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.