Autopilot Altitude "Hunting", Garmin G3X touch.

[bjdecker]

Reviving an older thread from RV-14...

My RV-7 will chase altitude by +/- 40ft when in straight and level flight in ALT mode. I've followed the G3X autopilot installation / calibration instructions to the letter multiple times. None of the 18 pitch parameters (Advanced mode enabled) seem to have an effect on the altitude hunting. TBH I haven't tinkered with some of them as there isn't a description of what behavior they control.

The controls in this aircraft are smooth, no binding, no side loading on the RE bearing/hinges.

System details:

Pitot tube is a GAP 26 - unregulated, located outboard of the last inspection panel on the left wing, just aft of the spar. Gretz mount.
Static ports are the machined aluminum clones of the SD-42-BSLF rivet (available from Steinair, Van's), spaced from the fuselage by .032". Location as specified by Van's RV-7 plans (DWG 28, Detail "E")
Pitot/Static lines are comprised of 1/4" O.D. Polyethylene tubing. 1 "break" in static line at the top of the "hoop" along the top of the F-708 bulkhead, Parker 362PLP-4 "Union Y" fitting. No leaks in Pitot or Static system.
All lines terminate at GSU 25 located on the back of the G3X Touch PFD.
Auto Trim is [Enabled].
G3X Touch System software version is 9.31

Current AP settings (Pitch):

Pitch Servo Max Torque [50%]
Pitch Servo Direction [Normal]
Pitch Servo Clutch Monitor [Enabled]
Pitch Servo Control Mode [Normal]
Pitch Correction Time [1.00]
Pitch Fine Adjust Amount [1]
Pitch Fine Adjust Time [.20sec]
Pitch Airspeed Gain Schedule [0.000]
Pitch Altitude Gain Schedule [0.000]
Min Airspeed Limit [50kts]
Max Airspeed Limit [180kts]
Pitch Servo Gain [0.50]
Vertical Speed Gain [1.00]
Vertical Accel Gain [1.00]
Vertical Speed Inertial Gain [0.00]
Airspeed Gain [1.00]
Airspeed Accel Gain [1.00]
Airspeed Track Gain [1.00]

General flight conditions:

6500ft
2500RPM
23.7" MAP
10.1 GPH FF
13°C OAT
30.10" local alt.
Flight weight is ~1700lbs
TAS 172 knots

Did I miss anything? If anyone would care to share their experiences or have suggestions (and as an aside RTFM isn't really helpful in this case ) I'd appreciate it.

Brian

 

[Brantel]

Send your logs to Garmin, they will help you get it sorted. Make very small changes and observe the result. Took me a while but I eventually got my AP and plane to play very nicely together. Unfortunately I don’t have a record of what the settings were. Plane was sold so I can’t check em.

 

[RWoodard]

I’m no expert having not completed or calibrated my G3x installation, but is there a chance that having the GSU 25 mounted to the screen might be allowing vibration to create an oscillation? I know many people install it this way (even certified) and have no issue, but I could see how each installation could be breaking new ground in relative harmonics and vibration.

I love your post, btw. The detail is perfect!

 

[PilotjohnS]

i had this happen to me too. I followed the G3X detail setting instructions. One first flight after, the plane would hunt, I had to turn down the altitude gain a lot. Now it is fine.

The only thing I need to change is the trim speed; i slowed it down too much. My RV has such a large speed range, the trim speed decrease with airspeed needs to be carefully set. with

 

[bjdecker]

I’m no expert having not completed or calibrated my G3x installation, but is there a chance that having the GSU 25 mounted to the screen might be allowing vibration to create an oscillation? I know many people install it this way (even certified) and have no issue, but I could see how each installation could be breaking new ground in relative harmonics and vibration.

I love your post, btw. The detail is perfect!

Rod,

Thanks - another engineer friend of mine posited that there may be a standing wave of sorts being set up in the static line...we just can't help it. (see Occam's Razor https://en.wikipedia.org/wiki/Occam's_razor).

Brian

 

[fl-mike]

I had to turn the gain way down from the recommended setting. I could see the elevator jittering in the mirror.

 

[bjdecker]

i had this happen to me too. I followed the G3X detail setting instructions. One first flight after, the plane would hunt, I had to turn down the altitude gain a lot. Now it is fine.

The only thing I need to change is the trim speed; i slowed it down too much. My RV has such a large speed range, the trim speed decrease with airspeed needs to be carefully set. with

John,

By Trim Speed are you referring to the Trim Configuration, Trim Speed option in the G3X Config mode?

I think I left mine at the default -- 80kt/150kt, then 100%/25% for all the trims. Airspeed source is "Indicated Airspeed"

B

 

[PilotjohnS]

John,

By Trim Speed are you referring to the Trim Configuration, Trim Speed option in the G3X Config mode?

I think I left mine at the default -- 80kt/150kt, then 100%/25% for all the trims. Airspeed source is "Indicated Airspeed"

B

Yes, I had it at 25% and it seemed twitchy at speed, so I turned it down to 10% and it doesn't hardly move at speed. Seems like there is not enough torque to overcome air loads or something. So back to 25%

 

[Southern Pete]

Also look at how much capacity there is in your static system, as in how much tubing and instruments. It may be that static system is being slow to react to altitude changes, it is very small pressure changes we are trying to measure (1mb). I also had to reduce the altitude gain from the initial setting to get smooth performance. You also could try ballasting the aircraft to move the CG forward in the normal range to reduce the pitch sensitivity.

 

[lr172]

You need to play with the altitude hold gain. Keep lowering it in increments until it gets lazy and doesn't react fast enough to altitude changes then back up to the optimum spot. My G3X is rock solid in alt hold, but took some experimentation with gains to get there. Also be sure that the trim is not too agressive for the airspeed as mentioned above. You want less trim authority at higher speeds. The AP uses trim and if too aggressive, can be over reacting.

This is FAR more likely to be a gain setting issue and NOT a static system issue. In the case of the later, all of your instruments would be bouncing around in straight and level flight.

 

[Longez]

My RV-7 will chase altitude by +/- 40ft when in straight and level flight in ALT mode. I've followed the G3X autopilot installation / calibration instructions to the letter multiple times. None of the 18 pitch parameters (Advanced mode enabled) seem to have an effect on the altitude hunting. TBH I haven't tinkered with some of them as there isn't a description of what behavior they control.

Brian,

That is a LOT of altitude hold deviation. Good job explaining your installation, but it would help to hear how things went as you methodically performed the in-air pitch servo configuration tuning.

As you know, you have to start with Pitch (PIT) mode tuning (the most accurate vertical mode) and only progress to the more complex Vertical Speed (VS) and Altitude Hold (ALT) mode tuning when PIT mode performance is near perfect. Set PIT=0 to perform this tuning.

Once PIT mode is perfect, move to VS mode by setting VS=0 and tuning for this mode.

At this point ALT hold mode should work well, but might need minor tuning.

Like others have said, I have some concerns that you may have a static system problem, but probably not based on what you have said. Hopefully your less than ideal ADAHRS mounting location is not negatively affecting PIT mode. If your PIT mode works very well, but the autopilot performance falls apart when you try to tune VS mode, you may have a static system problem.

I flew my G3X/G3X Touch autopilot for years before the pitch servo Vertical Speed Inertial Gain configuration parameter was developed. This parameter utilizes attitude information to further stabilize ALT hold. The autopilot was always very smooth and held altitude very well without benefit of the Vertical Speed Inertial Gain setting, but once I tuned that parameter it took my ALT hold performance to a new level and now holds within 5 ft most of the time. Admittedly, I have no idea how an RV-7 will respond to changing this gain parameter.

The Vertical Speed Inertial Gain varies from 0 (off) to 1 (maximum). I am using a setting of 0.98, but I caution you to not expect this parameter (by itself) to fix your large ALT hold deviations. You should first do the basic tuning to get acceptable ALT hold performance before trying to improve it with the Vertical Speed Inertial Gain parameter.

It probably goes without saying that you should disable auto-trim until you get altitude hold working the way you want.

Steve

 

[bjdecker]

I had to turn the gain way down from the recommended setting. I could see the elevator jittering in the mirror.

Mike -

Do you recall which gain you adjusted? There are 9...

Brian

 

[JDA_BTR]

I said this a while back….

Garmin G3X, Trim and Autopilot

After some troubleshooting I've made some changes to the auto-trim and wanted to report on it. What I found was that with the recommended autopilot settings for my G3X system, when slowing down and entering an approach the system would not settle down on pitch/speed/glideslope. I would try to...

vansairforce.net

 

[bjdecker]

You need to play with the altitude hold gain. Keep lowering it in increments until it gets lazy and doesn't react fast enough to altitude changes then back up to the optimum spot. My G3X is rock solid in alt hold, but took some experimentation with gains to get there. Also be sure that the trim is not too agressive for the airspeed as mentioned above. You want less trim authority at higher speeds. The AP uses trim and if too aggressive, can be over reacting.

This is FAR more likely to be a gain setting issue and NOT a static system issue. In the case of the later, all of your instruments would be bouncing around in straight and level flight.

Larry,

Thanks - except there isn't an explicit "Altitude Hold" gain; rather a collection of things that relate to, but not easy to derive their effect -- Vertical Speed, Airspeed, etc.

So if my altitude and airspeed "wander" in ALT mode, should I adjust the Airspeed Gain up? or should I adjust the Vertical Speed Gain up?

The "book" implies a correlation between AP modes (IAS, VS, ALT) and some of the gains - for example, if I am in IAS mode, then the Airspeed Gain would be the knob to turn, likewise for the VS mode - Vertical Speed is the knob. ALT is a bit indeterminate. Or I just can't find the chapter/verse in the manual.

<sigh>

Thanks as always.

Brian

 

[lr172]

Larry,

Thanks - except there isn't an explicit "Altitude Hold" gain; rather a collection of things that relate to, but not easy to derive their effect -- Vertical Speed, Airspeed, etc.

So if my altitude and airspeed "wander" in ALT mode, should I adjust the Airspeed Gain up? or should I adjust the Vertical Speed Gain up?

The "book" implies a correlation between AP modes (IAS, VS, ALT) and some of the gains - for example, if I am in IAS mode, then the Airspeed Gain would be the knob to turn, likewise for the VS mode - Vertical Speed is the knob. ALT is a bit indeterminate. Or I just can't find the chapter/verse in the manual.

<sigh>

Thanks as always.

Brian

Post above is correct. Too many years since I did this. Fly straight and level and engage AP it PIT mode. Adjust Pitch gain until the AP does a reasonable job of holding a relatively steady altitude. Not perfect, just no hunting. Remember that this is just holding a pitch angle, not an altitude, so you are observing the attitude indicator, not the altimeter.. PIT gain is the base line that dictates how aggressive the elevator moves under AP and the one all the others use, so want to do this first. Then engage ALT hold mode and adjust vertical speed gain until it holds the Alt well. Hunting can be too much or too little gain. If it is fast and twitchy, like it is chasing it's tail, it is too much. If it is very slow and lazy it is too little Then do a VS climb or descent and make sure it tracks well. May need to balance between the two as they share a gain setting. As mentioned above, ALT hold is really just maintaining a 0 VS, with cross checks on ALT target.

remember that the VS gain builds on top of the PIT gain. If the latter is off, the former can't really fine tune it and settle things out. As you know, in the RVs ptich sensitivity changes with airspeed, so do your testing at the speeds you normally use the AP or find a mid point that works for all.

 

[Thermos]

Static ports are the machined aluminum clones of the SD-42-BSLF rivet (available from Steinair, Van's), spaced from the fuselage by .032". Location as specified by Van's RV-7 plans (DWG 28, Detail "E")

Just curious…does this mean that you have a spacer or washer under the port to offset it by .032”? If so, that might be part of the problem.

If not, disregard.

Dave

 

[mburch]

Fly straight and level and engage AP it PIT mode. Adjust Pitch gain until the AP does a reasonable job of holding a relatively steady altitude. Not perfect, just no hunting. Remember that this is just holding a pitch angle, not an altitude, so you are observing the attitude indicator, not the altimeter.. PIT gain is the base line that dictates how aggressive the elevator moves under AP and the one all the others use, so want to do this first.

I agree with this so far, and this is what the installation manual tells you to do as well.

Then engage ALT hold mode and adjust vertical speed gain until it holds the Alt well.

Please don't do this... you want to use VS mode to properly configure your vertical speed gains, not ALT.

 

[bjdecker]

Just curious…does this mean that you have a spacer or washer under the port to offset it by .032”? If so, that might be part of the problem.

If not, disregard.

Dave

Correct -- during flight testing and subsequent air work, the airspeed seemed "off". I ran multiple tests with the GPS PEC (4 way, 3 way) and other tools to confirm the discrepancy.

Original location/configuration of static ports would yield an IAS of ~7kts slower than calculated. I added a .015" shim (washer) between the static ports and the fuselage which reduced the error by 4kts. An .032" shim/washer got the error down to ~2kts.

 

[bjdecker]

Brian,

That is a LOT of altitude hold deviation. Good job explaining your installation, but it would help to hear how things went as you methodically performed the in-air pitch servo configuration tuning.

As you know, you have to start with Pitch (PIT) mode tuning (the most accurate vertical mode) and only progress to the more complex Vertical Speed (VS) and Altitude Hold (ALT) mode tuning when PIT mode performance is near perfect. Set PIT=0 to perform this tuning.

Once PIT mode is perfect, move to VS mode by setting VS=0 and tuning for this mode.

At this point ALT hold mode should work well, but might need minor tuning.

Like others have said, I have some concerns that you may have a static system problem, but probably not based on what you have said. Hopefully your less than ideal ADAHRS mounting location is not negatively affecting PIT mode. If your PIT mode works very well, but the autopilot performance falls apart when you try to tune VS mode, you may have a static system problem.

I flew my G3X/G3X Touch autopilot for years before the pitch servo Vertical Speed Inertial Gain configuration parameter was developed. This parameter utilizes attitude information to further stabilize ALT hold. The autopilot was always very smooth and held altitude very well without benefit of the Vertical Speed Inertial Gain setting, but once I tuned that parameter it took my ALT hold performance to a new level and now holds within 5 ft most of the time. Admittedly, I have no idea how an RV-7 will respond to changing this gain parameter.

The Vertical Speed Inertial Gain varies from 0 (off) to 1 (maximum). I am using a setting of 0.98, but I caution you to not expect this parameter (by itself) to fix your large ALT hold deviations. You should first do the basic tuning to get acceptable ALT hold performance before trying to improve it with the Vertical Speed Inertial Gain parameter.

It probably goes without saying that you should disable auto-trim until you get altitude hold working the way you want.

Steve

Steve,

Thanks...and thanks. I need to do a better job with the calibration steps (job #1). Admittedly, I plugged in numbers, got the "debounce" set correctly and kept flying. I think it's clear, that I need to go through each step, and given my ADHD I should bring a friend.

On the topic of PIT mode, and ROL for that matter - what's the correct way to select this? Just push the AP button once a particular flight attitude is attained? (presumably) What if the A/C is climbing/descending a bit?

Cheers!

Brian

 

[Longez]

Steve,

On the topic of PIT mode, and ROL for that matter - what's the correct way to select this? Just push the AP button once a particular flight attitude is attained? (presumably) What if the A/C is climbing/descending a bit?

Cheers!

Brian

Brian,

When you first press the AP button on your autopilot control panel, it activates in Pitch (PIT) and Roll (ROL) modes. It will capture the current aircraft pitch, but you can then easily use the flight director bars and the pitch wheel on your GMC to select a pitch of zero. Each click of the pitch wheel changes the selected pitch up/down 0.5 degs.

As you say, you can also switch another active vertical mode (e.g. VS or ALT) to PIT just by pressing the button for that active vertical mode.

I always do all my climbing in PIT mode, so I am an expert at using PIT mode.

Steve

 

[Thermos]

Correct -- during flight testing and subsequent air work, the airspeed seemed "off". I ran multiple tests with the GPS PEC (4 way, 3 way) and other tools to confirm the discrepancy.

Original location/configuration of static ports would yield an IAS of ~7kts slower than calculated. I added a .015" shim (washer) between the static ports and the fuselage which reduced the error by 4kts. An .032" shim/washer got the error down to ~2kts.

Interesting…did you have the alt-hold problem before adding the shim? Reason I ask is that sharp edges around static ports have been known to cause problems.

I had alt-hold problems similar to yours and they seemed to get better when I switched from old-style pop rivets to the slightly-flatter threaded style that you’re using. There are a couple of other threads on VAF with details from builders who had similar problems and solutions.

Dave

 

[Walt]

Check the elevator trailing edge profile, a fat TE will cause the pitch to be overly sensitive which will cause the AP to hunt.

 

[bjdecker]

Interesting…did you have the alt-hold problem before adding the shim? Reason I ask is that sharp edges around static ports have been known to cause problems. I had alt-hold problems similar to yours and they seemed to get better when I switched from old-style pop rivets to the slightly-flatter threaded style that you’re using.

Dave

Dave -

Interesting data point. I didn't pay much attention to the AP behavior at the time as I was more focussed on the slowness -- The original ports were knife-edged and flat across the orifice. After painting, the knife edge transition from the fuselage to the port "plateau" all but disappeared -- read: it was FLAT....and it was incorrect.

I chucked the original ports and went with the Van's offering (not the SD-42-BSLF, I didn't like the tube-to-tube-RTV-slather interface). BMILLS and SCSMITH shared some feedback after they also used my original ports during the CF Taper Wing testing.

As I mentioned, the new dome shaped ports are great, and dialed in pretty well - I was thinking about putting a chamfer on the washer to ease the transition from fuse-to-port...we'll see.

Thanks for the feedback,

Brian

 

[bjdecker]

Check the elevator trailing edge profile, a fat TE will cause the pitch to be overly sensitive which will cause the AP to hunt.

Thanks Walt,

The skin on the elevator and trim tab is flat to within 3/32 of the TE (per Chapter 5) -- not puffy, not concave...



I think this is more of a case of PEBCAK or a problem with the MEAT-SERVO

 

[MConner]

My -10 was altitude hunting and I found the trim servo mount was allowing slop in the trim tabs. Tightened it up and problem solved.

 

[bjdecker]

My -10 was altitude hunting and I found the trim servo mount was allowing slop in the trim tabs. Tightened it up and problem solved.

Mark,

Good suggestion , and a CI item for certain!

My servo mount and trim tab have 0 free play. I deviated from the plans here and used MS20001 extruded hinge (not AN257) - there is 0 lateral play as well. I figured any looseness in the trim tab could have fatal consequences (e.g. Galloping Ghost).

 

[mburch]

It probably goes without saying that you should disable auto-trim until you get altitude hold working the way you want.

This really needs to hilighted. It's very important to follow the installation guidance and disable auto-trim until you have the autopilot working properly.