RV-6 composite cowl repair materials

Discussion in 'Composites' started by gtae07, Mar 17, 2019.

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  1. Mar 17, 2019 #1

    gtae07

    gtae07

    gtae07

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    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
     
  2. Mar 17, 2019 #2

    TFF

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    I’m inclined to say do the outside first. A mold depending on location can be made from thin aluminum or something a little nicer if complicated. I would also cut out a little more core and lay the fiberglass from the inside so the messy bits don’t have to be addressed. A 1 oz piece of cloth can be laid over the outside for sacrifice of sanding and stopping cracks in the paint after the patch is cured and dressed some. After the outside, I would do the core and inside. The outside is probably still the main structure over the cosmetics so it’s the part that needs the bigger attention.
     
  3. Mar 17, 2019 #3

    Toobuilder

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    How big is the section of core that needs replacement?

    If not too big, I'd consider attacking the inside first. Blend a "crater" as deep as required to not break through the OML, fill with really dry micro, fair and patch the IML as you detailed above. Once cured, feather the OML for the dime, nickel, quarter patch and sand to proper contour.
     
    Last edited: Mar 17, 2019
  4. Mar 17, 2019 #4

    gtae07

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    My thinking with starting from the inside is that, if the damage does indeed go all the way through, we'll have to trim out a complete hole. Bridging over the hole with wet hand layup isn't going to be all that pretty; at the very least it probably won't keep exactly to the original contour. I'd expect it to be a little flatter. By adding core next, and the second layup to the outside, we can shape the outside part of the core back to the original contour and get a good smooth layup there.
     
  5. Mar 17, 2019 #5

    gtae07

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    Very roughly (going by memory from the other day) about an 8" circle. At least, that's the part on the outside that's bubbled out. I imagine the core is probably shot there but he hasn't yet pulled the cowl off. I'll be back up there this week and will have a better idea of what he's looking at.

    On the bright side, Spruce is a 5 minute walk from his hangar...
     
  6. Mar 17, 2019 #6

    TFF

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    That’s why I would do the layup against the aluminum, even Ceco it to the cowl if you could not tape it on. A couple of holes ? You are already doing a composite repair. It would not be an open hole against the aluminum; of course with some bag or release film of some sort. I’m sure if the outside does not have to be perfect, cutting back what you can and laying and maybe just filling voids with epoxy is what I would do too. Stabilize it and go on.
     
  7. Mar 17, 2019 #7

    Himat

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    Is the outside shape of the cowl ok where the delamination is?

    If so, first thing then may be to clean, wax, add release agent and make a mould on the outside. Then remove, cut away delaminated area, chamfer the inside edge, prepare for lamination and fit the mould on the outside. Then build up the laminate against the outside mould from the inside.
     
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  8. Mar 17, 2019 #8

    gtae07

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    No, it's visibly bubbled outward.
     
  9. Mar 17, 2019 #9

    DeepStall

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    Pics, please! Any idea what the source of the delam was? Impact damage, or entrapped water freezing? 8" sounds very large. Have you tapped it out to try to establish the delam boundary?

    If your cowl is no longer to contour can you take an OML splash off another aircraft to facilitate OML repair from IML as others have suggested? Otherwise, still doable to repair OML from OML but maybe fussier bodywork.

    Your repair in the original post sounds reasonable to me. Correct to stick with foam+micro core, not wet layup over honeycomb core. I'd also add that after you get the damage out, measure the thickness of the facesheet in that area (and try to establish layup stacking sequence), and put back the same or +1 ply. 3/core/3 layup sounds reasonable, but match what you have. Wetting out the repair plies together on a plastic film transfer sheet on a table (not in place) and trimming shape to a paper template might help too.
     
  10. Mar 18, 2019 #10

    wsimpso1

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    gtae07,

    First things first, how it was damaged will say a lot about what to expect inside and then how to get a long lasting repair on it.

    If the outside shape is perfect, pulling a splash (mold - very generously sized you may not know how big the damage really is) off the outside is a real time saver...

    Next, if the outside shape is perfect, do you really want to (or need to) cut into that layer?

    After pulling a splash of the outside, I would start by removing the destroyed laminate and cores from the inside. Then, whether you need to cut the outside or not, you can rebuild toward the inside. If the outer skin turns out to be fine, leave it alone and built from there. I would try to vacuum press the new stuff to the original outer facing if I have it. If I had to remove the original outer facing, I would cut the outer facing back to clean solid material, cut back the core an inch and inner facing some more, then clamp the splash on and laminate a new outer skin lapping inside the old one with peel ply. After cure, in goes a carefully sized new core and cure. Then new inner facing.

    Materials?

    Ideally you would replace Nomex core with Nomex core, but in most repairs, the honeycomb just fills with resin. Personally I would go with Divinycel. As to dry micro as a core, it is heavy and will make a hard spot in the part, which can lead to the skin cracking at the edges of the micro core...

    Cloth should match what came out. If you can capture some of each facing as a coupon, you can id number of plies, ply orientation, and weight of cloth by gently burning away resin with a flame. You might use an extra ply in the repair. This would show on a repair working on the outside, but if you work from the splash to the inside, it will just show up as a slightly bumpy layer inside the cowl.

    Resin? What was it made with? That is the default. That said, West System is great for this sort of repair, is available in small quantities, etc.

    Dacron peel ply will save much work at each layer of work. You still have to sand, but the sanding goes better with areas where you used it.

    Dry micro in epoxy is the fairing compound of choice. See Wayne Hicks' blog on finishing a Cozy and George's curedcomposites web page.

    How will you keep your beautifully repaired cowling from being trashed again? I think you should revisit the circumstances and make sure that you cover the events well to prevent recurrence.

    Billski
     
  11. Mar 18, 2019 #11

    wsimpso1

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    Ouch. That complicates things a little. I am still going to recommend a splash. First step is to fair back the bubbled stuff, and bondo back up to correct shape, like it was a car body, then wax it and pull a splash for later. Then proceed with a repair from the inside against the inside of good outer facing and against the splash as I described above. You will have way less fairing to do... With the edge of the splash sealed with vacuum bagging mastic, you can even vacuum bag the various layers of the repair.

    Billski
     
  12. Mar 18, 2019 #12

    gtae07

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    On the ipad again so quoting is a right pain...

    I don’t know how the damage occured nor what the inside of the cowl looks like (yet). Dad just showed me when I was visiting last week and we didn’t have time to investigate; right now he and the airplane are 220 miles away. My guess based on location would be heat and/or oil saturation.

    As noted, material is prepreg epoxy and fiberglass, but beyond that I don’t know the details (it’s Van’s proprietary? Or at least I don’t think they give out the details) Anyone who’s built a “green” Van’s cowl has it. It looks like BID material as best I can tell from internet pictures and research.

    Vacuum bagging is probably not an option for him as he won’t want to fool with it or invest in the tools and materials. I would do it on my project, but he’s the one with the final say and he’ll be doing the work himself (I don’t live close enough to help with this in person; I’m trying to get enough together to give him a good “how to”). Best I can do is advise. I’’ll recommend the splash/mold but I don’t know if he’ll feel like doing it.

    I’ll try to get a few pictures when I’m up there tomorrow (Mon) through Wednesday or so.


    I’d feel more confident in this matter if I could get into the composites repair class at work but I haven’t been able to yet (engineers only get in on a space-available basis).
     
  13. Mar 18, 2019 #13

    wsimpso1

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    gtae07,

    All of what I recommended is doable without vacuum, it will just be a bit thicker towards the inside and a little heavier. It does put a premium on having the core near perfect, as vacuum bagging makes everything conform with the splash.

    If the cowl is compound curved through the repair region, you can curve Divinycel by putting it into a warm oven - being a good insulator, it will take a while to warm through - then place it in the splash with a sand bag. If it springs back too much, turn up the heat a little in the oven and repeat. Gotta look up the Tg and heat deformation temps on Divinycel for this work.

    Another option for core that is not too heavy is still Nomex honeycomb, but pour it full of Instafoam to keep it from becoming solid resin later. There are Nomex honeycombs that are stiff in bending, and those that have the honeycomb pattern a little messed up and conform to compound curves easily. Your process and core have to go together. You can shape it to fit one of several ways:

    Get the outer facing replaced, rough and final sand the inside surface, make the conformal honeycomb fit the space and comply with the curves, then fill with Instafoam and weight in place with visqueen and a sandbag or two. It will bond itself in this way. Then you can do any carving of the inner shape as you need;

    Get the outer facing replaced, rough and final sand the inside surface, make the honeycomb fit the space and comply with the curves, then line the space with visqueen, fill with Instafoam and weight in place with visqueen and a sandbag or two. You can pull it all free after the Instafoam cures, trim, sand etc, then bond in with thickened micro;

    Make a thick panel of stiff honey comb, fill it on a table protected with visqueen, then carve the side that goes against the outside facing, bond in with thickened epoxy, then carve the inside.

    All carving of this filled Nomex honeycomb will be big dusty work, and will probably require Roloc sanding discs and/or rotary knife on an angle die grinder. Nomex is tough, and the foam friable, cutting and sanding the combination is nasty work in overalls, hat, gloves, and a respirator. Much better IMHO to find a honeycomb that will conform with the compound surfaces and in the right thickness, then you only need to trim the edges and sand off excess foam.

    I was going to ask what Van's recommends for repair. Maybe it is worth asking... but be careful with the info, because they avoid working fiberglass as much as possible and seem to push their customers that way too. You know, "We do not recommend repairing them. We will sell you a new cowling for $X thousand dollars". And then you still have to fit it, make it perfect, attach all of the hinge halves, and then paint it. If you can get the honeycomb from them, by all means do so. I would fix the one you have and then you only need to fair, finish, and re-paint the one you fix...

    Billski
     
  14. Mar 18, 2019 #14

    Himat

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    Ok, one more question to check. Is the laminate itself ok on both the inside and the outside? And the cowl is non-structural?

    Depending on why and how the panel have delaminated it might be possible to drill some strategic placed holes, inject glue (epoxy) and pull everything back together with vacuum as the glue cure. A vacuum pump may not be necessary, in some cases a wet and dry vacuum cleaner can be used instead. (The vacuum cleaner must have separate air for cooling, if not it will burn out if operated without any flow.)

    Pulling vacuum on one hole should show if it is possible to pull the parts back together.
     
  15. Mar 19, 2019 #15

    gtae07

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    Thanks for all the suggestions evveryone... but it looks like Dad has just gone ahead and done this on his own. He was finishing up when I got in yesterday and it’s curing in the dining room.

    I don’t know what he did or how but I’ll probably find out later today.
     
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  16. Mar 21, 2019 #16

    wsimpso1

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    So, how did the damage occur, how did he fix it, what materials did he use? You have a bunch of folks roped in with you, give us some details. Maybe pictures too...
     
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  17. Mar 22, 2019 #17

    gtae07

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    We just got home yesterday. I had no pictures, unfortunately; he’d already finished and got it painted by the time I had a chance to see it.

    My understanding is it was caused by heat from the RH cylinders. His reflective heat shield had come loose and it damaged the inner face sheet.

    As he described the repair, he trimmed away the inner face. The core looked to him to be usable, so he “put some thin epoxy down inside” to make sure it was attached to the outer face, and then laid up a few strips of BID on top.

    You can still see a slight bulge on the outside if you look for it but it’s not spongy and doesn’t sound delaminated to my uncalibrated ear.
     
  18. Mar 22, 2019 #18

    wsimpso1

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    Tapping a quarter on edge will show delamination pretty clearly. Tap on the same part in places known to be good, then around the bulged area. If they sound the same, everything is attached. If they sound different, something is detached. The difference is usually pretty obvious with a quarter. This is called a coin tap test, and is common in searching for facing delamination.
     
  19. Mar 23, 2019 #19

    gtae07

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    I'm familiar with tap testing and that's what I was referring to... I'm just a rank amateur at it.
     
  20. Mar 23, 2019 #20

    BJC

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    Are you in Peachtree City today?


    BJC
     

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