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Jul 23, 2016, 09:43 PM
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Question

Tail boom repair for Hobby King DLG


Well, a bad launch and a weak tail boom lead to a very interesting launch today. Needless to say I need to repair my boom before any more flying.

What happened is a split formed long wise from basically under the wing trailing edge back 1/2 way to the horizontal stab. Call it 7" worth. The carbon fiber was soft there I noticed when handling the airplane so I somewhat expected this to be a weak spot. Couple this with poor launch technique and you have a downed airplane.

Question is, how best to repair this damage. Everything is still strong enough that I could javelin launch and get 20 seconds with full control.

First thing that comes to mind is to cut off the pushrod guides and add fiberglass over the tubing with as little epoxy as I think I can get away with. Then reattach or make new push rod guides. Re-balance with some weight in the nose and go fly.

Option two, I have small carbon fiber rods that were going to be spars for a 3D foamie. I could glue those to the broken fuselage tube to work as external stiffeners. I would use 4 around the top, bottom and both sides.

Option three, replace the tube with an arrow shaft. I would have to locate the correct OD and I bet they weigh more and will again require ballast to get the CG right. Also have to worry about all the alignment issues with drilling holes for the wing mount and slot for the vertical tail. If I am going to all that work I might as well build the plans I have and make a new airplane and not stick with the HK guts.

I am leaning towards option two. I have the rods in hand and I think it would need the least amount of work. I do not have experience with fiberglass so there will be a learning curve. I am not worried about looks, this is just a beater to learn with. Question is, will it withstand poor technique launches while I still learn.

How have you guys approached such damage?

James
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Jul 24, 2016, 04:56 AM
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rdwoebke's Avatar
James,


Can you post a quick picture?

If it is only a split length wise I would fix it with a fiberglass wrap on the bias.

More later.


Ryan
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Jul 24, 2016, 08:06 AM
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Pictures


Here are a couple of pictures, very hard to see the damage. Best is on the close up. You can see where the splits are, on the larger picture you get a better idea of the length of the splits. There are several around the diameter of the tubing. It was soft to begin with so I don't know if there was enough resin in this part, other parts of the tube feel stronger.
Jul 24, 2016, 10:39 AM
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Boom


Light wipe of epoxy on the boom. Tight Spiral wrap carbon tow in one direction, then the other. Light layer over that. Blot out excess epoxy with paper towel, toilet paper. Minimal added weight.
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Jul 24, 2016, 02:01 PM
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I'm not familiar with that model. Can you post a link?

I think it would be best to understand how those booms are likely made. By the pictures you posted I believe that boom is constructed by taking a metal mandrel, waxing it, then putting a layer of very light fiberglass on the bias down then a layer (or two) of uni directional carbon. And the order of those two might be different, might be carbon first then glass. The carbon is "uni directional" only goes in the direction of the split.

So that is a pretty strong thing unless it gets a length wise crack like that. If you imagine an empty soda can it is pretty strong, you can't twist it and you can't bend it. But if you cut a slit in an empty soda can length wise with a scalpel you can easily twist or bend the soda can. Same deal with your boom.

So based on your pictures and what I think you have there I think you can easily fix it by just applying 2 layers of 3/4 ounce fiberglass on the bias. If those pushrods are fully enclosed in housings I believe you can just lay the glass over the pushrods and all. I have done that in the past. What you will do is first I would wick thin CA in the crack to kind of stabilize the boom a bit. Then cut your bias cloth for 2 wraps, then you will paint on laminating epoxy with a small paint brush. Then wrap the wet cloth area with electrical tape. That helps to really get the glass down well. After the eploxy cures you can peel off the electrical tape.

If your boom has kind of spiral ridges in it, and it looks that way from the picture, the boom is made that way (not with electrical tape probably but with something more fancy). Like I explained the booms are likely uni directional carbon and bias fiberglass. The carbon carries all the bending strength. The bias fiberglass prevents or at least is supposed to splits. They wrap that whole thing with a kind of tape and let it cure then peel the tape off.


Ryan
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Jul 24, 2016, 03:55 PM
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Ryan,

I cannot get any more clear picture, trying to use my phone and the one closeup is about as well as I can do. However, your description is spot on. The tube looks like it was wrapped around a form and then held on the former with something that left small ridges.

Never worked with epoxy/fiberglass before so I am not sure of the exact terms. When you say on the bias, what exactly does this mean?

Also, you mention laminating epoxy. I assume this is different than the 2 part epoxy you would buy at a hobby shop or Lowes.

I can picture what I think you are saying but want to ensure I do things correctly.

James
Jul 24, 2016, 04:19 PM
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Bchirrick,

Thanks for the information and the picture. Looks like a neat and tidy way of doing things. I will have to look and see if I can find some carbon tow. Using the tow instead of fiberglass sheets it looks like will save a great deal of weight.

What type of epoxy will be used in this type of repair?
Jul 24, 2016, 04:24 PM
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James,

In a bit I will post some drawings explaining bias.

Laminating epoxy is a 2 part epoxy and you can probably get it at a hobby shop but not lowes. Personally I like EZ Lam which you can get from cst composites in bottles for 18 dollars and you can also get 3/4 ounce per square yard fiberglass there too. Often laminating epoxy isn't a 1:1 mix ratio and I use a little digital gram scale to get the ratio. You won't need much epoxy for this fix. Just a few grams and you likely will blot a lot of it up you put on

If you are going to be flying DLGs you'll probably be fixing stuff from time to time so getting some light fiberglass and some laminating epoxy is going to come in handy.

Ryan
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Jul 24, 2016, 04:34 PM
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rdwoebke's Avatar
Hopefully you can read my scribbles and this makes sense.


Ryan
Last edited by rdwoebke; Jul 24, 2016 at 05:22 PM. Reason: typo
Jul 24, 2016, 04:42 PM
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Easy to understand and exactly what I was picturing. Now just waiting for Wednesday when I can go get some fiberglass and resin.

EZ lam, is it really thin? I would guess so to get it to wet the glass properly.

Thanks again for the info.

James
Jul 24, 2016, 05:12 PM
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rdwoebke's Avatar
Quote:
Originally Posted by zekjet
Using the tow instead of fiberglass sheets it looks like will save a great deal of weight.
If this boom is as I suspect it is and the crack is as I suspect the lightest material would be the 3/4 ounce glass cloth I suggested. That is 3/4 ounces per square yard. Carbon tow I think is something like 3 ounces per square yard.


Ryan
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Jul 24, 2016, 05:17 PM
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rdwoebke's Avatar
Quote:
Originally Posted by zekjet
EZ lam, is it really thin? I would guess so to get it to wet the glass properly.
James,

It is pretty thin. You can definitely brush it on it is about the viscosity of paint. EZ Lam is a little thinner than typical Hobby Shop laminating epoxy and definitely way thinner than hardware store 2 part epoxy or typical hobby grade 2 part epoxy (stuff listed as 5 minute or 15 minute). A big reason laminating epoxy is thinner is that the chemical reaction that makes epoxy work isn't really a 1:1 ratio. But to make the 1:1 ratio for some of the basic hobby stuff or some of the hardware store stuff they put filler into the hardner portion of the 2 part epoxy to make it so that it mixes at a 1:1 ratio by volume. It makes it easier for the basic person to mix and for some applications you want some filler anyway (but the filler used in those epoxies isn't the strongest).

I almost never use those 1:1 ratio epoxys any more. I just use laminating epoxy and add filler if I need something thicker.


Ryan
Latest blog entry: Supergee wing mount pylons
Jul 24, 2016, 05:55 PM
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Sounds like I have a plan then. I can cut off the pushrod guides, one was bad anyway, and go with two layers of the fiberglass. I will also look for some EZ lam or equivalent.

I will post pictures of my repair for any critique in a few days.

Thanks again for the help,

James
Jul 25, 2016, 06:44 AM
Registered User
You got the most important advise already.

I would also recommend to use some thin or medium CA to close the cracks and stiffen the boom to make handling easier.
Then, before laminating, lightly sand the shine off the boom to ensure a good bond.

I would rather take one layer of 50...55g/m2 fiber glass than 2 layers of 25, it is easier to handle, and I believe it's also stronger.
In the center of the cracked area, I'd put an additional 1" wide strip to increase hoop strength.

To get the fiberglass pressed onto the boom you can use Video casette tape, and roll it onto the boom, shiny side facing the boom.

Good luck!
Jul 25, 2016, 07:34 AM
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rdwoebke's Avatar
Quote:
Originally Posted by surfgeorge
I would rather take one layer of 50...55g/m2 fiber glass than 2 layers of 25, it is easier to handle, and I believe it's also stronger.
I could be wrong but my understanding is 2 layers of X weight cloth (assuming both cloths are the same material and have similar thread weaves) is stronger than 1 layer of 2X weight cloth.

roughening up the surface of the boom is a good idea for the repair. A better method might be what Drela recommends to lighten booms and that is to take an exacto or a razor blade and scrape the boom. You scrape it length wise. Scraping the boom can take the ridges off the booms which are really kind of just unnecessary extra epoxy. Assuming the boom is made like a Supergee boom (that might be a big assumption) then as long as what you are scraping off is white or clear you are OK. If you start to scrape or sand off anything black you are removing carbon which you don't want to do.


Ryan
Latest blog entry: Supergee wing mount pylons


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