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3 Axis Autopilot and Trim

Everwild

Well Known Member
Working on the empennage and researching approach to trim options. Any insight would be very much appreciated!

I'm leaning toward Garmin 3 axis autopilot with yaw dampening. My understanding is you use aileron, pitch and rudder trim to remove any unbalanced control forces.

Once balanced, the autopilot servos take over and do the work of flying the plane and take the wag out of the tail if you opt for yaw dampening. Please correct me if I'm wrong but as I understand it, autopilot servos are NOT designed to adjust for an out of trim condition and the system will squawk at you.

Which leads me to my main question. Can the Garmin autopilot control all three axis of trim AND three axis of autopilot? If that's the case I would assume I would need three electric trim motors and all three autopilot servos?

When I looked up some specs on the GSA28 interface it shows two inputs for controlling trim. Not three. https://support.garmin.com/en-US/?faq=JgjMM09KZw3qmcMthIHNw9

Can the Garmin system in fact control all three axis of trim? Or does the pilot control the rudder trim manually? Options being the pilot meat servo on the pedals, a spring yaw system, or electric trim system option.)

Thanks in advance if there are any Garmin experts out there who can provide some insight into the best way to set up a three axis system.
 
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Garmin

I have the garmin g3x T system in my -10. I installed the yaw damper and aping bias rudder trim. The system easily auto trims the pitch and roll. I modified the spring bias to be controlled with a linear actuator, however, it is not wired for auto trim. I am fairly certain that had I wired it, it would auto trim the rudder as well.

That said, I find that I really do not use the rudder trim that much. I found a setting that is in the ballpark, and that is where it stays. The yaw damper works exceptionally well at stopping the tail wag. Not sure if the gfc500, though…
 
I have the yaw dampener installed and it’s works good. You get spoiled with the Garmin 3 axis system installed. I also installed The Aerosport rudder bias trim. It does two things. One, when the AP is off (so is the yaw dampener) you can set the bias trim. Once you find the sweat spot with the bias trim you rarely need to mess with it after that. The other thing is your rudder doesn’t get whipped from the wind or prop wash. There is no need for a separate electric rudder trim. The yaw dampener hold the bubble center when activated. The Aerosport trim takes care of the rest when the AP is off and can save your rudder from damage.
 
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3 Axis Autotrim

Working on the empennage and researching approach to trim options. Any insight would be very much appreciated!

I'm leaning toward the Garmin GFC500 3 axis autopilot with yaw dampening. My understanding is you use aileron, pitch and rudder trim to remove any unbalanced control forces.

Once balanced, the autopilot servos take over and do the work of flying the plane and take the wag out of the tail if you opt for yaw dampening. Please correct me if I'm wrong but as I understand it, autopilot servos are NOT designed to adjust for an out of trim condition and the system will squawk at you.

Which leads me to my main question. Can the Garmin autopilot control all three axis of trim AND three axis of autopilot? If that's the case I would assume I would need three electric trim motors and all three autopilot servos?

When I looked up some specs on the GSA28 interface it shows two inputs for controlling trim. Not three. https://support.garmin.com/en-US/?faq=JgjMM09KZw3qmcMthIHNw9

Can the Garmin system in fact control all three axis of trim? Or does the pilot control the rudder trim manually? Options being the pilot meat servo on the pedals, a spring yaw system, or electric trim system option.)

Thanks in advance if there are any Garmin experts out there who can provide some insight into the best way to set up a three axis system.

You are able to drive autotrim through a GSA 28 yaw damper just fine. Page 27-10 of the G3X Touch Installation Manual provides an example of how the GSA 28 would be wired to the yaw trim motor.

You are correct in that the GSA 28 yaw damper servo should not be relied upon to correct for an out of trim condition. The yaw damper dampens the yaw axis movements and makes the ride more comfortable, especially for those in the back seat in turbulent air, and while the yaw damper assists the yaw trim in keeping the ball centered, the yaw trim should do the heavy lifting.

You will need to ensure that the trim motor falls within the spec's of the GSA 28 trim output, namely drawing 1A or less. A trim motor that draws more than 1A will ultimately damage the GSA 28.

Please feel free to reach out to us with any other questions that may come up. You can reach us directly at [email protected], or 1-866-854-8433.

Thanks,

Justin
 
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The GSA28 servos can handle trim in all 3 axes. However the 10 doesn’t really need rudder trim and certainly not a yaw damper. Roll trim is worth doing and elevator trim a must.

I recently upgraded from the TruTrak GX Pilot to the GMC507 with the Garmin GSA28 servos. Very happy with it, super on coupled approaches.
 
Really?

“Certainly not a yaw damper”…it may not be NEEDED but is certainly a nice option, especially for the back seat passengers…
 
My personal opinion is a yaw damper is not needed. Others will argue the point but after 420 hrs I don’t see the need. Some have a Dutch roll but my 10 doesn’t exhibit it. Rudder trim, on the other hand is something to seriously consider. It’s not a must have but I can fly feet on the floor in cruise with the Aerosport spring bias system so I’m a big fan. Before I had to rest a foot on the right pedal— that gets old on a long X-country but it isn’t a show stopper. YMMV….
 
Right rudder needed (Very little of the time)

Although not much time in the 10 have about 200 hours in the 14 and they fly very similar. I have a yaw damper and that's pretty much it on the rudder. For the 7 minutes during the take-off to level flight phase I use right rudder (Really not much after I leave the runway) and it provides some exercise (which I need for my right leg) and after that zip for hours at a time. Even high bank angles don't need much rudder. (I'm a sailplane pilot so maybe I got use to using the rudder) Building a 10 and would not think of spending a minute adding anything more. The trim mods I've seen on the 10 and 14 rudder seem excessive for the time it's needed. In the end we are experimental builders and it's everyone's right to build what they want.
 
Although not much time in the 10 have about 200 hours in the 14 and they fly very similar. I have a yaw damper and that's pretty much it on the rudder. For the 7 minutes during the take-off to level flight phase I use right rudder (Really not much after I leave the runway) and it provides some exercise (which I need for my right leg) and after that zip for hours at a time. Even high bank angles don't need much rudder. (I'm a sailplane pilot so maybe I got use to using the rudder) Building a 10 and would not think of spending a minute adding anything more. The trim mods I've seen on the 10 and 14 rudder seem excessive for the time it's needed. In the end we are experimental builders and it's everyone's right to build what they want.

Same here. Simple wedge on the rudder and no damper. The 10 requires very little rudder when climbing at 130 Kts and have never noticed any tail wagging even in turbulence. Possible it's there, but have never felt it.

Larry
 
How often do you ride in the back?...:D

Me— never. But I’ve had plenty of pax in the back in all kinds of conditions and no one has ever mentioned it. My kids, in particular, would have let it be known without question and they were asked about it. Maybe my 10 is just lucky. All I’m saying is you can’t paint every 10’s performance with the same absolute brush—there’s just too many variables.
 
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Actually

Sorry, the ride in the back comment was supposed to be funny…most of us never ride in the back…

I find that the yaw damper does make a difference in turbulence; not so much when it’s smooth. I am glad I installed it…
 
Yaw damper

Yaw damper + 1 (Good for passengers and back seat guests I’m sure)

Stuff hanging off the rudder, why bother?
 
Be aware

Be aware that if you intend on using a servo to activate the spring bias system, you will need to find the neutral point by changing the springs…trial and error. The aerosport capstan allows for basically infinite centering adjustment. Using a servo will create a finite range of motion (unless you use a capstan servo). It will work out fine but you will need to find the correct springs to bring the loaded rudder center into the range of the servo motion…
 
Be aware that if you intend on using a servo to activate the spring bias system, you will need to find the neutral point by changing the springs…trial and error. The aerosport capstan allows for basically infinite centering adjustment. Using a servo will create a finite range of motion (unless you use a capstan servo). It will work out fine but you will need to find the correct springs to bring the loaded rudder center into the range of the servo motion…

Good point. It seems there is some flexibility in adjusting the length of the cables that attach the springs to the rudder pedal arms to find a good neutral starting point?

A small motor to drive the capstan is an interesting idea. Wonder if anyone has tried it?

Garmin mentioned the main limitation is you want to keep the draw under 1 amp or less or the yaw servo could be damaged controlling the trim servo/motor.

How much effort is needed to turn the knob on the Aerosport capstan? Anyone have a guess?
 
Aerosport trim rudder authority

Another quick question for folks who have used the Aerosport trim system. Does the Aerosport rudder trim have enough authority to center the ball on climb? Or is it mainly used for centering the ball in cruise ?

Not that I would mind using right rudder on climb but with a considerable investment in a 3 axis autopilot, you would want the system to have enough authority to trim yaw in climb as well.

This might be a good counter argument for a rudder mounted trim tab instead?
 
Good point. It seems there is some flexibility in adjusting the length of the cables that attach the springs to the rudder pedal arms to find a good neutral starting point?

A small motor to drive the capstan is an interesting idea. Wonder if anyone has tried it?

Garmin mentioned the main limitation is you want to keep the draw under 1 amp or less or the yaw servo could be damaged controlling the trim servo/motor.

How much effort is needed to turn the knob on the Aerosport capstan? Anyone have a guess?

My system uses a linear actuator to move a bell crank which the springs attach to. There is about 2" of throw at the end of the bell crank. I needed to play with changing K, the spring constant, to get the loaded rudder neutral to correspond with the center of the actuator range of movement. It works well but definitely took some trial and error to refine.

I also think that a stepper motor/capstan system may be a better way to go...

That said, now that the rudder trim system is centered and working, I really do not move the rudder trim much at all during flight...certainly less than I would have thought. Not sure the effort to build the system was really worth it.
 
Although not much time in the 10 have about 200 hours in the 14 and they fly very similar. I have a yaw damper and that's pretty much it on the rudder. For the 7 minutes during the take-off to level flight phase I use right rudder (Really not much after I leave the runway) and it provides some exercise (which I need for my right leg) and after that zip for hours at a time. Even high bank angles don't need much rudder. (I'm a sailplane pilot so maybe I got use to using the rudder) Building a 10 and would not think of spending a minute adding anything more. The trim mods I've seen on the 10 and 14 rudder seem excessive for the time it's needed. In the end we are experimental builders and it's everyone's right to build what they want.
I now have 1300 hours or so in the ‘10, no rudder trim or yaw damper.

Dampers weren’t available when I built. Never missed it even in turbulence but of course, no back seat experience. Like so many mods; if you did it, you like it, if you didn’t do it you’re fine as-is.

I saw a relatively simple DIY spring trim system in a ‘10 I copied a lot of stuff from. I built it then never finished setting it up because I didn’t need it. Some right rudder during climb then feet on floor. But I too come from sailplanes, then a Maule so I miss using the rudder a lot. Just doesn’t need much and is very well behaved in stock condition.

Now if I can just figure out who put a dent in my rudder 10’ off the ground!!?? I know it wasn’t that deer I hit the other night…
 
I am really unfamiliar with GA avionics and what the plan is 5 years from now.. for now I'm working on the empennage, just finished Horizontal Stab and building the elevators now... Is this something that I need to decide now so that I can install whatever is required for an autopilot?

I do not plan to do rudder trim or yaw damp.. I was planning to do that spring system behind the pedals.
 
G3X Logging data

Trying to gather some analytical data on the "ball" position in flight and thought it would be captured in the 118 parameters the G3X captures on the SD card. Can't seem to find it. Any help would be appreciated. I know rudder input dependent but feet off the pedal's comparisons would tell me something, I think.
 
Trying to gather some analytical data on the "ball" position in flight and thought it would be captured in the 118 parameters the G3X captures on the SD card. Can't seem to find it.

Look for the "Lateral Acceleration" column. I've found that 1 ball-width displacement of the slip/skid ball equates to about 1/8 of a G.
 
PIREP

I had a chance to fly in a -10 recently on a bumpy day that had the yaw servo. There was a noticeable difference in the amount of tail wag with it on vs off.

The builder also did a demo by putting in full rudder, then releasing. Without the yaw damper the tail oscillated a few times before they diminished to center. With yaw on, the tail immediately returned to center with no bobble.

Worth $885? Probably.
 
To the best of my knowledge all of the autopilots that work with a -10 connect the pitch servos to the push-pull tubes just behind the battery. Access while sitting in the baggage compartment is pretty good. It’s better with the fully built tailcone pre-attachment to the fuselage, but not that much better. Things change in 5 years, I’d wait.
Edit: this was meant to be a reply to post #20.
 
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RV-10 Garmin Yaw Damper

I installed a Garmin Yaw Damper in my RV-10. Mostly I wanted automatic rudder trim, as rudder trim varies with altitude and power settings in cruise situations. I really like the Yaw damper. It makes a noticeable difference in turbulence. Yaw Damper off there is a noticeable amount of swinging around and side to side upper torso motion. Yaw Damper on this is noticeably reduced with the turbulence acting more in the vertical axis. Last week I turned the Yaw Damper off for landing and a passenger pilot friend commented what a big difference it makes in ride quality. The automatic rudder trim is also excellent and I really enjoy the way it works.
 
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