Howto:Make an aircraft

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There are many things required to develop an aircraft for FlightGear. We encourage new aircraft developers to start their 'career' by modifying and enhancing existing aircraft. It is much easier to do and gives you a giant advantage by the time you create an aircraft completely by yourself.

The content in this article gives a summary of codes and can be found on most planes. So, for examples on how it's used, you can take a look at some random planes.

Additional information is available in a special forum topic.

Before you start

If you are a complete beginner, knowing the processes involved will help out quite a bit. Roughly 4 simplified steps, first creating your 3d model, then the FDM, followed by creating animations and finally to implement systems. Each aircraft in FlightGear has two main development areas:

1. The Flight Dynamics Model (FDM): This tells the computer how your aircraft reacts to various conditions. The FDM is composed of two main models most people use, either JSBSim or YASim. Or the lesser used model UIUC. Define the dimensions, mass-and-balance data. Tell FlightGear which part of your aircraft and where it may hit the ground. Where the gear is, if and how it travels. How it is propelled and how it flies if you move the controls. A subarea of the FDM are integral systems like the autopilot and fuel systems.
2. The appearence of the aircraft, which can be split in two areas:
   1. 3-dimensional model and all of its visual aspects: animations (These give visual feedback from your FDM. Move the aileron of your 3D model when the aileron control is moved. Same for the other aero surfaces, the gear. Than go ahead with the instruments, animate the needles, the digital readouts. Define hot spots and pick animations, so you can click in your 3D cockpit.), lighting and textures (images that are put in specific locations on the 3D model).
   2. auditory (sounds).

Introduction

There are lots of FlightGear aircraft that need cockpit work. This requires both artistic (create 3D models and textures...) and technical abilities (create XML, perhaps even nasal scripts and gathering information of the thing being modeled). This is also one area were your work is very visible to every day FlightGear users and this makes this effort one that has a major impact of the apparent quality of FlightGear.

This is also a very good place to start working on things in FG. So how do you go about doing this?

• Find an aircraft that is part of fgdata GIT that needs cockpit work that interests you. This is important the aircraft should be one that you have a fondness for. This work is a labor of love and that will not be the case if you just pick some random aircraft.
• Ask on the forum and perhaps also on the developers list who the current dev for that aircraft is.
• Contact that person and ask them if it is OK to work on that aircraft. They will probably say OK but they will also probably say that you have to work in a certain way and they may go as far as saying OK but I want you to work on <some particular instrument or control>. Don't take this as a snub. The aircraft dev has put a lot of effort into this aircraft and in the end they have to live with whatever you do to it so you have an obligation to do things their way.
• Try to start out with something very simple such as adding a switch (or a switch panel with a few switches) or a placard. This may seem trivial but you will learn a lot from doing something simple while at the same time minimizing the learning curve and your level of frustration.
• Also the aircraft dev will likely have to spend time mentoring you and it may actually take him more time to do this mentoring than it would have taken him to do the work. So stick around and do enough work to that aircraft that the mentoring effort pays off for the aircraft dev.
• Keep in mind that the learning curve is fairly steep and at first the work will take a lot of effort to do what would be trivial task for someone with lot of experience doing this stuff. You need to be ready to deal with the learning curve.

Rules and guidelines

• All images in FlightGear should be sized to powers of two (eg. 64*64, 128*256 or 16*1024). Since FlightGear 1.9, images no longer have to be saved in the .rgb format. Right now .png is most common used among FlightGear developers.
• Please note that we use spaces in our codes, some developers use rather tabs, to make our code easy(er) to read. Every line that starts a new tag, we press the space key once, so you get a kind of stairs. It does not really matter what method you use, as long as you use it consistently throughout all of your files. But XML files that are used by JSBSim FDMs must use spaces and not tabs.
• Filenames and directories are case-sensitive on most OS's. Windows is not case-sensitive, so when you are developing on a Windows machine, you won't notice any problems. On other OS's, there is a difference between 'Boeing' and 'boeing'. Since FlightGear is used on multiple platforms, make sure your code makes correct usage of capitals.

Tools of the Trade

If you realize this is harder than you expect, don't worry. For the first-timer, the FDM(aircraft.xml), aircraft-set.xml, and the engine.xml files are the easiest. Modeling will require considerable time and practice. You want to have the tools for the job, so here are some tools to help:

• Aeromatic (works only for JSBSim, gives you entire engine file and FDM (aircraft.xml) file.
• JSBSim Commander (Only JSBSim, helps with the multitude of tags, try this on your aircraft BEFORE you release it and AFTER you are finished with all the xmls)
• This wiki
• Flightgear Forum
• Design plans and specifications of your aircraft, including:
  • Three-View Plan (shows front view, top view, side view)
  • Technical Specifications (from Wikipedia, Boeing, or Airbus)
  • NOTE: Implement the specifications into the xmls
• A friend, and/or another developer, let's say you only want to do the FDM and XMLs. You'll need another person (a modeller) to model. Don't worry, the <author> tag has enough room. ;)
• 6 days for xmls + 2 weeks for modeling
• your patience

Directories and files

The data created during development results in many files, which are stored in several directories per aircraft. Each aircraft has its own directory in the $FG_ROOT/Aircraft/ directory. The first thing to do when you start working on a new aircraft is to make a directory for it. A short version of the aircraft name (eg. harrier), or its serial number (eg. 747-400) is prefered. More directories might be needed further on, but we will create them when we need them. They are explained in this article, but you can skip them if you are working on your first aircraft. For now we create one directory, namely:

• Models/ all files related to the appearence of the model are saved in this directory.

All paths are relative to $FG ROOT, with the exception of some that are explicit noted.

• Hint for Developers: You can pre-test all your .xml files without having to load FlightGear by opening them with Internet Explorer or Mozilla Firefox. These web browsers will parse XML documents and let you know if you made a mistake coding them.

It's worth noting that FlightGear doesn't expect any particular structure of folders or files, i.e. you could theoretically put things ANYWHERE, as long as you properly reference all files and paths in the top-level aircraft-set.xml file. However, it makes sense to follow some existing conventions (look at other well-maintained aircraft). Some more tips can be found here: Standard aircraft structure (needs to be updated as of 05/2012). A more recent discussion covering the pros & cons of the various approaches is to be found on the forum.

The .xml file (FDM)

This file contains the entire (or partial) Flight Dynamics Model of the aircraft. We have three different systems; they all have their up- and downsides.

• YASim, needed for: helicopters, towing over a multiplay network
• JSBSim, needed for: pushback (external forces)
• UIUC (rarely used these days)

With limited data available, YASim is generally considered to be the best way to go. When you have acces to real windtunnel data and/or require more flexibility, JSBSim might be a better choice.

The most important file describing the aircraft's dependencies is the Aircraft-set.xml file.

Engines/ directory

Only required for JSBSim aircraft. In this directory the engine and thruster/propeller files are stored.

Models/ directory

This directory contains all model related files; such as textures, models and animation files (.xml). In the -set.xml file we've set the path to one .xml file. That file should link (indirect) to each other model file (including a possible 3D cockpit) needed for the plane. The whole plane could be modeled into one file, but most developers prefer to split things up. A cockpit for example can be built up by several files, each existing of just one instrument or panel. This way it is easy to (re)move certain parts of the aircraft. You set up your model file like this:

 <PropertyList>
 
 <path>Aircraft.ac</path>
 
 <model>
  <name>Cockpit</name>
  <path>Aircraft/.../Models/cockpit.xml</path>
  <offsets>
   <x-m> -5.25</x-m>
   <y-m>  0.00</y-m>
   <z-m>  1.30</z-m>
  </offsets>
 </model>
 
 <animation>
  <type>rotate</type>
  <object-name>AileronLeft</object-name>
  <property>/controls/flight/aileron</property>
  <factor>-65</factor>
  <center>
   <x-m> 2.09</x-m>
   <y-m>-5.50</y-m>
   <z-m>-1.50</z-m>
  </center>
  <axis>
   <x>1</x>
   <y>0</y>
   <z>0</z>
  </axis>
 </animation>
 
 </PropertyList>

The cockpit.xml file can contain a similair structure, with all the instruments as seperate models. The amount of models is not limited, nor is the amount of animations.

A detailed article about animating and models can be found in Howto: 3D Aircraft Models.

A common subdirectory is the Liveries/ one, which holds the livery files. See Livery over MP for more information about that subject.

Nasal/ directory

All Nasal code specific to the aircraft is placed in this directory, with the exception of some system or instrument specific Nasal. If a certain Nasal script is usefull for all aircraft (eg. weather or multiplayer related) it can be placed in the $FG ROOT/Nasal directory. Nasal scripts that are useable on multiple aircraft (eg. air-air refueling) can be found in the $FG ROOT/Aircraft/Generic directory.

Sounds/ directory

What is an airliner without the sound of its mighty engines, or a glider without the sound of wind blowing around your face? Sounds are quite important to increase the feeling that you are actually into the simulation. In this directory all sound files that are specific for the aircarft are stored. Sounds that can be used on multiple aircraft (eg. the click of a switch or thunder) are available in the $FG ROOT/Sounds directory.

Tutorials/ directory

A helpfull feature in learning how to start the engines of a plane, flying a basic leg etc. is FlightGears tutorial system. It allows you to create a step by step tutorial that guides the user through a certain procedure. It can even indicate what switches should be pressed!

More about the tutorial system can be found at Tutorials.

Comments in XML files

We can place extra information in XML files, that will be ignored by FlightGear. Such as descriptions and explanations of certain XML markups, changelogs etc.

These will be mainly useful to other developers working with your XML code, for example when updating/maintaining your files, or when borrowing XML code from your work.

 <!-- Last update: 09-10-2007 -->

Beware that certain characters are not allowed within an XML file, even within comments, and will always cause problems. This include the characters:

 -- < &

Related content

• Howto: Request new aircraft
• Howto: Make a helicopter