went to southport air show a couple of saturdays ago
it was miserable
weather was cold and wet with an overcast sky the wind didn't help much either
at this time of year unless your lucky the chances are it's gonna be at best overcast and wors't raining
you have to account for white clouds and whatever colour the plane is
do you have a steady hand ?
i don't so i tend to go for the fastest shutter speed i can get a way with no less than 400th/s double if i can get away with it
so now your up to 400 iso at around 7/800th of a second at around f6 f7 ( i was on saturday )
the end result is not so good skies but planes with reasonable exposure
example
ISO 400 1/600th F6.4 at 280mm
ISO 400 1/800th F6.4 at 280mm
as you can see on jets a slow shutter speed to preserve propeller blur isn't important what is important is exposure
if you have an overcast sky as we have in the examples letting your camera make the decision is probably going to end up with nice skies and a silhouette for a plane and i have seen some fantastic photographs done that way but most of the time they don't turn out so well so your better off setting your exposure for the plane ( usually 1 - 2 stops over exposed ) and sacrificing the sky
where prop planes are concerned as said above if your hand is steady go for the slowest shutter speed you can get away with to keep the motion blur in the prop
if your as unsteady as me i forget about the blur and concentrate on trying to expose the plane properly
example
ISO 400 1/800th F6,4 at 100mm
have a look at the exif of other plane shots to see how others have done it
it might help you on your travels
good luck....