The RAF (CFS) teaching for rotary winged aircraft uses a vector diagram for the forces acting on a rotor blade. This shows the resultant of lift and drag on a blade segment as a the "Total Reaction". This is then broken down into "Rotor thrust" acting vertically up the rotor mast and "Rotor drag" acting perpendicularly to it. Rotor thrust lifts and propels the aircraft. Rotor drag is opposed by the application of engine Torque.
Similar thing to the prop theory given by Beags. It has to be done for rotating things because the lift and drag on a rotor blade don't act coincidentally with the rotor axis. I don't think it's necessary to do it for plank wings as it over-complicates the issue.