I wonder if FSUIPC can help me with this...

[kerke]

Hi Pete, as you know, MSFS 2004's autopilot uses elevetor trim to control altitude hold and basically all autopilot's vertical axis functions. My problem is that when I disconnect the autopilot, my FS 2004 aircraft are always out of trim and therefore they jump up and down until I manually recover them. This is not a specific issue with a specific aircraft, this issue happens with all FS2004 aircraft that use the default FS2004 autopilot, and NOT with those that use their own autopilot such as the PMDG 737NG.

Some real pilots recommend that I trim the aircraft before I activate the autopilot ALT control, and I do. But it really makes no difference because one you hit the A/P ALT button, FS 2004 uses "only" elevator trim to control the aircraft vertical funtions, as opposed to real airplanes that typically don't use elevator trim (or use very little of it) to hold ALT, climb and descend on autopilot ALT mode.

Is there a way to force the FS 2004 autopilot to behave like a real-world aircraft and don't use elevator trim as the primary way to control vertical functions? If not, is there a way to avoid my aircarft from suddently and abruptly climbing or descending when I disconnet the autopilot?

Thank you in advance,

*** I'm using the user-registered FSUIPC version 3.4.

Kerke

[Pete Dowson]

My problem is that when I disconnect the autopilot, my FS 2004 aircraft are always out of trim and therefore they jump up and down until I manually recover them.

Erno, I think that's wrong. As the A/P is actually managing to keep a correct attitude for whatever it is being told to do, the aircraft is stable at that pitch with that trim -- assuming your elevator controls are doing nothing, i.e. centred or left wherever you put them. When you then disengage the A/P the aircraft should, by definition, remain stable at that attitude. That certainly is the case here, for all default aircraft. In fact one "cheating" way of getting an aircraft into perfect trim in FS has always been to engage the A/P, let it trim, then disengage it. As far as i know that hasn't changed for many many versions.

Of course it is not realistic for it to use trim in this way -- a true autopilot pitch control would operate the elevator AND then trim out the forces, just like the pilot would. But this would be no good at all for FS, because it would, indeed have the result you say -- with the elevator non-centred for a particular pitch, when the A/P is disconnected, the user's auto-centred elevator axis would immediately cause the aircraft to be out of trim.

In other words, Microsoft have done it the way they've done it specifically to avoid the problem you say you've got!

Is there a way to force the FS 2004 autopilot to behave like a real-world aircraft and don't use elevator trim as the primary way to control vertical functions?

No, that would actually be rather catastrophic, except in very sophisticated cockpits where some sort of motor control can move your elevator axis for you. As I said, the way it is is not realistic, but is done to ensure you do NOT get the very problem you seem to think you get!

Think about it. If in a climb it pulled its elevator stick back and trimmed out, as it should, and you then disengage it and let your *centred* elevator suddenly take control, you'd be diving before you know it!

If not, is there a way to avoid my aircarft from suddently and abruptly climbing or descending when I disconnet the autopilot?

I really do not know how you are achieving that -- I cannot make it do it here, and when you think it though you must surely see that what you are saying isn't logical. As long as the A/P always uses trim only, the centred (or original, if not centred) position of your joystick should be correct when you release the A/P.

Regards,

Pete

[Pete Dowson]

My problem is that when I disconnect the autopilot, my FS 2004 aircraft are always out of trim and therefore they jump up and down until I manually recover them.

Erno, I think that's wrong. As the A/P is actually managing to keep a correct attitude for whatever it is being told to do, the aircraft is stable at that pitch with that trim -- assuming your elevator controls are doing nothing, i.e. centred or left wherever you put them. When you then disengage the A/P the aircraft should, by definition, remain stable at that attitude. That certainly is the case here, for all default aircraft. In fact one "cheating" way of getting an aircraft into perfect trim in FS has always been to engage the A/P, let it trim, then disengage it. As far as i know that hasn't changed for many many versions.

Of course it is not realistic for it to use trim in this way -- a true autopilot pitch control would operate the elevator AND then trim out the forces, just like the pilot would. But this would be no good at all for FS, because it would, indeed have the result you say -- with the elevator non-centred for a particular pitch, when the A/P is disconnected, the user's auto-centred elevator axis would immediately cause the aircraft to be out of trim.

In other words, Microsoft have done it the way they've done it specifically to avoid the problem you say you've got!

Is there a way to force the FS 2004 autopilot to behave like a real-world aircraft and don't use elevator trim as the primary way to control vertical functions?

No, that would actually be rather catastrophic, except in very sophisticated cockpits where some sort of motor control can move your elevator axis for you. As I said, the way it is is not realistic, but is done to ensure you do NOT get the very problem you seem to think you get!

Think about it. If in a climb it pulled its elevator stick back and trimmed out, as it should, and you then disengage it and let your *centred* elevator suddenly take control, you'd be diving before you know it!

If not, is there a way to avoid my aircarft from suddently and abruptly climbing or descending when I disconnet the autopilot?

I really do not know how you are achieving that -- I cannot make it do it here, and when you think it though you must surely see that what you are saying isn't logical. As long as the A/P always uses trim only, the centred (or original, if not centred) position of your joystick should be correct when you release the A/P

Regards,

Pete

[kerke]

> really do not know how you are achieving that -- I cannot make

> it do it here, and when you think it though you must surely see

> that what you are saying isn't logical. As long as the A/P always

> uses trim only, the centred (or original, if not centred) position

> of your joystick should be correct when you release the A/P.

Hi Pete, thank you for your quick reply.

I did not mention on my post that I don't use a joystick. I've been flying FS with the plain keyboard ever since the FS for the Commodore 64 days, and I've become pretty good at it.

To reproduce the autopilot disconnect issue, I take any MSFS GA aicraft, I move the elevator control all the way up, then all the way down, then center it (I do this a few times), I adjust my trim for takeoff, I set the autopilot ALT to, say, 2000 feet and my vertical speed to 700 or maybe a little more. I then takeoff and engage the A/P at 500' AGL, I hit the ALT button to allow the autopilot to climb my aircarft to 2000 feet, and I set my heading bug to fly straight and leveled, then I hit the HDG button. Once my aircraft is flying at 2000, leveled and straight, I hit the ALT button to disconnect the altitude hold, immediatelly my aircraft starts to descend or sometimes starts to climb, but it does not continue to fly straight. It gets "much much" worse once you land the plane and takeoff again: many times when I hit the ALT button to disconnect the A/P ALT hold, my airplane goes into a dive or sometimes it will start to climb real bad.

I actually tried to reproduce this very same scenario on this baby (http://www.kerke.us/avsim/09192004a/1.JPG), with the same A/P settings over at Tamiami Executive Airport KTMB Rwy 9R, we used the same settings (http://www.kerke.us/avsim/09192004a/3.JPG), and when we disconnected the A/P at 2000 on a leveled flight approach to Rwy 9R, the airplane continued flying leveled and straight as if the A/P ALT button was still ON.

Anyway, I've installed FS9.1 on my daughter's Dell computer and I did have the same problem.

Best regards,

Kerke

[Pete Dowson]

I did not mention on my post that I don't use a joystick. I've been flying FS with the plain keyboard ever since the FS for the Commodore 64 days, and I've become pretty good at it.

Aha! You should have said soyes, in that case you are probably correct. It isn't something I've ever investigated. Sorry.

How would you expect FSUIPC to deal with the problem when you disengage the A/P? By experiment here it looks as if the keyboard-set elevator value is retained when the A/P takes over. And in fact it seems that the keyboard elevator is still effective with A/P ALT control operating -- all that happens is that as you change the elevator via the 2/8 keys, the A/P compensates by changing the trim.

The problem seems to be that it doesn't zero the elevator (as a joystick centering would). Maybe if there were an option in FSUIPC to zero the elevator position automatically when A/P is engaged or disengaged, this would help? I'll try that here.

Regards

Pete

[kerke]

> The problem seems to be that it doesn't zero the

> elevator (as a joystick centering would). Maybe if there

> were an option in FSUIPC to zero the elevator position

> automatically when A/P is engaged or disengaged, this

> would help? I'll try that here.

Brilliant!!! Thanks again Pete, if you ever add that feature to FSUIPC, please let me know (my email address is jorge@kerke.us), I'd be happy to test it for you, and I would immediately know if the feature solved the A/P ALT disconnect issue.

Thinking to myself: The question now is at which point should the elevator be zeroed: (a) when the ALT button is engaged, (b) when the aircraft vertical speed reaches "0" (meaning the aircraft is perfectly leveled), or © when the ALT button is pressed to disengage ALT hold? The ideal point at which the elevator should be zeroed by an external module is probably when is does not cause any sudden up/down side effect. Then, when either the ALT button or the A/P are disengaged, the aircraft would continue flying straight because the elevator is at "0". I think you nailed it right in the head :wink:

I'm sure the addition of this feature to FSUIPC would be greatly appreciated by the many "keyboard" FSimmer out there :D

Regards,

Kerke

KTMB

[Pete Dowson]

if you ever add that feature to FSUIPC, please let me know (my email address is jorge@kerke.us), I'd be happy to test it for you, and I would immediately know if the feature solved the A/P ALT disconnect issue.

I'm sending an interim test version of FSUIPC for you to try. It seems to work well here -- I zero the elevator on AP Alt engagement and disengagement. Probably only the latter was needed, but I think it helps make sure the AP doesn't use stupid trim settings too.

Let me know how you get on please. Check the "Technical" options page for the new feature. It's only there for FS2004 -- but the facility is usable on earlier versions. For those you have to edit the FSUIPC.INI file (ZeroElevForAPAlt=Yes).

Regards,

Pete

[kerke]

Pete, I will test it tonight when I get home, then I will post my results on this forum thread. Thank you for your excellent tech support.

Kerke

KTMB

[kerke]

Pete, I tested your interim test version of FSUIPC and I can report that it did work! Your new "Center Elevator on AP ALT" feature fixed the FS2004 AP ALT disconnect problem -- Brilliant!

Thank you so much -- next time you're in Miami, I'll buy you lunch :lol:

Kerke

KTMB

[Pete Dowson]

Pete, I tested your interim test version of FSUIPC and I can report that it did work! Your new "Center Elevator on AP ALT" feature fixed the FS2004 AP ALT disconnect problem -- Brilliant!

Good. It'll be in the next official user release, which I was aiming for early next week but now looks though it may be a bit later (due to other things, not this). Please remember to update to an official release when it is available.

Regards,

Pete