Firewall penetrations, exhaust work

So this is a combination post for both today and yesterday. The reality is that I spent a lot more than three and a half hours on airplane stuff, but a lot of that was doing research and stuff inside…so due to my completely arbitrary decision to only count “shop time” when I accrue hours, that’s what the number is.

The first thing I worked on yesterday was the firewall penetrations. As mentioned before, I decided to ditch the plans method of using a plastic snap bushing cooped with sealant, in favor of nicer stainless penetrations. One concern with the penetration kits I bought was that they weren’t going to work as provided, due to how close the two firewall holes sit to the outside skin. So job #1 was to modify the mount flanges for to handle this. For now, I’m only working on the two 3/4” fittings – I also have a 1/2” one that’ll get used for the alternate air cable, but I’m not going to mess with that right now. Basically, I just cut one corner off each fitting flange. That leaves me with only three screws instead of four to secure these, but since they’ll be mounted with a layer of sealant underneath, I don’t think there will be an issue with proper sealing.

The more fun part of this was figuring out if the fittings were going to work. One downside of these new fittings is that they actually don’t accommodate as much stuff as the snap bushings. The snap bushings end up being maybe 11/16” ID, but these stainless fittings call for their ID to be lined with firesleeve to provide chafing protection – which reduces the usable space quite a bit. This isn’t an issue for the port-side fitting – it will only carry two power supply wires, one 6AWG and the other 10AWG. The starboard fitting is a different matter – the wire bundle there is already a decent size, and it doesn’t yet include the SDS ignition harnesses that need to be threaded back into the fuselage.

So I spent a whole lot of time today tinkering with that particular passthrough. Through trial and error, I verified that it was not possible to pass all my wires through the 3/4” fitting with the firesleeve lining in place. Well, that’s a problem. There’s a 1” fitting just like this one, but there’s not enough room between the side skin and the firewall angle to fit that – 3/4” is as big as I can go. I considered adding a 1/2” fitting below the first one, and splitting the bundle, but due to the way the flanges are laid out, it’d have to be probably 2” away from the other one. This would also cause problems for my cable management plans inside the forward baggage area.

It eventually occurred to me that I don’t have to use firesleeve inside the fitting – just something that works for anti-chafing. There will be an outer firesleeve sheath that provides actual fire protection; the internal bit isn’t really doing anything. I did an experiment where I just wrapped the wire bundle in silicone tape, which fit just fine, and I thought I had a decent solution – but this approach might make me hate myself down the road. It’s very possible I’ll want to add wires to this bundle down the road, and trying to remove and replace that tape in situ would be, to put it mildly, difficult.

What I needed was some chafe-resistant material that I could put inside the fitting, and have it essentially stick to the fitting, not the wires. After some thinking, I had the idea of putting some sort of tubing inside the fitting. Some time spent browsing McMaster-Carr yielded exactly what I needed – some thin-wall nylon tubing. With a little fine-tuning of the tubing OD, I should be able to press-fit a section into the fitting, thus providing my chafe resistance, while still leaving plenty of space for the wire bundle. In a sense, I’m ending up with something like one of those plastic snap bushings, but inside a far more fireproof fitting.

On the other hand, that means no more mucking with the penetrations until Yet Another Parts Order comes in. What to do now? I ended up completely switching gears. The build manual suggests starting the FWF layout with the exhaust, since it’s an immovable object that’s going to be emitting tons of heat – that is, it’s a pretty big consideration for routing everything else. I’d had the upper exhaust stacks on the engine for a while, but left the lower pipes off to keep them out of the way while removing/reattaching the engine repeatedly. Now that I’m (hopefully) done with that, I might as well go ahead and put those on, and start figuring out how the hangars and other stuff work.

It definitely looks a lot more airplane-like with the full exhaust system in place…even if it’s just sort of hanging in thin air for now:

I spent the rest of the afternoon wrapping my head around the exhaust hangar setup. There’s some bracketry that I’ll need to drill bolt holes in before I can actually work with the hangars…that’ll be another day. The other packaging consideration for the exhaust is the cabin heat muff – this is a somewhat bulky device that wraps around two of the exhaust pipes. Also, it’s kind of a royal pain to assemble, which is especially annoying when considering that the spot I temporarily mounted it today isn’t going to work…so I’ve got to pull it back off again.

That’s also a job for another day. Maybe tomorrow.

Oh, another consideration – before I do anything remotely final with the exhaust hangars, I probably need to drag out the lower cowl and at least hold it in place, to get an idea of clearance between the pipes and the cowl. That’s going to be fun, since I’ve sort of been using the finish kit crate as a table…actually the shop in general is in dire need of some cleanup and organization. Maybe that needs to be tomorrow’s job, instead of more building stuff.

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