A good place to start looking into making aircraft would be the
developer portal on the FlightGear wiki.
Usually the best way to approach aircraft development is looking at existing aircraft and copy the parts of them. [1] The main file of the aircraft are the
<aircraft>-set.xml file, all other files are linked to from that file, directly or indirectly.
Aircraft development can be both challenging and fun. You might be able to do a flyable exiting aircraft in a day or two, but making a really great one, that you have to fly by the manual, often take several months or years.
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[1] Though keep in mind that the aircraft of the official hangar, FGAddon, that you can browse in the launcher will require you to use the same license if you do so. They use the GNU General Public License, version 2 (GPLv2).