Can't get Autopilot setting adjustment for proper handling qualities

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flyinlouis
Posts: 4
Joined: Mon Mar 14, 2022 1:45 pm

Can't get Autopilot setting adjustment for proper handling qualities

Post by flyinlouis »

Hi. After Pitch and Roll Servo installation, I have a hard time adjusting the parameters to have good flight characteristics without oscillations or overshoot. In Roll, it goes pretty good with small oscillations however and those roll oscillations are similar to a dutch roll movement. Right now, when AP falls into Altitude hold upon reaching target altitude, the aircraft continues to climb and MFD ask for more trim up. I am confused and do not understand that situation. Could it be that Pitch servo gain is not high enough?

I started by setting gains and damping at 100% for roll servo and pitch servo. For now, best settings I have found was a gain of 89% and damping of 119% or so (not sure exact numbers from my memory).

-To perform Pitch and Roll Servo response test, should we place the damping setting at 0 first and increase them by step until getting no oscillations?
gain at 100%, damping at 0. Perform response test until getting slight oscillation and then increase damping to resolve them?

I find the procedure in the A/P install manual not so clear so this is why I am asking advice here.

Thanks for your help,
Louis

Pelican 914 on Amphib Floats
Horizon 10.1 SW 2.00 and Sport EX 7 with SW 4
EIS 4000 with dual FF red cube
GRT_Jeff
Posts: 802
Joined: Tue Dec 11, 2012 12:11 am

Re: Can't get Autopilot setting adjustment for proper handling qualities

Post by GRT_Jeff »

Trim alerts are always about forces and torque, not about gain. Gain is about how aggressive the autopilot is. If the autopilot is unable to level off that could mean it doesn't have enough torque or it could mean the trim is too strong for it.

You should always start the gains and damping at 100%. If that already oscillates a lot then you need to lower the gain with the damping at 100%. The damping amount scales with the gain behind the scenes so it generally doesn't need to be lowered from 100%. There may be cases where lowering the gain alone can't stop the oscillation but that's just something you have to find out.
flyinlouis
Posts: 4
Joined: Mon Mar 14, 2022 1:45 pm

Re: Can't get Autopilot setting adjustment for proper handling qualities

Post by flyinlouis »

"Trim alerts are always about forces and torque, not about gain. Gain is about how aggressive the autopilot is. If the autopilot is unable to level off that could mean it doesn't have enough torque or it could mean the trim is too strong for it."

Totally agree Jeff, but if it was a question of torque, I would expect the Autopilot to request for trim down to level up and stay at selected altitude. What I see is that system request for trim UP that bring the aircraft climbing furthermore and getting away from target altitude even when in ALT HOLD mode. This is what I do not understand. I will take some pictures / video next time I go fly with AP. Will probably redo a pitch servo force calibration also.

Thanks for the advice for the gains. I will test flight that way and I feel I need minor adjustments to get it to acceptable HQ.
flyinlouis
Posts: 4
Joined: Mon Mar 14, 2022 1:45 pm

Re: Can't get Autopilot setting adjustment for proper handling qualities

Post by flyinlouis »

Hi Jeff. I went into my General Setup-SerialPort Input and Output config setting and found that they were at standard servo. I changed it to GRT Servo Enhanced. I saw also that there was a GRT Servo Standard option. What is the difference between all these AP servo control profiles ?
GRT_Jeff
Posts: 802
Joined: Tue Dec 11, 2012 12:11 am

Re: Can't get Autopilot setting adjustment for proper handling qualities

Post by GRT_Jeff »

These configure how the autopilot control outputs are calculated and set the base level (100%) gains. GRT Servo Standard is from all the flight testing with our autopilot. The GRT Servo Enhanced has a few adjustments to localizer/glideslope tracking, mostly to make it more aggressive. The Other Autopilot setting was the previous tuning for TruTrak. Auto mode uses GRT Servo Standard for GRT servos and Other Autopilot if you don't have our servos.
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