A question about setting the integration step

[ujust4fun]

I met a problem with setting the integration step.

Expected your help!

[ujust4fun]

I met a question when I set the different time-step.

import jsbsim
import matplotlib.pyplot as plt
import print

fdm = jsbsim.FGFDMExec(None)  # Use JSBSim default aircraft data.
fdm.load_model('737')
fdm.run_ic()
fdm['propulsion/engine[0]/set-running']=1
fdm['propulsion/engine[1]/set-running']=1
fdm['fcs/throttle-cmd-norm']=1

fdm["fcs/left-brake-cmd-norm"] = 0
fdm["fcs/right-brake-cmd-norm"] = 0
fdm["fcs/brake-parking-cmd-norm"] = 0
fdm.set_dt(0.04)
mach=[]
time=[]
while fdm.run() and fdm.get_sim_time()<50:
    time.append(fdm.get_sim_time())
    print('Sim time:',fdm.get_sim_time())
    print(fdm["position/h-sl-meters"])
    mach.append(fdm['velocities/mach'])
plt.plot(time,mach,label='dt=0.04')
fdm.reset_to_initial_conditions(1)
fdm.run_ic()
fdm['propulsion/engine[0]/set-running']=1
fdm['propulsion/engine[1]/set-running']=1
fdm['fcs/throttle-cmd-norm']=1

fdm["fcs/left-brake-cmd-norm"] = 0
fdm["fcs/right-brake-cmd-norm"] = 0
fdm["fcs/brake-parking-cmd-norm"] = 0
fdm.set_dt(0.08)
mach2=[]
time2=[]
while fdm.run() and fdm.get_sim_time()<50:
    time2.append(fdm.get_sim_time())
    print('Sim time:',fdm.get_sim_time())
    print(fdm["position/h-sl-meters"])
    mach2.append(fdm['velocities/mach'])
plt.plot(time2,mach2,label='dt=0.08')
plt.legend()
plt.show()

the result is different caused by the different set_time(0.04) set_time(0.08). I am confused by this. Could you explain it to me？ thx

[ujust4fun]

By the way, fdm.reset_to_initial_conditions(1) is a reset function, but what the parameter means. I am confused by its value explanation

[bcoconni]

You are calling fdm.run_ic() without having properly set the initial conditions first.
If I had fdm['ic/h-sl-ft'] = 4.0 to your script:

fdm = jsbsim.FGFDMExec(None)
fdm.load_model('737')
fdm['ic/h-sl-ft'] = 4.0  # Spawn the aircraft 4ft above the ground.
fdm.run_ic()

By default, JSBSim uses an initial height of 0ft and that results in the landing gears being compressed by almost 4ft. When the simulation starts, the compression energy in the landing gears is released and the aircraft is unrealistically propelled to the outer space (more details are given in the discussion #819 albeit your case is a variation as you don't hit the ground but initiates the simulation as if the aircraft had just violently hit the ground).

Once I fixed the initial height parameter, I'm getting something that is more plausible

Of course, relaxing the time step results in a lower fidelity simulation.

[bcoconni]

By the way, fdm.reset_to_initial_conditions(1) is a reset function, but what the parameter means. I am confused by its value explanation

The value is meant to open new output files upon reset or continue logging data in the same output files:

• Setting the parameter to 1 means that the current output files will be closed and new output files (with their name appended with a counter _1, _2, etc.) will be used.
• Setting the parameter to 0 means that the simulation will continue logging its output in the same output files than before calling fdm.reset_to_initial_conditions.

[ujust4fun]

Still have a question when i want to output the data from the 737.xml.
I have set the output code like follows,

fdm.set_output_filename(0, 'this_one.csv')
fdm.enable_output()
while fdm.run() and fdm.get_sim_time()<50:
    time2.append(fdm.get_sim_time())
    h2.append(fdm["position/h-sl-meters"])
    mach2.append(fdm['velocities/mach'])
plt.plot(time2,h2,label='dt=0.08')
plt.legend()
plt.show()

and also , I copied the output property from the c172x.xml to 737.xml and commented the other output method in latter.

    <output name="JSBout777.csv" type="CSV" rate="10">
        <rates> ON             </rates>
        <velocities> ON        </velocities>
        <position> ON          </position>
        <atmosphere> OFF       </atmosphere>
        <fcs> ON               </fcs>
        <ground_reactions> OFF </ground_reactions>
        <propulsion> ON       </propulsion>
        <simulation> ON        </simulation>
        <massprops> ON        </massprops>
        <forces> OFF           </forces>
        <moments> OFF          </moments>
        <aerosurfaces> OFF      </aerosurfaces>
        <property> position/vrp-gc-latitude_deg </property>
        <property> position/vrp-longitude_deg   </property>
        <property> position/vrp-radius-ft       </property>
        <function name="velocities/pi-deg_sec">
          <todegrees> <p> velocities/pi-rad_sec </p> </todegrees>
        </function>
    </output>

However, there is none output . Where did i wrong? hope you can help me.

[bcoconni]

Output files are written to the output path that can be accessed using the following command:

fdm.get_output_path()

So your file should be located in the folder returned by this command.

If you need output files to be written elsewhere, use fdm.set_output_path()

Also to rename the output file from JSBout777.csv to this_one.csv, the command fdm.set_output_filename() should be called before fdm.run_ic() or it will be ignored. And you don't need to call fdm.enable_output() as it is enabled by default.

The first lines of your code should look something like:

fdm = jsbsim.FGFDMExec(None)
fdm.load_model('737')
fdm['ic/h-sl-ft'] = 4.0  # Spawn the aircraft 4ft above the ground.
fdm_set_output_path('folder/for/output/files')
fdm.set_output_filename(0, 'this_one.csv')
fdm.run_ic()