throughout their motion. This adjustment is in addition to any that is being applied deliberately to manoeuvre. Since every control system has some mechanical limit, as the aircraft speeds up it loses manoeuvrability. Drag is a function of the square of airspeed, so the same changes in speed cause the drag to vary by a factor of four. To reduce the net force as much as possible, helicopter blades are designed to be as thin as possible, reducing their drag. In the 1950s, helicopter blades were made in much the same fashion as fixed-wing aircraft wings; a spar ran the length of the rotor blade and provided most of the structural strength, while a series of stringers give it the proper aerodynamic shape. This method of construction, given the materials of the era, placed enormous stresses on the spar. To lessen the loads, especially the rapid changes, the rotor hubs included a system of bearings that allow the blades to move forward or back in response to drag, and up and down in a flapping motion in response to changing speed. These were in addition to the system used to change the angle of attack to provide control; rotor hubs tended to be very complex. Performance limits From an elementary aeronautical point of view, there exists two main problems regarding the maximum airspeed of a helicopter as imposed by its rotor system. 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. One solution to this problem is to increase the rotor rpm so that the relative airspeed of the retreating blades are higher. However, this solution also has its limits. As any airfoil approaches the speed of sound it encounters a problem known as wave drag. Airfoils designed for subsonic flight will experience a significant increase in drag if they are subjected to transonic or higher