Property Tree Servers
FlightGear have a built in servers that you can connect to for view and interacting with the property tree through telnet or through HTTP using a web browser or HTTP requests.
Web server
To view the property tree with a web browser, start FlightGear with the following option:
--httpd=5400
Where 5400 is the port number. You can pick any port number, but 5400 will probably work just fine.
Now on the same machine, fire up a web browser and open up the following URL:
Now you can browse the entire FlightGear property tree "live" (you will need to refresh the browser). You can even change most values if you like. You can for example configure autopilot modes and even set control inputs so you can literally fly the airplane from your web browser, although it is not the most convenient interface for doing that. ;-)
Telnet server
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FlightGear comes with a internal telnet server that can be used to read end set values within the property tree structure. The telnet server is activated with the --telnet=port command line option, where port is the number of the listening port that will be opened.
A connection to the server can be done using a telnet client or opening a simple socket from any program. Multiple connections (more below) are possible at the same time. Too start FlightGear and open the port 5401 as available telnet server use this command:
fgfs --telnet=5401
Props server
There is a similar interface minus the HTML wrappings that you can enable with the following option:
fgfs --props=5401
Once you have a "props" interface configured you can now
telnet localhost 5401
and interact with the property system (again live) and set and examine values using a command line style interface. This is covered more on the Telnet page.
Writing properties
The cool thing is that you can easily write scripts to access this --props=port interface.
Take a look at the following link for some example Perl scripts. flightgear/flightgear/next/scripts/perl/examples/
Note that there is no requirement that you do this with Perl. You could just as easily interact with a running FlightGear instance using Perl, C, C++, Java, Python, probably even Ack, Visual Basic or anything else that can do TCP/IP network communication ... Matlab? netcat?
Also note that the downside to this interface is that you cant blast a lot of data across it. It is fine if you want to monitor location and speed every second or 1/4 second and occasionally set some values (such as dump in a new weather configuration, reset the aircraft location, or read a set of values, etc.)
But if you need to track 100 different variables at 60 Hz, this is not the interface you are looking for.
Multiple servers
Note that you can setup as many of these as you want ... for instance, just to be obscene you could do:
fgfs --httpd=5400,5401,5402 --props=5403,5404,5405
Now there are 6 different network interfaces running that you can access from anywhere.