This document gives a TREMENDOUS amount of detailed information about an aircraft folder (the folder that contains everything for an INDIVIDUAL airplane within the FS9/aircraft folder) - what a model / texture / panel / sound folder does, the air and aircraft.cfg files.
This should be enlightening for all, especially for less experienced FS enthusiasts. It should help many people in adding in freeware aircraft, panels, sounds or textures. Study it as you have time and compare it to the aircraft folders and files you have in your aircraft directory. It has been a big help to me over many years.
This information also applies to FSX, with a few minor exceptions.
Have fun --