Cowl/fiberfrax modification

[Scott Hersha]

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:

[Kyle Boatright]

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.

[CJ in EP]

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.

[Scott Hersha]

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.

[Scott Hersha]

Originally Posted by CJ in EP

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.

[Bavafa]

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.

[Stevea]

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

https://www.aircraftspruce.com/catal...atedshield.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.

[Scott Hersha]

Originally Posted by Stevea

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

https://www.aircraftspruce.com/catal...atedshield.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.

[DanH]

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.
.

[BillL]

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.