In order to research the effect of rotor speed

blade spring stiffness and horizontal tail area on the controllability and stability of optimum speed rotor (OSR) helicopter

the flight dynamics model was established

and the induced velocity of rotor and aerodynamic interference of helicopter was calculated by free wake model. Finally the differential method was used to calculate the aerodynamic derivatives of the helicopter and control the derivatives matrix. The result shows that the controllability of the helicopter decreases with the decreasing of the rotor speed

but the change of stability about angle of attack was opposite. However the increase of blade spring stiffness in root can not only improve helicopter controllability

but also reduce the stability. The longitudinal stability of the OSR helicopter was decreasing without horizontal tail

and the handling qualities of sample helicopter are put at the third level in ADS-33E

but the period of instability oscillation cycle was longer

and the controllability of the helicopter was enhanced after cancelling horizontal tail. Thus

the scheme without horizontal tail for OSR helicopter is still controlled completely through the flight control system.