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RV-6 composite cowl repair materials

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gtae07

Well-Known Member
Supporting Member
Joined
Dec 13, 2012
Messages
2,637
Location
Savannah, Georgia
So I'm getting myself roped into helping my dad repair the cowl on his RV-6 (or at least, advising him on the process). He has a fairly large bubble of delamination on the external surface of the underside. Hated to break it to him but he's going to need to cut out the outer skin and eventually repaint. I don't yet know the extent of the damage on the inner side but I'm going to assume the inner face sheet is damaged as well. From what I can tell the cowl is prepreg fiberglass/epoxy with overexpanded nomex hexcell core. I'd guess 3 plies of BID on each surface (7-9oz/yard3 ?). For the core I'm thinking either foam or dry micro; the OX hexcell is much more expensive and probably more difficult for him to work with.

https://www.aircraftspruce.com/catalog/cmpages/rutan.php
https://www.aircraftspruce.com/catalog/cmpages/divinycellfoam.php
https://www.aircraftspruce.com/pages/cm/fillers/bubbles.php (<--if dry micro used as below)


Anyway, I'm trying to help him figure out what materials he needs to get from Spruce to do the repair. I have a decent idea of the repair process from specifying similar repairs at work (yes, I know making a drawing vs. doing are different things, but not the point); the trick is figuring out the exact materials to use given that the resources he has at his disposal are a little more limited than what we have at a repair station for large jets.


Here's what I'm thinking, assuming both surfaces are damaged:

Trim out delaminated areas and remove damaged core material, leaving a neat smooth hole. Remove paint and finish for approximately 2" around trimmed area, and gradually scarf material towards the hole. If present, remove oil contamination with acetone. Dry thoroughly.

Fabricate a caul plate/form for the inner surface if required. Lay up three plies on the inner surface, first one extending at least 1/2" beyond the trimmed area and each successive ply extending approx another 1/2". Coat caul plate (if used) with release agent and allow inner plies to cure.

After cure, scuff sand core side of the face sheet as required and clean. Apply dry micro to fill the area OR trim and form a piece of foam (Divinycell H45 of appropriate thickness) to fit, and bond in place, using weights as required to ensure it stays in place.

After core is cured, sand flush to ~.015 below desired external contour and clean. Lay up three plies on the outer surface, first one extending at least 1/2" beyond the trimmed area and each successive ply extending approx another 1/2".

After outer plies are cured, clean, sand, and refinish areas as desired.


The cowl is non-structural but it needs to stand up to air loads and heat.



TL;DR: Please review my material choices, and let me know if you think the dry micro approach to core replacement is a bad idea
 
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