Howto:Implement pushback

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Note  This system does not work over the multiplayer network.
A KLM Boeing 747-400 ready for pushback at EHAM. This is the Goldhofert truck.
A F-16 being towed at KLSV. This is the military truck.
A BA Boeing 777-200 being pushbacked at EHAM. 777-200 is YASim
A LH Boeing 727-230 being pushbacked at EHAM, here using the military truck

Currently, pushback can be used on JSBSim planes andYASim aswell, but the method differs a bit depending of type of fdm.

For JSBSim

Systems/pushback.xml

Create the following directory, if not present $FG ROOT/Aircraft/.../Systems and create a file named pushback.xml with the following content: 

 <?xml version='1.0' encoding='UTF-8' ?>
 <system name="pushback">
  <property>/sim/model/pushback/target-speed-fps</property>
  <channel name="Pushback">
 
    <switch name="systems/pushback/linked">
      <default value="-1"/>
      <test value="0">
        /sim/model/pushback/position-norm gt 0.95
        /gear/gear/wow == 1
        gear/unit[0]/wheel-speed-fps lt 50
      </test>
    </switch>
 
    <summer name="systems/pushback/speed-error">
      <input>/sim/model/pushback/target-speed-fps</input>
      <input>-gear/unit[0]/wheel-speed-fps</input>
    </summer>
 
    <pid name="systems/pushback/force">
      <input>systems/pushback/speed-error</input>
      <kp>/sim/model/pushback/kp</kp>
      <ki>/sim/model/pushback/ki</ki>
      <kd>/sim/model/pushback/kd</kd>
      <trigger>systems/pushback/linked</trigger>
      <output>/sim/model/pushback/force</output>
    </pid>
 
    <switch name="systems/pushback/force-output">
      <default value="0"/>
      <test value="systems/pushback/force">
        systems/pushback/linked == 0
      </test>
      <output>external_reactions/pushback/magnitude</output>
    </switch>

    <fcs_function name="Pushback Force X">
      <function>
        <cos>
          <toradians><property>fcs/steer-pos-deg[0]</property></toradians>
        </cos>
      </function>
      <output>external_reactions/pushback/x</output>
    </fcs_function>

    <fcs_function name="Pushback Force Y">
      <function>
        <sin>
          <toradians><property>fcs/steer-pos-deg[0]</property></toradians>
        </sin>
      </function>
      <output>external_reactions/pushback/y</output>
    </fcs_function>
 
  </channel>
 </system>

This is the "cruise control" for the pushback truck. You define the cruise speed at the dialog and the controller varies the external force in a way that the actual speed matches your given value. The working horses here are the summer, which compares actual and reference speed, and the PID controller which calculates the resulting force. A basic description is: "the more actual and reference speed differ, the more force is applied (kp) plus the more time passes until the reference speed is reached, the more force is applied (ki)". The switches check, if the pushback truck is actually linked to the aircraft.

.xml (FDM)

After the </propulsion> tag we add a reference to the pushback system: 

  <system file="pushback"/>

And at the end of the FDM, the following is needed to attach the forces of the pushback to the aircraft. The location should be somewhere near the nose wheel of the aircraft (where the pushback truck is connected). 

 <external_reactions>
 
  <force name="pushback" frame="BODY">
   <location unit="IN">
    <x>  -139 </x>
    <y>    0.0 </y>
    <z>  -71.0 </z>
   </location>
   <direction>
    <x>1</x>
    <y>0</y>
    <z>0</z>
   </direction>
  </force>
 
 </external_reactions>

-set.xml

Between the <model> tags in the -set.xml file add the definitions for the speed-hold controller: 

 <model>
  <pushback>
   <kp type="double">100</kp>
   <ki type="double">25</ki>
   <kd type="double">0</kd>
   <position-norm type="double">0</position-norm>
  </pushback>
 </model>

The values for kp, ki and kd control how the cruise control reacts and may be adjusted depending on the mass of the aircraft.

Note: if the model tags are already there, you do no need to add them again. Just place the stuff between them. If you want the truck to be connected on startup, position-norm to 1.

Below the </sim> tag, we add our menu dialog. A generic one is available at $FG_ROOT/gui/Dialogs/pushback.xml. Add the following lines to include it in the Equipment menu of the aircraft. Note: if the menubar and default tags are already there, you do no need to add them again. Just place the stuff between them. 

 <menubar>
  <default>
     
   <menu n="5">
    <item n="10">
     <label>Pushback</label>
     <name>pushback</name>
     <binding>
      <command>dialog-show</command>
      <dialog-name>pushback</dialog-name>
     </binding>
    </item>
   </menu>
 
  </default>
 </menubar>

Models/ .xml

In your planes model file, we add a reference to one of the generic pushback models, or a custom made truck, shipped with your plane. Edit the offsets to fit your plane; the z offset should be at ground level, while the x offset is at the nosewheel (center).

Currently four different types are available:

Goldhofert
Used on medium sized to large airliners (for example 737, 747)
Douglas
Used on medium sized to large airliners
Military
Used on military and small aircraft (for example F-16, Hansajet)
Forklift
Used on general aviation or private jets

Both models can be found in the $FG_ROOT/Models/Airport/Pushback directory. The following example uses the Goldhofert, but by changing the path to (Douglas|Military|Forklift).xml the other trucks can be implemented.

Renders of the different models (order: Goldhofert, Douglas, Military, Forklift):

Pushbacks.png 
 <model>
  <name>Pushback</name>
  <path>Models/Airport/Pushback/Goldhofert.xml</path>
  <offsets>
   <x-m>-25.0</x-m>
   <y-m>0</y-m>
   <z-m>-4.0</z-m>
  </offsets>
 </model>

For YASim

For YASim we have to fake the external forces with the thruster. A thruster in YASim is a very simple "thrust only" engine object which does not consume any fuel etc.

.xml (FDM)

So we add the following lines to our yasim.xml: 

  <thruster x="19.246"  y="0.00" z="-2.7" vx="1" vy="0" vz="0" thrust="20000">
  <control-input axis="/sim/model/pushback/target-speed-fps" control="THROTTLE" src0="-1" src1="1" dst0="-1" dst1="1"/>
  </thruster>

For the coordinates use the same as the coordinates of the nosegear. The thrust itself is in lbs- don't set it too high! 1/3 of the aircraft's empty mass should be enough!

Then we just have to add our menu dialog. A generic one is available at $FG_ROOT/gui/Dialogs/pushback.xml. Add the following lines to include it in the Equipment menu of the aircraft. Note: If the menubar and default tags are already there, you do not need to add them again. Just place the stuff between them. 

 <menubar>
  <default>
     
   <menu n="5">
    <item n="10">
     <label>Pushback</label>
     <name>pushback</name>
     <binding>
      <command>dialog-show</command>
      <dialog-name>pushback</dialog-name>
     </binding>
    </item>
   </menu>
 
  </default>
 </menubar>

Models/ .xml

And like in JSBSim, in your planes model file, we add a reference to one of the generic pushback models, or a custom made truck, shipped with your plane. Edit the offsets to fit your plane; the z offset should be at ground level, while the x offset is at the nosewheel (center). 

 <model>
  <name>Pushback</name>
  <path>Models/Airport/Pushback/Goldhofert.xml</path>
  <offsets>
   <x-m>-25.0</x-m>
   <y-m>0</y-m>
   <z-m>-4.0</z-m>
  </offsets>
 </model>

Note: If the aircraft author didn't use gear/gear[0]/steering-norm for ground steering, the pushback model won't be turned with the nosegear

Done! Now you have a Pushback on your favourite YASim-Model!

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