JSBSim Simulink Function

[kvgkrish]

Hi all,
This is Gokul doing a Master of Aerospace Engineering at TUM Germany. Currently working on master thesis at Chalmers University of Technology.
I am working on a Bio-Inspired UAV dynamics system. I am new to JSBSIM and I have set up all the files and worked fine with available flights and scripts. If I use a new flight configuration I have simulated the model. It shows trim failure and sometimes flight scripts are not used.
Can you help me to resolve this issue?

[seanmcleod]

Are you sure the aircraft should be trimmable at the initial conditions you're specifying for the trim? Or there could be an issue with the FDM file you've created.

If you share the FDM file and the conditions you're using for trim others could potentially take a look and help.

[kvgkrish]

I have attached the flight data, script, and initial conditon for your reference.

aircraft is StratoSurfer, Scipts/Test_StratoSurfer_Cruise, and the Initial condition is initCruise.

https://github.com/UASLab/OpenFlightSim/blob/master/Simulation/aircraft/StratoSurfer/StratoSurfer.xml

[seanmcleod]

Just had a very quick glance. Is the engine definitely on and running? I see the throttle being advanced, but it's not 100% clear that the engine is running.

What is the output in terms of thrust and power and h-agl-ft below?

    <event name="Trim">
      <condition> simulation/frame gt 1 </condition>
      <set name="simulation/do_simple_trim" value="1"/>
      <notify>
        <property>velocities/vtrue-kts</property>
        <property>position/h-agl-ft</property>
        <property>aero/alpha-deg </property>
        <property>aero/beta-deg </property>
        <property>propulsion/engine[0]/thrust-lbs</property>
        <property>propulsion/engine[0]/power-hp</property>
        <property>attitude/phi-rad</property>
        <property>attitude/theta-rad</property>
        <property>attitude/psi-rad</property>
      </notify>
    </event>

[kvgkrish]

I have attached the engine power and new script for your review.
Even after this modification, model has failed du to trim error.

[seanmcleod]

The trim routine will adjust the throttle while trying to calculate a trim solution. So it doesn't matter what you set the throttle position to.

But as I mentioned, I'm not sure that the engine is on/running, in which case it doesn't matter what the trim routine does in terms of setting the throttle command, if the engine isn't on it will produce 0 thrust.

Look at the output you produced just above, it shows the thrust as being 0.

[kvgkrish]

Thanks for the response. How do you make sure the engine is on or not?
I faced this issue while editing the aircraft file and getting this error.

[seanmcleod]

Take a look at this example of using the <running> element.

jsbsim/aircraft/737/rudder_kick_init.xml

Line 11 in 4b3ec67

<running> -1 </running>

The -1 means all engines that the aircraft has.

[kvgkrish]

Now I am getting Very high thrust and again trim failure.

[seanmcleod]

Well then there is something wrong with your engine, thruster model etc. With that sort of maximum thrust and a vehicle with a mass of only 3lbf, the trim routine probably can't find a small enough throttle command like 0.000001 to us to get a trim solution.

[kvgkrish]

I have reduced the command, but it is still getting very high value.

[seanmcleod]

No, no, no.... As I mentioned, the trim routine will try different throttle commands when trying to calculate a trim solution, so it doesn't matter what you set the throttle command to in the Motor Start event!

The point is it appears the engine power is way too high for this sort of vehicle. And the trim routine has probably some minimum throttle increment that it tries when trying to find a solution which is way too much in terms of thrust.

Think about it in terms of a rough sanity check, should a 3lbf vehicle have a motor of 350W, producing some enormous amount of thrust?

[kvgkrish]

It is a very small motor. I have attached the details of it.

[seanmcleod]

Compare the amount of thrust you're seeing to the thrust measured for this engine for different propellers etc.

https://www.innov8tivedesigns.com/images/specs/Cobra_2217-8_Specs.htm

[kvgkrish]

Yes, it is very high compared to the tabulated value.
How to resolve this issue.

[seanmcleod]

Step through the JSBSim code where it calculates the thrust and see if you spot what input is making it so large.