All wings require a certain amount of air to flow over its surface in order to generate lift. The inherent flight mechanics of a helicopter not at hover will result in a portion of the rotating blade disk to "see" a lower airspeed relative to the direction of travel. As the speed of the fuselage in the commanded direction increases, there will be a decrease in relative airspeed of the retreating blades. A conventional helicopter will reach a hard limit in terms of maximum speed when the relative airspeed of the retreating blades decays to near zero thus resulting in retreating blade stall.