Matt Fritz offers a set of fast play miniatures rules for World War II Aerial Combat.
This post on making WWI aircraft from clothespins is not really a set of instructions, but the photos show clearly enough what has to be done. Brilliant bit of model making.
Fokker is a one-page set of rules for playing miniature games with World War I aircraft.
Fokker Fodder is a set of free wargames rules for World War One aircraft battles. The rules are designed for both single battles, and for campaigns.
Knights of the Sky is a set of free wargames rules for WWI Aircraft.