The Targetware sim is free and a work in progress. Several parts of it deal with WW1 aviation - 'Richtofen's Skies' and 'War Over Poland' (the latter using WW1 aircraft in a post-WW1 theme). Just bear in mind that it's flight-modelling is realistic and so not exactly easy
Off-line, the sim features no AI aircraft, so to get the most out of it you have to fly on-line.
Targetware :: Unlimited Expansion Flight Sims