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Trim via GSA28 not via motor
- Thread starter eld
- Start date
As far as I know you can't. You can't have one GSA28 serve multiple purposes (as a primary flight control autopilot servo AND a trim servo). You can't use a GSA28 installed as an A/P servo as a trim servo because that would require the servo be under constant load which isn't how they're meant to be used. To use a GSA28 as a trim servo requires the installation be done in a way that the servo doesn't directly experience flight loads (I.E. adjusting a jackscrew). To do what I think it is you're wanting, you need to install a separate electric trim such as a Ray Allen trim system, then you'd have the option to wire it though your pitch GSA28 (which I'd highly advise you do) which adds a lot of capability, it 1) allows your autopilot to automatically re-trim for speed changes to unload the pitch servo 2) adds an A/P disconnect function if you attempt to manually trim while the A/P is engaged 3) allows you to use your A/P disconnect switch (if installed) to stop a runaway trim situation regardless of the A/P being on or off.
There are plenty of certified installations where you can add a GSA28 for electric trim as part of an autopilot system, but all those are installed in a cable driven trim system, where a cable loop turns a jackscrew to adjust the trim tab, and the servo is just integrated into the existing cable loop with a bridle cable. In a system like this the servo is isolated from flight loads. I've never seen an RV with a cable driven pitch trim (not impossible just incredibly rare), 99% of them have a Ray Allen trim system, or the older ones with manual trim have the vernier cable.
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Me too. I’d be interested in your transistor design
Tanks for your replay,
I understand your comments; I implemented it myself in the way you described. I simply added a small electronic component to avoid putting all the power into the trim control switch when the autopilot isn't in use.
My question today, regarding this method of using a GSA28 in trim mode, concerns a friend's aircraft (not an RV) which has cable control (so the servo is isolated from flight loads).
I haven't found anything in the Garmin documentation about how to connect the trim control switch via the GSA28, without a motor behind the GSA.
I understand your comments; I implemented it myself in the way you described. I simply added a small electronic component to avoid putting all the power into the trim control switch when the autopilot isn't in use.
My question today, regarding this method of using a GSA28 in trim mode, concerns a friend's aircraft (not an RV) which has cable control (so the servo is isolated from flight loads).
I haven't found anything in the Garmin documentation about how to connect the trim control switch via the GSA28, without a motor behind the GSA.
You'll want to connect your trim switch to both the primary autopilot servo as well as the trim servo. There's a brief mention of this in section 16.1:
Note to RV builders: The type of installation being discussed here is not applicable to any RV - just amplifying that for clarity.
Note to RV builders: The type of installation being discussed here is not applicable to any RV - just amplifying that for clarity.
Does this mean the ray Allen servo is wired in parallel to the trim switch/relay and the GSA28 pitch servo? I've seen a steinair diagram showing only the connection to the GSA28
OK, thanks for the good page,
Garmin can add a drawing for more clarity for connection switchs to 2 servos
because in this mode, the switch no longer appears to be a bipolar inverter, but just a simple up and a down switch
No, you still need a DPDT switch here:
No, this has nothing to do with a DC trim motor. This is in reference to using a second GSA 28 with a capstan drive to actuate a cable-driven trim system, which has nothing to do with any RV application.
yes, but on garmin doc ?
