Unmamned rotorcraft requires automatic control system to adjust transient flight behaviors properly, and real-time computation of aerodynamic forces is also in demand. Finite-state inflow model is one of the most efficient models which can fulfill such a requirement. When a rotorcraft is flying close to ground, a strong downwash impinges on the ground surface and reflected airstream influences the flow field of the main rotor. Such a physical phenomenon makes the aerodynamic computations complex, and previous studies have solved such a problem by simulation ground effect with a ground rotor. But it still requires numerical integration to compute the flow field of main rotor downstream, and the computational efficiency degrades evidently. In this paper, adjoint theorem is proposed to calculate induced velocity in the rotor wake with closed form equations. The hover and forward flight conditions will be examined for different moment of inertia of blade and forward flight speeds.