Spandau and Lewis is a set of free wargames rules for WWI aerial dogfighting.
aircraft
Fast Play Jet Combat
Fast Play Jet Combat is a set of free wargames rules for 1/600 aircraft on a hex grid. Interestingly, it requires a d100 die. If you don’t have one handy (and who does), you can do the same thing with two d10s in different colors.
Fokker Fodder
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.
Crimson Eagles Pulp Air Combat Rules
Crimson Eagles is Michael Reitz’s mash-up between the old FASA Crimson Skies pulp aircraft rules and the Canvas Eagles WWI rules.
Knights of the Sky WWI Aircraft Rules
Knights of the Sky is a set of free wargames rules for WWI Aircraft.