Firewall Part 3 plus a Flame Test
This kit uses a sheet of galvanized steel as the firewall and is further covered by a heat barrier to insulate the cabin from the engine heat (and noise). The galvanized steel is good as a fire protectant (per FAR 23.1191), however, I have seen several other builders dismiss the included insulation as being rubbish. The provided material is constructed of a thin piece of foil on top of foam. In the photo sequence below that spanned maybe about 5 seconds, I performed a brief burn test of the insulation using a butane torch. The foil burns off nearly instantly. The foam backing also caught on fire but self-extinguished once I removed the heat.
I know this foam insulation is not designed the provide the level of protection the galvanized steel sheet provides; nonetheless, I was pretty disappointed with how easily the foam burned. I am ordering a different material to use instead, a Thermotec 14001 aluminized heat barrier. The Thermotec is made of woven silica with a flexible aluminized finish, and is rated to withstand 2000°F, which is about the temperature of a butane torch. I will repeat this test for comparison once I receive the new material.
Back to the firewall. I’m working in earnest to finish up preparations to the firewall as well as the canopy at the same time. Before I can mount the engine, I need the firewall completed, and I need the canopy installed, since the integrated cables for the parachute attach to a bracket on the firewall that shares a fastener with the engine mount. First up, I installed this 100A current shunt for the Garmin G3X. It was early April when I discovered that this little part isn’t included with the firewall kit but is called out on the installation drawing. A visit to Aircraft Spruce showed that the part was backordered until end of May. I just received the shunt last weekend. Here’s a link to the part on Aircraft Spruce.
In a previous post I showed off a custom heater, based on ideas I borrowed from an automotive design. It was working nicely, but then I got around to testing power draw. I was a bit surprised to find out the blower motor pulls over 20 amps at full speed. A typical car alternator can supply somewhere in the 50-100A range, so a 20A blower is no problem. But on this Rotax 915 engine, the generator that supplies power to the airframe maxes out at 420W, just under 30A. With all the other avionics and lighting, I just don’t have 20A to spare for a blower motor.
Trying to find a different blower proved to be difficult, as most DC blower motors made for cars don’t make their electrical characteristics easily available. After considerable time trying to find another blower, I decided to go back to the heater that Sling already chose for the TSi, a Sciroco Tenere Performa. Luckily, the Sciroco has a much smaller blower, maxing out at about 6.4A current draw at full speed. I will upgrade this heater from a fully manual system into the automatic system I set out to create originally.
Since I’m starting with the same heater used by Sling, I drilled the holes for the coolant lines in in the same firewall location. The holes need to be 1.25″ in diameter to accept AN631-14-20 grommets.
I am well into the design of the revised heater system. I was able to leverage what I learned from my Mark 1 design, and iterate the Mark 2 design more quickly. Initial fit checks look good. I am utilizing all of the existing mounting points for the TSi design so that this automatic system can be retrofit onto other Sling 4 / TSi’s.