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onemilehigh
12-06-2016, 01:19 PM
I am interested in having both a manual trim wheel and electric trim in a G3X system. I assume some sort of trim servo with a clutch is needed to pull this off?

Could a GSA 28 servo be dedicated for elevator trim since it has a built in clutch?

Boomer506
12-07-2016, 12:54 PM
I am wiring my GSA28 pitch and roll servos now and have done some reading on the interface with elevator trim.

One of the great things about these autopilots is that they will drive your trim tab servos to assist with control forces. So IMHO you are going to want to include a normal trim system that your GSA28 can drive while autopilot is engaged. One of the side benefits is that when you disengage AP, the airplane is already trimmed up so you don't have a big un-commanded pitch or roll at disconnect. Also, if you somehow were to get your GSA28 servos to provide a continuous elevator force to provide a trim like input, your servos would be providing a torque output all the time, which may lead to early wear and failure.

I've got a regular MAC trim servo for pitch and roll that my GSA28s drive, and when AP is disconnected, I can still trim with my stick mounted coolie hat. The regular trim system is not that expensive, and when coupled with your GSA28, really takes advantage of the full capabilities of your AP.

onemilehigh
12-07-2016, 02:04 PM
Thanks for the feedback Boomer, but what I am really wanting to know is it possible to use the GSA 28 as the trim servo. This would be in addition to GSA 28 servos for elevator and aileron.....and maybe even yaw.

The primary purpose of wanting the GSA 28 in lieu of say a Ray Allen, is to take advantage of the internal clutch. This way I can have a manual trim wheel and electric trim.

Pat Hatch
12-07-2016, 02:46 PM
Thanks for the feedback Boomer, but what I am really wanting to know is it possible to use the GSA 28 as the trim servo. This would be in addition to GSA 28 servos for elevator and aileron.....and maybe even yaw.

The primary purpose of wanting the GSA 28 in lieu of say a Ray Allen, is to take advantage of the internal clutch. This way I can have a manual trim wheel and electric trim.

I think I understand what you're trying to do. Why not just have the manual trim on a second trim tab on the other elevator? This sounds really unconventional, but I'm assuming you're trying to do something like have manual trim for aerobatics and then electric trim for autotrim and autopilot operation. I've never heard of anyone doing this but that doesn't mean it hasn't been done or that it can't be done!

I can't imagine a scenario under which the GSA 28 can be used as a trim servo. In any event, doing it my way would certainly be a lot less weight and less complex than having a third GSA 28--not to mention the additional cost of the servo.

rvsxer
12-08-2016, 09:09 AM
I think I understand what you're trying to do. Why not just have the manual trim on a second trim tab on the other elevator? This sounds really unconventional, but I'm assuming you're trying to do something like have manual trim for aerobatics and then electric trim for autotrim and autopilot operation. I've never heard of anyone doing this but that doesn't mean it hasn't been done or that it can't be done!

The RV-6 demonstrator (Ol' Blue?) had that setup in the early 90's because back then the Ray Allen servos had no speed control and were hard to use. I have heard it's common to use a lot of nose-down trim during an aerobatic sequence so you might even use a trim lever like the RV-4. A friend of mine told me when he entered the box in his Pitts the throttle and trim levers went full forward.