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Downloaded the Q400F external model but no read me to tell me what to do with it.
Can you point me in the direction of some instructions please?
Put the .mdl file into the same directory as the original, default one.
If you open aircraft.cfg, you can see a line "model=" for each livery.
For freight liveries, you can change it to "model=F" so it uses the freighter model.
E.g. FedEx livery from avsim: https://library.avsim.net/download.php?DLID=182905
description=Majestic Software Dash-8 Q400 FEDEX by Peter Brown
This is just an external model without windows. It does not change the load manager or anything else. So when loading your Q400 freighter, you have to use an "85kg passenger" unit of mass for your cargo :-) . No big deal, though.