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  #1  
Old 03-16-2020, 10:04 AM
martinair martinair is offline
 
Join Date: Jul 2015
Location: germany
Posts: 22
Default cracks in upper cowling

Hi, I have a problem with the upper cowling of my wonderful RV4. It had cracks on the right side, directly behind the spinner/prop. Last winter I grinded everything out, placed many, many new glass-fibre mats on/in it, used a lot of filler, painted everything. it looked really nice- but after 2 hours the cracks are there again. Any recommendations for the next repair? Thanks!

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  #2  
Old 03-16-2020, 10:49 AM
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DanH DanH is offline
 
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Made a repair for an RV-4 owner recently. Same problem, with several previous repair attempts. Installed some new structure inside the cowl to change the load path. Probably not enough hours on it yet to declare the mod a success.

Most don't appreciate how much load the cowl must bear.
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  #3  
Old 03-16-2020, 04:51 PM
Taltruda Taltruda is online now
 
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Location: Las Vegas, NV
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Default Fiberglass

Quote:
Originally Posted by martinair View Post
Hi, I have a problem with the upper cowling of my wonderful RV4. It had cracks on the right side, directly behind the spinner/prop. Last winter I grinded everything out, placed many, many new glass-fibre mats on/in it, used a lot of filler, painted everything. it looked really nice- but after 2 hours the cracks are there again. Any recommendations for the next repair? Thanks!

[IMG][/IMG]
You mention glass-fiber mats.. are you using shopped strand glass fiber? Or aircraft weave? I wonder if grinding the back side and putting 2 layers of carbon fiber, then grind out the crack and scarf in the fiberglass weave on the top would be the way to go?
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  #4  
Old 03-16-2020, 05:45 PM
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DanH DanH is offline
 
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Quote:
Originally Posted by Taltruda View Post
I wonder if grinding the back side and putting 2 layers of carbon fiber, then grind out the crack and scarf in the fiberglass weave on the top would be the way to go?
Remember, carbon is a high modulus fiber...it has no "stretch" compared to the typical low modulus glass fabric. So, the load path would be from glass to carbon to glass through a single bond line; the new glass fabric scarfed into the outer surface would not bear much of the load, if any. The loaded bond line is carbon to polyester glass; consider the joint mechanics (modulus again) and bond strength to polyester.
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  #5  
Old 03-16-2020, 09:58 PM
Scott Hersha Scott Hersha is offline
 
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Location: Cincinnati, OH
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Iíve had similar cracks in wheel pants, usually caused by mechanical damage, but Iíve always fixed these by cleaning/grinding out the crack, filling the crack with epoxy/flox, and then laying a couple layers of 9oz bidirectional fibergalass (compound surface) on the interior and exterior of the crack area - a couple inches on each side of the crack. Never had a crack come back. The cloth on both sides makes a difference, as does the flox, which fills the void and structurally transfers the load to the adjacent somewhat flexible fiberglass layup. As Dan said, an inflexible carbon patch will only lead to cracks at the edges of the carbon patch.
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  #6  
Old 03-17-2020, 07:32 AM
sblack sblack is offline
 
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I was told by a very experienced composites guy that the older cowls were vinylester and that it produces a waxy buildup on the surface. To get epoxy to stick to it you need to make sure you remove all the waxy stuff or it wonít bond. I have has layups just peel right off which is discouraging. So that means really roughing it up well and scrubbing vigorously with mek or acetone. I donít know if that is the real issue here but any repair will need that kind of treatment if it is to survive.
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  #7  
Old 03-17-2020, 08:15 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by sblack View Post
I was told by a very experienced composites guy that the older cowls were vinylester and that it produces a waxy buildup on the surface. To get epoxy to stick to it you need to make sure you remove all the waxy stuff or it won’t bond. I have has layups just peel right off which is discouraging. So that means really roughing it up well and scrubbing vigorously with mek or acetone. I don’t know if that is the real issue here but any repair will need that kind of treatment if it is to survive.
I think that's spot on, see below.

Found some photos on the home hard drive. You can try this experimental approach, and if you do, a long term report would be appreciated.

This was an old RV-4 cowl with previous repair attempts. Part of the problem was poor bonding and delamination of those repairs. The delaminated material had to come out; prep is really important.





Photo borrowed from the web. New approach was to add a support band inside the cowl, extending from screw flange to screw flange.



Start by inserting a thin foam block in the end of the inlet ramp, merely as a support for the new glass layup....



...installed as the next step.



Exterior was refinished with 3 cap plies. Forget scarfing in a section over the crack. Just cap the whole nose. Note how the three plies do not end at the same place. Staggered drop-offs reduce stress concentration at the transition to the old glass.

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  #8  
Old 03-17-2020, 10:24 AM
martinair martinair is offline
 
Join Date: Jul 2015
Location: germany
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Thank you, Dan, i think I will try next winter- of course I sanded/grinded everything inside & outside before I started to work. Inside a quite a few layers of normal glass-fibre/epoxy mats you can buy in every car supply. didn't help. So I will use just normal duct-tape (3 layers) inside for support, is that what I understood? And on top of that band I use again Epoxyand mats? Sorry for me as a german these explanations are difficult to understand...
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  #9  
Old 03-19-2020, 08:07 AM
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DanH DanH is offline
 
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This may help.

Cowl inflates like a balloon. Load is approximately 60 pounds per square foot at cruise speed, higher at VNE. Center of upper and lower cowls is tied together at inboard end of intake, behind spinner. Load path crosses a gap at end of upper intake.

New structure creates direct path across the gap. E-glass tape is easy to use for this application. It may lay best if installed as left and right sections with a center overlap.



https://www.aircraftspruce.com/pages/cm/tape/etapes.php

7781 8-harness satin weave will conform to compound external contours (last photo, previous post).

https://www.aircraftspruce.com/catalog/cmpages/7781.php

As noted previously, this mod is experimental. In-service feedback is appreciated.
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Last edited by DanH : 03-19-2020 at 08:13 AM.
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  #10  
Old 03-20-2020, 01:46 AM
martinair martinair is offline
 
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Thank you Dan, it's clear now, thank you very much. I am planning to do it in winter- however, if we are facing a ban for VFR flights like already in Belgium, I might do it earlier.
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