Retreating blade stall is a hazardous flight condition in helicopters and other rotary wing aircraft, where the retreating rotor blade has a lower relative blade speed, combined with an increased angle of attack, causing a stall and loss of lift. Retreating blade stall is the primary limiting factor of a helicopter's never exceed speed, VNE. Retreating blade stall occurs at high forward speeds, and should not be confused with rotor stall, which is caused by low rotor RPM and can occur at any forward speed. Advancing vs. retreating blades A rotor blade that is moving in the same direction as the aircraft is called the advancing blade and the blade moving in the opposite direction is called the retreating blade. Balancing lift across the rotor disc is important to a helicopter's stability. The amount of lift generated by an airfoil is proportional to the square of its airspeed (velocity). In a zero airspeed hover the rotor blades, regardless of their position in rotation, have equal airspeeds and therefore equal lift. In forward flight the advancing blade has a higher airspeed than the retreating blade, creating unequal lift across the rotor disc. A fuller treatment is provided in dissymmetry of lift. Compensation Most helicopter designs compensate for this by incorporating a certain degree of vertical "flap" movement of the rotor blades. When flapping, a rotor blade will travel upward during its advance, creating a lesser angle of attack (AOA) and therefore lesser lift. When the blade retreats, the blade falls downward again, increasing the AOA and therefore generating greater lift. There are three general designs. The earliest, and by far, least common design today, is the fully rigid rotor system; the blades are rigidly fixed to the rotor hub but made of a flexible material that allows some degree of flap. Semi-rigid rotor systems have a horizontal hinge at the base of the blades that allow flap as they rotate. By necessity they always have an even number of blades, as each opposing pair is mechanically connected to prevent vibration. Fully articulated rotor systems use a combination of flapping and a horizontal motion that moves the retreating blades forward slightly and moves them back again on the advancing side, thus creating more relative airflow and lift on the retreating side at the expense of the advancing side. In all cases, the pilot may compensate the induced roll with left or right