A/P servo wire cleanup, load testing
I took a few minutes to improve the stability of the wire exits from the connector shells on the autopilot pitch servo and the magnetometer. The wires were a little loose in the shell exits so I added some silicone tape to make them a nice snug fit. I sure wish I had allowed about another inch of clearance behind the magnetometer for the connector, but it will be fine. It doesn’t rub. As you can see, I’m using a bunch of stick-on cable tie anchors. I’ll keep a close eye on them to see how they hold up. The plan for when they let go is to put a dab of “shoe-goo” on them, and they’ll be good to go. I actually already did this on some of the ones that are a bit harder to get to.
I also did some electrical load testing and am prepared to post my results. Builders (including myself) like nothing more than real measurements as opposed to calculations.
First up, I’ll report my EFIS AUX battery test. This battery is a 5Ah gel cell battery mounted next to the AHRS that is wired to an aux switch position on my EFIS power switch. It drives DU1,2,3, the AHRS and magnetometer when selected (DU 3 can be disconnected separately for max conservation).
I started with the battery at 13.2V with its charging circuit disconnected. I powered up only all of the EFIS equipment as described above. The battery status at runtime is as follows:
11.6V @ 35min.
11.5V @ 40min.
11.2V @ 47min.
9.2V @ 55min. (began rebooting. Voltage too low)
I think that is very cool data. It will run even longer than that if the charging circuit is left connected to the main battery. So, worse case of electrical failure, provided that the EFIS, AHRS, and magnetometer are still ok, we have at least 45 minutes of completely disconnected power for attitude, air data, direction, and even navigation with the internal GPS in DU2. Even if the aux battery isn’t fully charged for some reason, there is still plenty of time to get on the ground. I’m very happy with this setup.
Next up are my load results for all of my panel equipment. I have a digital variable power supply that can put out 20A that is connected exactly how the alternator(s) will be. This allows me to select a voltage input into the aircraft electrical system and read the current output on the power supply. This is fun for measuring real load of stuff in its installed state with all the busses, wires, and switches in between. Not to mention minimum operational voltages.
I set the power supply to 13.4V and left the main battery contactor open (main batt disconnected). Then I turned on everything that is currently installed in the panel. The whole panel draws 8.3A (with the A/P active). Then I began turning off equipment and noting the current difference as follows:
TruTrak A/P with pitch servo on 1.3A
GTX330 Transponder 1A
SL40 .4A
PMA8000B Audio Panel .3A
GNS430 GPS1/Nav/Com 2A
GRT EFIS DU3 1A
GRT EFIS DU1,2,AHRS 2.2A
GRT EIS .1A
This is all critical stuff to know and have tested before the wheels leave the ground. It is neat to see all of the hard work in electrical design and calculation come together to match the calculated design within a few tenths of an amp.
