As basic researches to clarify aero-mechanical behaviors of a tilt rotor, the blade flapping motions during low speed transition state are analyzed using the proposed inflow model for a rotor being subjected to uniform flow with arbitrary attitude angle from 0° (rotor mode) to 90° (propeller mode). The inflow model is formulated by extending acceleration potential theory for a circular wing so aerodynamic load and corresponding induced velocity distributions as to be consistent with the flapping motions obtained by dynamic running test with the model rotor. Comparisons between calculated and measured blade flapping motions for the rotor both at fixed attitude and tilting conditions are made, thus ascertained usefulness and reasonability of proposed inflow model.