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 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.