aknowledged Autopilot Trim Hat CSS causing permanent pitch input.

[Bunny Clark]

I've discovered that if trim hat CSS is used to bias pitch hold in either ATTH or A/P mode, it induces a permanent pitch input relative to the location of the TVV to the horizon line at AP disengage. This does not seem to be caused by stick CSS inputs, nor does it seem to effect roll using either method. Once it happens it seems to be irrecoverable, engaging another autopilot mode will stabilize the aircraft as expected, but the unwanted pitch input will return once autopilot is disengaged. Neither T/O Trim nor FCS Reset will correct the issue in the air. Reengaging an attitude hold program and using the trim hat to set a new attitude will cause a new bias when autopilot is disengaged, but seems to do so in a way that compounds, rather than overwriting, the previous bias.

 

I attached two tracks.

 

In the first track I engage ATTH and use the trim hat to set a nose low attitude. I disengage autopilot with the paddle switch with my hands completely off the controls. The aircraft pitches down. I pull the aircraft back to level and again release the controls, and the aircraft pitches down. I pull the aircraft back to level and engage BALT, which holds level flight. Again disengaging autopilot with the paddle switch, the aircraft pitches down.

 

In the second track I engage A/P and use the trim hat to set a nose high attitude. Just like the first track I disengage autopilot with the paddle switch without my hands on the controls, this time the aircraft pitches up. Like in the first track I pull it back level and let it pitch up again. I engage BALT which levels the aircraft, and disengaging autopilot again causes a pitch up.

 

Tracks were made on current Stable version, 2.5.6.55960

Hornet A-P 1.trk

Hornet A-P 2.trk

[oldcrusty]

I used the trim in UA mode once or twice only as an experiment. In a mild pitch up, say 10 deg. I was able to trim down to .9 G (or .8G, whichever it was) to stop this 1G creep. The climb was pretty stable and when I leveled off, I was able to trim it back to 1G. It took quite a few seconds. I watched the G on the HUD. I was at low altitude and low weight, so the STABs on FCS page were at 1.1 NU to maintain 1 G.

As far as ATTH is concerned, I used it once in a while in a spiral descent or climb. I found it a little tricky ( not as good as FPAH in the 'other' sim). If I put the jet in a 5 deg. climb and a 45 deg. bank, engage ATTH, the nose will drop to 2.5 deg. with bank unaffected. I had to pull the nose at around 1.8G, maintaining the bank, back to 5 deg. and it would stay there. Pulling on the stick too much will trip the A/P.

Well, you must've had a good reason to set the pitch bias in a UA mode. I stay away from it, it's easier to push the stick slightly in steep climbs or descents to stop the 1G creep.

 

I didn't have a chance to d/l your tracks.

Edited October 20, 2020 by Gripes323

[Bunny Clark]

I'm going to necro this old post, because the problem remains on the current 2.7 Open Beta, despite unspecific changes to CSS. Others have posted in other threads about having the same issue, and for me it remains unresolved on both Stable and a fresh Open Beta install. 

[cofcorpse]

Could you provide a new track in a fresh OB, please?

[Bunny Clark]

10 hours ago, cofcorpse said:

Could you provide a new track in a fresh OB, please?

Yup, sure can. 

 

Here's a simple short track. Enable ATTH, set a +10° attitude using the trim hat, hit the paddle switch. No pressure is exerted on the stick the whole time. 

 

F-18C AFCS.trk

[cofcorpse]

Thank you for the track!

I see the behaviour but can't tell right now if it is correct or not. We will investigate and correct if needed.

 

For now I can suggest to change attitude in ATT or AP modes with the stick, not the trim, or trim down after autopilot disengagement.

[Bunny Clark]

On 5/24/2021 at 10:07 AM, cofcorpse said:

For now I can suggest to change attitude in ATT or AP modes with the stick, not the trim

Yup, that's exactly what I have been doing for months now. 

 

Thanks for looking into it!