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Trutrak Gemini autopilot porpoise

SwimmingDragonfly96

Well Known Member
Working with trutrak Andrew on this one but I’m away from the plane and wanted to see if anyone else could chime in. My autopilot occasionally porpoises when trying to capture an altitude, and occasionally while it’s turning due to GPS commands. It seems to be trim induced by the auto trim module, and when I cancel the autopilot, the plane is severely out of trim. I manually trim it and the proposing is gone upon re engaging the autopilot. I know it could be an issue with auto trim speed, but this autopilot has been installed and functioning properly for 4+ years and no settings have been changed, so I’m not sure why too fast of an autotrim would cause me problems now? Anyone experience something like this before? The builder ferried the plane with me and it occurred repeatedly throughout the flight and on every flight since. He said it had happened one other time about 6 months ago. He’s a really honest dude, and the autopilot worked great on both mine and the A&P who did the pre-buy’s test flight, so let’s just assume he’s being honest. Also, he’s a fixer, so if it was a persistent issue before, he would have certainly tackled it.
 
Working with trutrak Andrew on this one but I’m away from the plane and wanted to see if anyone else could chime in. My autopilot occasionally porpoises when trying to capture an altitude, and occasionally while it’s turning due to GPS commands. It seems to be trim induced by the auto trim module, and when I cancel the autopilot, the plane is severely out of trim. I manually trim it and the proposing is gone upon re engaging the autopilot. I know it could be an issue with auto trim speed, but this autopilot has been installed and functioning properly for 4+ years and no settings have been changed, so I’m not sure why too fast of an autotrim would cause me problems now? Anyone experience something like this before? The builder ferried the plane with me and it occurred repeatedly throughout the flight and on every flight since. He said it had happened one other time about 6 months ago. He’s a really honest dude, and the autopilot worked great on both mine and the A&P who did the pre-buy’s test flight, so let’s just assume he’s being honest. Also, he’s a fixer, so if it was a persistent issue before, he would have certainly tackled it.

Data point for you. I have the GX pilot, a TT product and it will do the same porpoise on engagement if trim (manual) is out. It may also occur if the AP was turned on during a bumpy taxi/air, so I usually recycle at cruise in smooth air. Standard operation is to switch on just after start while stationary. I installed a switch to lower my trim motor speed (Ray Allen) for cruise (over 100KIAS). For landing (below 100), the faster setting seems more comfortable. YMMV

IMO Experience/time in the RV is probably a more relevant factor than honesty.

On another subject, where is your plane based?
 
I have similar on my Gemini which I believe is due to static system lag. The Gemini installation manual (page 2) mentions that if the Gemini instrument is at the tail end of the static line of several instruments, that long line causes a delay in change in static pressure getting to the Gemini causes porpoising. I guess you can think of the static as a giant bellows with a time delay from end to end due to air compressibility. Sensing aircraft change is the main thing that limits the instability level allowed in a fighter jet as I have dealt with for most of my professional career.

Next time I am under the panel I plan on putting tee in static line to make the Gemini not near end of the line.
 
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Porpoise

I have Dynon equipment with auto-trim but their A/P can exhibit similar behavior if the A/P is configured to too high a level of sensitivity. It can hunt for a heading or altitude. That may not apply to the TruTrack but it is probably worth a quick thought.
 
I have similar on my Gemini which I believe is due to static system lag. The Gemini installation manual (page 2) mentions that if the Gemini instrument is at the tail end of the static line of several instruments, that long line causes a delay in change in static pressure getting to the Gemini causes porpoising. I guess you can think of the static as a giant bellows with a time delay from end to end due to air compressibility. Sensing aircraft change is the main thing that limits the instability level allowed in a fighter jet as I have dealt with for most of my professional career.

Next time I am under the panel I plan on putting tee in static line to make the Gemini not near end of the line.

According to the operations manual you can tune the static lag impact, page 25 of the manual Rev B. I do not have the problem noted in the thread with my Gemini (just passed the IFR cert pitot/static test) but I do not have the autotrim module installed. The autotrim module manual does say that too fast autotrim can cause the problem mentioned (If rate setting is too fast then aircraft will hunt in pitch), could it be a static system problem?
Figs
 
According to the operations manual you can tune the static lag impact, page 25 of the manual Rev B. I do not have the problem noted in the thread with my Gemini (just passed the IFR cert pitot/static test) but I do not have the autotrim module installed. The autotrim module manual does say that too fast autotrim can cause the problem mentioned (If rate setting is too fast then aircraft will hunt in pitch), could it be a static system problem?
Figs

It is a balance between all the time delays in the system and the speed at which the autopilot and auto trim react. Ideally you want the aircraft attitude and position detection to be faster than (inside the loop) the control inputs reaction time. Unfortunately a computer will be operating much faster then the physics of compressible fluid flow, aircraft flex, etc. I have thought of tuning down the lag adjustment but first want to work the easy to fix lags in the system.
 
Thanks for all your replies! How about the fact that it’s starting now and it was fine before? Is it possible for original settings to fall out of spec?
 
Speed it up.

I installed the TT autotrim module about two years ago. I purchased it from a friend who installed the TT autopilot but because his aircraft had manual trim couldn’t use the autotrim module.

I won’t say the autotrim module loses its settings but I do believe my tolerance for an out of trim condition has tightened over time. I suggest you consider speeding up the autotrim module setting using the procedure outlined in the TT autotrim manual (it’s easy) and see if that doesn’t address your problem. (I think it will)

One test you can do prior to resetting your module speed is to fly an instrument approach with flap setting changes. If your aircraft is slow to compensate with trim for the configuration changes then you definitely need to speed up the autotrim module. Mine’s set near the upper speed limit.

You can download the manual through the Bendix King website. My two cents.
 
I installed the TT autotrim module about two years ago. I purchased it from a friend who installed the TT autopilot but because his aircraft had manual trim couldn’t use the autotrim module.

I won’t say the autotrim module loses its settings but I do believe my tolerance for an out of trim condition has tightened over time. I suggest you consider speeding up the autotrim module setting using the procedure outlined in the TT autotrim manual (it’s easy) and see if that doesn’t address your problem. (I think it will)

One test you can do prior to resetting your module speed is to fly an instrument approach with flap setting changes. If your aircraft is slow to compensate with trim for the configuration changes then you definitely need to speed up the autotrim module. Mine’s set near the upper speed limit.

You can download the manual through the Bendix King website. My two cents.

Interesting you feel I need to speed up the autotrim. Everyone has said I need to slow it down, but I will give this a try if slowing it down doesn't work. Appreciate the words!
 
Gyro reset

I installed an Xcruse for a friend. It worked great for 6-8 months. One day it started to act weird. It would porpoise and turn away from course. Andrew or Bendix told him to do a gyro reset. He did and it fixed the problem. I believe if you start taxiing before turning On the AP and not let it do the self test before moving it can throw the gyros off.
 
I installed an Xcruse for a friend. It worked great for 6-8 months. One day it started to act weird. It would porpoise and turn away from course. Andrew or Bendix told him to do a gyro reset. He did and it fixed the problem. I believe if you start taxiing before turning On the AP and not let it do the self test before moving it can throw the gyros off.

I’ve seen this mentioned a couple of times in different posts. I mentioned it to the builder and he referred me to the trutrak Gemini manual where it states the plane doesn’t need to remain stationary for booting up. Is this information actually incorrect in the manual?
 
Porpoising

During IFR practice this weekend I noticed the porpoising mentioned in the thread during turns in holding patterns. It was gusty and moderately turbulent but the porpoising was noticeable. Did anyone come up with a fix?

Also when ATC is providing vectors for an approach what solution does anyone have to emulate a "heading" mode versus GPS track. I currently use the CWS mode switch in GPS track to change to the designated heading.

Learn something new every day.
Figs
 
I went thru about 10 tanks of fuel trying to tune the porpoise out of a GX Pilot in my RV7 back in 2013. Solved it by removing and replacing with Garmin.
 
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