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  #1  
Old 01-21-2012, 08:24 AM
DanH's Avatar
DanH DanH is offline
 
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Default Extending a Cowl Edge

A common question in my PM inbox is how to extend the edge of a cowl that has been trimmed a little too short. The subject has been covered very well in the past, but perhaps a dedicated thread will search better.

There are two methods. A small extension (like 1/8", maybe 3/16") can be done with a flox/epoxy mix. A longer extension is done with multiple plies of glass fabric.

Both methods are based on a scarf joint. Block sand the scarf with coarse grit paper. The scarf should be 3x wider than the glass thickness, minimum. The wider it gets (5x, 10x thickness) the stronger the joint becomes.



The backing plate can be aluminum, smooth stiff cardboard, almost anything handy. Cover the face of it with some slick tape so it will release from the epoxy. I would simply cleco it into place and fill the holes later.

Extension with fabric is straightforward. Lay enough plies to equal the thickness of the original glass, let it cure, then sand the outside flush and trim the edge to the new length.



Flox is milled cotton fiber. In a flox/epoxy mix the fiber provides isotropic tensile reinforcement. Cured flox is quite tough, which is why, with an appropriate scarf, you can use it alone for a short extension. Same procedure as above.

No need to suffer the pain and embarrassment of ugly cowling gaps. You can scarf and add flox or fabric one evening, then shape the edge the next evening. Have fun!
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Last edited by DanH : 07-17-2018 at 12:16 PM.
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  #2  
Old 01-21-2012, 11:50 AM
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Default

Dan,

You have great timing because I've been noodling on that very subject. Thanks so much, now I know how,,,,
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  #3  
Old 01-21-2012, 01:59 PM
Zero4Zulu Zero4Zulu is offline
 
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Default

A great way to make the backing as Dan describes is to use Wax Sheets with adhesive backing. Available at McMaster-Carr.

http://www.mcmaster.com/#wax-sheets/=fwlsrk
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  #4  
Old 01-21-2012, 02:25 PM
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Dan, your tips are invaluable for "How to xxxxx in fiberglass."


Thanks!
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  #5  
Old 01-21-2012, 02:56 PM
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RV7Guy RV7Guy is offline
 
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Default Good stuff!!!

Dan's work is spot on. I would only add not over think this stuff. It is relatively simple. I've helped a couple of people "recover" lost pieces of cowl

No need for a fancy backing strip. I use various thicknesses of balsa sheets that are covered with clear packing tape. They can be taped or clamped in place. Very pliable for the contours.

Excellent Dan!!!
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  #6  
Old 01-21-2012, 03:18 PM
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Wax is pretty expensive. I use scrap aluminum sheet for a backer. Cover the face that the glass will lay-up on with clear packaging tape. No need to even wax it or use a mold release agent,,,,its not gonna stick. Then I use good two way carpet tape to hold it in place. Like RV7Guy says,,,,don't over think it.
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  #7  
Old 01-28-2012, 05:41 PM
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I just came in from the shop doing a very similar operation. I did my best to channel my inner Horton. Fiberglass and I have always had an uneasy competition. I figure if I'm going to make it through the showplanes fastback canopy, we must learn to work together.
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  #8  
Old 09-03-2014, 12:56 PM
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Question

Quote:
Originally Posted by DanH View Post
.....

Extension with fabric is straightforward. Lay enough plies to equal the thickness of the original glass, let it cure, then sand the outside flush and trim the edge to the new length.



No need to suffer the pain and embarrassment of ugly cowling gaps. You can scarf and add flox or fabric one evening, then shape the edge the next evening. Have fun!
Resurrecting an old thread...

How many layers, and of what weight cloth can I be expected to use on an old white cowl?

The cowl thickness is about 0.110 inches at the edge I need to extend.

It seems that would be 8 or 9 layers of the West cloth tape I have been using - what would be a better cloth material?
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  #9  
Old 09-20-2017, 10:50 PM
E. D. Eliot E. D. Eliot is offline
 
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Default Thanks again, Dan

I appreciate that you gave me a personal lesson. I did my layup using an approximately 8/1 scarf, backed up with a scrap aluminum piece cut to size.

First layup, I used three layers of fg on the outside of the cowling and three layers laid inside of the cowling. These joined by fg epoxy about 1/2 inch 'outside' of the desired new edge while everything was wet.

Used peel ply fabric material on the outside and inside. This produced a very straight layup with almost enough thickness as the original cowling material.

Second layup was two more layers on each side. Left overnight to cure. Easy, strong, and straight fix after sanding.

This is my first cowling and I like the results. I suppose that I took a lot longer to achieve the nice results that I have but it is worth it. Learning a new and worthwhile skill is gratifying. I'm not trying to win any awards but I learned a long time ago to try and do the best work that I can. If I had left that corner of the cowling as it was, I know that I would regret it every time I did my pre-flight.

I really like fiberglass work and greatly appreciate your help. I appreciate. best to you, Dan, Ed Eliot RV-12 #120188
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  #10  
Old 09-21-2017, 09:01 AM
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You're welcome Ed.

If I understand correctly, you elected to place a few plies on the inside of the cowl edge. Let's look at that.

Given that most edge extensions are only an 1/8" or less, a single sided scarf as previously described is more than adequate. Technically it does have a weak spot, as marked, but very short extensions just can't generate enough bending load to worry about it.

If a builder wishes to be really conservative, or the extension is somewhat greater, the always-safe bet is a double-sided scarf with an included angle of around 20 degrees. However, it's tricky to do in the home shop because it doesn't lend itself to the use of a backing plate (a "caul plate" in the pro composite world).

A single-sided scarf with some additional plies added to the back side after cure roughly approximates the symmetrical double scarf, but it adds thickness, shimming the cowl edge outward so it may not be flush with the aluminum skin.

So how to approximate the strength of a double scarf while using a caul plate for a nice straight edge, without gaining additional thickness? First do an ordinary single-sided scarf as previous described. After cure, use the edge of a 2" or 3" sanding disc in your die grinder to cut a dish-shaped depression in the back side. Now do a layup to fill the dish, and after cure, sand it flat.

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Last edited by DanH : 11-14-2018 at 07:05 AM.
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