It depends on a number of factors. Lighting, speed of the aircraft and what is in the background. I typically shoot in the range of 1/60 to 1/400 for props. When I get to jets, I bump up to 1/100th, maybe more depending on available light. You don't have to worry about stopping any moving parts on a jet. With the Aeronca landing and the prop almost stopped, the engine was near idle because he was landing, so it wasn't spinning as fast and I basically stopped it at 1/320.