In basic terms, a helicopter’s main rotor system is what provides lift and thrust. The rotational speed (rpm) is held constant and the pitch angle of the blades is varied to change the amount of lift and thrust. Engineers look at many different variables when designing a rotor system and one is the direction the rotor spins, clockwise or counter clockwise (when viewed from above). Unlike the tail rotor, from an aerodynamic efficiency standpoint there is no difference between the two directions. As it turns out, in most cases the direction of rotation can be associated with the country of origin. For example, as a general rule, helicopters manufactured in the United States (and some other countries like Germany) turn counter clockwise, while French and Russian designed helicopters spin clockwise.
However, from a piloting viewpoint there are some differences. The biggest one comes from Newton’s third law that states for every action, there is an equal and opposite reaction. As such, the torque applied to the rotor system causes the helicopter’s fuselage to spin the opposite direction. This is the primary reason for the tail rotor, or more accurately called the anti-torque rotor. It applies a force opposing the main rotor torque to stop the fuselage from spinning. The more power the pilot demands, the higher the torque and the more thrust the anti-torque rotor must produce to control yaw. In the cockpit, it is the pedals that control the anti-torque rotor’s pitch and therefore its thrust. What this means to the pilot is in a clockwise turning main rotor, right pedal must be added as power is increased and left pedal when power is reduced. For a counter clockwise spinning rotor system it is just the opposite.
Some of the other differences that are not as noticeable to the pilot are translating tendency and dissymmetry of lift. Translating tendency is the tendency of the helicopter to drift in the direction of tail rotor thrust. A clockwise turning rotor will cause the helicopter to drift to the left. Dissymmetry of lift refers to a difference in lift across the rotor system as one blade advances into the wind (headwind) and the other side retreats (tailwind). Again, depending on which way the rotor spins the advancing side will be on the right or left side of the rotor disc.