How to take off manually in a real-time simulation?

[hesetone]

Hi:
Thanks for your great work in JSBSim. I am trying to get a real-time simulation. In my project, I setup a workbench program named wbControl by Qt, which could send commands to JSBSim.exe in the form of set fcs/elevator-cmd-norm 1,set propulsion/engine[0]/set-running 1,set fcs/throttle-cmd-norm etc. My purpose is sending commands to control a take off activity, I want to control its take-off just by commands when the plane is on the ground and at rest.

1, I choose jsbsim/aircraft/737/737.xml as the test aircraft, jsbsim/aircraft/737/reset00.xml as my initial files.

2, I write commands in the order below, these will be send to JSBSim.exe via socket(IP:5139) in time order :

Time 5.0 s:
Send "set fcs/steer-cmd-norm 0.5"
Send "set fcs/elevator-cmd-norm 0.5"
Time 7.0 s:
Send "set fcs/steer-cmd-norm 0.0"
Send "set fcs/elevator-cmd-norm 0.0"
Time 15.0 s:
Send "set propulsion/starter_cmd 1"
Time 20.0 s:
Send "set fcs/throttle-cmd-norm[0] 0.6"
Send "set fcs/throttle-cmd-norm[1] 0.6"
Time 50.0 s:
Send ""set fcs/steer-cmd-norm 1.0"
Time 55.0 s:
Send "set fcs/steer-cmd-norm 0.0"
Commands above is extract from B737_Runway.xml. But when these commands are sent to JSBSim.exe, the vc-kts always stays ZERO and position/h-agl-ft stays 49.0 inches all the simulation process. it isn't meet my need, some log file is below:

----------------------JSBSim Execution Beginning----------------------
Start: Monday November 21 2022 15:25:56 (HH:MM:SS)
simulation elapsed time: 0.0083 vc-kts: 0.1589 Distance: 0.0000 ASL: 1.2446
simulation elapsed time: 5.0000 vc-kts: 0.0000 Distance: 0.0081 ASL: 1.1247
Receive() returns: set fcs/steer-cmd-norm 0.5
[set] [fcs/steer-cmd-norm] [0.5]
ready to reply successful.
Receive() returns: set fcs/elevator-cmd-norm 0.5
[set] [fcs/elevator-cmd-norm] [0.5]
ready to reply successful.
Receive() returns: set fcs/steer-cmd-norm 0.0
[set] [fcs/steer-cmd-norm] [0.0]
ready to reply successful.
Receive() returns: set fcs/elevator-cmd-norm 0.0
[set] [fcs/elevator-cmd-norm] [0.0]
ready to reply successful.
simulation elapsed time: 10.0000 vc-kts: 0.0000 Distance: 0.0081 ASL: 1.1247
simulation elapsed time: 15.0083 vc-kts: 0.0000 Distance: 0.0081 ASL: 1.1247
Receive() returns: set propulsion/starter_cmd 1
[set] [propulsion/starter_cmd] [1]
ready to reply successful.
simulation elapsed time: 20.0083 vc-kts: 0.0000 Distance: 0.0081 ASL: 1.1247
Receive() returns: set fcs/throttle-cmd-norm[0] 0.6
[set] [fcs/throttle-cmd-norm[0]] [0.6]
ready to reply successful.
Receive() returns: set fcs/throttle-cmd-norm[1] 0.6
[set] [fcs/throttle-cmd-norm[1]] [0.6]
ready to reply successful.
simulation elapsed time: 25.0083 vc-kts: 0.0000 Distance: 0.0081 ASL: 1.1247
simulation elapsed time: 30.0083 vc-kts: 0.0000 Distance: 0.0081 ASL: 1.1247
simulation elapsed time: 35.0083 vc-kts: 0.0000 Distance: 0.0081 ASL: 1.1247
simulation elapsed time: 40.0167 vc-kts: 0.0000 Distance: 0.0081 ASL: 1.1247
simulation elapsed time: 45.0083 vc-kts: 0.0000 Distance: 0.0081 ASL: 1.1247
simulation elapsed time: 50.0083 vc-kts: 0.0000 Distance: 0.0081 ASL: 1.1247
Receive() returns: set fcs/steer-cmd-norm 1.0
[set] [fcs/steer-cmd-norm] [1.0]
ready to reply successful.
simulation elapsed time: 55.0083 vc-kts: 0.0000 Distance: 0.0081 ASL: 1.1247
Receive() returns: set fcs/steer-cmd-norm 0.0
[set] [fcs/steer-cmd-norm] [0.0]
ready to reply successful.
simulation elapsed time: 60.0167 vc-kts: 0.0000 Distance: 0.0081 ASL: 1.1247

In here:
ASL is taken from function FDMExec->GetPropagate()->GetAltitudeASLmeters(),
Distance is taken from function FDMExec->GetAuxiliary()->GetDistanceRelativePosition(),
vc-kts is taken from FDMExec->GetAuxiliary()->GetVcalibratedKTS().

so :
1, how can I take the 737 off manually by commands?
2, how to write the correct command to take the plane off when an aircraft type be specified next time?
Sincerely looking forward to your help, thank you.

[seanmcleod]

Have you logged the engine thrust to confirm that the engines are actually running and producing thrust?

You seem to have missed the propulsion/cutoff_cmd command when you copied the existing script?

jsbsim/scripts/B737_Runway.xml

Lines 26 to 32 in 8fb7741

	<event name="Advance Throttle">
	<condition>simulation/sim-time-sec ge 20.0</condition>
	<set name="propulsion/cutoff_cmd" value="0"/>
	<set name="fcs/throttle-cmd-norm[0]" action="FG_RAMP" value="0.6" tc="1.0"/>
	<set name="fcs/throttle-cmd-norm[1]" action="FG_RAMP" value="0.6" tc="1.0"/>
	<notify/>
	</event>

[hesetone]

Ohhhhh,thank you, Sean, but there is another question. when i add propulsion/cutoff_cmd on, it really works. but it seems that the plane will not hold the altitude, it will climb continuously from 220s or 230s approximately. The process is plot as below image(paint by gnuplot). Now, I don't want use ap/autopilot-roll-on, ap/altitude-hold or anything else here. Can I hold the altitude just by sending command like set fcs/xxxxxxxx 1?

The image above is the result of 737 test:
vc-kts is taken from velocities/vc-kts
h-agl-ft is taken from position/h-agl-ft
theta is taken from attitude/theta-rad

Actually, I did another same 737 test, in which I sent a command set fcs/elevator-cmd-norm 0.125 to JSBSim.exe between 220s and 230s. then the plane began to dive sharply, and it seems the plane is out control.

Back to the same question , I have a same test on aircraft OV10, the commands I sent is below:

Time 1.0 s:
Send "set simulation/do_simple_trim 2"
Time 2.0 s:
Send "set fcs/right-brake-cmd-norm 0"
Send "set fcs/left-brake-cmd-norm 0"
Send "set propulsion/starter_cmd 1" 
Time 8.0 s: /// I guess `propulsion/engine[0]/n2` will increase over 15 8.0s later.
Send "set propulsion/cutoff_cmd 0"
Send "set fcs/throttle-cmd-norm[0] 0.4"
Send "set fcs/throttle-cmd-norm[1] 0.4"
Time 13.0 s:
Send "set fcs/throttle-cmd-norm[0] 1"
Send "set fcs/throttle-cmd-norm[1] 1"

Then I got result as below:

it seems that aircraft OV10 didn't climbed at all. How can I take the plane OV10 off and hold its altitude just by commands?
Thank you very much for your help.

[seanmcleod]

In general, think what a human pilot would do. So for the OV10 case how about moving the elevator once you build up enough speed? It looks like the aircraft just continues to run along the ground at about 170kt for ever.

In terms of the 737 and holding altitude, think again what a human pilot or an auto-pilot has to do to hold altitude. They basically have to adjust the throttle and the elevator in order to trim the aircraft, and keep it in trim since as you fly fuel will be burnt off so the mass of the aircraft will decrease and the cg will shift, so you'll need to keep adjusting both throttle and elevator to keep it in trim.

So either you need to make use of an existing auto-pilot implementation, or write your own. Your only other option is to get JSBSim to perform a trim operation, which will be valid for a while until the fuel burn starts to make a noticeable difference.

[hesetone]

Thank you, Sean, I have a better understanding of this now.