If you are shooting flying aircraft from the ground - low level passes in front of the grandstands etc. - use the longest glass you can hand hold and keep the target in the view finder while panning. I've had good results with a 300mm on B25s, Mustangs and Cosairs, just remember to only shoot when they're close.
Because of the back light from the sky, take an incident meter reading or meter off the back of your hand with your in-camera meter and set the fastest shutter speed you can get.