Anything below about 15 frames per second (fps) is close to unusable, or at least, will not be enjoyable. Additionally, the more the framerate decreases, the more other features of the simulation will suffer. The current FlightGear software architecture is such that long rendering times (slow framerates) may prevent other parts of the flight simulator, such as the autopilot, from having sufficient CPU time to respond correctly in the context of the simulation.
FlightGear's framerate is influenced by various factors, including:
- scenery complexity (terrain,clouds, trees, particles, random 3D objects, multiplayer/AI traffic)
- cockpit complexity (2D vs. 3D)
- environmental options (eg. visibility, precipitation)
- FDM update interval
- AI scenarios
- aircraft speed (scenery paging)
You can disable most features that often adversely affect framerate. Here is a collection of settings that you may change to improve your framerate. Please note that some of these will only take effect if they're specified at startup and cannot be changed after the simulation has started.
Contrary to what many people believe, the impact of high vertex-count models on framerate is fairly minimal. For graphic artists, the key things that should be avoided are:
- high resolution textures
- textures containing alpha
- textures themselves
- unwant polygons casting shadows
AI-traffic is computer controlled traffic, which includes traffic over the MP network. By turning off this function you're not able to see other aircraft. Most stuttering, or lag, in FlightGear is caused by AI traffic. Especially heavy models need quite some time to load, eg the F-14 or the B-1B. Every time a new aircraft enters your local area, your computer has to load the model for that particular aircraft. The time required to read the data from the drive creates a stutter, or lag, which can range from a minor annoyance to a crash. The busier the place you fly, the more lag you'll get; eg flying around KSFO will generate more lag than a flight near EGLL, since there is more traffic near KSFO.
Operating System Specific
Many newer Linux distributions, like openSUSE 11.2, come with KDE4 as a desktop environment. While it allows some fancy desktop effects, it is known to consume enough computing power from the CPU and the graphics card to slow FlightGear down by 10 frames per second or even more. Choosing another window manager from the login screen, like xfce results in higher FlightGear performance. Setting "Compositing" to "off" in KDE4 might also help.