Check the file which you have downloaded, I had the same problem with the Viking as well. I unzip the file had it on the desk top and moved it into the aircraft folder try to fly the Viking and nothing in the game. Turns out the file were really a file with in a file, for example the file I had put in the aircraft file was named some like this [1234] Viking that was the folder I put in the game. Well that was wrong, what I need was the file contained within, it was just named Viking. So I made a copy pasted it into the aircraft file and it worked. I hope this helps you, sorry if I didn’t do such a great job explaining what to do, I don’t know the technical names for much when it comes the files I learned everything I know from trial and error.