Space Planes! I only recently began experimenting with the space plane features of KSP, but they offer a wealth of information about drag and air resistance. The steering of these planes is so sensitive, that you really see how rudders and pitching affect flight. Also, placing aerodynamic caps on engines rather than leaving them flat noticeably alters acceleration and top speed capabilities of a plane. Trying to build a plane is also complex, as you have to consider how much wight the wheels can support, how to balance weight versus fuel, and aerodynamics. Space Planes offer a great insight into the physics of airplanes and how complex the operation of these machines is, and I have a new perspective regarding planes for the next time I'm on a flight.