Time: 2.5 hrs
The aluminum I ordered for the TCU bracket and to build the shelf for the avionics came a few days ago so I decided to make the mount for the TCU. This was pretty straight forward, just had to cut the channel to size and drill a few holes for some screws. I’ll be mounting the TCU on the cabin side of the firewall so I needed to find a suitable place close to where the cabling passes through the firewall. I wanted to keep the ambient pressure sensor mounted on the engine side since in my opinion that would be the best place to read the pressure that the engine is experiencing, not in the cabin. However the Rotax provided cabling between the TCU and ambient pressure sensor is very short, maybe a foot long and the Rotax manual says to mount it in the cabin so I guess I’ll mount it in the cabin near the TCU.
I’ve also been thinking a bit about how to connect the two alternators (well one alternator and one generator) into the system. TAF seems to connect them both to the main bus, but from all that I’ve seen this isn’t a good idea. You’re not really going to get increased capacity from the second alternator like you think. One alternator, the one with the higher voltage output will be the one that gets taxed the most. The second one won’t really kick in until the first one has pretty much maxed out. Typically you want to run them at 80% capacity so that you don’t damage them. So to see any real added capacity you’re having to run one of the alternators very hard. My idea was to maybe split the main bus into two buses. One with most of the equipment and the other with the main fuel pump and a few other high load items, like the pitot heat. The tricky part is how to keep them isolated, but also allow some fail over should one of the alternators fail. I did end up working out a pretty nice arrangement using 2 SPDT relays which I may end up implementing. But I’m wondering if all the equipment is really going to draw more then 40A. Once I get the panel built and the other avionics installed I can do a test using an ammeter to see what the current draw will be, then add maybe 10A for comm transmits. If it’s pushing 50A then I move some things around and implement the dual relays arrangement.
I was going to mount it a bit lower on the left side, but it looks like the rivets would hit the reinforcing channel on the engine side of the firewall so I had to scout around for another place. Amazingly it fits really well on either side o the firewall support channel.
Looks like it will all fit nicely. Note the ambient pressure sensor mounted on the right. I’m glad I didn’t put the front top skin on yet… this would have been a pain to mount with the skin on. So now I’ll disassemble this and spray it with some etch primer and then some grey paint.