Do I have an AP Pitch tuning problem

[RJ]

My Aircraft hunts up & down in a range of about 20' to 50' is this the best I can get re pitch control. I would love to get it to hold closer but am I expecting to much. Any suggestions as to what settings I can tweak to stop or reduce the the hunting up & down. I seem to be on the throttle a lot more than I expected re my 600Kg High wing / 912iS engine. I usually have no problems at cooler much higher Altitudes or in calmer cooler air at lower Alt. One or two persons = same performance
As soon as the Air warms up = Hunting
Thanks for any AP tuning help you may have
Cheers Hawke

 

[morhall]

I am experiencing similar hunting for altitude on my D100 with the SV42 servos. Not sure why it started this behavior, it used to hold quite nicely. Lately, it's all over the place.

I tried tweaking gains and sensitivities to no avail. I have to think that something in my overall system has changed, but I don't have any smoking gun. The control system has the same heavy spring loading it always has in pitch. The pitot static system was recently tested out OK for up to 20k.

The firmware is the same as I've previously had zero issues with.

I put the autopilot in test mode and it moves the control surfaces just fine.

I haven't checked the status page in flight to see about slipping. That's probably the next bit of data to collect.

Overall it hunts and overshoots pretty dramatically. 50-100ft, although at 170kts it takes very little pitch change to produce a pretty rapid rate of climb.

I also experience a wandering in HDG mode and I've noticed a fair bit of lag in my compass. I suspect that lag is manifesting itself as wandering as the plane overshoots, overcorrects, overshoots and slowly builds up a PIO(APIO?).

NAV mode is stable and TRK as well. Just HDG and ALT are having trouble. There just isn't enough information in the Install Guide to troubleshoot this kind of problem.

Hopefully the Dynon moderators are back from OSH and see this update on this thread.

Morgan

 

[Raymo]

If tweaking the AP settings do not help, I would recommend having the static system checked. It may have moisture or restriction somewhere in the system.

 

[dynonsupport]

Any time we hear an AP has been working well and then stops, the first question is always if the shear screw has broken. Morgan, have you checked for this?

 

[morhall]

Somehow missed this response. Maybe my notifications aren't turned on.

A year later and for a while the problem was resolved, now it is back. The shear screw was fine and the status showed no pitch slipping either.

For a while I did find that the AP worked better if I flew slightly out of trim. Not so much that the servo complained about trim, but enough that it required a little pressure to hold altitude. As though the dead spot with neutral trim was causing the AP to hunt.

Updates in the last year. AP was working OK, when flown slightly out of Trim. In May, we broke a shear screw on the pitch servo. Replaced that a week or so ago.

Last week I was flying up north at 10500 and level. Smooth conditions and the airplane trimmed for hands off level flight. Engaged ALT HOLD. Plane almost immediately starts to wander in a PIO of up to 100+ feet off assigned altitude.

I tried increasing and decreasing settings for VSI gain, ALT gain, sensitivity, pull rate and nothing seemed to resolve the issue.

Looking at the display, the rate of climb is stable at or near zero when AP is off. Same with the pitch display. So I don't think that I'm getting erroneous static input data. Or maybe I am, but it's not indicated as quickly as the AP is responding?

I'm at a loss. It's very frustrating and completely unusable in flight right now. The Glasair is very pitch sensitive, in the sense that even the slightest change produces a fairly large VSI change at cruise speed.

I have tried to follow the tuning guide for the sky view, but there seem to be a number of settings present that the D100 doesn't have.

I'm just not sure what to try as adjusting the gains doesn't seem to make any difference.

I'm not seeing slippage, but should I move to the inner bolt holes on the servo arm? Give the servo finer grained control over the elevator?

At a loss.

Thanks for suggestions.

Morgan

 

[Dynon]

If you're sure you don't have a broken shear screw, you should definitely use the dedicated AP tuning guide on the documentation area of our website as your first resource for addressing performance issues.

 

[jakej]

Morgan,
What I do with some aircraft is disconnect the static line, just vent it to cabin & then do a test flight - it’s a simple check to eliminate static system issues.
Is it possible for you to email me a picture of your servo & mount method ?
I’ll pm you.

Jake
Glasair IIS FT. 2400hrs
Australia

 

[morhall]

Thanks Jake.

I went for a test flight yesterday to give it a test at low level (2000msl) since the last time I remember it working ok was relatively low and suspecting a static issue, I thought maybe at lower levels it just worked better. Nope, same problem of wandering and blowing through altitudes. Next step was going to go to ambient cabin for the static, although all of my other static instruments seem stable.

Got back to the hangar and ran a servo calibration/test and noticed it seemed to be slipping, but no slippage on the status page.

Engaged ALT Hold and went back to wiggle the elevator and could feel it slipping.

For some reason, it broke another shear screw without ever slipping on the Clutch.

I pulled the aft bulkhead out to get access to the servo and could feel some wobble in the control arm.

When I got the replacement screw 2 weeks ago, my partner in the plane installed it. I've been mending a broken shoulder and couldn't make some of the reaches. I guided him through the replacement, but didn't feel things myself. I think the castle nut may have been a touch too loose. I don't think he over tightened the shear screw, he was very careful.

I believe the shear screw broke on the very first flight after installing it. I guess I'll get a new one and then reinstall myself so that I can be confident that the castle nut is installed properly. I'll also turn the torque down from 70% to 40 or something small until I'm seeing slippage. My plane has pretty heavy springs on the pitch circuit, but the servo doesn't seem to have any problem moving them.

I don't have a picture of the servo handy. It's mounted on the pilot side, behind the aft bulkhead on the floor. Pushrod goes forward to a bellcrank to engage with the elevator pushrod.

I could move the pushrod connection on the servo to the inside hole, I don't need much throw in my plane, but since the clutch isn't slipping, I don't think that is the reason it broke.

Morgan

 

[jakej]

FWIW I use the SV42T on the Glasair, just need to carefully align it so the pushrod travel is parallel to the for/aft travel of the elevator bell crank. Having the auto trim option is the best way to go with the Glasair due to the ‘heavy’ trim spring bias ( no trim tab).

 

[mmarien]

I had a some trouble with my pitch servo. Mostly in rough air. I did a couple of things at the suggestion of Jake. The first was to add pitch trim and better align the servo with the bell crank. I was using a spacer on the servo arm to align it but that put too much stress on the bearings in the servo.

I also changed to the SV42 and use the middle hole I believe. You don't normally use the full pitch travel but the servo shouldn't stop the pitch from full travel or go over center.

The shear screw is extremely weak in torque. Put some thread lock on it and finger tighten it.

I put about 35 hours on since the changes. I believe it made a difference.

 

[Dynon]

Morgon - if you've had a shear screw that was recently obtained from us break, please let our support team know about it via support at dynonavionics dot com or 425-402-0433. They've gone through some changes that should prevent most fatigue-related breaks. We'd be interested in learning more.