Suppose I have an engine with propeller. I know there is aerodynamic force pushing the propeller opposite direction the engine is turning it. One component of this force is creating lift, but second is braking the prop. Now these forces are applied at some distance from engine shaft, thus they're creating torque, which is turning the whole engine if it wasn't fit somewhere.
And now my problem: how to describe what happens, after fitting this engine out of mass center. For example take a ruler, that has pivot somewhere and at the end of it, there is this engine with propeller. How do I describe momentum at the center of mass - the pivot, instead at the engine (engine has no weight in this example)?