Re: [Jsbsim-devel] Longitudinal trim: stab or elevator

[Tony P. <ton...@co...>]

You're right about the user of the stabilizer.  Further, small airplanes
often do pitch trim with an elevator tab rather than the elevator itself.
Now, that said, changing the position of the elevator tab changes the
position of the elevator, so the c172 model sums the pitch trim and elevator
commands to emulate that.  (which is not entirely accurate ...)
 
The original trimming routine is set up to trim with pitch trim by default.
It can be configured to do other things at the C++ level, however.
 
For FG integration, using the trim systems is highly desirable as FG has no
way of setting the user's controls (joystick, throttle, ...)

  _____  

From: jsb...@li...
[mailto:jsb...@li...] On Behalf Of Fabian
Grodek
Sent: Sunday, April 27, 2008 5:13 AM
To: Development issues
Subject: [Jsbsim-devel] Longitudinal trim: stab or elevator


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