When shooting prop aircraft, you need to watch your shutter speed. Unlike jets where you want to stop all the action, you do want to have some prop blur to convey the motion of the aircraft. Otherwise it will look like it's floating in midair.
A shutter speed of 1/400 is probably the maximum that you'd want to shoot and still maintain adequate prop blur. Typically I shoot in Shutter-Priority mode and set my shutter speed to 1/250 - 1/320.
Notice the difference in the photos below.
1/800 f/7.1, ISO 200
1/320 f/10, ISO 200
