In order to fly, a typical helicopter turning its blades has whichever blade is spinning 'forward' travelling through the air faster than the aircraft is flying since it needs to keep circling the main shaft. As you get faster and faster, the end of that blade will ALREADY be going supersonic at its end. The shock wave from that blade's tip breaking the sound barrier will damage the aircraft. The speed of sound is about 1100 feet per second, which was a problem with some of the WW2 bombers. If the tips of their propellers went supersonic, the resulting turbulance affected the wing and the rest of the plane. Using a rotor, the theoretical maximum speed for helicopters is just over 250 mph.
The way that Hollywood got around that little detail was explained in the pilot episode. The rotor disengaged and the aircraft was powered by the 'Turbos' in the engine exhaust pushing it. The lift was developed owing to the fuselage being shaped as a 'lifting body', much the same way that some of NASA's research ships were made to fly without anything resembling wings.
BTW, the world record for the fastest helicopter is 249.1 mph.
