Could somebody explain the physics of the rotational force acting on an an aircraft when Vmca is reached.
I was talking with a mate (neither of us are MEP) regarding reducing rudder authority as Vmca approaches with yaw towards the dead engine.
is the Yaw enough to turn the A/C on its back?
The use of terms such as "rotational force" and "yaw" are contradictory in your question, and may lead to confusion.
Can an assymetrical thrust situation in a multi-engine airplane cause a roll, or cause the airplane to roll onto it's back? Yes, easily. Shut down one engine in a light twin, push the other up to takeoff power, then go do stalls. See what happens.
The goal in flying an airplane, of course, is to prevent that from happening. This may be gained in part by increasing airspeed, in part by reducing thrust on the good engine(s) to lessen the assymetrical thust, and in part by the loading and configuration of the aircraft.
In a light piston twin airplane, assymetrical thrust does several things, including producing unequal lift between the left wing and right wing, as well as a yawing force and a rolling force. When insufficient control remains to offset these forces, a roll or control departure can, and generally does result.