ark

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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+1 I find it so bad now that I now use VS initially to climb, was fine before V2.8.

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Does anyone have problems with the altitude modes?

ALT HLD is all over the place for me and I just can't get it to work with the default logic.

 

 

My only working solution/workaround after way too much time spent with this is replacing the default behavior with another custom PID controller. It seems to work much better, also when combined with ALT SEL, but one drawback is that initial capture may be a bit rough at high vertical speeds. There might also be a bit of permanent target altitude lapse of +/- 50 ft at worst due to the inherent steady-state error, but that might be more realistic than not (it's at least less than a tenth of the RVSM limit of +/- 500 ft).

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Something has clearly happened  (probably inadvertent error or unintentional consequences)  between L35 vers 2.7 and 2.8. Ver 2.7 seemed to work fairly well as far as the altitude modes are concerned, so I have gone back to ver 2.7 while adding in a few of the more minor ver.2.8 changes, like altitude alerting at 1000ft vs 300ft and the orange attitude indicator.  My guess is the ver 2.8 altitude modes problems have to do with changes in the pitch trim system area.

 

Btw, one of the listed ver 2.8 changes is:  Altitude lower deflection indicator now is for localizer only.  For some reason I don't understand what this means -- help please?

 

Thx,

Al

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I have no idea what this means, but it surely isn't relevant to the autopilot issues.

 

 

VS hold mode was the next thing to fix. My first attempt using attitude hold mode and continuous target pitch adjustments didn't work too well, as did a PID controller, so now I'm using the bog standard VS hold function with unreachable target altitudes. Handoff to ALTSEL/ALTHLD modes still needs to be implemented.

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:  Altitude lower deflection indicator now is for localizer only.  For some reason I don't understand what this means -- help please?

 

Agree, this is counter intuitive. In Localiser there is no altitude guidance, except in the GTN 750 when using an RNav approach.

 

The Alt Hold only works for me when at selected altitude, or approaching selected altitude, in VS and <50 fpm?

 

Makes for a bit of swearing in the flight deck.

 

Regards

 

David.

 

I was just thinking what a well supported aircraft this is now, FSFX, REX WX Weather Radar, GTN 750 and MCE, so if we can get the AFCS right she's a little gem.

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I've had the Lear 35 since Flysimware first released it, and have upgraded it whenever a new

version has been released.

 

I've never had any issue with the Altitude Hold; yet for the life of me I've never been able to

get the SPD mode to work reliably, regardless of which upgrade I've flown.  After much trial and

error I've worked out a set of procedures that work.    I don't know how realistic they are, but they're

the only ways I've been able to fly the airplane in a credible fashion, without causing my "passengers"

dire distress, or injury.

 

I never use the SPD mode upon initial climbout.   The result is nearly always that I lose control

of the airplane; the jet pitches up drastically to nearly vertical unless I respond quickly; easing

back on the power only causes it to drastically pitch down, and it is a handful to manage to avoid

going into a fatal dive. 

 

Upon takeoff, once airborne, I engage the autopilot, and its heading mode; ALT SELECT is also

on from the beginning.   Once the aircraft is at the attitude and rate of climb I want, with the initial

power reduction set and the flaps are retracted, I engage the VS mode.    That procedure works

flawlessly for me.

 

I've found the SPD mode to be great for adjusting pitch during climbs. but worthless for maintaining it.

I immediately engage  VS once I'm at the attitude/rate of climb I want.  That procedure also works quite

well for me.   My experience is that the SPD mode is simply too unreliable and touchy to keep engaged

through any climb phase.

 

Where the SPD mode works very well, in my experience, is during descents.   I am able to maintain negative

pitch control with it quite nicely.    That's the other mode of flight where I've found it to be of real help.

 

Just my two cents...

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tatfsn,

What sim do you use? I ask because SPD Hold works fine for me in v2.7 with the box version of FSX, and I know this is also true for a friend I fly with. So I'm wondering if the use of different sims may be a factor that helps to explain some of the differences that are being reported.

Thx,

Al

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ark,

 

When the Lear 35 was first released I was using FSX (boxed version).    I've since switched to Prepar3d

(version 3.3) and now use that exclusively.

 

I've had the same issue with SPD hold in both versions of the sim, and in every version of the Learjet

that I've tried to use it.    Again, I've found SPD hold to be useful for the purposes that I mentioned

earlier, but those clearly seem to be not the use for which it was intended.

 

Oh, well, I still love Flysimware's latest work of art.   Their Lear and their MU-2 are just about the only planes

in P3D that I fly these days. 

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SPD hold in the Lear v2.8 still disastrous for me... I've abandoned SPD mode in v2.8 in favour of VS. Whenever I select SPD mode during a climb (2000ft/m @220kts), I immediately pitch up steeply, almost to a stall, before pitching down so steeply I either hit the ground or overspeed. It definitely worked fine for me in V2.7 though. I'm using P3DV3.4

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Hi Folks,

 

Yeah - ditto Andy - I think we're all in the same boat...

 

Best course of action is to submit tickets to Flysimware - they do respond... I've been meaning to on this issue but just haven't gotten to it yet...

 

Regards,

Scott

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I really want to buy this Lear since I have the GTN 750 but hearing about the autopilot issues is not encouraging... is this still worth buying? 

Alex

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