AI Systems

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Revision as of 01:34, 26 June 2006 by (Talk) (AI Models)

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FlightGear has a simple Air Traffic Control system.

For airports with a tower frequency, if that frequency is tuned in to your radio, you may contact ATC.

To obtain the tower frequencies for an airport within range of ATC, go to the ATC/AI menu, choose Frequencies to display the dialog. If there are any airports within 40mn range, a button with the airport designation will appear. Click see the frequencies. Tune the tower frequency in on the COM1 radio and then hit the single quote key to open the ATC window.


  • Untowered airports are not supported in any way by ATC.
  • You must be within 40 nautical miles of an ATC facility (towered airport with tower frequencies) to contact ATC.

AI Air Traffic


AI Models

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 they are defined in a "scenario file" only. The types of AI objects you can have are airplanes, ships, thunderstorms, thermals 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/AI directory. You select which scenario files you want to use by naming it in the preferences.xml file. The preferences.xml file has an entry that looks like this (FlightGear versions newer than 0.9.4, including CVS):

 <ENABLED type="bool">true</ENABLED>

The above bit of XML enables the AI system and selects a scenario file called aircraft_demo.xml.

The scenario file contains one entry for each AI object. The entry specifies what kind of object to create, what its initial conditions will be, and optionally (for aircraft and ships) a flight plan. The entry for a sailboat could look like this:

 <SPEED-KTAS type="double">12.0</SPEED-KTAS> 
 <ALTITUDE-FT type="double">0.0</ALTITUDE-FT>    
 <LONGITUDE type="double">-122.33333</LONGITUDE> 
 <LATITUDE type="double">37.61667</LATITUDE>
 <HEADING type="double">20.0</HEADING>

Most of the parameters are self-explanitory. The "type" of object can be one of "aircraft", "ship", "storm", "thermal", "ballistic" or "static". The rest of the items give the AI object a model, a starting location, and a starting speed and direction. You use the <MODEL> 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). Here is an example of how to create an aircraft AI object:

 <ALTITUDE-FT type="double">7000.0</ALTITUDE-FT> 
 <LONGITUDE type="double">-122.6</LONGITUDE> 
 <LATITUDE type="double">37.9</LATITUDE>
 <HEADING type="double">210.0</HEADING> 
 <BANK type="double">-15.0</BANK>

It looks much the same as the ship AI code. There are two differences, the <CLASS> item and the <BANK> item. If the class is set to "tanker" the airplane will allow you to refuel if you can get close behind it. The bank is of course similar to the ship's rudder. 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. The flight plans are kept in data/Data/AI/FlightPlans. To create an airplane with a flightplan do this:


To make a thunderstorm, use this:

  <SPEED-KTAS type="double">20.0</SPEED-KTAS> 
  <ALTITUDE-FT type="double">4000.0</ALTITUDE-FT>
  <LATITUDE type="double">37.3917</LATITUDE> 
  <LONGITUDE type="double">-122.2817</LONGITUDE> 
  <HEADING type="double">90</HEADING>

There's not much to it. No, they don't turn :) To create a thermal, use this:

 <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</DIAMETER-FT> 

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. A model can be assigned to the thermal, and usually this will be a small cloud to mark the thermal's location. To create a sink, just give a "thermal" a negative strength, and give it a null model. Please see the demo scenario (thermal_demo.xml) for examples.

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

 <SPEED-FPS type="double">500.0</SPEED-FPS> 
 <ALTITUDE-FT type="double">50.0</ALTITUDE-FT> 
 <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> 

The AI storm objects can be displayed on weather radar. See the Aircraft/Instruments/wxradar.xml file for details. The AI aircraft objects can be displayed on radar. See the Aircraft/Instruments/radar.xml file for details, or fly the T-38 for a demo.

  • Using Interpolation Tables

Interpolation tables are very handy for effecting animations that are non-linear in relation to the property they are referenced to.

They save the use of factors, offsets and min/max values.

For example; relating flap extension to airspeed of an AI model. Typically an aircraft will extend flaps on final approach to control Indicated Airspeed (IAS) and stall speed to affect a low speed controlled landing. Upon touchdown the extra lift efficiency introduced by the flaps is no longer required or desirable, hence the flaps will be retracted ASAP after touch down.

This is relatively simple in a sim aircraft as the /surface-positions/flap-pos-norm property is a normalised indicator of the flap setting chosen by the pilot.

AI aircraft have no pilots to control flaps nor does the flight plan <flaps-down>true/false</flaps-down> parameter effect the /AI property tree parameter, a relationship to the IAS is the next best choice.

To effect this relationship (IAS/flap-position) using factors/offsets and min/max would be quite difficult and non-intuitive. Using interpolation tables allows the following scenario to be setup very easily and intuitively;


Max 123knts Cruise 90knts Stall 50knts (no flaps) Flaps 0, 10, 20, 30 deg. Approach @ 90knts 10* flaps @ 90knts 20* flaps @ 70knts 30* flaps @ 60knts Flare & touch down @ 50knts Brake @ 45knts Retract flaps @ 45knts

Table IAS/flap extension IAS(knts) Flaps >90 0 70-89 10 60-69 20 45-59 30 <45 0

When the pilot extends the flaps "one notch" they will extend to "10*" etc. In the property tree, these 4 steps will typically be normalised to 0.00, 0.33, 0.66, 1.00. This will differ from aircraft to aircraft. If the aircraft does not have equal extensions for each "notch" of flaps, the values observed may be; 0.00, 0.10, 0.30, 0.66, 1.00. Modeling this non-linear extension using "factor, offset, min/max" would be extremely difficult if not impossible. Modeling it using interpolation tables is very easy, as you shall see.

For sim animations, the actual physical rotation of the control surface in the real world needs to be researched; this may reveal a linear or non-linear relationship between "nominal flap extension indicator" and physical rotation of the surface. That is, "10 degrees of flap" might only involve rotating the flap surface 5 degrees around its axis in the wing structure. In our 0,10,20,30 scenarios, assuming a linear relationship, 30 degrees of flap would result in the flap being rotated 15 degrees around its wing axis. Therefore the normalised /surface-positions/flap-pos-norm property would have a factor of 15 applied to the rotate animation

The way to change a linear relationship that uses "factor, offset, min/max" in the sim animation to a interpolation table in the AI animation is best understood by examining how "factor, offset, min/max" approach works;

1) Take normalised value of the flaps (0=retracted, 1=extended) 2) Apply the factor. 3) Apply the offset 4) Apply the min/max values

e.g. factor=60 offset= -30 min=-10 max+10

these figures are nonsense but are used to illustrate a point;

flaps retracted(0*) = 0 x 60 = 0, (offset=-30) = -30, (min= -10) = -10 flaps extended(10*) = 0.33 x 60 = 20, (offset=-30) = -10, (min= -10) = -10 flaps extended(20*) = 0.66 x 60 = 40, (offset=-30) = 10, (min= -10/max=10) = 10 flaps extended(30*) = 1.00 x 60 = 60, (offset=-30) = 30, (max=10) = 10

more realistically, offset and min max are not used for flaps, only a factor.

say 27*, this represents the maximum rotation of the 3-D component in the model around its defined axis.

flaps retracted(0*) = 0 x 27 = 0 flaps extended(10*) = 0.33 x 27 = 9 flaps extended(20*) = 0.66 x 27 = 18 flaps extended(30*) = 1.00 x 27 = 27

from this it can be seen that the 3-D object will be rotated 0, 9, 18 & 27* to represent the 0, 10, 20 & 30* deployment of the flaps. Relating this back to the speeds above;

IAS(knts) <rotate> value 0 0 44 0 45 27 59 27 60 18 69 18 70 9 89 9 90 0 100 0

this gives a stepped effect, where the movement is limited to 1knt of airspeed. That is the 3-D object will linearly move from 9* to 18* while the aircraft looses speed from 70knts to 69knts. This behaviour will make the need for an upper and lower limit of a stepped value obvious.

a simplified table of;

IAS(knts) <rotate> value 0 0 45 27 60 18 70 9 90 9 100 0

will cause the flaps to start extending at the rate of 1/45 x 27 degrees per knot of airspeed gained until 45 knots when the rate of change will adjust to the new gradient. This is not how flaps behave, but can be used to good effect with "tail dragger" animations where at a certain IAS the tail starts rising, maybe at an increasing rate, until flying attitude is reached when it stops rising any further. The tail rising is not a stepped function of IAS.

Now we have the basic ideas behind interpolation tables, the question is how are they implemented…answer;…easy ….an example of the stepped flaps animation above




Here endth Interpolation tables 101

-D ene


Submodels are AI ballistic objects that emanate from, fall from, or launch from the user aircraft. They are presently used to model smoke, contrails, flares, tracers, bombs, drop tanks and flight path markers.

Submodels are controlled by the submodel manager. The manager reads a submodel configuration file at the start of the sim session. This configuration file is written by the aircraft author and defines all the submodels for that particular aircraft.

As an example examine the submodels file in the Aircraft/737-300 directory. This file creates two submodels which will become the airplane's left and right engine contrails. Each contrail needs its own submodel definition because the contrails begin at different locations. Each contrail consists of a train of individual "puff" models that are released in rapid succession as long as the "trigger" property is true. We ensure an unlimited supply of puffs by setting the "count" parameter to -1. The individual puffs, being AIBallistic objects, will follow their own ballistic paths once released. In this case we have used the "bouyancy" parameter to negate gravity in the ballistic path. The puffs have been given a life span of eight seconds. At cruising speed the 737 will thus have about 400 puffs behind it at any moment.