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L35 SPD Hold

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Since installing L35 ver 2.8 I have noticed that the AP SPD Hold function hunts up and down in pitch much more than with previous L35 versions. I see fairly large pitch swings with the a/c seemingly trying to catch up with the FD.  Has anyone else noticed this? Flysimware says the SPD Hold code has not been changed, so I'm wondering if something else, like a change in the pitch trim limit, might be contributing to this? Or if something in my setup is at fault. I use FSX.

Thx,

Al

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Scott -- thanks for the feedback . A fellow I fly with regularly also has this SPD hold hunting issue.

 

Gerry -- I do use the Yaw damper, but don't really understand how it might impact the SPD Hold. I don't see any roll problems.

 

And just for the record, the VS Hold works fine.

 

Al

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In our L35 FSX/P3D world, does the AP make pitch corrections just via the trim system?

Thx,

Al

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I ran the following SPD Hold experiment tonight using different values of elevator_trim_limit as specified in the aircraft.cfg -- all under the same calm and clear wx conditions.

 

- Using the ver 2.8 elevator_trim_limit of 10 degs, setting SPD Hold at about 220Kts after takeoff resulted in repeated pitch swings of up to + - 20 degs as the a/c tried to catch up to the FD. Reminded me of a classic under-damped control response.

 

- I then set the elevator_trim_limit to 30 degs which was the value used in the previous L35 version. At normal trim setting the a/c just about leaped off the runway and it took a lot of forward yoke pressure to keep the pitch under control, and if not careful the a/c would pitch up towards a stall. However, if kept under control then when SPD Hold was activated the a/c settled down fairly quickly -- no 20 degree pitch swings.

 

- I then tried elevator_trim_limits of both 20 and 15 degrees.  The 20 degree response was not too bad, and the 15 degree response better than with a 10 degree setting, but not very good.

 

So I conclude from this admittedly simplistic experiment there is likely an issue involving the trim system which is causing the poor SPD Hold response in ver 2.8. I don't think just changing the elevator_trim_limit is the answer since other pitch response factors are involved and just changing the elevator_trim_limit will likely have unintended consequences.

 

If someone would be willing to try some version of this experiment to either corroborate or refute my results that would be useful information to have.

 

Thx,

Al

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Hi Al,

 

Yeah - sorry - I was jumping off a train when I last responded - hence my brevity... Concur - definitely see considerably more slop when trying to use "SPD Hold"... If I can manage some time this weekend - I'll ping you for testing...

 

Regards,

Scott

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I only tried the v2.8 Lear yesterday as I was waiting until for the WX Advantage Radar to be updated to make it compatible with the P3Dv3.4 hotfix2. First impressions - "SPD Hold" mode not performing as it did before for me either. I was shooting for 10,000ft at 220kts and 1500ft/min climb rate - got to 10,000ft, overshot, pitched down dramatically towards the ground, overshot, pitched sharply upwards to point it almost stalled and I had to disable the autopilot and intervene. I only had a light fuel load and 4 passengers. I'm also sure I had to set my trim value higher than previously to extinguish the 'Take-Off Trim' light on the annunciator panel (4 instead of 3). I also tried an ILS approach using NAV and GS and nearly flew into the ground due to dramatic pitch swings while the AP tried to stay on the glide slope. I only did one flight before calling it a night. Will try again when I'm sober.

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While I haven't tried it, I suspect some minor tweaking of the AP PIDs would resolve this issue.

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FSW's SPD hold code is kind of spaghetti and hard to fathom.

 

I've written proper, custom PID controllers and they do seem to work (with a bit of lapse during climbs or descents), but I need to update my 2.7 mods first before I release it. Maybe this weekend...

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've written proper, custom PID controllers and they do seem to work (with a bit of lapse during climbs or descents), but I need to update my 2.7 mods first before I release it. Maybe this weekend...

Bjoern,

Where in a typical a/c sim is the AP PID code located?  Said another way, for a default a/c like the Lear45, where would I find the AP PID code?  And in the case of the L35, I see where the AP Modes are defined, but where is the existing AP PID code that your PID would replace?  I'm missing the big picture.

Thx,

Al

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Where in a typical a/c sim is the AP PID code located?  Said another way, for a default a/c like the Lear45, where would I find the AP PID code?  And in the case of the L35, I see where the AP Modes are defined, but where is the existing AP PID code that your PID would replace?  I'm missing the big picture.

 

My PID controller runs in a 2D gauge and writes directly to the elevator trim variable. Therefor, nothing gets replaced.

 

Nav and glideslope hold controllers may be tweaked in the aircraft.cfg.

https://msdn.microsoft.com/en-us/library/cc526949.aspx#mozTocId942782

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Same problem here. SPD hold doesn't work now, but it did in V2.6 for sure.

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