Cowl delamination, RV-6

[rv6n6r]

The epoxy honeycomb cowl on my 24 year old RV-6 has developed a large delaminated bubble on the outside, and matching one on the inside. Some questions:

1) Why - is it simply age? The area is not particularly close to exhaust pipes and shows no signs of excessive heat on the inside, and SCAT tubes in the same area and even closer to the pipes show no burning of any kind.

2) Suggestions on the repair? I built the plane and did all the fiberglass work, but I haven't done a repair of this type. I guess the whole area will need to be cut out and patched - just with layups or would I put in some kind of honeycomb or foam substrate?

3) Has anyone else with an aging (or not) RV experienced this sort of thing?

 

[Desert Rat]

In A&P school MANY years ago, I remember doing a honeycomb repair as a shop project. It really wasn't that difficult and didn't require cutting a hole all the way through

The process was to take a router with the bit depth set so that it would eat through the back side and most of the honeycomb, but not so deep that it got into the back side of the surface layer.

once you have a cavity slightly bigger than the damaged area, fabricate a plug of an appropriate thickness out of balsa, then fill in the cavity with epoxy. You have to make sure the balsa stays put because it will tend to get pushed up out of the hole by the epoxy.

Top it off with a couple of plies on the inside where nobody will see it except you.

Don't know if this would work for you or not, but it's worth considering as it seems like it would be a lot less labor intensive that having to patch a big hole.

 

[David Paule]

Be sure to seal any possible places where moisture could get into balsa, if you're using balsa. Balsa can get soggy and soft, depending on the moisture level. I'd recommend getting some honeycomb instead.

Dave

 

[JonJay]

Bummer Randall. Yours might be one of the first epoxy cowls. Not sure when that transitioned. My concern with a repair might be that whatever caused this pops up again. My guess is a manufacturing defect. Just took this long to show up.

Can you take a bright light and see if your cowl can transmit enough light through it to see if there are any inconsistencies elsewhere or the extent of the edges of this delamination?

Let me know if I can help. I’ll pop up there.

 

[Desert Rat]

Be sure to seal any possible places where moisture could get into balsa, if you're using balsa. Balsa can get soggy and soft, depending on the moisture level. I'd recommend getting some honeycomb instead.

Dave

100% agree about moisture intrusion, but if it's completely encapsulated in epoxy I don't see how it would be a problem.

I also agree that honeycomb would be better, but it's not exactly cheap and for a repair like this that isn't using prepreg, how do you keep the cavities from filling up with epoxy?

Like I said, I haven't done anything like this since A&P school, so I'm not trying to start a debate, I'm just genuinely trying to learn something.

 

[PaulvS]

An alternative to balsa and honeycomb is closed cell foam core sheet, available in various thicknesses from suppliers that sell fiberglass products.

 

[gasman]

In the early 70's I was based at Travis AFB and we did repairs like that on C-5 delaminations. I can't remember
much about the repair other that it went very well and we used a surface vacuum bagging process.

It did involve the honeycomb to be replaced due to damage. If there is no damage to the honeycomb, It should
be able to stay... Airframe repair was way more fun on the C-141.

Do some research. There is a proper fix for this.

 

[phloidtrout]

I had the same issue pop up on my 6 last year (1st flight was in 2000).
I believe that mine had delaminated due to insufficient insulation on the interior of the cowling, allowing the exhaust pipe heat to compromise the material. Vans also felt this was the cause, and sent me a picture of recommended insulation coverage that is apparently in current build plans. I don't believe there were such guidelines when I built it in the 90's.
Anyways, what I did was carefully trim away the interior skin up to the edge of where it had separated from the core, with a razor blade, then sanded it a bit further into the area where it was still bonded to the core. (Leaving the core intact)
On mine, the debond of the interior skin was more extensive than the exterior skin debond area, so the trim of the inside skin went beyond the debond of the outside.
I shaped some supports to lay the cowl on from sandbags & towels..., and positioned it so the exterior was supported in the area of the delam. I mixed up some West Systems epoxy with micro ballons and applied enough to the honeycomb (from the interior) to insure it weeped down to the exterior skin.
Then I Laid up two plies of fiberglass, extending 1/2 and 1" over the trimmed interior skin, repainted the interior, and applied the more extensive insulation footprint and went flying.
The repair is solid, and the exterior paint is fine. A keen eye can spot the slight miscontour on the bottom of the cowl if they are looking for it..., but I'm ok with it.

 

[blaplante]

Fiberglass ... Boats

Years ago... I bought a book "This Old Boat". Looks like it's still available, cheap, used. It had a whole section on repairing fiberglass boats where the structure had water intrusion and had started to delaminate. I don't remember the details, but spongy decking is a common problem. So many deck fittings screw down or penetrate the deck; giving easy water ingress if not well sealed.

May be worth a read. An interesting & enjoyable book as I recall.

 

[rv6n6r]

Thanks all

I believe that mine had delaminated due to insufficient insulation on the interior of the cowling, allowing the exhaust pipe heat to compromise the material.

I have the S/S sticky but it doesn't cover the area where it's delaminated. It still seems like that shouldn't be it but who knows and I will be adding more S/S just to be sure.
Anyway thanks for all the responses, that's some worthwhile info to go on.
And I'll be getting to this by and by (and yes Jon I'll be taking you up on that offer if only to get a chance to reconnect!)
I'll try to post updates for the benefit of anyone else this happens to.

Not a bad project for fall / winter on the Oregon coast anyhow.

 

[phloidtrout]

Here is a shot of my delam before repair, and the insulation guidance from Vans

 

[JordanGrant]

Same

FWIW, I have a similar delamination on my cowl as well. Completed in 2006. Thanks for the advice from others - I'll need to repair that at some point.

 

[Jimmy Cox]

Cowling repair

We use ParaBeam for our core at James Aircraft. It is a 3 dimension fiberglass that lays into compound curves easily. We can sell you small quantities if you want to try it.
JamesAircraft.com

 

[penguin]

Blistering is usually due to heat, I had some with a 2000 6 with no SS sheet.
I would cut away one skin at a time to leave the core and the other side providing some shape, probably starting on the inside. Fill the honeycomb with stiff micro and sand with coarse paper (60 or 80 grit) to get the desired shape, might take 2 or 3 applications of micro. Then lay up a couple of layers of light cloth. Then do the same on the outside paying more attention to the shape. Lay up another couple of layers of glass. Might take a few days as there are 6 or 7 resin processes involved

 

[Richard Connell]

I had the same issue pop up on my 6 last year (1st flight was in 2000).
I believe that mine had delaminated due to insufficient insulation on the interior of the cowling, allowing the exhaust pipe heat to compromise the material. Vans also felt this was the cause, and sent me a picture of recommended insulation coverage that is apparently in current build plans. I don't believe there were such guidelines when I built it in the 90's.
Anyways, what I did was carefully trim away the interior skin up to the edge of where it had separated from the core, with a razor blade, then sanded it a bit further into the area where it was still bonded to the core. (Leaving the core intact)
On mine, the debond of the interior skin was more extensive than the exterior skin debond area, so the trim of the inside skin went beyond the debond of the outside.
I shaped some supports to lay the cowl on from sandbags & towels..., and positioned it so the exterior was supported in the area of the delam. I mixed up some West Systems epoxy with micro ballons and applied enough to the honeycomb (from the interior) to insure it weeped down to the exterior skin.
Then I Laid up two plies of fiberglass, extending 1/2 and 1" over the trimmed interior skin, repainted the interior, and applied the more extensive insulation footprint and went flying.
The repair is solid, and the exterior paint is fine. A keen eye can spot the slight miscontour on the bottom of the cowl if they are looking for it..., but I'm ok with it.

I just went through this process recently. There was no doubt as to the cause as there was obvious heat damage to my inside skin.
I largely followed this process exactly with one key difference.
Because I was concerned about the outward deformation of the cowl, before I laid up the new glass on the inside, I spent some time with a heat gun to make the outward deformation concave from the outside. That way, when the interior fiberglass was repaired and the cowl was structurally sound, all that was needed was to fill the exterior rather than remove a proud bubble - with all the associated glass work.

 

[9GT]

Blistering is usually due to heat, I had some with a 2000 6 with no SS sheet.
I would cut away one skin at a time to leave the core and the other side providing some shape, probably starting on the inside. Fill the honeycomb with stiff micro and sand with coarse paper (60 or 80 grit) to get the desired shape, might take 2 or 3 applications of micro. Then lay up a couple of layers of light cloth. Then do the same on the outside paying more attention to the shape. Lay up another couple of layers of glass. Might take a few days as there are 6 or 7 resin processes involved

Spot on advice here.

 

[BillL]

The epoxy honeycomb cowl on my 24 year old RV-6 has developed a large delaminated bubble on the outside, and matching one on the inside. Some questions:

1) Why - is it simply age? The area is not particularly close to exhaust pipes and shows no signs of excessive heat on the inside, and SCAT tubes in the same area and even closer to the pipes show no burning of any kind.

2) Suggestions on the repair? I built the plane and did all the fiberglass work, but I haven't done a repair of this type. I guess the whole area will need to be cut out and patched - just with layups or would I put in some kind of honeycomb or foam substrate?

3) Has anyone else with an aging (or not) RV experienced this sort of thing?

While aging may have some influence it is likely a void aggravated by internal heat.

I use Soric to make plenums by infusion, the Soric makes it 5X more stiff and stronger too. It does not soak resin like foams and will easily lay in the 3D shape.

If you have a blister inside and out, I think I would choose to cut back the outer, use some disposable filler to make the area match the contour, then spray on some release and cover the outer side with a couple of layers to use is a mold. It will be removable so you could drill a few holes for clecos, remove it clean out the entire area to be finished. Reinstall the mold and do your layup inside with 2-3 layers, lay the core and 2 more glass where you get the core to engage with the old core and leave the inside/outside finished to the original contour.

It should all result as an invisible repair once final painted.