Abstract: In this study, a novel multi-mode flight control strategy for unmanned helicopter, in presence of model uncertainty, atmospheric disturbances and handling qualities specification requirements (as in ADS-33E-PRF), based on multi-loop control structure is presented. Robust H-infinity D-stabilization optimal control technique is utilized in inner loop, which ensures the stability of flight control system in case of change of helicopter model uncertainty and effectively eliminates effect of gust disturbance on helicopter states and collective/cyclic inputs. Sufficient condition for the solvability of robust H-infinity D-stabilization controller is derived in a form of Linear Matrix Inequality (LMI) for inner-loop control design. By formulating command tracking problem as state feedback stabilization problem of augmented system, proportional-plus-integral (PI) tracking configuration is used in outer loop to improve the dynamic and static operation characteristics of the helicopter system. Based on the proposed control strategy, multi-mode flight control for the helicopter is designed and the flight performance is verified. Analysis and simulation results show that level 1 handling requirements as defined in ADS-33E-PRF are accomplished.