SimplePlanes is a physics based sandbox game where you design and fly your own planes. The editor is fantastic and lets you make all sorts of customizations like the shape and size of your wings, engine type, and even weapon systems. There's a system to share your designs and I'm very impressed with how creative others are, flying around in a functional 355 DeLorean really made my day.

Personally I found most planes incredibly difficult to fly if your going any faster than 20% throttle, but the sandbox nature of the game lets you abuse the win conditions. AI controlled planes can't beat me in a race if I shoot them down from the start.