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Heatshielding Question

ppilotmike

Well Known Member
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?
 
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.
 
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KISS.....

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

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
 
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Horizontal foil seam?

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!
 
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

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.
 
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



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.

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

Dan,

Below is my source for the 0.010" SS sheet:

Frank Li
American Metals Company
PO Box 41
200 Obringer Lane
Sarver PA 16055
Phone 724-625-8666
Email: [email protected]

Please let me know what style of pop rivets you used. I've change my mind about using the structural rivets that are part of the actual firewall. I will likely put some SS pop rivets in on say a 6" OC spacing, just to hold the sandwich to the firewall. Also, I'm planning to vent the Fiberfrax off-gas via the edges of the sandwich and also the seams around the recessed box.
 
I installed a temperature limit switch near nose gear leg attachment. I set it at 230*F. Normally, 220*F is about max operating temp across the switch unless at 16,500 near gw in high AOA and in thin air. The NC switch opens and energizes an LED on the panel in case of fire. My fuel flow max warning is set at 27 gph. I installed an 1/8" layer of fiberfrax on inside of firewall and tunnel bottom. The best thing to do is build, inspect and maintain so that you lessen the chance of fire. Have a warning or two so that you are not flying along for 5 minutes while flames lap at your aluminum/fiberfrax/stainless before starting an emergency descent. We are way more likely to die in a LOC accident than an in-flight fire.
 
Firewall frax thickness and edge attachment.

I have a couple of questions for the crew. I did my belly insulation using 0.005" TI (from Titaniumgoat) and 1/16" frax, using 3/32" rivets around the perimeter and Mk319BS monel rivets in the center. The perimeter was also sealed with sikaflex pro to limit the chance of water egress, and the aluminum belly skin was primed externally prior to the application. The center rows of rivets were wet riveted in place using firebarrier 2000+. I was pretty happy with how things turned out. I'm now working on the firewall, and have a couple of questions.
1. Using the fourier's law, it appears that the doubling of insulation thickness results in a halving of heat transfer into the cabin. At a 65degC engine compartment temperature and 20degC cockpit temperature, for 1/16" frax 1.55kW is transferred into the cabin, and 0.77kW for 1/8" frax. The only downside is that I see the 1/8" frax is going to be much harder to handle during the installation, especially sealing the perimeter and in the firewall recess. Has anyone used 1/16" frax instead and if so, what were the results? I figure if it is ok to protect the aluminum belly, then it must be adequate to protect the aluminum rivets in the firewall. I figure the thinner insulation would give a neater finish by limiting pillowing and thus the risk of catching a tool on it in the future and tearing it.
2. For a new build, could you just sandwich the foil around the perimeter between the flanges and the cowling hinges? I figure this would eliminate the problem of having the risk of the firebarrier 2000+ separating around the perimeter and the foil coming away in the future.

Below are some pics of my belly skin install.
2vif1b8.jpg

2cnjnzk.jpg
 
[
Please let me know what style of pop rivets you used. I've change my mind about using the structural rivets that are part of the actual firewall. I will likely put some SS pop rivets in on say a 6" OC spacing, just to hold the sandwich to the firewall. Also, I'm planning to vent the Fiberfrax off-gas via the edges of the sandwich and also the seams around the recessed box.[/QUOTE]

Mike,
I used 1/8 large head SS pull rivets. (I will have to get the exact detail of the rivets at the hanger). Also got 1 inch steel washers plated with 1/8 hole (Lowes) through which the rivets were installed. That distributes the force of the rivet nicely. I did not need 6 inch spacing and used much larger spacing. Also the items that mount to the firewall serves as anchors of the sandwich.
Johan
 
RV-10 Belly Shield

Here's some pictures of how my belly shield turned out. I think it looks great and will virtually disappear when it gets painted. I used 1/16" fiberfrax materials, sandwiched between 0.010" stainless steel sheet and the aluminum belly skin. My heat shield comes back about 18" from the firewall, it's sealed with 3M Fire Barrier 2000 and is riveted, using slightly longer rivets (to allow for the SS thickness and FF thickness) to the understructure, during my build. With the 1/16" FF and the 0.010" SS sheet, there was very little pillowing, as you can see.

AF1QipOAUXE53hd80GZGpLOdl7Z7FfrrKVSxguN4p75P

AF1QipPj_0ytugQydmcHys00373FOT9ISzalWBFIuWhO


I plan to use 1/8" thick Fiberfrax and the same 0.010" SS sheet to make a heat shield on my firewall (engine side, of course!), per Dan Horton's suggestions. It will be held in place using SS pull rivets, spaced out at 6 to 9" OC, plus all the other connections through the firewall, and sealed up with 3M Fire Barrier 2000. I will also be using SS heat box valves, in lieu of the aluminum ones Vans provides. All in all, this should provide a bit of extra protection, in case of an in-flight fire, for not much added cost, work or weight, IMO.
 
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Johan, to your point, my perimeter rivets are just regular AN426AD3-3.5 rivets, and the two rows of interior rivets are Mk319BS as they are monel and thus much higher temperature capable than the AN426 rivets. The holes were first drilled in the 0.040" belly skin, then the titanium was sandwiched between that and some particle board flooring, pressing down hard on the skin so that the holes drilled cleanly. The layer was clecoed to the board as I went along. After drilling the center holes to 7/64", they were then dimpled using a #40 dimple die (as that was the only size I had that fitted) but they worked fine.
So how about sandwiching the stainless foil between the cowling attach hinges and the firewall flanges? Anyone tried this approach?

Tom.
 
So how about sandwiching the stainless foil between the cowling attach hinges and the firewall flanges? Anyone tried this approach?

I'm sure it can be done, but 6 years in service says a fillet of Firebarrier around the perimeter (2nd drawing, post 5) works fine to fasten and encapsulate the edge of the sheet.

SS foil is razor sharp. Forming a good 90 degree flange to trap under the hinge material may involve bloodshed. For sure you don't want an exposed edge anywhere when finished.
 
Belly Shield Attachment

Anyone see a downside to attaching a belly shield with #6 stainless screws such as MS21207 100? FLAT HEAD TYPE B SCREWS? My 6 is flying and I'd like to do the attachment from the outside only. Don't see readily available stainless pull rivets smaller than 1/8". The #6 screws would require a smaller hole.

Cheers, David
RV-6A KBTF
 
Anyone see a downside to attaching a belly shield with #6 stainless screws such as MS21207 100° FLAT HEAD TYPE B SCREWS? My 6 is flying and I'd like to do the attachment from the outside only. Don't see readily available stainless pull rivets smaller than 1/8". The #6 screws would require a smaller hole.

Cheers, David
RV-6A KBTF

My plan is to put some sealed nutplates around the perimeter and down the center of the shield.

Also, .010 stainless sheet is on McMaster: https://www.mcmaster.com/#standard-shims/=15vvzw3

Will a 24" x 50" sheet work for the firewall and belly shield of an RV-7?
 
Firewall Recess Area

Anyone have any photos there willing to share on how they covered inside the firewall recess neatly using the Fiberfrax and SS foil. I've done a sample with cardboard and don't care for the look.
Any thoughts or reasons why you cant just cover the recess for a simpler install especially if you have a right angle oil filter adapter? I don't have any penetrations coming thru it now.
 
Anyone have any photos there willing to share on how they covered inside the firewall recess neatly using the Fiberfrax and SS foil. I've done a sample with cardboard and don't care for the look.
Any thoughts or reasons why you cant just cover the recess for a simpler install especially if you have a right angle oil filter adapter? I don't have any penetrations coming thru it now.

I'd be very interested in knowing this one as well. If I can leave the recess off entirely and just use a flat cover on the opening I'd prefer to do that.

What goes into that space in a normal installation?
 
Besides the "end-on" oil filter already mentioned, my understanding is the recess accommodates a constant-speed propeller governor mounted to the accessory case.
 
Besides the "end-on" oil filter already mentioned, my understanding is the recess accommodates a constant-speed propeller governor mounted to the accessory case.

So basically there's no reason the recess cant be covered if your not intending to use a constant speed prop and you have a right angle adapter for the oil filter? Alternators mounted on the accessory case should not be a factor because of the location right? I'm going from an auto engine to a Lycoming so I am eyeballing locations before I insulate the firewall and hang the engine.

Still would love to see pics of someones insulting job in and around the firewall recess.
 
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Besides the "end-on" oil filter already mentioned, my understanding is the recess accommodates a constant-speed propeller governor mounted to the accessory case.

Hmm. I'm planning on a remote oil filter but the governor may cause a problem.

Are parallel-valve O-360 family engines with forward-mounted governors common?
 
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