daRock, Cratecruncher, both have it pegged. If you have these figured out, that's the first step of not only designing and building a plane, but also understanding how, and why, it flys. The thrust line, as stated, is the imaginary line the engine's prop shaft would follow if uninhibited... no matter the attitude of the plane. The datum line, also as stated, is the imaginary line that equally divides the aircraft's balance while in flight, particularly.