Simplified c172_cruise script: altitude is not hold even with autopilot

[kevung]

Hello Jsbsim community,

In order to learn to use the c172 with autopilot, I a simplified version of the script the c172_cruise_8K.xml, with only heading and altitude autopilot commands.

I expected the altitude to stay constant to 4000ft but it decreases with time. Am I doing something wrong?

Thank you very much for your help.

The script I use:

<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xsl" href="http://jsbsim.sf.net/JSBSimScript.xsl"?>
<runscript xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:noNamespaceSchemaLocation="http://jsbsim.sf.net/JSBSimScript.xsd"
    name="C310-01A takeoff run">

  <description>Flying a plane in a straight line</description>
  <!-- <use aircraft="c172x" initialize="airborne0"/> -->
  <!-- <use aircraft="c172x" initialize="reset00"/> -->
  <use aircraft="c172x" initialize="reset01"/>

  <output name="straight_line.csv" type="CSV" rate="0.1">
  <!-- <output name="straight_line.csv" type="CSV" rate="60"> -->
    <rates>            ON </rates>
    <velocities>       ON </velocities>
    <position>         ON </position>
    <atmosphere>       ON </atmosphere>
    <fcs>              OFF </fcs>
    <ground_reactions> OFF </ground_reactions>
    <propulsion>       OFF </propulsion>
    <simulation>       OFF </simulation>
    <massprops>        OFF </massprops>
    <forces>           ON </forces>
    <moments>          ON </moments>
    <aerosurfaces>     OFF </aerosurfaces>
    <coefficients>     ON </coefficients>
  </output>

  <run start="0.0" end="2000" dt="0.008333333">

     <event name="Start engine">
        <condition>
           simulation/sim-time-sec  ge  0.01
        </condition>
        <set name="fcs/throttle-cmd-norm" action="FG_RAMP" value="0.50" tc="0.05"/>
        <set name="fcs/mixture-cmd-norm" action="FG_RAMP" value="1.0" tc="0.05"/> 
        <set name="propulsion/magneto_cmd" value="3"/>
        <set name="propulsion/starter_cmd" value="1"/>
        <notify/>
     </event>

     <event name="Trim">
        <condition>
           simulation/sim-time-sec  gt  1.0
        </condition>
        <set name="simulation/do_simple_trim" value="1"/>
        <notify>
           <property> velocities/u-aero-fps </property>
           <property> velocities/v-aero-fps </property>
           <property> velocities/w-aero-fps </property>
           <property> attitude/psi-rad </property>
           <property> attitude/theta-rad </property>
           <property> attitude/phi-rad </property>
           <property> position/h-agl-ft </property>
           <property> accelerations/pdot-rad_sec2 </property>
           <property> accelerations/qdot-rad_sec2 </property>
           <property> accelerations/rdot-rad_sec2 </property>
           <property> propulsion/engine/thrust-lbs </property>
           <property> propulsion/engine/propeller-rpm </property>
           <property> velocities/p-rad_sec </property>
           <property> velocities/q-rad_sec </property>
           <property> velocities/r-rad_sec </property>
           <property> fcs/elevator-cmd-norm </property>
           <property> fcs/aileron-cmd-norm </property>
           <property> fcs/rudder-cmd-norm </property>
           <property> fcs/elevator-pos-norm </property>
           <property> fcs/right-aileron-pos-norm </property>
           <property> fcs/rudder-pos-norm </property>
           <property> fcs/throttle-pos-norm[0] </property>
           <property> fcs/mixture-pos-norm[0] </property>
           <property> inertia/weight-lbs </property>
           <property> velocities/vc-kts </property>
           <property> velocities/ve-kts </property>
           <property> velocities/vtrue-kts </property>
           <property> atmosphere/T-R </property>
           <property> propulsion/engine/fuel-flow-rate-gph </property>
           <property> propulsion/engine/power-hp </property>
        </notify>
     </event>

  <event name="Hold heading and altitude">
     <condition>simulation/sim-time-sec ge 5</condition>
     <set name="ap/heading_setpoint" value="200"/>
     <set name="ap/heading_hold" value="1"/>
     <set name="ap/altitude_setpoint" value="4000.0"/>
     <set name="ap/altitude_hold" value="1"/>
     <notify/>
  </event>

  </run>

</runscript>

The graph of the altitude

[kevung]

Is it due to the length of the simulation? Is it due to aircraft fuel capacity?

[seanmcleod]

This is what I see when I run your script.

And zooming in on the altitude it doesn't look like the autopilot is very stable.

And looking at the airspeed and elevator.

And round the 328s mark things go pear shaped resulting in a stall.

So my initial guess is the PID gains aren't very well tuned, and maybe there is an integrator error that builds up eventually resulting in full elevator deflection leading to a stall.

I'm not familiar with this particular auto-pilot, so I'm not sure if it used to be more stable in the past and something has changed or whether it's always performed like this.

[kevung]

This is what I see when I run your script.

And zooming in on the altitude it doesn't look like the autopilot is very stable.

And looking at the airspeed and elevator.

And round the 328s mark things go pear shaped resulting in a stall.

So my initial guess is the PID gains aren't very well tuned, and maybe there is an integrator error that builds up eventually resulting in full elevator deflection leading to a stall.

I'm not familiar with this particular auto-pilot, so I'm not sure if it used to be more stable in the past and something has changed or whether it's always performed like this.

Thank you very much for your help! I'm quite new to JSBSim and to aircraft control and that was the reason why I chose to use the autopilot for a first introduction and the c172x plane seemed to be the most basic/simple one :) . I'm going to have a look into the reference manual if autopilot tuning is tackled.

Thanks again and all the best for the new year.