Thursday, December 29, 2016

Trim Wiring

My tail section came with a Ray Allen trim servo for pitch trim, continuing that I recently ordered another trim servo and roll trim bracket. I also recently ordered my stick grips with a 4 way high-hat and now it is time to start planning what wires I will need to put in place to control everything. I think I will be going with a G3X system (to include autopilot) as well as a VP-X electronic circuit breaker controller.

With this mix of electronics it is time to figure out what controls what and what else do I need from a trim and autopilot perspective. My first question was do I need a safety trim? In looking at the G3X system it looks like the preferred approach is to wire the trim servos through the GSA28 autopilot servos. While the VP-X system has safety trim built in, per their install manual, when you install GSA28s with a VP-X you configure the VP-X to disable the variable speed trim functionality. If you dig deeper into what the GSA28s are capable of everything makes a little more sense.

The Garmin servo provides a built-in interface to drive trim servos at no extra cost. When the autopilot is off, the servo provides speed scheduling for the manual trim commands. When the autopilot is on, the servo adjusts to constantly keep the aircraft trimmed. 

What does all of this mean? When the power is removed from the servos, speed scheduling is disabled and the trim commands are passed directly through the servos to the trim motors. Auto trim is not functional when the autopilot is disengaged, and manual electric trim via the stick hat is never disabled, but if you attempt to manually trim the plane when the autopilot is engaged, the autopilot will disengage to honor your request.

It is always your job to trim the plane when the autopilot is disengaged regardless of whether the servos are powered on (and providing speed scheduling) or off and the trim motor drive is passed through without speed scheduling. In general, with these systems, your servos will always be powered, even when you are not using the autopilot.

That seems pretty straight forward and confirmed by the VP-X install manual:

So I started thinking more about that and I got a fairly uncomfortable feeling. Not that I don't trust Garmin, but I don't really trust anyone. I especially don't trust electronics that I did not assemble. If you look at the wiring diagram above you quickly realize that in the event of an autopilot power failure you loose control to your trim. On the RV-10 there is no manual trim wheels so you would be SOL. How can that be? Well... Reading more into the Garmin manual I found this:

In the event that power to the GSA 28 is removed, a fail-safe system connects the trim input switch directly to the trim motor. In this condition, the trim switch powers the trim motor directly and the motor runs at its full speed when the switch is pressed. The same condition also occurs if a trim switch and motor are connected to the GSA 28, but the trim control function is disabled.

Well that makes sense. The next question is how does one wire in the trim switches. I would prefer having both control sticks have full control. I am not sure why other than I like symmetry. In the GSA28 install section you will find a the following note:

So looks like I should be thinking about a GAD 27 as I select my avionics. It is a $500 dollar unit that appears to have a bunch of functionality that I do not need. I went back to the VP-X manual and it looks like there is a trim mixer in that device. So assuming I disable all of the VP-X trim safety systems perhaps I can use the VP-X to mix my trim and G3X to provide auto-trim and speed scheduling. Other than that I still have to check to see if the G3X system has any form of runaway protection.

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