Learning about autopilot

I've tired researching, but I cant find the answer.

How does a Garmin servo move the elevator? Does it connect to the trim or to the actual elevator control?

It seems to connect to the elevator control, but if it does this, how does an out of trim airplane affect it? it seems like it would cause undue stress on the servo if you don't have it trimmed well.

G3X can control the trim motor (separately). Autopilot uses both elevator and aileron servos to turn/pitch but can also trim the elevator or call for pilot trim input.

Yes, the pitch servo is connected to the elevator control. Additionally it can drive the pitch trim servo automatically.
If it is not connected to the trim servo, you would get a message on the pfd to manually trim the elevator. If not and the force on the pitch servo becomes to high, the internal clutch would open. This would reduce the performance of the ap system and could lead to an oscillation in the pitch channel.
We've installed the Servo with the kit from Garmin and it is connected to the trim servo. The system works very well.

So does this mean manual trim is not an option, I need electric trim?

Robin8er said:

So does this mean manual trim is not an option, I need electric trim?

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Re-read the post before yours. If you have manual trim, then the autopilot will not trim. You have to see the "trim cues" and manually trim. If you don't and the forces get too large the autopilot will either disengage or have its clutch slip.

Thanks. Where can I read up on auto pilots? it seems relatively simple, but I want to read up on it if I can.

Go to <trioavionics.com>, select "trio pro", then on that page click on "video" (highlighted in yellow) in the top left corner. You can watch several videos on autopilot operation. I think there's good reading on the TruTrak web site, too.

Garmin Servos 101

Robin8er said:

I've tired researching, but I cant find the answer.

How does a Garmin servo move the elevator? Does it connect to the trim or to the actual elevator control?

It seems to connect to the elevator control, but if it does this, how does an out of trim airplane affect it? it seems like it would cause undue stress on the servo if you don't have it trimmed well.

Click to expand...

Hello Robin8er,

Thank you for your interest in Garmin servos. As usual, most of the responses posted above are great, but a few corrections are needed. The GSA 28 is the most advanced autopilot servo on the market, so it isn't surprising that many may not fully understand it.

The servos do indeed connect to the control surfaces (e.g. elevator). The autopilot may be used in aircraft that only have manual trim and in this case "Trim Up" and "Trim Dn" messages pop up on the PFD when the torque monitoring servos detect an out-of-trim condition. It is fairly tolerant of out-of-trim conditions and if the pilot is doing a pretty good job of re-trimming the aircraft when transitioning from climb to cruise, and cruise to descent, it will be rare to see these messages. In other words, it doesn't nag much.

For aircraft where the trim motors are connected to the GSA 28 servos, the servos correct for any out-of-trim condition by driving the trim motors as necessary when the autopilot is engaged. This works really well because while the servos can always detect an out of trim aircraft when the autopilot is engaged and they are flying the plane, the pilot can't feel this and is just guessing and reacting to messages provided by the autopilot. This has the added advantage of always having the plane in trim when the autopilot is engaged so there is no slight jerk on the stick when the pilot disengages the autopilot and the aircraft responds to any slight out-of-trim condition pulling on one side.

We routinely test fly our software in G3X equipped homebuilts with both manual trim and auto-trim. The autopilot works really well in both installations and I can't say that I mind the slight extra workload of flying the manual trim aircraft with the autopilot engaged.

There is speculation above about the effect of large out-of-trim conditions that the autopilot will disengage at some point, or a clutch will open, or a clutch will slip. None of these are true.

The GSA 28 has no mechanical slip clutch that can need adjustment or wear out. It also has no stepper motor that can lose steps and jerk the controls when over-stressed. The GSA 28 does have a simple no-slip engagement "clutch" that couples the planetary gear box to the brushless DC motor.

One of the many things the GSA 28 does really well is monitor and control output torque. When configuring the GSA 28 autopilot servos, the user establishes a maximum torque setting for manual over-ride. If the autopilot is engaged and the pilot needs to quickly over-ride the servos to avoid something like a bird, the servos will never push back any more than this maximum torque setting. This is done through electronic torque limiting by a very fast processor, not a mechanical slip clutch.

Getting back to your original question, if a manually trimmed aircraft gets significantly out of trim because the pilot was ignoring the "needs trim" messages on the PFD, the servo simply pulls/pushes as hard as it is allowed to based on the max torque setting, and it will do that as long as is necessary.

Let us know if you have additional questions. There is a great deal of information about autopilot installation and operation in our installation manual and pilot guide.

Thanks,
Steve