Air War 1917 is a barebones — and free — version of Wessex Games’ AirWar: 1918 biplane combat rules.
Flying Circus is a set of free wargames rules based on the old Avalon Hill Mustangs game.
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.