I enjoy sketching planes as a warm up exercise before I start to really sketch. Planes include most of the essential elements such as ellipses, perspective and offer a quality subject for quick renders. They are also a good outlet to get my creative juices flowing and sketch something down to earth or far out. For example this plane uses a "fusion warp-drive" which would explains the blue flame and exhaust. I'm still in the prototype phase of that system ;).