Heatshielding Question

[ppilotmike]

Not wanting to resurect older threads about the topic of heatshielding (both firewall and belly near exhaust exit), I want to ask the following:

The Plan:
My plan to insulate my firewall and also the belly of my airplane, near the exhaust, is to create a sandwich of materials. The firewall will consist of the Van's stainless steel firewall (structural), then a layer of 1/8" fiberfrax insulation, then a sheet of 0.010" stainless steel sheet, for protection more shielding. The belly of the plane, near the exhaust, maybe 2'-0" back, will consist of Van's outer aluminum skin (structural), then a layer of 1/16" fiberfrax insulation, then a sheet of 0.010" stainless steel sheet. Everything will be riveted per the plans, using aluminum rivets, but I'll be adding some extra, longer rivets (to allow for depth of sandwich), sporadically to hold the assembly together.

The Question:
My concern is that the aluminum rivets have a much lower melting point than the assembly, and that adding additional ones to secure the sandwich of material, might have detrimental effects on the structure. Is there another, better way to secure this sandwich of materials, like an adhesive, that would protect the normally installed aluminum rivets of the firewall/belly skins, while still providing resistance to "coming apart" in the heat of a fire? Stainless steel rivets?

[krw5927]

For the "extra, longer rivets", if they're truly extra, you could use Monel blind rivets. Ensure at least 4D spacing to existing rivets and at least 2D edge distance to any part edge in the stack-up.

[rleffler]

KISS.....

Stainless screws, nuts, and washers work just fine. You don't need many to hold the material in place.

[David Paule]

Mike, look up DanH's postings about firewall insulation. He suggests using stainless foil tape instead of sheet. On the inside bottom, he suggests aluminum tape.

If you run any structural firewall rivets through the Fiberfax, the thickness and softness of the Fiberfax will essentially ruin the joint. If it were me, I'd use different fasteners for the Fiberfax and foil tape than the structural rivets. DanH has suggestions for fasteners for attaching those materials, too. I'd try to use the Fiberfax/foil tape to protect any structural rivets, if that's a concern.

Dave

[DanH]

Originally Posted by David Paule

Mike, look up DanH's postings about firewall insulation. He suggests using stainless foil tape instead of sheet. On the inside bottom, he suggests aluminum tape.

Easy to get confused. The stainless steel sheet overlay can be quite thin; 0.002" at a minimum and perhaps 0.010" max. It is commonly listed as "foil", and seems to be available in rolls 24" wide.

That width is fine, because the desired installation incorporates an overlap seam down the center of the firewall:



In the stackup above, the only tape is 2" wide solid aluminum duct tape, available at your local heating and AC supply house. The overlap has a specific purpose...it's a vent. The underlying fiberfrax has a binder (a glue, so to speak) that holds the felt together; it will outgas when heated. That outgas is flammable. We want to vent it into the engine compartment, where it won't matter. We do not want it to vent into the cabin via the firewall perimeter flutes.

So, the aluminum tape seals the overlap during normal operations, but melts almost instantly when exposed to fire, providing a vent. The physical overlap of the stainless provides the fire protection. The perimeter is sealed like this:

If you run any structural firewall rivets through the Fiberfax, the thickness and softness of the Fiberfax will essentially ruin the joint.

Agree. Note that Dave and I are both talking about trying to share structural 470 or 426 rivets with the fiberfrax/stainless overlay...don't do it. Use a separate set of stainless blind rivets or similar to fasten both the firewall overlay and any belly overlay.

[ppilotmike]

Originally Posted by David Paule

Mike, look up DanH's postings about firewall insulation. He suggests using stainless foil tape instead of sheet. On the inside bottom, he suggests aluminum tape.

If you run any structural firewall rivets through the Fiberfax, the thickness and softness of the Fiberfax will essentially ruin the joint. If it were me, I'd use different fasteners for the Fiberfax and foil tape than the structural rivets. DanH has suggestions for fasteners for attaching those materials, too. I'd try to use the Fiberfax/foil tape to protect any structural rivets, if that's a concern.

Dave

Dave,

The foil Dan used is 0.005" thick, which seemed a little too thin for me (could be ripped when working in the engine compartment). The sheet I'll be using is double the thickness of the foil, at 0.010", but is still very thin and light. My intention is not to use the structural rivets required for the firewall assembly to hold the "sandwich" in place. The whole idea is to have the sandwich protect the underlying structure, independantly. My worry was that the extra penetrations (for sandwich attachment) might somehow weaken the firewall. I like the idea of some monel blind rivets, although they sound pricey. With tape, I feel like I wouldn't get the "hold" needed. Then again, all the pass-throughs will also help to keep it in place, as will the mounts for various things on the "hot side." Perhaps those things are enough to keep the sandwich in place...?

[ppilotmike]

Originally Posted by DanH

Easy to get confused. The stainless steel sheet overlay can be quite thin; 0.002" at a minimum and perhaps 0.010" max. It is commonly listed as "foil", and seems to be available in rolls 24" wide.

That width is fine, because the desired installation incorporates an overlap seam down the center of the firewall:

Dan,

Thanks for the helpful diagrams. I like your "vent" idea. I had planned on an installation without seams, thinking it would provide greater protection, but hadn't considered needing any more venting than what I'd get through the "attachment points." My friend found a source that provides SS 304 "Foil" 0.010" thick in sheets that are 36" wide x whatever length you need. I ordered a sheet 36" x 60" since the RV-10 firewall is large and I wanted some for the belly too. In your search for relevant information on the Fiberfrax (specifically) offgassing, did you surmise that such a large vent would be absolutely required (i.e. lots of offgassing)? The binder appears to not be very toxic. I would think a good seal, preventing smoke into the cabin would be adequate, but maybe a few holes in the sheet, covered with tape would also be advisable.

[Lynnb]

Am I wrong in thinking the Rivets will tear through the foil? Also, do you use a closed end rivet?

The diagrams/drawings are extreamly helpful, thanks.

Lynn

[cattflight]

Is there any reason the foil overlap and seam cannot be horizontal across the firewall? Dimensionally, to use the material off the roll best, the Fiberfrax on my FW would have its own seam or overlap vertically. Thinking the stainless foil seam could be perpendicular to minimize the seams intersection. If I were to do this, would I want the upper foil to overlap the lower foil or vice-versa? Thinking the shingle effect will keep hot fluids from entering the Fiberfrax layer.

I welcome the collective wisdom here. And, DanH, thanks for all the effort. Amazing work and guidance beyond the RV community!

[DanH]

Originally Posted by ppilotmike

I like your "vent" idea. I had planned on an installation without seams, thinking it would provide greater protection, but hadn't considered needing any more venting than what I'd get through the "attachment points." My friend found a source that provides SS 304 "Foil" 0.010" thick in sheets that are 36" wide x whatever length you need. I ordered a sheet 36" x 60" since the RV-10 firewall is large and I wanted some for the belly too. In your search for relevant information on the Fiberfrax (specifically) offgassing, did you surmise that such a large vent would be absolutely required (i.e. lots of offgassing)? The binder appears to not be very toxic. I would think a good seal, preventing smoke into the cabin would be adequate, but maybe a few holes in the sheet, covered with tape would also be advisable.

Mike, please do post a source for the 0.010" SS sheet. It could be handy in some applications, in particular a belly overlay.

It's not that the outgassing requires a huge vent...see below

Originally Posted by cattflight

Is there any reason the foil overlap and seam cannot be horizontal across the firewall?

A subtle detail is that the aluminum tape is expected to melt in the event of a significant fire. The hot part of the firewall will be the centerline, down low, near the cowl exit, as fire tends to follow airflow. Thus, you kinda want some of the SS foil overlap seam in that location, so the aluminum tape has the opportunity to melt and open a vent.