A helicopter gets its aerodynamic lift the same way a fixed-wing aircraft does, the only difference being that it has only rotor blades instead of fixed-wings. The angle of attack (the angle at which the leading edge meets the airstream) determines the amount of lift. As the rotor turns, it traces out the circle in the air which is known as the rotor disc. If a helicopter is just hovering, the speed of the blades through the air is constant at all points around the rotor disc. When the helicopter is moving forwards, however, the speed of the blade changes relatively as it travels around the disc. When a blade is traveling towards the front of the disc, its airspeed is its speed due to rotation plus the speed of the helicopter. When a blade is traveling towards the rear of the disc, its airspeed is reduced by the speed of the helicopter, which at that point is effectively traveling in the opposite direction to it.
At high speeds, this effect amplifies manifold. The drag on the forward-moving blade is greatly increased and it begins to lose lift because of the break-up of the airflow over it. The backward-moving blade loses lift because of its relatively low airspeed. The helicopter can not sustain itself in the air. Therefore, no helicopter can fly at more than 400 kilometers per hour. The world record speed was achieved by a Westland Lynx helicopter (photo, above), which reached a speed of 400.87 kilometers over Somerset, England on August 11, 1986.
Westland Lynx (Wikipedia)