Last week, Eurocopter took the wraps off its developmental X3 helicopter, with a claimed top speed of 220 knots. The "speed barrier" for conventionally designed rotorcraft has always been defined by the speed differential between the advancing rotor blade(s) set off by that of the retreating blade(s). The faster the aircraft flies, the greater the speed of the blade on the advancing side of the disk — and all the associated problems of drag, structural requirements etc. In 2008, Sikorsky flew its X2 with coaxial contra-rotating rotors. Since the two sets of rotors were spinning in opposite directions, the problem of advancing-vs-retreating blade speed differential was minimized. The Sikorsky has been clocked at 250 knots in test flights — a good 100 knots faster than normal helicopter speeds. The newly unveiled Eurocopter X3 uses a different configuration — fixed forward facing propellers (in contrast with the tilting 'prop-rotors' of a tiltrotor) and stub wings that share the lift responsibilities with the main rotor disk when the aircraft is in high-speed flight.