Dear Andre,
I agree with your equation in the case of a rigid rotor (all blade and/or teeter degrees-of-freedom disabled). The rotor inertia, “J_Rotor”, is computed and output to the FAST summary (.fsm) file. However, if the rotor is not rigid, “J_Rotor” will change with blade deflection and/or rotor teeter during the course of a FAST simulation, so this calculation will only be exact for a rigid rotor (for a flexible rotor, the calculation will work when averaged over some length of time). More discussion on this topic is available in the following forum topic: Aerodynamic Torque.
Best regards,