User Manual for do_simplex_trim

[LFGsim]

Hello,

Happy to be back to JSBSim after more than a decade!
Where can I find details about the different trim cases that can be done with the "simulation/do_simplex_trim" function?
For example, how does a "Custom" (option 4) trim is done?

Thank you.
Fabian

[seanmcleod]

Do you specifically mean do_simplex_trim, since this is the only reference to it in the repo.

https://github.com/JSBSim-Team/jsbsim/search?q=do_simplex_trim

Or do you mean do_simple_trim?

https://github.com/JSBSim-Team/jsbsim/search?q=do_simple_trim

jsbsim/scripts/737_test.xml

Lines 41 to 50 in 375f5be

	<!--
	For "do_simple_trim" (Classic trim):
	0: Longitudinal
	1: Full
	2: Ground
	3: Pullup
	4: Custom
	5: Turn
	6: None
	-->

[LFGsim]

In fact, my question is for both, do_simplex_trim and do_simple_trim.

do_simple_trim:

• Which option should I use for, say, a trim with OEI for straight and level flight. I guess it should be 1: Full, correct?
• What is the option "4: Custom" for?

do_simplex_trim:

• What is the difference with do_simple_trim?
• Are the same six options available?

I apologize for so many questions.
Thank you.
Fabián

[seanmcleod]

So JSBSim over the years has accumulated 3 different trim implementations, see - #227 (comment) however only 1 of them is currently enabled, i.e. the do_simple_trim legacy trim routine by Tony Peden.

The do_simplex_trim refers to Goppert's trim routines, but it isn't currently enabled. What was added back fairly recently was the linearization code of Goppert.

In terms of the various trim modes for do_simple_trim take a look at:

jsbsim/src/initialization/FGTrim.cpp

Lines 801 to 855 in ebb770e

	void FGTrim::SetMode(TrimMode tt) {
	ClearStates();
	mode=tt;
	switch(tt) {
	case tFull:
	if (debug_lvl > 0)
	cout << " Full Trim" << endl;
	TrimAxes.push_back(FGTrimAxis(fdmex,&fgic,tWdot,tAlpha));
	TrimAxes.push_back(FGTrimAxis(fdmex,&fgic,tUdot,tThrottle ));
	TrimAxes.push_back(FGTrimAxis(fdmex,&fgic,tQdot,tPitchTrim ));
	//TrimAxes.push_back(FGTrimAxis(fdmex,&fgic,tHmgt,tBeta ));
	TrimAxes.push_back(FGTrimAxis(fdmex,&fgic,tVdot,tPhi ));
	TrimAxes.push_back(FGTrimAxis(fdmex,&fgic,tPdot,tAileron ));
	TrimAxes.push_back(FGTrimAxis(fdmex,&fgic,tRdot,tRudder ));
	break;
	case tLongitudinal:
	if (debug_lvl > 0)
	cout << " Longitudinal Trim" << endl;
	TrimAxes.push_back(FGTrimAxis(fdmex,&fgic,tWdot,tAlpha ));
	TrimAxes.push_back(FGTrimAxis(fdmex,&fgic,tUdot,tThrottle ));
	TrimAxes.push_back(FGTrimAxis(fdmex,&fgic,tQdot,tPitchTrim ));
	break;
	case tGround:
	if (debug_lvl > 0)
	cout << " Ground Trim" << endl;
	TrimAxes.push_back(FGTrimAxis(fdmex,&fgic,tWdot,tAltAGL ));
	TrimAxes.push_back(FGTrimAxis(fdmex,&fgic,tQdot,tTheta ));
	TrimAxes.push_back(FGTrimAxis(fdmex,&fgic,tPdot,tPhi ));
	break;
	case tPullup:
	TrimAxes.push_back(FGTrimAxis(fdmex,&fgic,tNlf,tAlpha ));
	TrimAxes.push_back(FGTrimAxis(fdmex,&fgic,tUdot,tThrottle ));
	TrimAxes.push_back(FGTrimAxis(fdmex,&fgic,tQdot,tPitchTrim ));
	TrimAxes.push_back(FGTrimAxis(fdmex,&fgic,tHmgt,tBeta ));
	TrimAxes.push_back(FGTrimAxis(fdmex,&fgic,tVdot,tPhi ));
	TrimAxes.push_back(FGTrimAxis(fdmex,&fgic,tPdot,tAileron ));
	TrimAxes.push_back(FGTrimAxis(fdmex,&fgic,tRdot,tRudder ));
	break;
	case tTurn:
	TrimAxes.push_back(FGTrimAxis(fdmex,&fgic,tWdot,tAlpha ));
	TrimAxes.push_back(FGTrimAxis(fdmex,&fgic,tUdot,tThrottle ));
	TrimAxes.push_back(FGTrimAxis(fdmex,&fgic,tQdot,tPitchTrim ));
	TrimAxes.push_back(FGTrimAxis(fdmex,&fgic,tVdot,tBeta ));
	TrimAxes.push_back(FGTrimAxis(fdmex,&fgic,tPdot,tAileron ));
	TrimAxes.push_back(FGTrimAxis(fdmex,&fgic,tRdot,tRudder ));
	break;
	case tCustom:
	case tNone:
	break;
	}
	//cout << "TrimAxes.size(): " << TrimAxes.size() << endl;
	sub_iterations.resize(TrimAxes.size());
	successful.resize(TrimAxes.size());
	solution.resize(TrimAxes.size());
	}

[LFGsim]

Excellent!
Thank you Sean!

Fabián