As far as I know, only one WW1 combat flight sim has ever featured that core activity of Great War air operations, the artillery shoot - Dawn of Aces. Forget the latest Flyboys travesty, it's the worst so-called upgrade of any flight sim ever made, grab the 2.75 version and fly it in '3D' mode. It's not the latest fun 'n' effex by any means but it IS a halfway-decent attempt to reproduce some of the thrills of WW1 fighting in the air, whereas other sims are mostly just very pretty 'Knights of the Air' bolt-ons.
DoA 2.75 is free, only an 18Mb download and can be flown head-to-head for free via direct link 'tween a host PC and friend. Give it a spin.
Cheers!