I wondered too; here's some information I found: https://aviation.stackexchange.com/questions/16948/do-helicopter-rotor-tips-regularly-go-supersonic/16958#16958

I wondered too; here's some information I found: https://aviation.stackexchange.com/questions/16948/do-helicopter-rotor-tips-regularly-go-supersonic/16958#16958 https://aviation.stackexchange.com/questions/16948/do-helico... From the middle of that answer: > In normal operations, and design aims to achieve this, the rotor tips do not go supersonic since when they do, there is a sudden and large decrease in performance with more power required, higher blade loads, vibration and noise. > Think about a helicopter flying forwards. The advancing blade at its most perpendicular position experiences a relative airflow which is equal (ignoring all kinds of minor side effects) to the forward speed plus the speed of the blade. The retreating blade is experiencing a relative airflow equal to the speed of the blade minus the speed of the helicopter. > If the blades rotate so fast that the tips are supersonic, then the main lift generating part of the retreating blade, the outer two thirds of the span, would experience such a low airspeed, for some of the span it will even be negative, that the blades will stall causing a catastrophic roll into that side. It is this phenomenon which ultimately limits the rotational speed of the blades and the maximum speed of the helicopter. I also found this: https://www.quora.com/Does-every-helicopter-main-rotor-blade-tip-move-at-the-speed-of-sound/answer/Tom-Farrier https://www.quora.com/Does-every-helicopter-main-rotor-blade...