New Lava-Like Coating Can Stop Fires In Their Tracks

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DanH

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I have gone back to just putting a layer of Fiberfrax and another layer of plywood on the pilots side of the firewall.

Be careful with fiberfrax 970 on the cabin side. The fibers are glued together with an organic binder. When exposed to flame, the binder outgasses. The outgas is both flammable and a smoke source.

Again, insulate firewalls on the engine side. The best cabin side is bare stainless, not even painted.
 

DanH

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The DarkAero design team of engineer/brothers did a deep study on their firewall materials choice

They are very bright kids, and I am sincerely flattered.

Started publishing firewall insulation tests over on VAF more than a decade ago. My rigs use a flat black radiant heat target 6" behind the test subject to judge heat transmission to the pilot's feet, but the marshmallow man (backed with a thermocouple) is a great touch.

Feb 2005, early test rig. Look familiar? The current one is more attractive.

First Test Rig.jpg Early Test Rig 2.jpg
 

DanH

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Be careful with fiberfrax 970 on the cabin side. The fibers are glued together with an organic binder. When exposed to flame, the binder outgasses. The outgas is both flammable and a smoke source.
Again, insulate firewalls on the engine side. The best cabin side is bare stainless, not even painted.

Follow up....

Watch the DarkAero video. Note the smoke generated by the ceramic felt (i.e. Fiberfrax 970) when heated. Not a problem when sealed away from the cabin, as it is in the DarkAero design. Very bad on the cabin side of a stainless firewall in a metal airplane.
 

wsimpso1

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I think what is BEHIND the stainless steel/ gal steel/ titanium firewall is as important as the firewall.

First comment - People do not understand FIRE. Let's get into the details.

My comments here pertain to composite airframes and firewalls. Between the firewall and humans is a bad place to put the insulation. Composite airframes are easily compromised by temperatures that are quite modest compared to typical combustion products. We have fire at somewhere around 1800F to 2000F.

Long held and well demonstrated practice is Fiberfrax between the fire and firewall plus a thin layer of metal to protect the Fiberfrax until the fire comes. The composite resins usually go soft around 200F. That is a long ways below 2000F. A typical composite firewall is either plastic foam or plywood cored, and will quickly be charred and fall apart when exposed to flame gases at 2000F. Beside the awful thought of the fire intruding into the cabin, there is the issue of the engine and mount departing the airframe, resulting in a tumbling airframe and crash at whatever terminal velocity impact orientation that produces.

Put simply, the epoxy must be kept cool to keep the airframe together. 200F is probably a good steady state target. Having done the heat transfer calculations, to keep the 2000F flame from raising the composite firewall above 200F in a steady state case, four plies - 1/2" - of Fiberfrax is appropriate. Three plies was not enough. Why not do the transient case? First, your recognition of the failure may take more time than we would all like, then you have to do an emergency descent and landing. IIRC, the FAA standard is 2000F for 15 minutes, and that is well along the transient case and looking pretty close to steady state to this engineer.

Fiberfrax is fragile in day to day living, and needs a layer of something to protect it from being abraded or wiped off during service work as well as to keep it from being loaded with liquids common in aviation that would put the flame much closer to the structural firewall. Yes, a nice 0.015 stainless or titanium sheet will do and keep any flame out of the Fiberfrax. A 0.015 aluminum sheet will protect the Fiberfrax from day to day, will keep flammable liquids out of the Fiberfrax, and will melt local to a 2000F fire, but its job when installed this way is done once the fire starts, and you just saved some weight. There are other folks coming to the same conclusion. But if you feel better with a 0.015 ferrous sheet (~4-1/2 pounds) instead of aluminum (~2 pounds), go for it.

Our friends using pusher prop designs have historically used 1/8" to 1/4" of Fiberfrax then a thin metal covering. They also have the advantage of airflow running away from the firewall, not toward it. With a substantially less severe heat dose on the insulation, this may indeed be completely adequate.

Get into a metal airframe structure with a ferrous firewall, and all of this may change...

(a) Don't make anything critical in the engine bay out of aluminum, including the prop- control or cowl flap hinge.
Hmm, this might be difficult to achieve. Things like cases for prop governors, carburators/throttle bodies, and other components are usually made of low melting point metals. VW cases are magnesium alloy. This must scare the poop out of some folks, but I have yet to hear of major issues with them.

(b) Don't use foam/ fiberglass sandwich behind the firewall, I only use plywood now. Much stronger for longer.
Tests have been done with plywood cored laminates. While you may get more time out of plywood cores, the composite resins loose strength at only a little above 200F and the wood chars quickly and loses all structural integrity at temperatures far closer to 200F than the flame at 2000F. Protecting and structures from fire requires significant insulation. Once we have that, I doubt that the firewall will be easily compromised.

(c) Put a fire resistant and heat insulation blanket or material on the pilot side of the firewall and use extra cover over the engine bolts and intrusions.
I do recognize that the fire on the steel engine mount will tend to transfer heat to the firewall itself. I think most of us are counting on usually only burning the firewall in one place with connections holding (because we applied suitable fireproof insulation to the firewall). A layer or two of refractory material over the engine side of the mount bolts and tubes may have real benefit here.

(d) use a stainless steel strip in the cowling layup to support the cowl fasteners.
Harump. You think the cowling will stay put if it sees these kinds of temperatures? I suspect that a ply of Fiberfrax, other commercial heat shield, or even tumescent paint on the inside of the cowling will do a lot more for keeping the flame products going out the intended cowl outlets.

Billski
 
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Be careful with fiberfrax 970 on the cabin side.
Out gassing isn't the only problem. Per the MSDS for this class of insulation it is classed as an lung carcinogen due to airborne fibers.
Take a look at compounds called rigidizers as one method to contain the fibers. They are generally colloidal silica (AKA Cabosil) and good to 2300F.

Cutting and installing should be done with appropriate PPE.
 

wsimpso1

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The DarkAero design team of engineer/brothers did a deep study on their firewall materials choice.

Having done the analysis, I gotta think that the graphite-epoxy layer on the front side of the honeycomb is toast after 15 minutes with the stainless sheet seeing 2000F. I figure that 1/8" Fiberfrax in contact with the carbon-honeycomb is about 1200F when the stainless is 2000F.

I also have to wonder if they have considered an insulator over the connection between engine mount and firewall on the engine side. Might be a really good idea if they are as serious about meeting the intent as well as the letter of CFR Part 23.

Bill
 

J.L. Frusha

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For homebuilts, would we necessarily NEED some fancy-schmancy name brand, or can we use something homemade that works? Sure, for ultralights, probably, but why not something homemade and affordable (cheap, even)...?

Intumescent 'Cookie-dough' refractory...?

8 parts Flour
4 parts Corn Starch
4 parts Powdered Sugar
4 parts Baking Soda
1 part Borax

Simply add water to make a cookie-dough like mixture... Too little and it's crumbly, too much and it will stick to your hands like wet clay.
 
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robust

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Rhino

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Carbon felt, melting point 1500*C
The graphite felt is actually better. The problem with this is that melting point doesn't tell us everything we need to know. How durable is it? Although it's harmless as it normally sits, does it outgas or produce any hazards when burned? How can it be adhered to a firewall? Does it require a protection layer?
 

DanH

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A 0.015 aluminum sheet will protect the Fiberfrax from day to day, will keep flammable liquids out of the Fiberfrax, and will melt local to a 2000F fire, but its job when installed this way is done once the fire starts, and you just saved some weight.

Bill, based on a burn run with direct flame exposure, I don't think an aluminum cover sheet is appropriate. Fiberfrax 970 is made of short fibers held together with, well, glue. When the glue burns away, there is little to hold the fibers in place. The sheet falls apart. The sheet in the photo could not be handled after the burner was extinguished.

Given the windy/shaky environment, I think the fiberfrax fibers will simply blow away when the aluminum cover sheet melts.

Break.

The second photo is a test of 970 in an aluminum foil envelope (see "lung carcinogen" above), on the cabin side of a 0.019" stainless panel. There is a 2000F burner running on the "engine side". The open flame is jetting from the space between the stainless and the now-melted aluminum foil. That's the burning glue, ignited by contact with the glowing stainless. Just a finale really; prior to flame there was lots of smoke.

This was one of a great many burn runs with a variety of materials on the cabin side...mostly to convince folks to quit putting stupid stuff on the cabin side of firewalls.


P9280016.JPG P9280003.JPG
 

robust

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The graphite felt is actually better. The problem with this is that melting point doesn't tell us everything we need to know. How durable is it? Although it's harmless as it normally sits, does it outgas or produce any hazards when burned? How can it be adhered to a firewall? Does it require a protection layer?
Answers on this site. Translate with google
 

rv7charlie

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From the website:
Carbon fiber does not melt, and this determined its wide application as thermal insulation at temperatures above 1500 C in an inert environment.

So, 'inert environment'? Perhaps this is just an issue with translation, but the cowl is hardly an inert environment. Most forms of carbon will burn when oxygen is present.
 
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