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Cowl/fiberfrax modification

Scott Hersha

Well Known Member
There are some areas on the lower cowl on my RV6 build that are pretty close to the exhaust pipes. I would like to protect these areas with 1/8” fiberfrax and the aluminum self adhesive cowl shield I got from Vans. Question for those who have done this - what did you use, if anything, to attach the fiberfrax to the cowl? My cowl surface is still unprotected. My plan is to seal it with bilge paint from my local boating shop. I’ve tried it on the inside of my wheel pants (old kit), and it seals very well, and is made to protect the fiberglass bilge area of a boat from oil, fuel, hydraulic fluids, etc. Should the fiberfrax/aluminum shield be put on over this bilge treatment, or before?
I’m unsure what would stick to fiberfrax, or if being held down by the adhesive back aluminum is enough.

Picture of the bilge paint:

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Scott,

I didn't think big chunks of fiberfrax would hold their shape if I simply laid a bigger piece of tape over them. So I applied the fiberfrax in small strips - maybe 3" wide, covered with the aluminum tape. Then I overlapped another piece of fiberfrax right up to where the first layer of fiberfrax ended, and covered the new strip of fiberfrax with the aluminum tape, overlapping the previous layer of aluminum tape. Basically like shingling a house.

Ultimately, the only excess material and weight was in the form of an extra layer or two of aluminum tape in a few places.
 
Try a small dab of bondo

I think it was Bruce Hill that documented his use of small dabs of bondo to secure the fiber, and once set (which is only 15 minutes or so) cover with foil tape. It worked great for me.
 
Kyle,
Thanks, that makes a lot of sense. I have two areas I want to protect that are 4” wide, and two shorter areas that are 6” wide, so two 2” strips in each of the narrower areas, and 3” strips in the others. Should work fine and I’ll put down the bilge paint first, since the aluminum shield will stick fine to that.
 
I think it was Bruce Hill that documented his use of small dabs of bondo to secure the fiber, and once set (which is only 15 minutes or so) cover with foil tape. It worked great for me.

I might do that too. I’ll rough up those spots with 80 grit before applying the bondo - not sure bondo would stick well to bilge painted fiberglass.
 
All my three RVs have had a full 1/16" layer of fiberfrax on the lower cowl covered with Aluminum from VANS. Spruce sells a glue for fiberfrax but it was worthless for my use and did not stick. But if you take care and extend your aluminum table a good 1.5" beyond the fiberfrax, it will not come off. Between the three planes I have well over 1000 hours and it stayed on with no issues. The cowl remains very cool and easy to touch right after landing.
 
exhaust pipe heat shields

So why not use one or more exhaust pipe heat shields instead?

https://www.aircraftspruce.com/catalog/eppages/exaustheatedshield.php

Way easier than bonding fiberfrax to the inside of the cowl and covering it with the AL foil.

I only have the AL foil on my lower cowl (no fiberfrax). Heat shields were used where exhaust pipes were too close to fluid lines and wire runs. Maybe my exhaust doesn’t come as close to the cowl as yours does.
 
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So why not use one or more exhaust pipe heat shields instead?

https://www.aircraftspruce.com/catalog/eppages/exaustheatedshield.php

Way easier than bonding fiberfrax to the inside of the cowl and covering it with the AL foil.

I only have the AL foil on my lower cowl (no fiberfrax). Heat shields were used where exhaust pipes were too close to fluid lines and wire runs. Maybe my exhaust doesn’t come as close to the cowl as yours does.

I will have a couple of those shields for a couple areas, but they won’t fit where the tail pipe ball joint/springs come the closest to the cowl. Fiberfrax/aluminum is only planned for certain areas on the bottom cowl. There’s a couple places where the exhaust tail pipe/ball joint is about 1 - 1.5” away from the cowl at the edge of the chin scoop radius.
 
No need for a large area of fiberfrax. Just shadow the pipes. In strips, nothing is needed to hold the fiberfrax except the aluminum foil cover.

In general, the adhesive on foils and tapes always sticks better to a smooth painted surface. Scott, a simple test. Cut two equal width strips of foil. Stick one to the fender of your truck, and the other to the bilge paint. Peel them. If they require the same effort to peel, the bilge paint is a good surface for tape.

The installation below has been in place over 1000 hours.
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The 8 has more room down there than my 7, and I did use a larger area for the fiberfrax, I used 3M spray contact cement and with 200 hrs it still sticks well. I do have a couple of areas that have lots of acoustic noise (or vibration) and fractures the aluminum overlay, but the insulation stays in place.

YMMV.
 
Bilge paint

I used bilge paint on the inside of my cowl and did the fiberfrax/aluminum tape shield. The tape will stick to the paint; not as well as gorilla tape but mine has not lifted in 150 hours on my -10. If you choose to lift it, it will come off…
 
Test

...<snip>
Scott, a simple test. Cut two equal width strips of foil. Stick one to the fender of your truck, and the other to the bilge paint. Peel them. If they require the same effort to peel, the bilge paint is a good surface for tape.

Love it Dan!
Maybe slap that foil tape sample on the wife's car for added quality control?:D
 
Love it Dan!
Maybe slap that foil tape sample on the wife's car for added quality control?:D

I could be done with my “experiments” if I tried that. It would probably peel the paint off her car! I’ll do the experiment on the cowl in a couple days. I put the first coat on today. Tomorrow I’ll sand with 220 grit (per directions on the can), clean it up, and put on another thin coat (first coat wasn’t so thin because the cowl was sucking it up like sponge. You can recoat in 24 hours, but full cure takes a couple days.
 
Won't hurt any kind of decent paint. I've peeled plenty aluminum foil tape from both the inside and outside of the cowl.
 
I tried the peel test today on my wheel pant because it’s fully cured. The aluminum stuck to the bilge painted fiberglass and didn’t peel off when I removed the tape, which took a pretty good pull.
 
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