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Fiberfrax on Firewall Question...
I was considering putting fiberfrax on the engine side of the firewall with a thin layer of steel over it to protect and keep it in place as was recommended in some other threads. I had read several people say they wish they did this before mounting items on the firewall (such as battery tray, contactors, etc.). They also talked about creating quarter inch stand-off's for these items. My question is how would mounting these items on 1/4" stand-offs effect the integrity of their mounting? I am concerned that it would compromise the security of them. For instance, the battery tray is currently flush on the firewall and bolted in three locations, if each of those bolts had a quarter inch stand off behind them it would not seem as stable.
Could you guys please give me your opinion on this? it would be nice to hear from someone who has actually used the stuff or researched it as well. Thanks. Matt |
It is true that more fiberfrax thickness means greater insulation value. However, two layers of 1/8" fiberfrax (1/4" total) may be overkill.
Remember, this is a system. Performance is a function of both the stainless foil and the fiberfrax. They each have a different purpose, and neither would do a great job alone. The foil is a reflector and a hot gas seal. Think of it as the layer which turns back most of the energy before it can ever reach the fiberfrax. The fiberfrax itself merely reduces the quantity of heat transferred from the stainless foil to the firewall structure. Point is, the stainless foil does the heavy lifting. I doubt doubled fiberfrax thickness is twice as good Standoffs and fasteners: One interesting test observation was the effect of fasteners. They create a small point hot spot on the cabin side, transferring heat by conduction and bypassing the insulation air space. A fastener assembly with more cross-section area (like a button standoff) would conduct more heat. The point heat source effect of fasteners is why I don't think it wise to "double insulate", i.e. place fiberfrax/foil on the engine side and believe it is then safe to place some stupid plastic, rubber, or polyester fiber insulation choice on the cabin side of the firewall. If the firewall is bare (nothing, not even paint), the hot fastener head merely radiates a very small quantity of heat into the cabin. If that same fastener head is allowed physical contact with a flammable material, it can ignite it via conduction. So, a standoff is unlikely to create a practical problem if the backside is bare, but will make things worse if the backside is in contact with something dumb. Take this thought a little further. A builder might think that nice thick insulation on engine side (requiring standoffs) would allow him to safely place sound insulation on the cabin side.....and that's not necessarily true. § 23.1191 Firewalls. (f) Compliance with the criteria for fireproof materials or components must be shown as follows: (1) The flame to which the materials or components are subjected must be 2,000 ±150 °F. (2) Sheet materials approximately 10 inches square must be subjected to the flame from a suitable burner. (3) The flame must be large enough to maintain the required test temperature over an area approximately five inches square. (g) Firewall materials and fittings must resist flame penetration for at least 15 minutes. § 23.1182 Nacelle areas behind firewalls. Components, lines, and fittings, except those subject to the provisions of §23.1351(e), located behind the engine-compartment firewall must be constructed of such materials and located at such distances from the firewall that they will not suffer damage sufficient to endanger the airplane if a portion of the engine side of the firewall is subjected to a flame temperature of not less than 2000 °F for 15 minutes. |
Dan;
Like Matt, I'm planning the same firewall insulation and I'm wondering the same problem. In your consideration, if I understand well, the .125" fiberfrax plus stainless foil should be enough like heat insulation. In this case I don't need to make stand off for battey, rele et... but would be nice if I bolt all together (ex battery rack + stainless foil + 1/8 fiberfrax + firewall) like sandwich, so I'll be able to avoid further fastener or glue, but I'll need just some sealant (like 3M fire barrier 2000) around holes and firewall edges? If not, could you show us some pic where we can see how to keep this insulation to the firewall? Thanks a lot for your time |
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I used standoffs for the battery box (3x #10's) and fuel flow transducer (4x #10's on an angle) and everything is pretty solid feeling. If you were really nervous I think you could use large diameter teflon or delrin standoffs to spread the load more without being overly heavy (they might melt in a big fire, but a saggy battery box doesn't matter at that point). DO remember to mount EVERYTHING you want mounted on the firewall before permanently installing the insulation, I don't think it'd be possible to cut a clean hole after that. I glued the standoff's to firewall with RTV and a cleco, and that was more than adequate to allow them to live under the insulation, then later get a bolt in them. I closed all the seams in the stainless with aluminum foil/speed tape, looks good and should seal. It might come off in small areas in a big time fire, but most seams are overlapped, so I don't think the insulation would be greatly compromised. Buy some good mechanics gloves. |
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I was wondering if you could do this. Thanks for mentioning it. |
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I would suggest you drill all holes for cables, wires, bolts, screws or any other protrusions you will need in the firewall prior to installation. "Dry mount" everything on the firewall to make sure everything will work properly. Then remove all of those things and install the insulation, cutting out for all the needed penetrations. Then methodically bolt everything in place with the insulation in its proper place. I am sure there will be no way to completely avoid the sharp edges of the foil but I do believe this method will be much better than what I had to endure. |
....and the....??
Guys;
Very interesting your experience about skin cuts I'll follow your advice :) but I've some more question Ground in the engine side: the battery, engine case and other few things needs some metal contact directly on firewall so ground forest inside, I'm wondering if just the pass thru bolt should be enough to make a solid ground, or you left some part off insulation, how did you solve this question? Fuel; For the fuel line pass thru, did you fit the usual AN833 bulkhead sandwiching the insulation? or.... I ask so because I'm pretty fearful about fuel :confused: Alu tape; The edge of fiberfrax and steel coil, ends really where the firewall make the radius flange .... or you left 1" or 1/2" or... to allow the tape grip? Control cables; I've eyeball in aluminum TTP but, before decide to fit this kind of insulation, I tough to switch to steel one, someone said "its firewall!!!", so now that kind of cable pass is designed for thin bulkhead.... You? Any advice or suggestion will be appreciated Thanks |
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Aluminum tape will melt instantly in a fire, so tape is not used to hold anything. Tape is only used to seal foil overlaps which would be fireproof even without the tape. Quote:
BTW, cutting a hole in stainless foil is easy. Just use the corner of a razor blade to make a series of connecting punctures...poke, poke, poke. |
...poke, poke, poke.
GREAT Dan Thanks |
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