Re: [Flightgear-devel] Feeding FlightGear data through the Protocol Pipe

[Bruce J. <br...@ja...>]

Date: Wed, 29 Jan 2014 17:02:51 +0100 (CET), Anders Gidensta <and...@gi...> wrote:
> 
> 
> On Wed, 29 Jan 2014, Jon S. Berndt wrote:
> 
>> Would it be possible to use net_fdm to send named properties over to
>> FlightGear?
> 
> IIRC net_fdm is just a fixed format packet. Changing that would be a major 
> change and would need measures to preserve backwards compatibility.
> 
> The quickest way to your goal with least (if any) source changes would be 
> to switch to a generic protocol - with a JSBSim output specification you 
> should be able to stream a comma separated list of states (rows) to FG.
> You then set up the generic protocol for FG to take each element in the 
> rows and put it in the right FG property.
> 
> This way the set of properties to transfer would be defined by XML files 
> both on the JSBSim and on the FG side.

Jon,

That's how we're driving FG for the same project you're working on: custom protocol.

We've made up an arbitrary property tree hierarchy and are shoveling control deflections and other bits into those properties; on the other end, we're using those properties to drive a custom HUD, and I guess Gene H's 3D model is doing the same thing for control deflections.

In our custom protocol XML file, items like this appear:

  <!-- Upper left body flap deflection -->
  <chunk>
    <name>ulbf-pos-norm</name>
    <node>/surface-positions/ulbf-pos-norm</node>
    <type>double</type>
  </chunk>

and 

  <!-- Outer glideslope flight path angle, deg -->
  <chunk>
    <name>ogs-gamm-deg-hud</name>
    <node>fcs/ogs-gamma-deg-hud</node>
    <type>double</type> 
  </chunk>

The first chunk is used by the 3D model, and the second by my custom HUD (which required a custom HUD.xml file and some mods to HUD.cpp)

To hook up the surface deflections, a custom XML file for the aircraft that included things like:

  <PropertyList>
    <path>DC.ac</path>               <!-- this is the exterior geometry file -->
    <animation>
      <type>rotate</type>
      <object-name>ul</object-name>  <!-- I expect this matches some geometry structure name in the .ac file -->
      <property>/surface-positions/ulbf-pos-norm</property>  <!-- matches protocol property list tree -->
      <offset-deg>0</offset-deg>
      <center>
        <x-m>12.052</x-m>
        <y-m>-1.402</y-m>
        <z-m> 3.576</z-m>
      </center>
      <axis>                     <!-- I assume this is the rotation axis -->
        <x>0</x>
        <y>1</y>
        <z>0</z>
      </axis>
    </animation>
   .
   .
   .
  </PropertyList>

Give me a call or text and I can try to share the details.

-- Bruce