother with this thru the wall stuff in the first place is a mystery to me.

CharlaineC said:

I do not honestly know much about high power but I have had this happen on a few lpr rockers and mpr more recently. I like to use a small diamond dreamel bit and cut off the remainder of the fin. then carefully drill out the old fin ever 1/8 or 1/4 inch. Then taking a small cold chisel and fine nose break off the tabs and bull them out slowly. then clean off the epoxy with the chisel. and sand lightly.

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You use a cold chisel or a wood chisel ?

Hi Guys,

I have exactly the same problem with a Loc Caliber. When I finally get the ##*$% fin out would it be ok to replace it with G10 rather than ply? Im not sure if the diffrence in weight would affect stability etc?

Many thanks

Mike

why not just fiberglass a plywood fin? I was at the grocery store today and saw the new Reynolds food saver system. Ten bucks. They didn't have the gallon bags, but even the quart bags would work for the Caliber. Get yourself some 6 to 8 oz weight FG and some mylar film. Here I think the reynolds no stick aluminum foil would also work (note only one side is no-stick!).

Cut two pieces of FG a bit larger and in the general shape of to the fin. Wet out the FG with a good grade of epoxy--here West or Aeropoxy is best, but even hobby epoxy would work--30 min or longer. Then wrap the fin and FG in mylar, put in bag, and pull the vacuum as far as this little gizmo will allow. You may need need to "fingerpaint" the fin a bit to get it ridge free. Set aside until the FG is rubbery/leathery--then trim the remnants. (The mylar should pull free but stubborn areas can be simply sanded off). You will have a fin much stronger than the stock ply, and barely heavier than the other two. My guess is you will never go back to bare ply.

Personally I have come to believe G10 is not that good as fin material, at least in raw form, preferring say the GLR honeycomb products finished with FG or CF.