HVAC System and Panel Progress

This is a long overdue update on my project, but I’ve been quite busy on a few different aspects.

First major update is that the custom HVAC system is about 90% complete. I’ve gone through so many iterations of the design over the last 3 years, but earlier this year, I finally locked down the design and committed to manufacturing the parts that would be installed in the plane.

I originally designed and printed the parts using PETG, which is probably my favorite type of 3D filament. However, recent events at work got me thinking about fire safety. Although I’m not required to comply with FAR 23.853, I decided to design to it, using materials that are flame retardant. PETG is typically not rated to be flame retardant, though there are some PETG variants with additives to improve its properties (eg Prusament PETG-V0).

I decided on using a polycarbonate blend with flame retardant additives, made by Polymaker. This filament doesn’t come with FAA test data (very few do), but instead has been tested/rated to UL 94V-0, which has similar criteria.  In an informal test, I verified this filament does not sustain a flame, and furthermore it develops a crusty carbony outer layer when it burns, which is how it prevents a flame from being sustained.  Other plastics typically melt into a gooey glob or start dripping, and those gooey drips can be quite flammable, helping to spread a fire.

PC is a strong and high-temperature tolerant plastic, but is notoriously hard to print because it warps easily. I had to make some upgrades to my printer, including a heated build chamber, to successfully print the large parts that make up the HVAC system design.  I’m very happy with the results, the PC printed parts are noticeably stronger than anything I’ve made with PETG.

Air distribution section
Intake plenum
Fully assembled unit
Fit check

I’m awaiting a printed circuit board for the controller for this HVAC system and then I can complete its install in the airplane.

The other major part I’ve been working on is the instrument panel and all the electrical interconnects. I gave a lot of thought to the electrical system architecture, considering failure modes and how to mitigate them, therefore choosing to have essential and non-essential buses (for load-shed in an emergency), simplified controls and a few bits of automation to reduce the number of switches needed. I’m glad I’ve already flown in the Sling TSi in Torrance CA, it gave me a good appreciation of the operational flow from startup to shutdown. I know I can automate even more, but all these custom parts take time to design and build, and I’m ready to get this airplane flying already!

To facilitate easier installation and maintenance of the panel, I routed all signals and power between the panel and the rest of the aircraft into two large bulkhead connectors. I chose to use TE Circular Plastic Connectors (CPC) because they use the same style of machined pins as the other avionics components, so I didn’t need any new wire crimping tools. There are over 100 power or signal connections spread over these two connectors.

It’s wild to see how complicated the wiring can become even for a light single engine airplane.  There are still many legacy electrical standards in use like ARINC 429 & RS232, even with the modern G3X avionics suite.  CAN and a proprietary form of Ethernet called High Speed Data Bus (HSDB) are also used, but sparingly.  For example, HSDB is used in a single point-to-point connection between the GTN650 navigator and the GTX45R transponder, which seems like overkill. Pretty much any component that is connected via CAN is also connected via RS232 or A429 as a backup – which makes for a lot of cables!

Wires and cables galore
Trusty Brady label machine to make sure all wires are identified
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