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How should I repair this fiberglass crack?

Craig23

Well Known Member
Hi everyone,

I've been working on the canopy skirt for my RV-8. I glassed the two halves together at the back and got the surface prepped. I had a friend of mine with a body shop paint the inside so I could rivet it to the canopy and canopy frame. While clecoing the skirt to the frame tonight I saw a crack at the back where the two halves had been glassed together. The inside of the skirt is now painted, and I'm wondering what would be the best way to repair the crack. It is full thickness at the top. I imagine I should stop drill the crack, but what are your suggestions after that? Will I need to add layers of glass over the crack? I've included a picture of the crack below. Thanks for any advice.

canopyskirtcrackrp8.jpg
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crack

Craig

I would not bother with stop drilling, I don't think it will matter in fiberglass. Just sand down the top surface into the glass and do a layup over the crack. I would do it while it is in place so that there is no stress on the parts after it cures. Just make sure you protect the canopy well.
 
I'd consider repairing it as one would repair a plaster wall. Open the crack line into a v-shape, fill with epoxy/flox, and lay-up with fiberglass.
 
Maybe this will help:



A one-sided scarf (not shown, but same principle) would work in the case of a fairing skirt joint. A two-sided scarf is far more durable in the case of a flexed part. Adding plies to the outside only is just a patch. It will work long-term only if the part never sees any stress.

Whatever you choose, take it back to bare glass before you add anything.

Addendum: relative strength by joint type, credit Niu:

 
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Maybe this will help:



A one-sided scarf (not shown, but same principle) would work in the case of a fairing skirt joint. A two-sided scarf is far more durable in the case of a flexed part. Adding plies to the outside only is just a patch. It will work long-term only if the part never sees any stress.

Whatever you choose, take it back to bare glass before you add anything.

Dan has it exactly correct. Dont try to save a bit of time by doing this quick and easy, you will only be re doing it later.
 
Dan is the MAN! Good images too.

Go to a hobby shop (Hobby Town is our local chain) and buy some of their lightweight BID FG cloth and use that. This stuff is thin and will let you get a couple of layers on there. Lay them 45 degrees to each other to take up any stress you might encounter.
 
A lttle confusing....

Maybe this will help:

<pic snipped>

A one-sided scarf (not shown, but same principle) would work in the case of a fairing skirt joint. A two-sided scarf is far more durable in the case of a flexed part. Adding plies to the outside only is just a patch. It will work long-term only if the part never sees any stress.

Whatever you choose, take it back to bare glass before you add anything.

...on the pictures.

Dan's pictures show repair techniques for a fibreglass composite structure with an internal core. Techniques are quite different, and the lower picture probably woud not be a factory approved repair on a German sailplane.

However a scarf joint is definitely the correct technique here.

My certified sailplane repair manual in front of me uses 30:1 as a correct scarf angle when a single layer of glass is involved - as is the case here.

This is probably just over an inch of scarf on each side of the crack and should be easy to do. Lay up the same number of layers as the original work, and add an extra layer. Make the first layer the size of the repair area after scarfing, and every subsequent layer a little smaller. This will aid the final sanding and finishing. Use Saran wrap or a peel ply so excess resin can be squegeed out.

This repair would classify as the same strength as the original on a certified plane, and the glider guys may be better at this since they have been selling (and repairing) composite planes since the mid-60's....:)

I can scan some pictures from the manual if you want, but have lost my web space in my switch to DSL...:(
 
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<<Dan's pictures show repair techniques for a fibreglass composite structure with an internal core. >>

Whoops, not what I intended. I'm sorry Gil, my fault, poor captions. Shouldn't have used the word "core". I'll repost an improved version this evening when I get back to the home computer. For now, please substitute "original laminate" for "core" <g>

You're entirely right about cored structure...pot in new core, scarf the skins.

<< My certified sailplane repair manual in front of me uses 30:1 as a correct scarf angle when a single layer of glass is involved - as is the case here.>>

Works for me.

<<Make the first layer the size of the repair area after scarfing, and every subsequent layer a little smaller. >>

I'd reverse the order; small first, largest last, not that it matters in a non-structural fairing. My reasoning is best illustrated with a picture, again later. However, I'll bet you have a reason too. How about it?
 
<<Make the first layer the size of the repair area after scarfing, and every subsequent layer a little smaller. >>

I'd reverse the order; small first, largest last, not that it matters in a non-structural fairing. My reasoning is best illustrated with a picture, again later. However, I'll bet you have a reason too. How about it?

Contact area of single layer, versus contact with many edges of layers. The first large piece ties all the sub layers together at the exposed ends of the scarf joint.
 
Great Info

Guys,

Thanks for all the responses. I looked at the crack again in the daylight and think I figured out what happened. When I glassed the two halves of the skirt together I did not use a scarf joint. I put three layers of glass along the inside portion of the joint, then used epoxy and microballoons to fill in the gap on the butt joint. As the piece flexed while moving it around during sanding and painting, the filler material cracked. it does not look like the glass layers holding the two pieces together are affected.

I plan to take the advice and put a scarf joint at the exposed seam of the two skirt halves. It sounds like I should sand down to the skirt glass in the shape of an open V, about an inch or so to each side of the crack. Then I need to lay in a few layers of cloth, starting out with a narrow piece and gradually increasing their width to cover the scarf. Does that sound right?

Thanks again. The Forums are outstanding!
 
I was just reading...

DanH said:
......
I'd reverse the order; small first, largest last, not that it matters in a non-structural fairing. My reasoning is best illustrated with a picture, again later. However, I'll bet you have a reason too. How about it?

...the repair manuals...:)

They all seem to use the order of bigger patch first.

As I think more about it, the reason (my speculation...:)..) might be concerned with having excess (i.e., non-structural) resin at the edges of each of the patch layers. It you do it the manual suggested way, the edges of each layer will be at the surface, and excess cloth edges will feathered in during the final finishing.
 
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Craig,
<<When I glassed the two halves of the skirt together I did not use a scarf joint. I put three layers of glass along the inside portion of the joint, then used epoxy and microballoons to fill in the gap on the butt joint.>>

Yep, that would be crack-prone for sure. Always balance layups (same # on each side of the original). Your plan to scarf the topside of the original glass should work out fine.

<<concerned with having excess (i.e., non-structural) resin at the edges of each of the patch layers. It you do it the manual suggested way, the edges of each layer will be at the surface.>>

Gil,
Thanks, good theory. I want to check a reference tonight, more later.
 
Interesting issue.

My favorite composite design reference is Composite Airframe Structures by Michael Niu, circa 1992. Most of the data appears to come from his work at Lockheed's Composite Development Center. Here's the illustration for what he describes as the most structurally efficient patch repair:



Last December Mike Dooley supplied a link to DOT/FAA/AR-03/74, which explores two different airline repair methods, both of which are also small ply first (illustration page 1).

However, Gil's older sailplane repair manual clearly outlines a large patch first scarf, as does Rutan's Moldless Composite Sandwich Homebuilt Aircraft Construction:



Maybe later research changed the rules, or maybe there is another factor. Have a theory going; tell me what you think.

Both the later examples are aerospace repairs; bullet holes in fighter planes, baggage cart dings in airliners. The repairs would be done by trained people. The repair itself would probably include a vacuum bag, and perhaps prepreg and a heat blanket.

The earlier methods are both wet layup, no vac bag, and assumed to be field repairs by ordinary folks. I suspect the structures are much less loaded too.

Consider this illustration:



In the "small ply first" example the plies are nice and straight, which is good for structural reasons, in particular in compression. However, some void areas are possible, which might reduce tensile strength if unfilled. Cured with heat and a vacuum bag, that wouldn't be a problem; the voids would be consolidated and fully filled.

In the "large ply first" example, there are no unfilled voids. Seems like a real advantage for a simple, unbagged wet-layup field repair where bubbles, voids and air are common issues. However, the fiber isn't straight, and that's not optimum from a structural standpoint.

So which to use? I now think Gil's method might be a better choice for the sort of work we're doing here. The glass on an RV is all non-structural; none of it is subject to loads requiring optimum composite performance. The "large ply first" schedule seems to offer the best chance of a nice tight laminate, which is really what we want for our purpose.
 
Good theory Dan..

Interesting issue.

...
However, Gil's older sailplane repair manual clearly outlines a large patch first scarf, as does Rutan's Moldless Composite Sandwich Homebuilt Aircraft Construction:
.....
Maybe later research changed the rules, or maybe there is another factor. Have a theory going; tell me what you think.

Both the later examples are aerospace repairs; bullet holes in fighter planes, baggage cart dings in airliners. The repairs would be done by trained people. The repair itself would probably include a vacuum bag, and perhaps prepreg and a heat blanket.

The earlier methods are both wet layup, no vac bag, and assumed to be field repairs by ordinary folks. I suspect the structures are much less loaded too.
.....

It should be emphasised that the 30:1 scarf I quoted was for "regular" structural repairs. The repair manuals specifically do not allow repair stations to scarf more highly stressed parts, such as wing spar caps.

These are usually made with much more unidirectional rovings/cloth, and IIRC either 60:1 or 100:1 is a scarf joint number for these locations (Experimental aircraft repairs only)

Your theory and pictures would apply to more advanced engineered conposites with much more directional orientation of the fibers.

The sailplane fus. and wing skin repairs usually involve several layers of cloth (and usually curved sections) and the fiber orientation is probably a little less critical. The manuals just say to copy the glass weight and orientation of the damaged part.

It is no surprise that Rutan uses the method I mentioned, since he started his operation at Mojave airport in a hangar next to Fred Jiran, who was one of the few early repair facilities in the US for fiberglas sailplanes...:)

However, one point....

The picture you show from the 1992 manual seems to imply a scarfed patch of an already cured laminate (not a core) bonded in place (shows "adhesive") and the surface plies are just to fair the two parts together. I don't think it's a wet layup method - more like a scarfed patch to a wooden wing skin.

A little straying from the original post, but an interesting discussion...:)
 
<<It should be emphasised that the 30:1 scarf I quoted was for "regular" structural repairs.>>

The link Mike posted last December is a good read, research specific to scarf angle, smaller vs larger.

http://www.tc.faa.gov/its/worldpac/techrpt/ar03-74.pdf

For RV-level glass repairs (fairings, cowls, wheelpants), I think we agree precise scarf angle isn't critical. 30:1 works for me. If new builders simply know a decent repair or joint includes a scarf angle, they'll be ok. Certainly would have saved Craig a re-do.

How much to scarf? For a single-sided scarf, measure the thickness of the original laminate, multiply by 30, and mark that distance from the edge. Sand flat from the mark down to the backside ply.

FWIW, that's a lot less scarf width than the "1" per ply" found in the old Rutan stuff....which is 100:1 if you assume 0.010" fabric and 166:1 for 0.006" fabric. Sure would keep those "big patch first" plies flat. <g>
 
I think that...

.....
FWIW, that's a lot less scarf width than the "1" per ply" found in the old Rutan stuff....which is 100:1 if you assume 0.010" fabric and 166:1 for 0.006" fabric. Sure would keep those "big patch first" plies flat. <g>

...the early Rutan hand laid up stuff was very epoxy rich. I doubt if the finished thickness of each layer actually measured 0.010...

Heck... the actual thickness of the 7725 cloth is 0.0093, so 0.010 assumes essentially no excess resin. Since these were hand squeeged lay-ups with no vacuum bagging, I think the actual scarfs recommended by Rutan might be closer to 30:1 number than the 100:1 assumption...:)

http://www.fiberglassflorida.com/products/datasheets/msds/reinforcements/Hexcel 7725.pdf
 
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