MOD A modern electrical power design for the Carbon Cub EX-3/FX-3

[ve6yeq]

With all the recent discussions about how the default electrical power configuration for EX-3/FX-3 with "IFR" panels is not living up to expectations, I am thinking through a design modernization that takes advantage of recent building blocks. I am posting my current design concept here to see if anyone has any comments on what I have missed or can improve on.

My long-term goal is to use my 2024 Carbon Cub EX-3 to explore Northern Canada, Greenland, and Iceland. This will require some long legs with >2h from the nearest possible diversion location, which will require both additional fuel (belly fuel pod and/or ferry tank) and upgrades to the electrical system.

My current research has led me to a design concept that uses a single critical systems backup battery (EarthX EBBS), a dual generator (from Monkworx), and an "accessory" deep-cycle battery (e.g., a "powerbank").

This design concept is inspired by RVs, and Mike Patey's "Scrappy" with an "accessory" battery to both take advantage of deep-cycle, high-capacity batteries and enable use of Starlink, USB-C device charging, etc., when not in flight, without draining the main CCA-optimized starter battery. The dual generators (2 x 30 Amps) will provide redundancy and extra power for future updates, such as Starlink, XeVision LED lighting, etc. The unified "critical systems" bus is powered by an EarthX EBBS or IBBS 6 Ah to provide backup power for precautionary landings without the complexity and maintenance (e.g. short lifetime of AGM battery) of the existing IBBS and manual AGM ignition backup.

The overall system is not that different from the default Carbon Cub EX-3 w/ "IFR" panel wiring harness, so I hope it will be more of a reconfiguration than a complete replacement of the current wiring harness.

 

[Cactus Charlie]

Some random, and possibly conflicting, thoughts as I wake up.

Considering your mission I think I'd like to see COM and IFR approach capability on the critical bus. They can always be shed if not needed.

I don't see the GAD 29 which must be present to convert GNC ARINC buses to CAN.

I don't think G5 needs to be on Critical bus. Its backup battery should last longer than any other battery in the system and it's another bit of wiring that could take critical bus down.

I like to see isolation between the two generators. perhaps one feeds critical bus and the other main bus. One generator should be able to supply useful power if the other has its output shorted.

With the problems I have seen with GSU 25 I'd like to have two and at least one of them should be the new -55 version.

 

[ve6yeq]

In my thinking, the addition of an accessory battery that can optionally supply the main bus almost eliminates the need for a critical system battery. However, as the accessory battery might be a removable "power bank", I think that a critical systems battery is still required as a regulatory requirement. The critical systems battery with the automatic fail-over controller contained in a IBBS or EBBS should remove some of the startle effect from a main bus issue and provide power while trouble shooting.

Thinking through the possible failure scenarios around the avionics bus, the only reason to have a separate avionics bus is to provide isolation from voltage drops during starting for avionics that do not have a secondary source of power. I have seen buck-boost dc-dc converters designed to boost voltage drops during starting to remove the need for manual switching but all that would allow for is the GNC-355A to GPS lock before engine start.

The main bus has three power sources - the two generators and power from the accessory battery. Each generator is completely independent with separate regulators, breakers, wiring to main bus, other than they are both powered by the same engine. The accessory battery can easily power the main bus, it just does not have enough current to power the starter. So when the main starter battery fails (e.g. BMS disconnects), the generators can power the main bus (no source of field required) except in the case of landing where the engine RPM will not be high enough. For that, an optional direct connection between the main bus and the accessory battery will be required for landing lights, etc..

From a ADAHRS standpoint, the current GSU-25C (I should check the hardware revision that I have) provides the full set including electronic compass and OAT and the G5 provides backup ADAHRS source (as well as backup autopilot) just without electronic compass, OAT and AOA. A magnetic compass does not really work up north and is not used at all as northern Canadian airspace is all true north not magnetic north and the Canadians are leading an effort to do away with magnetic direction completely in aviation.

Finally, I am including the GAD-29 within the navigation portion of the GNC-355 as it is only used with the navigator functionality and currently shares the same power source and circuit breaker as the navigation portion of the GNC. I will add up all the current requirements on the critical systems bus so maybe the GNC navigation portion could be connected to the critical bus not the avionics bus.

 

[Cactus Charlie]

The main bus has three power sources - the two generators and power from the accessory battery. Each generator is completely independent with separate regulators, breakers, wiring to main bus, other than they are both powered by the same engine.

A short between main bus and airframe may kill all of them. While that is improbable it should perhaps be considered in the FMEA.

Finally, I am including the GAD-29 within the navigation portion of the GNC-355 as it is only used with the navigator functionality and currently shares the same power source and circuit breaker as the navigation portion of the GNC.

I used separate breakers for my GAD 29 and GPS 175. The GPS 175 (and I assume the GNC 355) has RS-232 bus in and out that are independent of the GAD 29. GPS 175 RS-232 out feeds both the GDU and the G5 in my FX-3.

From a ADAHRS standpoint, the current GSU-25C (I should check the hardware revision that I have) provides the full set including electronic compass and OAT and the G5 provides backup ADAHRS source (as well as backup autopilot) just without electronic compass, OAT and AOA.

One advantage of 2 GSU is that the G3X Touch design allows manual selection of which GSU drives the PFD. The G5 cannot be manually selected as the PFD source. The G5 will drive the PFD if G3X considers GSU data to be bad but it takes about 20 seconds for that reversion to happen. I have told Garmin that I think this is far too long but no change yet. Reversion of attitude is independent of reversion for air data so the PFD sources can be split between the GSU and the G5.