There was a "competitor" called "TK" (can't remember what it stood for) that started off with korean war jets with AH style realism, but I think it petered out after a few years of development and testing. While it lasted they made a ton of progress, but it never got to a full release or pay to play. As a pure air combat flightsim it was pretty darn good though, better in some ways than AH.
Edit: I remember now, it was called "targetware", and they had "Target: Korea", "Target: Vietnam", and maybe WWI/WWII too. Pretty high fidelity flight model, great aircraft models and skins, as much realism as you could handle for aircraft systems including the option to have significantly more realistic engine modelling than AH, which would get a dedicated gamer a slight advantage over a player flying in "easy mode".
