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Jul 07, 2006, 01:49 PM
Thread OP

How repair carbon boom? Broke at v-tail joint so can't wrap with carbon tow....

Hi. I'm new to carbon material in airplanes and just crashed my glider.

It has a 30 inch carbon boom that is glued to the fuse. The tail of the boom is slotted vertically for a distance of 3 inches. A 3-inch long combination v-tail mount / tail skid is slid in to this slot in the tail and gluded with what looks like thin CA.

One half of the 3 inch slotted tail is cracked; the other is hanging by a carbon thread.

I wish it had broken away from the v-mount, closer to the fuse, so I could just wrap it, but it cracked right at the beginning of the v-mount so using carbon tow would be more of a laying-on parallel-to-the-boom process.

Is this fixable? I thought about epoxy alone. Then epoxy and carbon tow. Don't want to add much weight to the tail, of course, so I wondered if thin or medium CA could work by itself?

Thanks for your help,
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Jul 07, 2006, 03:41 PM
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davidfee's Avatar
Can you post a picture?

In general, it's not a good design practice to cut slots or notches in tubes. A proper mount would have included a ring section to fit over the boom.
Jul 07, 2006, 09:17 PM
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Erk's Avatar
Sounds to me like your best bet is to reinforce the boom with some CF flat strip on either side, front to back, then wrap that with tows. Or if the boom's got room inside, you might get away with an internal repair done similarly, except you'll still want to wrap the outside.


Jul 07, 2006, 09:21 PM
Thread OP
Sorry I can't do pictures yet. It is a 4 inch slot with one half broken almost free; the other side is cracked. A single piece of carbon made with a v-mount on top and skid on the bottom slips into the up-down slot.

I wonder if I could drill five holes in the v-mount/skid piece parallel with the broken boom, and then lace with thread and CA. Sort of "sandwich" the two sides of the slotted boom against the v-mount piece. Then coat the broken edges with tow and epoxy.

Or maybe cut away the front of the v-mount for 1/2" (it's 4 inches long now), just above the boom, so I can slip in a couple of wraps of tow around the boom and over the cracks on each side of the boom.

BTW, this is a Poly Pulsar ARF, so boom, v-tail mount, and skid came as one piece. Flys great, though.
Jul 07, 2006, 09:27 PM
Thread OP

Thanks a good idea. There's room inside for a 1/8" flat (diameter of boom at that point is 5/8".

- Use epoxy glue for the carbon flat and tow?
- The trick is wrapping tow on the aft-side of the break, as the v-mount and skid are in the way. Wrapping the boom in front of the break is no problem. Any ideas for that?

Jul 08, 2006, 01:32 PM
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davidfee's Avatar
I did a search on the Poly Pulsar and found some pics, so now I know what we're talking about.

Is it possible to completely remove the v-tail mount/skid at this point? That would give you the best access to the boom for the repair. Because the boom has failed at the front of the tail mount, this area really needs to be reinforced. The best-looking solution would be to make a clean saw cut at the front of the tail mount and fabricate a sleeve that will fit inside the boom to join the front & back sections. That might be delicate work, however, with the pushrods installed. It is possible... I've done it by slitting the sleeve so it can slide over the pushrods.

The next best solution is to wrap the outside of the break with carbon or glass fabric. This will be much easier without the tail mount in place.

It's not a bad idea to wrap kevlar thread or carbon tow around the boom at the front and back of the slot. To get at the back, drill holes through the v-mount at the top and bottom of the boom.

Use epoxy for everything... the slower the better. 5-minute epoxy is useless for something like this.

Prep all bonding areas by scuff-sanding (e.g. 220 paper) to remove gloss and promote adhesion.

Clean all surfaces with alcohol or acetone prior to bonding.

good luck,
Jul 08, 2006, 02:58 PM
Thread OP

That's the answer

Thanks David,
I popped the v-mount/skid free without additional damage. Have plenty of access inside the boom. Will:
- buy a 1/2" O.D. sleeve for inside the boom, about 1" long
- knotch the front of the v-mount 1/4" to allow 1/2" worth of tow wrapping on the outside of the boom, over the break, in front of the v-mount
- drill holes in the v-mount, above and below boom, to secure the back of the boom against the mount with thread
- 2 hour epoxy, 220 sand paper, and alcohol.

LAST Question:
Where can I buy carbon tube (or aluminum) with an O.D. of 1/2"? Only need 1 inch long, but my LHS doesn't stock that large.

Last edited by 4444; Jul 08, 2006 at 04:06 PM.
Jul 08, 2006, 04:28 PM
Registered User
davidfee's Avatar
Personally, I would make the sleeve... since you don't need much (1" length sounds about right). It can be as simple as... wax the outside of the boom and use that as the mandrel... wrap fiberglass/epoxy around the waxed boom... a two or three layers of 4-6oz should do it... break free when cured. Cut a slot in the sleeve to allow you to collapse it to the correct (smaller) diameter.

Make sense?

Jul 08, 2006, 04:38 PM
Registered User
Tapered Tubing
45102 .550 Taper .55-.25 .035 .032 36 stock rw cello-wrap $21.79

020017 .472 " 48 .472 .353 87.0g $9.18

Either 2" cut off or replace the whole boom. For the whole the boom, don't cut slots or drill !!

Design V-tail mounting. Mount the V-tail on a short (1/2 to 1") pylon.

Control horns can be below the V-tail control surface.

An advantage of custom made control horns, which you make from 1/32" aluminum sheet, is that the bases can be angled to the standing parts so that the standing parts are parallel and the mounting bases can be closer to the center. This minimizes both the interference and splaying issues.

Another issue is that the place on the control horn that the pushrod connects to rotates in an arc which is not in a vertical or horizontal plane. Under these circumstances, the best connection between the horn and pushrod is a ball joint rather than a clevis. The pin of a clevice will have a cone shaped interface with the hole in the control arm. You can get away with the clevice if a thin aluminum control horn is used so that the difference between a cylindrical hole and a cone shaped hole is minimized.
Last edited by Ollie; Jul 08, 2006 at 04:50 PM.
Jul 09, 2006, 05:54 PM
Thread OP

Make my own...

I appreciate the websites (and the time you took to look them up). I've got them saved for possible later use. Fortunately the Poly Pulsar has pretty good horns and I just used z-bends to connect to them. Then, with 30 minutes of creative bending, I finally got the push rods to swing on an arc as it travelled back and forth that fairly resembles the swing of the horn, so no binding. Plus, light weight in the tail.

Clever idea, and appeals to my "builder's pride" (and budget). The boom tapers smaller at the end so using the end for a wrapping mold might result in a sleeve that fits inside without cutting the slot you mention; however, since gluing it inside anyway, a slot shouldn't hurt the strength.
You said fiberglass... how about 1/4" carbon tow? I've got yards of that. Wind it 4 layers deep? Otherwise, I have 1.7oz FG... wrap 7 layers deep?


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