I couldn't agree more with Johnny L.A. The CGI in the most recent crop of period air combat films, Flyboys and Red Tails, were huge disappointments. The aircraft look completely un-natural in aerial combat - the speed at which they bank and turn remind me more of a video game than actual flight, likely because that's where most of these guys got their start.
And it's not like the guys who create these sequences don't have any reference material. Films like The Dawn Patrol, The Blue Max, Tora! Tora! Tora! and Battle of Britain all used real aircraft in their combat scenes. And that's over and above the actual combat footage from documentaries like Memphis Belle, which was shot during an actual mission.
Even the combat sequences in Star Wars are more "realistic" than some of these films. Mind you, I seem to recall hearing that Lucas did look at aerial combat footage when doing the research for the final battle for the Death Star and determining how it should look.