Matt Unsworth offers a set of free wargames rules called The Few that he uses with his students to simulate the Battle of Britain.
Nigel Lancaster has written a set of abstract modern air war rules called Instant Thunder, and has graciously permitted MiniatureWargaming to host them. This is a major work, with 28 pages of rules, aircraft and missle lists, and designers notes.
The Battle of Britain is a complete strategic game for refighting the epic battle of 1940.
Fantasy Flyers is a set of free miniatures wargames rules for aerial skirmishes. You can construct your own flyers
I think it would be a good alternate game if you own some of those old Man O’War flyers — the Dwarf Gyrocopter, Manticore Riders, Wyvern Riders, etc. You also could use 15mm flyers (although I can’t recall if I’ve ever seen a good variety of those), or possibly 25mm fantasy figures (but I think the distances may have to be modified for this).
In Clouds of Glory is a set of free wargames rules for World War One aerial combat.