making a diffFM is exactly the same as adding a new fmd to the buttons. The only difference is that you only need to add the fmd and emd file that your plane needs. The name of this new file is up to you and in the class file you need to point towards the new file created. example:
Property.set(class1, "FlightModel", "FlightModels/SM81AR.fmd:SM81AR_FM");
here a link to the latest version of ntrkwizard that works with any game version:
https://www.sas1946.com/main/index.p...c,42832.0.html