airspeeds. If the rotor rpm were to be increased in an attempt to alleviate retreating blade stall, the helicopter will face a maximum speed induced by extreme drag of the rotor' advancing blades as their tips approach supersonic relative airspeed. Thus, in summary; If main rotor rpm is too low, the speed at which the retreating blade sections stall will be the maximum speed limit. If the main rotor rpm is too high, the speed at which the advancing blades encounter supersonic airflow would be the maximum speed limit. Even to the casual observer it should be clear that the designer should aim for a balance between these two limits. It should also be mentioned that in addition to these two addressed issues, there are a host of others which also contribute to maximum airspeed limits. Derschmidt's solution The basic problem inherent in rotor design is the difference in airspeed for the advancing and retreating blades. Among the many effects this causes is one of interest; the blades rotate forward and backward around the hub as drag increases and decreases. Consider a blade as it reaches the rear of the aircraft and starts to rotate forward; during this time the relative airspeed starts increasing rapidly, and the blade is pushed further and further back by the increasing drag. This force is relieved by a drag bearing. During the brief period while it rotates around this bearing, the overall speed of the blade is decreased, slightly offsetting the speed due to forward motion. Derschmidt's rotor design deliberately exaggerates this rotation to offset the increase and decrease in speed throughout the blade's rotation. At the same point of rotation as the traditional blade above, a Derschmidt rotor has advanced the blade considerably to an angle of about 40 degrees compared to its rest position straight out from the hub. As the blade continues advancing, a linkage swings the blade from 40 degrees forward to 40 degrees rearward, slowing the tip by about 1/2 the rotational speed. This process is reversed as the blade reaches its forward-most position, increasing the speed of the blade as it retreats. The resulting motion helps smooth out the relative airspeed seen by the blade. Since the effects of the forward motion of the helicopter are reduced, or even eliminated at lower speeds, the rotor can be spun at a high speed without fear of reaching the wave