If you are looking for MP WWI simulator, I'd recommend Targetware. TW is beta graphics engine with open structure, available for free, so the community made a lot of mods for it, covering WWI, WWII (September 1939 campaign in Poland, Western-European, Mediterranean and Paciffic theatres), Korean War and acrobatic aviation. FM's are good, same as 3D modells. Graphic is a bit old, but nice. There's allso a mod called "War over Poland 1920", covering war between Poland and Bolschevik Russia in 1920. Not WWI, but close to it, as there were WWI airplanes and tactics still in use at that time. You can read more at:
BTW: Emp_Palpatine, "Richthofen's Skies" mod was originally named as "Target Flanders", this shows, what region does it focus on.
Of course all mods are available for free, such as the main application.
Just found a topic about RS on Combat Ace: http://forum.combatace.com/index.php?showtopic=28168