Autopilot hunted 4 degrees in TRK-ALT mode

[wayne4242]

When using "Level" mode, the autopilot holds the plane dead true, but when using TRK+ALT mode, it constantly, gently rocks through a 4 degree arc.  It will head to 314, then roll gently back to 310, then roll gently back to 314 etc, etc.  Ideas?

 

[mmarien]

The level uses the ADAHRS gyro to fly with zero pitch and zero roll angles. TRK follows a path on the ground and probably uses the compass. Was the compass calibrated?

 

[Dynon]

TRK actually uses GPS ground track as its source, and isn't influenced by heading changes.

 

[led]

Just a couple clarifications -

LEVEL holds zero bank angle and zero vertical speed (after raising the nose above the horizon.)

TRK (as has been said) uses GPS ground track, but some turn rate is involved. If it's rocking and rolling you may want to revisit the roll axis portion of the tuning guide. (This assumes that there's no slop or servo slip, because LEVEL is holding well.) A very slow GPS track update rate might also be the culprit.

HDG uses the compass; rocking and rolling there may be tuning and it may be miscalibrated compass. The best clue that the compass is a problem is if HDG hold is worse than TRK, and/or the compass shows a change in heading with a bank angle change while the nose is held on the horizon with rudder.

Also, be aware when watching performance that watching the compass rose in TRK mode will look funny if there's wind. =)

--
Lawrence Doan
Aeronautical/Autopilot Engineer

 

[DBRV10]

My idea is make sure ALL your linkages are rock solid. if there is any sloppy controls, even a rod end with some slack in it, you will get this.

The maths in the AP can't solve mechanical issues. It will deal with what it has as best it can. Like a broken shear screw, that still partly grabs enough. See that before too.

 

[mmarien]

When I was working on one of my google projects someone sent me this flight path. When I asked him if that is the way the plane flew he said it did. What it was, is that the GPS source he was using was truncating the lat/long positions. Upload your data to SavvyAnalysis and have a look at the flight path or use this software to convert the log to a Google Earth kml file and have a look there.