I have been building a (highly accurate) flight simulator.
Basically, I'm taking a geometric approach to the flight dynamics model (similar to how x-plane works, except they're using... older techniques/technology. I'm using much more advanced stuff [working on seeing if I can simulate the airflow for the entire craft, as opposed to doing it by sections and then integrating those together]). This is in contrast to things like MS Flight simulator and FlightGear (though, flightgear does have a poorly documented and rather inaccurate geometry based fdm - but most people use the table-based one, which is far more accurate than the geometry-based fdm they have implemented), which use lookup tables to guess how an aircraft would perform.
The problem I actually originally set out to solve was that xfoil and xflr5 suck to use (importing/exporting plane data is... either you can't, or you shouldn't), and fuck paying for the more expensive design testers. (this is why I have the focus on accuracy - if this was just going to be a simple game, I'd have spent far more time making it look pretty) However, I figured that I could also make testing be more fun by adding an interactive mode (i.e. I want to be able to do hardware-in-the-loop type stuff, as well as just manually flying), and at that point, it just is a scriptable flight simulator.
I'm still working on the flight dynamics model, been teaching myself fluid dynamics so that I somewhat understand what all is going on there (as much as anyone who hasn't spent years studying this can understand...), and I've been working on writing code to run on the gpu (yay, opencl) in order to do this. It's been fun.