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TruTrak Heading Hold Issue

Toobuilder

Toobuilder

Well Known Member
I've been flying behind a DGSV II for several years and thousands of miles of rock solid performance until yesterday. The 1 hour flight to Vegas found the roll wandering constantly up to about 5 degrees either side of level. This oscillation took about 2 seconds to cycle. It only does this when tracking a flight plan, and I did try an in flight reboot.

I'm thinking there is a setting I can change, but my real question is why did this behavior show up? Nothing has changed on the airplane other than re - rigging the rudder hinges.

Thoughts are appreciated.
 
What GPS source are you using? I encountered a horizontal oscillation issue a couple months back. Turned out there was a firmware update for the Garmin GPS that fixed the issue.
 
Out of curiosity, did you try ground based nav sources or simply heading mode? Results?
My thinking is this. If the aircraft exhibits the same symptoms in all AP modes then I would be looking for issues at the AP (computer, servos etc). If the symptoms only appear while tracking GPS based nav I would be looking at the GPS itself or the data bus from the GPS to the AP. Another very simple possibility is there could have been a GPS coverage issue. Not uncommon in that neck of the woods.
 
Check your control system and A/P servo mountings for anything that may have become loose. If something is loose, the servo will be constantly adjusting back and forth to compensate for the slop in the system.

Jon D.
 
So, you're seeing this ONLY in GPSS mode? (also shown as EXT LS on the AP display). What does it do in TRK mode?
 
Check the servo

Mine was doing this exact same thing recently and it turned out to be the servo. Lucas and gang fixed it rather quickly. :)
Lucas is right to have you check it in both GPSS and TRK mode. That confirmed it for me.

Vic
 
Well, it is far more pronounced in GPSS mode. Yesterday on a nice long straight segment I simply let it go to heading mode and it calmed down a bunch. The air was a little bumpy, so the minimal servo action may have been fighting that. When I reached my waypoint turn (about a 60 degree heading change), I engaged GPSS again and it turned and flew the ship home fairly well. Still couldn't quite find center, but was better than the day before.

I don't know off hand what my settings are today, but I do know that after the "blue screen update" a few years ago, the roll response was noticably more sluggish. As an attempt to speed things up a bit, I cranked most of the settings up near the max. It was still too slow, but not enough to really worry about. It has been like that for all this time with no other changes.
 
Actually, I think I DO know of a linkage change that might account for it. Had to read Jon D's post again and it clicked...

When messing with my rudder hinges, I went looking for the long shank rod ends and I found them in the control torque tube. I had them in and out a few times, and I did re-rig the torque tube to clear all structure at full deflection, but it is possible it is not in the exact same spot as before. The torque tube was never removed, nor was the servo linkage changed, but I'm guessing there is a null position on the servo which is now off.
 
There is no null position on the servo. Did you turn the GPSS Gain setting up high as well? If so, try knocking that back down to 16.
 
Yep, I see in the setup instructions that there is no null position... I will check my setting on the gain tonight.
 
Check the servo shear pin

On our RV-7A we had the pin break on the roll servo. The behavior was as you describe. It's easy to check - with the autopilot engaged push on the stick. If you feel minimal resistance the pin could be sheared. If it takes force to overcome the servo resistance - you can usually feel it "cog" when you do this - it's something else.
 
Thanks for that input - I have sheared the roll servo pin in the past, but this does not feel like that experience.
 
OK, I think I have the initial problem figured out. When I put the control torque tube back together I added a little bit of preload in the system, which shows up as a slight bind. Almost unnoticed on the ground, in the air on the way to Big Bear this morning it was pretty "sticky".

So I have corrected that issue, but an old concern has resurfaced. The roll servo does not look "right" to me. With the ailerons centered, the servo output arm is at the 12 oclock position, and the torque tube arm is at 3 oclock. Looks like things would work a whole lot better if both were at 6 oclock. As it is, the servo arm is breathtakingly close to the seat pan and would easily jam with any distortion, and there really is pretty poor leverage for the servo.

Is my current 12 and 3 oclock correct, or should it be 6 oclock for both?
 
Toobuilder said:
When I put the control torque tube back together I added a little bit of preload in the system, which shows up as a slight bind.
Click to expand...

I had a similar issue with the elevator on my 10 after re-assembly when I picked it up at the paint shop.

I had a very slight drag in the elevator, and the A/P would not hold level flight for more than a few minutes.

Once I got home and had the needed tools and parts to reset the elevators, the slight drag was eliminated, and the A/P stopped its porpoising.
 
I can Lucas, but after doing a search it appears to match your drawings and pictures of other RV-8 installations.

It just seems counterintuitive the way it is set up, but apparently works fine.

1zny91t.jpg
 
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That's interesting Mike - I have always seen the servo mounted on the port side (that's how mine is), and I think the geometry turns out a bit different. I'd have to dig WAY back in my archives for a picture though....
 
Yes, other examples show the opposite side mounting - but they are common in the mostly straight up servo arm and side mounted torque tube arm. That's the aspect that seems odd to my feeble brain.
 
Seems to work better now as well. Took a quick evening flight up to Tehachapi to get gas and it seems to track well even in the chop I experienced. Very snappy response now - Its amazing how just a little control system friction will change things.
 
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