Rise of Flight has the most detailed and accurate sim for WWI yet created. The 3D world is one of the most sophisiticated and they are building their sim in intelligent steps, you get the world files and the game files for free and then pay for individual planes or plane packages. You get missions for free, but the planes needed are what you acquire. The research and detail of the inidividual planes is superb and it is a remarkable sim. they have very detailed 3D world elements and are adding more. the world map is limited, no Channel yet or ocean, just France and the front lines, and not all of that..but I expect they will keep expanding the map as time goes on, You get two free planes so you can start out of the blue with campaigns. All and all the GUI and the presentation is elegant, and sophisticated and the best flight sim yet in many regards and esp for the WWI era.
It is very demanding spec wise because of the detail and complexity of the engine.