Firewall insulation [7.5 hours]


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Now that the upper forward fuselage assembly is riveted on, I can go ahead with the firewall insulation, using the Titanium foil and 1/8″ Fiberfrax I had previously prepared.

In order to prevent the Titanium from puckering, I used 1/8″ stainless steel spacers (available from McMaster Carr) for all #8, #10, 1/4″ and 5/16″ holes, and stainless steel washers for larger sizes. I used RTV to hold the spacers “in place”, a few swizzles of Fire Barrier 2000+ to hold the Fiberfrax in place, and then put on the Titanium. I used machine screws to retain the Titanium in place, these will later be replaced by whatever their respective position calls for. There’s a few pulled rivets across the bottom, and for most of the pass-thru’s I had left one rivet position open so that retaining rivets can be used here as well. The rivets I used are stainless steel, and have a closed end cap, so they should seal up quite well.

I installed the A/C pass-throughs and steel AN fittings for the duplex fuel system, and riveted on the oil cooler mount. I previously made up a Titanium insert for the center recess. I subtracted 1/8″ all around from the recess dimensions, and made the insert accordingly. The thing’s a work of art, but it turns out I should have allowed more wiggle room so I’m going to toss it and make up another one.

I went on a bit of a campaign mounting various items on the firewall, to get them off the shelf and out of the way. Finally, the engine mount went on and that’s another large item no longer on the floor.

I moved the paint booth out and tossed it in the farm shed. I don’t really have much use for it in the coming months, and moving it out clears up a lot of room in the workshop. At the very least it needs re-lining, it’s more like a dark room these days. It might get torn down, I think its usefulness is over after four years of dedicated service – an entire slow-build RV-10 got primed in that little paint booth!

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    1/8" spacers RTV'd to firewall
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    All spacers and washers RTV'd to firewall
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    Trial fit of Titanium, taped together in correct alignment
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    1/8" Fiberfrax in position
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    Titanium in place, oil cooler mount riveted on
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    Fuel fittings, heater box holes
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    Engine mount on
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    Moving paint booth out
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    Moving paint booth into farm shed

Firewall insulation [31.0 hours]

I’ve been working on the firewall insulation. There’s a lot of material on VAF about this, I won’t repeat it here. Suffice to say I’m using an insulation material called Fiberfrax, 1/8″ thick, on the outside of the firewall. In order to hold it in place, a thin metal foil layer is required. I’m using 0.008″ thick Titanium for this layer (I previously bought some 0.002″ Stainless Steel foil but decided I couldn’t work it without having it crinkle up and/or tear. Impossible to drill a hole through it). The main purpose of firewall insulation is to give me some time to get the aircraft on the ground in some sort of controlled manner, in the highly unlikely event of an engine fire. A secondary purpose is to minimise the amount of heat that can be transferred into the RV-10 tunnel.

I found the best tool to cut the Titanium foil was an ordinary pair of scissors. Drilling small holes wasn’t a problem, but enlarging them with drill bits is not possible. I used a series of reamers for the smaller holes, the angled end of the flutes works well. To drill a #12 hole, for instance, I would first drill a #42 hole, then #39, #35 using drill bits, then #30, #19 and finally #12 reamers. The back of the foil must be supported of course.

For larger holes, a step drill works, but to finish the hole or enlarge anything beyond around 9/16″, I used a 1/2″ round sanding attachment in a die grinder. These wear out quickly, I went through around twenty of them. Doing these large holes with the die grinder worked well, as long as the foil was supported right up against the stainless steel firewall.

In an ideal world, all of the firewall nutplates and pass-through positions would be known when the firewall was laying on a bench, before ever being attached to the fuselage. That doesn’t happen, so I had to find a way of accurately drilling holes through the foil for all of the nutplates etc. To do this, I 3D printed a lot of disposable drill guides. For any purpose, I designed a drill guide, printed it, and used it to accurately drill a #42 hole through the center of whatever I needed to. In this way, I worked around the firewall and made all of the holes required in the Titanium foil. I used #6 screws through the AN3 nutplates to hold the foil in place while I worked on it, and clecos where appropriate.

With the foil prepared, I cut the Fiberfrax to shape, sandwiched it between the foil and firewall, and then worked my way around all of the holes, cutting the Fiberfrax as necessary with a sharp modelling knife. For the engine mount points, I completely removed the foil and ‘frax. All of the gaps will be filled in later with Fire Barrier 2000+.

I then set the Fiberfrax and foil aside. I can’t attach it permanently until the upper forward fuselage assembly is riveted in place. In fact, all I seem to have done for months now is to prepare parts, and set them aside.

One of the downsides of adding firewall insulation like this is that it makes future firewall modifications difficult. It is possible to drill through the foil and firewall, but deburring is a problem. I tried to anticipate everything I could, and for pass-throughs I put in more than I needed, it’s easy enough to plug up unused pass-throughs. I also made a separately mounted plate that attaches to the right side of the firewall, for mounting electrical components on. At some future time, if the electrical requirements change, I can simply make up a new mounting plate for them, rather than rely on firewall mounted nutplates for each individual component.

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    3D printing a drill guide
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    Drilling holes in Titanium foil
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    Drill guide to go through AN3 nutplate
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    Drilling center hole through Titanium, from the rear
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    Drilling holes for oil cooler mount
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    Marking out upper Titanium foil sheet
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    Cutting upper Titanium foil sheet
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    Holes cut out in top Titanium sheet
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    Titanium foil insert for center section
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    Titanium foil insert for center section
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    All the drill guides used for Titanium foil
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    All holes and cutouts done, except for center
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    Engine mount cutout
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    Engine mount cutout
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    All holes and cutouts done, except for center
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    Fiberfrax stored for later use
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    Pass-thru's riveted on