Using AI objects in FlightGear

Starting with FlightGear version 0.9.4 you can place AI objects in the "FlightGear world". In version 0.9.4 the AI objects can be defined in the preferences.xml file, or in an airplane's *-set.xml file. In later versions of FlightGear they are defined in a "scenario file" only. Types of AI objects available are ships, airplanes, thunderstorms, thermals, aircraft carriers, static and ballistic objects.

AI objects have some things in common: The have a location in the "FlightGear world", they can have an associated exterior 3D model, and they can move according to an internal FDM (flight dynamics model). As of now, these objects are created at simulator start-up by adding some XML code to a scenario file. The scenario file must be in the data/Data/AI directory. You select which scenario file you want to use by naming it in the preferences.xml file. (Note: As with everything in the preferences.xml file, the entries can be overridden by an aircraft's *-set.xml file, or at the command line.) The preferences.xml file has an entry that looks like this (FlightGear versions newer than 0.9.4):

<ai> <enabled type="bool">true</enabled> <scenario>demo_scenario</scenario> </ai>

The scenario contains one entry for each AI object. The entry specifies what kind of object to create, what it's initial conditions will be, and optionally (for aircraft and ships) a flight plan.

Ships

The entry to create a sailboat would look like this:

<entry> <type>ship</type> <path>AI/Aircraft/sailboat1/sailboat.xml</path> <speed type="double">12.0</speed> <altitude type="double">0.0</altitude> <longitude type="double">-122.33333</longitude> <latitude type="double">37.61667</latitude> <heading type="double">20.0</heading> <rudder type="double">-3.0</rudder> </entry>

Most of the entries are self-explanatory. The "type" of object can be one of "aircraft", "ship", "storm", "thermal", "carrier", "static" or "ballistic". The rest of the items give the AI object a model, a starting location, and a starting speed and direction. You use the <path> item to give the object any valid exterior model. You can even make the ship look like an airplane if you want! The "ship" type can also have a <rudder> value specified, which will cause the ship to move in a circle (HINT: use small values, five degrees or less, and right rudder is positive).

Aircraft

Here is an example of how to create an aircraft AI object:

 <entry> <type>aircraft</type> <class>jet_fighter</class> <path>Aircraft/a4/Models/a4-blue.xml</path> <speed type="double">320.0</speed> <altitude type="double">7000.0</altitude> <longitude type="double">-122.6</longitude> <latitude type="double">37.9</latitude> <heading type="double">210.0</heading> <roll type="double">-15.0</roll> </entry>

It looks much the same as the ship AI code. There are two differences, the <class> item and the <roll> item. If the class is set to "tanker" the airplane will allow you to refuel if you can get close behind it (so far supported by JSBSim only) The "roll" will cause the airplane to fly in a circle. In the above example the A-4 will be orbiting to the left at 15 degrees of bank. You can also create a ship or airplane with a flight plan. In this case the object will follow the flight plan and then delete itself when it reaches the end, or (new since version 0.9.8) the object can be set to endlessly repeat a flight plan. The flight plans are located in data/Data/AI/FlightPlans. To create an airplane with a flightplan do this:

<entry> <type>aircraft</type> <class>jet-transport</class> <path>Aircraft/737/Models/737.xml</path> <flightplan>KSFO_ILS28L.xml</flightplan> </entry>

In this case you don't need to specify initial conditions because they are already defined in the flight plan. The AI aircraft objects can be displayed on radar. See the Aircraft/Instruments/radar.xml file for details.

Thunderstorms

To make a thunderstorm, use this:

 <entry> <type>storm</type> <path>Models/Geometry/thunderstorm.xml</path> <speed type="double">20.0</speed> <altitude type="double">4000.0</altitude> <latitude type="double">37.3917</latitude> <longitude type="double">-122.2817</longitude> <heading type="double">90</heading> </entry>

There's not much to it. No, they don't turn :) New since version 0.9.8 is the ability to add lightning and turbulence to a thunderstorm. For an example of this see the scenario file called "bigstorm_demo.xml". The AI storm objects can be displayed on weather radar. See the Aircraft/Instruments/wxradar.xml file for details.

Thermals

To create a thermal, use this:

 <entry> <type>thermal</type> <latitude type="double">37.61633</latitude> <longitude type="double">-122.38334</longitude> <strength-fps type="double">8.33</strength-fps> <diameter-ft type="double">4000.0</diameter-ft> <height-msl type="double">6000.0</height-msl> </entry>

The AI thermals don't move, they are invisible, and they don't "lean" downwind. The <strength-fps> defines the maximum vertical velocity of the airmass at the center of the thermal. The strength decreases to zero at the thermal's edge. <height-msl> defines the top of the thermal.

Ballistic objects

A ballistic AI object starts with an initial location, azimuth, elevation and speed, then follows a ballistic path from there (with air resistance included). Try this:

<entry> <type>ballistic</type> <path>Models/Geometry/rocket.xml</path> <speed type="double">500.0</speed> <altitude type="double">50.0</altitude> <longitude type="double">-122.39</longitude> <latitude type="double">37.62</latitude> <heading type="double">200.0</heading> <azimuth type="double">70.0</azimuth> <elevation type="double">45.0</elevation> </entry>

There is not much call for rocket launches in FlightGear, but the ballistic AI object is the basis for FlightGear's submodels system.

Aircraft Carriers

The aircraft carrier AI object is based on the "ship" object but is much more complicated. See the scenario file called nimitz_demo.xml for details.

Dave Culp, updated 23 Oct 2005
davidculp2@comcast.net