I have two computers that I use for Flight Simulator. Thanks to FreeSync I managed to reduce stuttering noticeably on both. Tips follow.
Make sure that the monitor has FreeSync/GSync enabled. I had to use an on-screen menu on one of my monitors to get that done.
I set a high refresh rate in Windows instead of the default value of 60 FPS. This may not be necessary for FreeSync to work properly, but I haven’t tested FreeSync games with the desktop running at default 60 FPS.
Disable VSync on Flight Simulator. With VSync enabled I get stuttering unless I drop the refresh rate to 30 FPS on the Flight Simulator settings. It seems that VSync prevents FreeSync from doing its adaptive frame rate shenanigans. It may be different for Nvidia users.
To avoid over-stressing my GPU, I use a feature on AMD software called Radeon Chill, which allows me to cap FPS to 60 or less while VSync remains disabled in-game. I rather have a quiet graphics card and a smooth constant frame rate than very high refresh rates and lots of stuttering in Flight Simulator. I expect a similar feature to be available for Nvidia users.
Your CPU is essential to avoid stuttering. There is an FPS monitor available on Flight Simulator when developer mode is enabled. If it tells you that the game is limited by the main thread, then CPU is your bottleneck. Try running the simulator in medium settings, or cap the frame rate at a lower value.
I increased the rolling cache size in Flight Simulator, although I am not sure that it helps.