Autopilot Calibration

[AlanR]

I have twin D1000's, A/P Module with Autotrim in a Bristell NG5. Owned since 2016.
I recently had to change the pitch shear screw which had broken ( Pitch oscillation) and following this I have re-configured and recalibrated the Autopilot (twice). The Autopilot now though in flight wants to be in a permanent slightly nose high attitude for some reason and I can't quite understand why.
I have checked that all the A/P settings are exactly as they were prior to changing the shear screw. Previously the A/P and Auto trim all worked fine and have done so for the past 7years.
So, I am at a bit of a loss to know why this is happening and the only thing I can think is that when doing the re-calibration the aicraft may have not been in a spot-on perfectly level position.
So my question is, is it necessaary for the aircraft to be in a perfectly level position in all axis when doing the calibration? I personally don't wouldn't have thought this mattered because the pitch of the aircraft on the round and in flight will always be slightly different anyway but maybe i am wrong?

All thoughts appreciated.

Thanks,
Alan.

 

[Rhino]

There are two different calibrations (servo plus auto-trim), and inflight tuning. Which have you done?

 

[Marc_J._Zeitlin]

The Autopilot now though in flight wants to be in a permanent slightly nose high attitude for some reason and I can't quite understand why.

What does this mean? If you trim for a particular IAS, and then engage the A/P to hold altitude, there's only one pitch attitude that will accomplish this. How could the plane possibly be flying at a different AOA at the same IAS as before, with no changes to the airframe?

 

[AlanR]

There are two different calibrations (servo plus auto-trim), and inflight tuning. Which have you done?

I have done the ground configuration and calibration only. All I have changed is a broken Pitch shear screw. So I am confused as to why the previous last 7years in-flight A/P tuned settings need to be different to what they were before?..When all I have changed is a Pitch shear screw.
Over the last 7 years I have tweaked slightly at different times the A/P in flight settings and had got the autopilot and autotrim settings virtually 'spot-on' so I am reluctant at this stage, unless completely necessary, to start from scratch again with A/P in-flight default settings. I am just trying to understand why this should now be necessary as I say having merely changed out a shear screw.
Out of autopilot mode the aircraft flys in smooth air 'hands off' perfectly straight and level in all axis. But into Autopilot on (In simplified mode) and the aircraft attitude immediately changes to an above AI auto trimmed 'pitched up' attitude and consequential 5knot speed reduction.
I can't help but feel the problem must somehow be connected with the ground calibration I have now done (done it twice now)so my question is.:-
Is it important the when doing the ground calibration the aircraft is in a perfectly, in all axis, absolutely perfectly level, as in flight position? ...and does this matter?...I can't find any mention of this in the manual.

 

[Rhino]

Is it doing this via the autopilot itself, or is the auto-trim doing it? No sense wasting time with the autopilot if the auto-trim is the problem.

Marc makes an excellent point. The aircraft should be attempting to climb if it pitches the nose up when you engage the autopilot. Does it climb?

Are these push-pull or capstan servos? It sounds like the flight controls may not have been in neutral position when the flight control rods or cables were attached to the servo.

We may be hung up on terminology. I was asking asking about calibrations, which are different from system configuration (setup). Obviously it's configured (setup) in the system, or you would have no autopilot at all. Have you performed the servo calibration?

Was the servo test procedure performed after the servo calibration?

Was the ADAHRS up and running when you performed the calibration and test? That's how the system knows what level is. If the ADAHRS is properly configured, having the aircraft perfectly level shouldn't be necessary, but it wouldn't hurt. Also wouldn’t hurt to confirm ADAHRS configuration.

Were all flight controls in the neutral position when starting these procedures?

Was the auto-trim calibration performed? Shouldn't be necessary if these were autopilot servos, but it doesn’t hurt.

Was inflight tuning of both autopilot and auto-trim accomplished after performing all of the above?

Does this occur in multiple autopilot modes? If not, in which mode does it occur?

 

[AlanR]

Thanks for the input..
All Push Pull servos. Lengths of Pitch rod not altered or changed from previously. I just disconnected the pitch arm from the Servo arm when I changed the Shear screw.
All flight controls are neutral when starting performing servo calibrations although as i said previously the ground on which the aircraft stands is not particularly level hence my question on this. I then followed on screen instructions as per on screen instruction. Calibrations carried out using both screens. On-screen confirmation of calibration obtained.
ADHARS all up and running. Have also done ADHARS config.
Servo test procedure done after servo calibrations.
As I have said previously I haven't yet done any further in any flight tuning because the Autopilot worked perfectly for the last 7years and all I have done is change the pitch shear screw. Why would it now need different in-flight tuning?
I have checked that the tuning settings I currently have showing are exactly the same as the ones used before changing the shear screw.That is what i don't understand.
It occurs with A/P in Simplified mode HSI and Tracking. I haven't tried it yet in any other modes.
Yes, thinking some more about this it just must be Autotrim that is the problem. It is just the attitude that changes when A/P is engaged.
If left to its own devices with the pitch up situation it obviously begins to start to climb but I haven't pursued this because as soon as it pitches up it auto-trims itself to a nose up attitude up and I have then pressed A/P disconnect.

I am now this coming week going to completely start from scratch again with config and calibrations and see where it leads me.

 

[Rhino]

Confirm the pitch arm was reconnected to the same hole on the servo arm. If not, that could be one reason why you need inflight calibration/tuning.
Before you do that though, I'd try this with auto-trim disabled. Your troubleshooting possibilities will be significantly narrowed if you can identify exactly which system is the problem.
I'd try some different modes too.

 

[AlanR]

Just to put this to bed...
I went back and started from scratch and on this and re-checked the servo pitch arm position and rechecked all physical servo connections. All good. Checked and double checked that I had all original, pre shear screw change, Autopilot settings inputted into both D1000 screens. Re-configured the servo's yet once again and recalibrated the Autopilot yet again and afterwards did a calibration test and guess what!...hey presto for some magic reason all is now good!
Both autopilot and auto trim work perfectly in flight absolutely spot on as they always have done before changing the shear screw!
I don't believe I did anything different this time to what I had done previously, and it is all a bit of a mystery why I have had a problem with this but hey who cares, now all is good!

Thanks again for the all the thoughts/ideas and input to this.

Alan.

 

[Rhino]

Gremlins. They're included at no extra charge.

 

[Dynon]

So just for the record, the only things the autopilot detect/calibrate routine does is identify the servos, what axis they're on, and what direction of travel is correct. It does not calibrat and e or store the throw amount (the AP has no permanent notion of throw or endpoints) and only sets the direction of travel. The only time you should need perform a servo calibration is if you've 1) changed a servo for a new serial number or 2) for some reason, have reversed its installation orientation (ie, built a new bracket or something for some odd reason). If you're replacing a shear screw, it should work the same after a mechanical reinstall.

That said, doing a redetect/calibration should've caused no harm either. So I'm also stumped as to why you were experiencing that behavior.

Note that changing the tuning values WILL change AP behavior but I realize this isn't what we're talking about here.

 

[AlanR]

Dynon,
Thanks for the input and clarification re servo calibration. I was so surprised when all I changed was the Shear Screw and things went haywire.
I guess we will never know now why or what actually caused my post screw fitting issue.
Anyway now all good!
Thanks again,

Alan