I’ve been thinking a lot about the firewall insulation. Best practise in the RV community seems to be the use of Fibrefrax on the outside of the firewall, held in place with stainless steel foil. To that end, after a US trip last year I brought back some 1/8″ Fibrefrax and a roll of 0.002″ stainless steel foil. A secondary but no less important issue is what to do about the floor in the area immediately behind the firewall. In the event of an engine fire, temperatures could reach 2000 degrees F in the engine bay, and cowling airflow will direct hot gases / flames out the bottom and under the front seat floor. While I’m trying to get the aircraft on the ground, the floor could literally melt out from under my feet, since the melting temperature of Alclad is 500F. It is interesting to note that the Cessna 172’s and 206’s I’ve trained in have basically nothing in terms of insulation/protection on the firewall forward floor area.
Commonly used ‘flameproof’ floor insulation materials such as Soundex are a non starter as far as I’m concerned, since they will in fact catch fire and expel all sorts of noxious fumes in the event of an engine fire. Again, best practise in the RV community seems to be external ceramic insulation. In the case of the RV-10 with a Showplanes cowl, the cooling air outlet spans the entire width of the lower forward fuselage skin (see picture). I obtained a roll of 18″ wide, 0.005″ thick Titanium and some 1/16″ Fiberfrax ceramic insulation to do this external insulation. The Titanium is secured in place with a combination of Monel pulled rivets, which will not melt at fire temperatures, and regular AN426 rivets on the outside edges, which will melt at fire temperatures. The rivets on the sides and rear will not be exposed to these extreme temperatures, so only the rivets along the firewall bottom flange would be an issue. The monel rivets will hold the Titanium in place, and I’m applying a layer of 3M fire barrier 2000 sealant to this forward edge as well, the combination of these should be adequate to keep the front edge of the insulation in place long enough to effect a forced landing. Or at least give me a fighting chance.
I sandwiched the Titanium foil between the F-1072 bottom skin and a piece of MDF, and drilled through the assembly for Monel rivet lines, match drilled AN426AD3 rivet holes, and platenut positions for the Showplanes cowl mount points. After that, I was able to de-burr and dimple the skin and the Titanium foil, which I put aside for later assembly. I primed the skin and then reverted back to the regular plan assembly steps. This is a long section of the build, and I’m currently at the point where I have mated the mid fuselage and forward fuselage assemblies. Now I have to prepare spacers to go between the two main spars and a set of bolts to hold the spars together. The skin overlaps and a few other parts are then match drilled, I pull the assemblies apart to de-burr and dimple, and will then mate them together for the final time.
The combination of the forward and mid fuselage assembly is quite long, to which gets added the empennage. I’m going to have to build a roll-around stand for the assembly so that I can get my workbenches back.