FlightGear runs best with current OpenGL video drivers. OpenGL 1.2 is required, and OpenGL 2.0 is preferred.
If the FlightGear video frame-rate seems slow, make sure the video card has a current OpenGL driver installed. Using a video card without hardware acceleration might produce frame-rates around several seconds per frame. On the other hand, a 3-Ghz CPU with a hardware-accelerated nVidia GeForce video card should produce frame-rates near 60 fps in most situations. The frame-rate will vary with the complexity of the scene, which is constantly changing, depending on the number of models and surfaces within view at each moment. See 3D Video Introduction for more information why FlightGear requires an OpenGL-capable video card.
OpenGL Drivers for Windows and Mac
Most modern computers these days ship with hardware accelerated 3D cards. If you are having trouble running FlightGear, visit your card manufacturer's web site and download and install the latest drivers for your card. Appendix C of The FlightGear Manual has some further information on configuring your machine to run OpenGL applications.
OpenGL Drivers for Linux, FreeBSD, Etc
Nvidia and ATI cards are all pretty well supported by the PC Unixen. Depending on your video card, your distribution, and your sys-admin experience, getting accelerated drivers running can be more or less of a challenge. We have assembled some information and pointers to information for configuring your 3d card in Appendix C of The FlightGear Manual.
Some video cards only support Direct3D, which is not compatible with FlightGear. Some 3D cards can't function if you set the display resolution too large or the pixel depth to something that card won't support. Some OpenGL drivers silently drop back to software-only rendering under those circumstances.