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The reason it is identified as a generic USB joystick is probably because the name it report to FlightGear do not match the name of any of the devices there is a configuration file for.
In the Joystick dialog (Main menu > File > Joystick Configuration) you can see the name that it reports to FG.
Make a copy of a configuration file for a similar device and rename it (you can find them in the directory data/Input/Joystick in your FG installation).
Open your new configuration file and add the
exact name reported in the joystick dialog with <name> tags near the other ones.
Restart FG and open the joystick dialog to make sure that FG have found the new configuration file, and begin tweaking it to suit your needs.
FlightGear is extremely flexible when it comes to configuration and you can do a lot more than the joystick configuration dialog can help you do.
For more details, see the readme file data/Docs/README.Joystick.html in your FG installation and the FG wiki article
Input device (
perm).