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Default B738 Trim (?)

J

Jetliner

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78
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unitedstates
Sorry if this is the wrong place but, it's the closest match I could find.

I'm having major issues with my default B738 in that, upon nearing FL200, my autopilot goes nuts. Actually, it's not entirely the autopilot - it's the whole stinking aircraft!

I use a standard climb profile: 250 below 10,000 and 280 above. 1800 FPM climb rate, ALT, HDG or VOR and IAS Holds engaged. But everytime I near FL200, it starts to pitch violently like the climb rate is zipping up and down. Even when I disengage the AP and try to manually level out, the trim is just too sensitive.

I was doing a mission that measures GF with audio cues and all I kept hearing was, "positive... negative... positive... negative..." Drives me insane!

I posted over at Flightsim but got very little so far; stuff like try reconnecting the controller. Anyone here know what's up? I seem to recall something similar in FS9 - runaway trim was it called? Not sure but, I've done searches and found nothing on it.
 
Above FL200 you are insisting on speed of 280 and a climb rate of 1800fpm. I'm afraid you are pushing the boundaries too far, the aircraft simply cannot maintain that speed and climb rate and in trying to you are getting the unpredictable pitching. Try switching to Mach and allow the aircraft to select its own climb rate.
 
I just did a FRAPS video of it - it's uploading right now so I'll post it when it's done. The thing is, even after disengaging the AP, the controls are WAY too sensitive. I cut all AP holds and the master and took manual control just trying to level out - it never happened. It just kept see-sawing all over the place. The slightest movement of the yoke or trim would send it (excuse the pun) flying! But I will try dropping the climb rate. 280 is the standard climb speed for the 3-7 above 10k.

Here it is: notice that as I pass 18,000, the VSI needle starts going berserk. I actually saw it nail 14,200 FPM!
http://www.youtube.com/watch?v=9bSbEb1-eaA

I should also point out that the annoying audios ("Positive," "Negative") are indicating excess G-Forces, as I was running this in a mission I'm working on which monitors them, among other things like excess bank angle, excess pitch, etc.
 
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A video does not help, you cannot maintain 280kts TAS and 1800fpm at that altitude whether you have manual or autopilot in control.
 
I just did another one - after 10k, I set 280 on the speed and manually set 800 FPM. (It was still on 1800) This time, around 16k it started going nuts. The pitch was less than 5%, maybe 3% and it still started. Switching to Mach didn't help either. And once again, without AP was the same thing. No matter what I did with speed/attitude, the plane will not level out - period. I can't help but think there must be something in the air file or the config that needs fixing but, I haven't done anything to them to cause this.

Someone at FlightSim asked me to post a screenshot of the aircraft when it started so, I just did a video instead to show what controls were doing what.

The trim is just, all wrong, I'm guessing, because when I took control, I got the throttles set and leveled out but, I couldn't trim it out. Everytime I gave a click of trim, it would pitch like I was jamming the yoke. Maybe I need to modify my elevator_trim_effectiveness value? Right now it's 1.0
 
I don't have any problem with the standard aircraft as installed in either FS9 or FSX.
 
I was not trying to be funny about it. I have just never seen the aircraft behaving like that, but it would be expected behaviour if you were asking for too high a climb rate at too high an airspeed at that altitude. To maintain the airspeed the aircraft needs to climb more slowly but once the climb rate drops the autopilot asks for greater pitch to increase the climb rate. Then the airspeed drops and around you go again until the aircraft becomes completely uncontrollable.
 
Ah - HA! I found the culprit!

Me. :eek:

I forgot that I had done a few mods to the Aircraft.cfg. Nothing in terms of performance and control values but, I did alter the max weight to the actual value of a B738, that being 174,200 pounds (up from the default 155,500). I also had neglected to load any pax so, I was flying empty. Maybe that played a tiny role - not sure but, I think the max weight did because now it's fine. I cruised right on up to FL340 without a hitch. So, now I have to figure out what the heck happened in the cfg.

Thanks for the input anyway!
 
Well I am glad you got it sorted out. Altering anything inside the aircraft.cfg can have dramatic effects and you should in no way try to alter figures to the real life ones, it just doesn't work.

You will find that some of the figures interact with others in an unpredictable way and for real life you would find a large number of the figures in most .cfg files are way off.
 
Oh my aching back! Sorry for this thread - it was so stupid!

I had edited the station points because I was working on making a standardized format: Main Cabin Fore, Main Cabin Mid, etc. Anyway, I had everything shifted forward 1 section so my center of gravity was in the cockpit!! :eek: No wonder the thing kept trying to act like a rocket!

Well, that's fixed now. I'll take that dope-slap anytime. :rolleyes:
 
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