GRT autopilot gain sanity check (8A)

[skirting_virga]

I had a long XC flight yesterday to fiddle with the autopilot. The builder had roll gain set near the default, 100%, which I found was ok in smooth air but exacerbated even the smallest amount of turbulence.


Starting with the servo response test, I had to work roll gain down to 7%, and roll gain damping very high (600-1100%) to get the system to behave. The result seems to be that it is able to intercept and track a course in even bumpy afternoon thermals. It could be a little more proactive, but it's honestly pretty good compared to before.

Does this make any sense? I know the 8 is lively in roll but I wasn't expecting to have to turn it down so low. Does anyone have GRT gain settings from a 7 or 8 to share?

Pitch is acceptable for now with lowish settings and I'm not motivated to further optimize it yet.

 

[clam]

GRT equipment in RV-8. My roll gain is set at 84% and damping at 100%. What I have found to be more influential, is the XTK gain. My AP roll performance was satisfactory in all modes, except LNAV. When tracking an LNAV track, especially on an approach, the roll (course) correction was continuous large amplitude corrections, near 25 deg bank angle corrections all the way down final. I don't recall exactly what the XTK gain is set now, but it is pretty low. It is not 7% kinda low, but I believe down in the low 80/high 70 range.

There are a few other super smart GRT guys on here that will likely add more color and accuracy for you.

I'll add this little bit of experience too, although probably not related based on your low gain settings ... but, I had a pax who fell asleep on a long xctry. I believe that during a course change, his leg had fallen against the stick keeping lateral pressure on it. The roll servo was trying to program in the turn, and torque built up enough to shear the screw. I never got a TRIM alert that I recall on that event. But the roll response then acted unsatisfactorily, and after some troubleshooting, I found the shear screw sheared. Replaced and all was well.

 

[rv8ch]

I don't recall my numbers but they are not far off the recommendations from GRT for RVs - I know the main guy there has a RV-6.

One thing I have seen is that the "180 degree get out of weather emergency button" didn't behave like I expected, and I'd suggest that you test this in good weather with a safety pilot before relying on this feature in an inadvertent VFR into IMC scenario.

The recommendation from GRT - which I have not yet tested - is:

"You can limit the bank angle for all autopilot operations in A/P Maintenance. As for the loss of altitude, the EFIS will use the normal altitude hold response to react to an altitude change caused by a turn. You could try increasing the Altitude Hold gain in General Setup. If it's 100%, try something like 150% or 200% to see what the effect is."

 

[skirting_virga]

The roll servo was trying to program in the turn, and torque built up enough to shear the screw. I never got a TRIM alert that I recall on that event. But the roll response then acted unsatisfactorily, and after some troubleshooting, I found the shear screw sheared. Replaced and all was well.

This is insightful - no idea if it applies to my situation but it raises the idea that I should inspect the servo.

I will note that in the servo response test, the external control loop isn't involved. I did tweak xtk and heading gains a little but most of my time was spent with the roll gain and damping.

My bank angle limit is 20 degrees at the moment

 

[skirting_virga]

Does anyone else have any insight as to why my plane seems to require 7% roll gain and high roll gain damping to eliminate roll oscillations (even in servo response test mode)? Something must be off...

 

[BH1166]

Been behind GRT AP for 800 hours in 6 yrs. All my settings are 100 or 200. I hand fly in really rough air as I don’t want to shear the pin grabbing stick in a tense moment. Flew today, pretty calm air, AP via heading and enav rock solid. Form up a question with data/settings and submit to [email protected] they see it all. I fly 6a…different but I got no settings like yours.

 

[NelsonME58]

I’m hoping an RV-9A owner with an installed GRT A/P may be monitoring this thread. I’ve attached photos of my pitch servo installation and was hoping someone could help me determine if it is correct. The install was completed by a maintenance shop hired by the previous owner. I suspect the rod end bearing installation on the servo control arm may be attached to the wrong side resulting in the angled installation. Any advice is greatly appreciated.

 

[skirting_virga]

I’m hoping an RV-9A owner with an installed GRT A/P may be monitoring this thread. I’ve attached photos of my pitch servo installation and was hoping someone could help me determine if it is correct. The install was completed by a maintenance shop hired by the previous owner. I suspect the rod end bearing installation on the servo control arm may be attached to the wrong side resulting in the angled installation. Any advice is greatly appreciated.

Great idea - can any 7/8/9 owners post pictures of their GRT roll servo install? I want to see where the pivot is on the bellcrank and the servo arm to figure out if mine is set up in a "typical" fashion or not. Given that the pitch servo does just fine, I'm thinking this could be the issue...

 

[rv8ch]

I’m hoping an RV-9A owner with an installed GRT A/P may be monitoring this thread. I’ve attached photos of my pitch servo installation and was hoping someone could help me determine if it is correct. The install was completed by a maintenance shop hired by the previous owner. I suspect the rod end bearing installation on the servo control arm may be attached to the wrong side resulting in the angled installation. Any advice is greatly appreciated.

I have the GRT servos in my RV-8, and I mainly used the Dynon instructions to install them. The Dynon instructions are very good, and the servos are very similar, physically. I also see they now have some videos.

dynon autopilot servo installation - Google Search

 

[skirting_virga]

This is a photo of my roll servo. The dynon instruction appears to specify 2". Mine is about 1-5/8". This might just be the problem.

 

[NelsonME58]

This is a photo of my roll servo. The dynon instruction appears to specify 2". Mine is about 1-5/8". This might just be the problem.

I’m experiencing too little torque in the roll axis even with the roll servo torque setting set to maximum of 15. I assume this is the “problem” you are experiencing as well? I am planning to move my servo arm attach point to the center position in order to increase torque. Haven’t measured the position of the rod end attached to the aileron bell crank yet. Here is a couple photos of my installation….

 

[skirting_virga]

I’m experiencing too little torque in the roll axis even with the roll servo torque setting set to maximum of 15. I assume this is the “problem” you are experiencing as well?

No, I'm having extreme sensitivity that required cranking down the roll servo gain to 7% before it would behave in turbulence. I think it has plenty of torque, but it oscillates in the servo response test mode at almost any gain, regardless of damping.

 

[AlphaCharlieBravo]

I’m hoping an RV-9A owner with an installed GRT A/P may be monitoring this thread. I’ve attached photos of my pitch servo installation and was hoping someone could help me determine if it is correct. The install was completed by a maintenance shop hired by the previous owner. I suspect the rod end bearing installation on the servo control arm may be attached to the wrong side resulting in the angled installation. Any advice is greatly appreciated.

That pitch servo is mounted 100% incorrectly!! The pitch servo mounts on the left side of the mounting bracket and the rod end bearing attaches to the right side of the bell crank. I am installing mine as we speak. Like someone suggested above. Go to the Dynon site and look at the install pictures. They are depicted in CAD.

A

 

[NelsonME58]

That pitch servo is mounted 100% incorrectly!! The pitch servo mounts on the left side of the mounting bracket and the rod end bearing attaches to the right side of the bell crank. I am installing mine as we speak. Like someone suggested above. Go to the Dynon site and look at the install pictures. They are depicted in CAD.

A

View attachment 69876

Hallelujah. I just received the same document from Jeff at GRT and realized the pitch servo was installed on the wrong side of the bracket. Can you take a look at the roll servo pictures I posted and tell me if that installation is correct? It doesn’t appear that the bell crank is stacked the same as the other pictures in this thread.

 

[AlphaCharlieBravo]

Hallelujah. I just received the same document from Jeff at GRT and realized the pitch servo was installed on the wrong side of the bracket. Can you take a look at the roll servo pictures I posted and tell me if that installation is correct? It doesn’t appear that the bell crank is stacked the same as the other pictures in this thread.

Looking at your pictures for the roll servo I would be extremely concerned with the amount of threads showing on the rod end bearings. Looking at the end of the aileron pushrod appears to show far to many threads and actually looks bent!

I apologize but I'm working on the inside of the fuselage for the next day or two and won't get to the roll servo until the weekend.
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[NelsonME58]

Looking at your pictures for the roll servo I would be extremely concerned with the amount of threads showing on the rod end bearings. Looking at the end of the aileron pushrod appears to show far to many threads and actually looks bent!

I apologize but I'm working on the inside of the fuselage for the next day or two and won't get to the roll servo until the weekend.
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View attachment 69885

Okay, saying something looks bent caught my attention. Are you referring to the servo control rod or the aileron push pull actuating arm? I agree there are a lot of threads showing on the servo arm at both ends. I will inspect closer. Thanks!

 

[AlphaCharlieBravo]

Aileron push rod threads into the brass colored rod , top left of your 2nd pic. That whole assembly looks suspect. Since you are not the builder I would suggest having someone who is involved in RVs to have a look at ALL of your control connections. The fact that your pitch control servo was installed incorrectly and the install of your roll servo looks sketchy.
The torque seal only indicates torque seal not how many threads are engaged. ( You can lead a horse to water and all that stuff) Seriously, have someone experienced in RVs have a look see at your control connections...all of them.

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[NelsonME58]

I’m looking at that rod end now. The witness hole is completely filled with threads and the rigging checks out so I’m confident of that part of the install. I measured the servo rod- 5.5 inches. Any one know if this is a reasonable length? Too short, too long?

 

[AlphaCharlieBravo]

I’m looking at that rod end now. The witness hole is completely filled with threads and the rigging checks out so I’m confident of that part of the install. I measured the servo rod- 5.5 inches. Any one know if this is a reasonable length? Too short, too long?

You should have 5" c/c from rod end bearing to rod end bearing and the rod should be around 3" long. Rigged correctly?

A

 

[NelsonME58]

My rod is 3 3/8 inches and rod end center to center is 5 1/2 inches so it is too long.

 

[rv8ch]

... That whole assembly looks suspect. Since you are not the builder I would suggest having someone who is involved in RVs to have a look at ALL of your control connections. ... Seriously, have someone experienced in RVs have a look see at your control connections...all of them.

Totally agree with this suggestion. There are probably 50 builders in a 15 minute radius of your airport. Check with the local EAA chapter and see if you can find someone who works on RVs professionally, or a kind soul who will have a look.