F-86F Sabre Rocks Side to Side between waypoints on autopilo

[Turnit]

I have been flying it a bit lately, and there is a thing it does between way points while it is on autopilot. It will rock side to side, and I have to slow it down to about 170 to 200 knots to get it to stabilize then speed back up. This will always happen only at cruise altitude, not if you switch way points while climbing or decent. Does anyone else fly this and see this?

[wkitty42]

i can't say that i've flown this craft but the description sounds like what i see the AI craft doing all the time... that is constantly twitching to stay on the flight path... in your case, it is probably more noticeable at high speeds than slower ones... does it do the same and is it as bad in the latest version 2016.4.3??

[Turnit]

I actually haven't tried it in the latest version. I typically cruise at 300 knots, slower speeds don't have the problem it seems.

[sanhozay]

I had a quick look at the files and It's using the generic autopilot, which obviously isn't tuned to any specific aircraft. It's quite an aggressive autopilot and what you describe sounds like instability in the roll axis controllers. When it makes a correction, it over corrects and overshoots where it wants to be, so then it corrects back the other way, but over corrects and so on ad nauseum.

If you want to try tuning it, you should copy the generic autopilot from fgdata into the aircraft's Systems directory and set it up as the autopilot in the -set.xml file, rather than change fgdata. I'd probably start with the wing leveler; halve the Kp to 0.25 and double the Ti to 20. Give it a test at 300 and if that sorts it, do the same to the other roll modes -- heading hold, etc.

[Turnit]

Okay, I will try those changes next time I fly it and I will report back if they work. Thanks for the help, and have a happy holiday!

[Turnit]

Someone can correct me if im wrong, but the F-86 has no systems folder in he files....

[Johan G]

You can add one.

Note: IIRC some things (pitot systems come to mind) are hardcoded to use "Systems" (with a capital "S"). It might not be used now* but may come in later

* Or rather, it is borrowed from the generic aircraft in, if I recall correctly, $FG_ROOT/Aircraft/Generic/Systems when an aircraft is loaded ($FG_ROOT being the "data" directory of the FlightGear installation).

Getting off topic here, but... Configuring for example pitot systems in the aircraft will override the properties of the generic aircraft. Things one can configure are for example parallel pitot systems so that one pitot/static system will drive the instruments for the pilot and one autopilot, a second one will drive the copilots instruments and second autopilot and a third one will drive the standby instruments, allowing for more "interesting" failure scenarios. For more info on that see $FG_ROOT/Docs/README.systems.

[Turnit]

There is a autopilot in the generic folder, but not under the systems folder. Thanks for the help.

[Turnit]

Do you guys think any editing in this code in the -set file is needed to fix the problem?

Code: Select all

<autopilot>
   <config>
     <min-climb-speed-kt type="float">70.0</min-climb-speed-kt>
     <best-climb-speed-kt type="float">75.0</best-climb-speed-kt>
     <target-climb-rate-fpm type="float">500.0</target-climb-rate-fpm>
     <target-descent-rate-fpm type="float">1000.0</target-descent-rate-fpm>
     <elevator-adj-factor type="float">4000.0</elevator-adj-factor>
     <integral-contribution type="float">0.01</integral-contribution>
     <zero-pitch-throttle type="float">0.45</zero-pitch-throttle>
     <zero-pitch-trim-full-throttle type="float">0.14</zero-pitch-trim-full-throttle>
   </config>
 </autopilot>


[wkitty42]

not unless it is integral-contribution, no... i don't see anything there that would indicate the accuracy for following the flightpath...

my 2 centavos as a still learning newbie

[Johan G]

Since you might possibly be making the autopilot from scratch, I think this wiki article might be of help (though you might already have seen it): Howto:Design an autopilot (perm).

PID controllers can be approached in many ways and can be both very frustrating and joyful to tweak. It may be a good idea to look at YouTube some tutorials on PID-tuning.

[Turnit]

I made the tweaks mentioned earlier to half the kp value and double the ti value and was connecting everything up in the -set file, and saw that in the code. Ill take a look at that article. Im a newbie at all of this, so thanks for all your help.

[Johan G]

You are most welcome.

[Octal450]

In a basic statement for Kp:
If the plane overcorrects himself constantly... reduce Kp slightly, until it stops.
If the plane won't react enough, increase Kp slightly, until it works optimally.
The Ti should be fine 10-20 in most cases. You can always add a max-rate-of-change filter, to reduce load, and give the AP some wiggle room.

[Johan G]

You can always add a max-rate-of-change filter, to reduce load, and give the AP some wiggle room.

Hmm, that is an interesting technique. Thanks for the inspiration.