Both the Sikorsky X2 and the Eurocopter X3 have propellers and both are targeting higher cruise speeds; however, their respective designers have not been working to the same principles.
The X2 is effectively a semi-compound helicopter, having contra-rotating main rotors and one aft propeller, while the X3 is a true compound with a single main rotor, wing stubs and two side-mounted propellers.
Moreover, they are aimed at different markets. After having hit the X2’s 250-knot cruise speed target last year, Sikorsky is now applying the technology to the military X2 Raider, an armed reconnaissance prototype. Eurocopter prefers to limit the speed to around 220 knots, for economical reasons, as it focuses on commercial uses.
Also, the X2 has fly-by-wire controls, while the X3 has conventional controls. This may be one of the reasons why Eurocopter reached 180 knots in three months last year, whereas Sikorsky reached that speed, in 2010, after 21 months of developing a more complex rotorcraft. Despite the difference in development schedules, it took about the same number of flight hours–between 11 and 12–for the two aircraft to reach 180 knots.