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Old Sep 18, 2004, 04:09 PM
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KurtMc's Avatar
Sacramento, CA
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Suggestions on how to fix a broken CF boom?

My "rock" plane unfortunately lived up to its name recently and I ended up with a snapped CF boom. The plane is an older-model Feather and I'd appreciate any suggestions on how to repair the fuse as opposed to buying another one? Wrap it in Kevlar thread and epoxy? Make a CF splint? Any ideas would help.

Here's a pic with inset of fracture site. TIA

Kurt
[IMG]broken_boom2.jpg[/IMG]
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Old Sep 18, 2004, 04:50 PM
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Coopersburg, PA
Joined Sep 2001
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Kurt,

The boom on my XP-3 snapped about an inch behind the pod when I had a radio failure and it dove into macadam from about 100 feet last year. I ended up getting a piece of tailboom from a "well known supplier" and used it as an internal splint. Since the booms are tapered it worked perfectly. I cut my piece from the area of the boom with the right diameter so that it slid in from the pod end and stopped sticking halfway out the broken end. I epoxied this in and let it set, then epoxied the tail end of the broken boom on. What was nice was that everything lined up automatically. It's held up all summer so far with many hundreds of discus throws.

Randy
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Old Sep 18, 2004, 06:20 PM
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Sacramento, CA
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Thanks Randy, I'll look into an internal tube as that seems like it would be structurally strong. Anyone tried a piece of copper or CF tubing externally?

My boom's the same diameter the whole length, the fracture is dead center, and with control cabling and antenna wire going down the middle, it's a pretty tight fit already.

Kurt
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Old Sep 18, 2004, 08:22 PM
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Fort Wayne, IN
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Kurt
You will need to make a jig to keep things square. Use the angle iron (steel) you can get at the hardware store that has the holes in each side (it is used to reinforcement broken items). It has holes down each side of its length and can be cut out so you have an open section in the middle to work with. Just cut from the pointy part of the angle to the holes and then down the length for 2-3 inches. Then make another cut from the angle in, and the center three inch angle will drop out and leave a nice open center section and straight rails connecting each end. Use electrical ties and wood blocks to hold both ends of the boom in place on the angle iron and line everything up. Use a small amount of epoxy to join and reinforce the inside of the boom and the break, Wait for the glue to dry. Then remove the angle iron jig from the now one piece boom and wrap the outside of the boom with fiberglass (or Kevlar) and epoxy to finish the fix. I hope I have explained it ok so you can figure it out. It has worked great for me and things come out straight and square.
John
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Old Sep 18, 2004, 11:01 PM
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Hi Kurk,

You may try making an open carbon sleeve to go over the outside of the broke part like what's shown in the diagram. The carbon sleeve can be made by the method shown in http://www.windandwavemodels.com/rollfuz.html. The internal diameter of the carbon sleeve should be slight smaller than the external diameter of the boom so that the spring action of the sleeve will make a tight fit. It's super easy !

Hope it helps,

Y C Lui
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Old Sep 19, 2004, 12:26 AM
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Y C Lui
Why not use a jig to glue the boom back together and then wrap with carbon, FG, or Kevlar?

John
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Old Sep 19, 2004, 02:29 AM
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Sacramento, CA
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Thanks for the ideas, John and YC. I'm probably going to give both ideas a try. I like the aesthetics of YC's external tube and it's probably going to be easier than trying to fit an internal splint.

I know kevlar's strong stuff John - how many times should do you wrap it around once you've got it lined up in your jig?

Thanks again guys, there's hope for this bird yet.

Peace

Kurt
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Old Sep 19, 2004, 02:52 AM
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Kurt,

You can get the same finish by using John's method. After applying carbon and epoxy, wrap a piece of mylar over it and secure with tape. You will need to pull the tape pretty hard in order to squeeze out bubbles and excessive epoxy so do make sure that the joint will have sufficient handling strength. That was how I fixed the boom of my Salome DLG.

Y C Lui
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Old Sep 19, 2004, 06:47 AM
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Quote:
Originally Posted by KurtMc
I know kevlar's strong stuff John - how many times should do you wrap it around once you've got it lined up in your jig?


Kurt
It depends on the Kevlar you use. The stuff I have is thick (maybe 2oz.) so I do not use more that two wraps. I used two wraps of .070 FG on my Art Hobby Colibri and it has held up fine. If you cut the cloth so the second wrap is just a little wider than the first you get a more smoothed, tapered edge. The tape wrapping is a good idea but I have not used it yet. I used a plastic food wrap once but it did not give as good of a finish as just letting it dry with out the wrap. I do try to blot off the extra epoxy to save weight. Donít forget to re-trim and reset the CG after you are done.

John
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Old Sep 19, 2004, 07:00 AM
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P.S. If you use FG it almost disappears when it is wetted out.
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