Digital Combat Simulator

Digital Combat Simulator World (DCS World) is a free to play combat flight simulator. It includes a free Sukhoi Su-25T attack aircraft and TF-51D Mustang, and is expandable through additional DCS modules as well as user-made add-ons and mods. While the Su-25T and TF-51D are free, modules produced by third parties and Eagle Dynamics are only available with a payment. source: wikipedia.org