[Jsbsim-devel] Longitudinal trim: stab or elevator

[Fabian G. <fab...@gm...>]

Hello,
Both original and new trim algorithms perform the longitudinal trim with the
elevator deflection. This is done by iterating with elevator-cmd-norm in the
cost function until ~zero pitching moment is achieved.

Since not all aircraft trim the pitching moment with the elevator (indeed,
most airliners and business jets trim in pitch with the horizontal
stabilizer) a more general algorithm would be to do the iterations with the
pitch-trim-cmd-norm. The user could then use this pitch-trim-cmd-norm in the
FCS block as input to either the elevator or to the horizontal stabilizer,
depending on his aerodynamic database buildup components and/or aircraft
flight control system.

I succeeded in making this modification to my local JSBSim branch for the
longitudinal trim case (the TODO comments of Agostino de Marco helped me a
lot!); now I can run a script in which I trim the aircraft with the
horizontal stabilizer and after a few seconds I perform some maneuver with
the elevator while the stab is kept fixed.
I put the aircraft on the air at some altitude and speed, make
a do_trim_analysis (longitudinal), and then pull up with the elevator. It's
really nice!
I'm still having problems when starting with the aircraft on the ground
ready for takeoff - I still need to debug that.

My proposal is to discuss the possibility of making this modification to the
official code.
So far I see the following advantages and also question marks:
1. Flexibility to trim with either the elevator or the stab.
2. How would this affect JSBSim-Flightgear integration? - I'm having some
trouble there, too.
3. Would this change affect something that I do not see right now?
4. There must have been some reason that motivated the use of the elevator
for trim (maybe the lack of tail-off aerodynamic data availability?, or
downwash?).

Comments?

Regards,
Fabian