GRT Avionics

In the general setup, there is Roll and pitch gain.
In the AP setup, there is roll and pitch servo gains.

One does not change the other. SO my question is, what is the difference?
I was having what I call the 'tic toc tic toc' gentle roll back and forth at about 1 second intervals in level flight.
I could not make much progress on the General setup roll gain, but have been making progress in the AP roll servo gain number.

Mike

The Gains for the Autopilot that are found on the Set menu, General Setup page, Give the Autopilot an exact target to shoot for. The Servo Gains on the Set Menu, Autpilot Maintenance page, can be thought of as the tolerance the servos use to maintani the point given by the normal gains. The full and technical explanation can be found here--http://en.wikipedia.org/wiki/PID_controller I like my explanation better.

Hope that helps.


Ben

OK so knowing that Ben, how would you recommend we start in adjustments? I assume first in AP gain. Then fine tune in general setup gains?

The gains can't be completely separated, and all of them are reacting at the same time to changing sensor data. Can you see what the EFIS is reacting to? If you don't know what's causing the problem, you have to experiment.

You might find changing a more overall gain improves the performance in one mode but makes another mode worse. Changing the servo gains will affect all modes. If the problem only affects one mode, such as heading, GPS course, or localizer, focus on the gains that are more specific to those modes.

Before adjusting the gains, make sure your (General Setup) Autopilot Control Profile is not set to Other Autopilot. For the GRT autopilot, it should be set to Auto or one of the GRT options.

Decreasing Heading Gain will make the EFIS react less to the heading moving away from the selected heading. However, it will also roll less to turn to a new heading.

Decreasing Roll Gain will make the EFIS react less to the track moving away from the desired track and drifting off the course line (cross track error). Again, the cost is less roll to change to a new track or intercept a new course.

Cross Track Gain (XTGain) is a more specific gain that affects the reaction to cross track error. If the plane keeps moving from one side of the course line to the other, lowering this gain should help. The time to intercept or get back on course will increase.

Localizer Gain functions like Cross Track Gain and affects localizer mode. The Cross Track Gain and Localizer Gain are multiplied together in localizer mode. If your localizer mode is performing as you want it and you change Cross Track Gain for some other reason, you will need to change Localizer Gain to get the localizer mode back to the way you want it.

Change the gains by doubling or halving at first, then by smaller amounts to fine tune them.

Jeff. Thanks for the useful description.
I would recommend that a quick start guide, or more detail guide, on configuring these would be useful.
Tell the user exactly how to start, and how to proceed in dialing in these numbers for the various settings.

Thanks Ben, Your last sentence was worth a bunch. (doubling or halving them) I never knew how much to start changing the percentages of gain. It's starting to make sense after reading this.

I have been flying behind my HXr for a couple of years now and flying coupled autopilot approaches into a glideslope driven off an SL30. I have tuned the gains for normal flying at cruise speed – that is for Roll , Xtrack, etc and I have them all working fairly well, but I am still not totally happy with the autopilot following the glide slope, although it does fairly well on the Localiser. Part of the problem is that it is hard to fly the glide slope long enough to verify the results but in reading some of the older posts on this forum it is possible that I don’t know what I am doing exactly. If the system is basically a PID controller, I am guessing that there is no I – Integral component and it is a PD controller with gains on the Proportional Band and the Derivative. What gains am I adjusting when it says Glide Slope gain – is it Kp or Kd or (Kp+Kd) or some multiplier for each gain. I also found this statement in one of the previous posts
“Localizer Gain functions like Cross Track Gain and affects localizer mode. The Cross Track Gain and Localizer Gain are multiplied together in localizer mode. If your localizer mode is performing as you want it and you change Cross Track Gain for some other reason, you will need to change Localizer Gain to get the localizer mode back to the way you want it.”

This lead me to wonder what is the error variable that the Glide Slope is following, Angular error from the glide slope angle ( how does it calculate it?) or Rate of descent, or positional error off the glide slope or some combination.

The glideslope error is the glideslope deviation. This error is integrated. We know the desired angle from the database when you arm the approach, otherwise we assume 3 degrees. The gain settings in the SET MENU are multipliers for the internal gains. In the case of the Glideslope Gain setting, all of the internal glideslope gains are multiplied by that one setting.

I'm planning on setting up my AP tomorrow. I have a theoretical question after reading the Wiki page on PID controllers. Since the EFIS knows pretty much what's happening probably better than the pilot, wouldn't it be just a SMOP to set the gains and dampening automatically?