How do you change the frame rates?
You can't just "change" them, they are dictated by your hardware and what settings you are using in FS, along with the type of plane and how complicated it is.
if you go into settings - display - hardware to can change the target frame rate.
Your system will try to achieve the target but it depends on your system specs.
Try setting it to 25 FPS and pres shift + Z twice while in flight and see what you get.