The Eurocopter X3 (or simply as the "X-Cubed") was a developmental rotary-wing platform intended to validate a helicopter design for speeds exceeding 250 miles-per-hour (typical operating speeds for modern helicopters range from 150 to 160 miles-per-hour). The design was based on a highly-modified Eurocopter EC155 airframe yet differentiated by the addition of side-mounted pusher-propeller engine units and fuselage protrusions not common to the EC155 design. The X3 undertook flight-testing with the prospect of the design being used in a future commercial- or military-minded rotary-wing platform (though weapon placements on wingstubs would have proven problematic in the X3's standard engine arrangement). The X3 was set to usher in something of a new age for helicopters considering its high-speed qualities.
A first-flight of the Eurocopter X3 was completed on September 6th, 2010 and, on May 12th, 2011, the X3 undertook a recorded flight with speeds in excess of 267 miles-per-hour. For its time, the X3 was billed as the "World's Fastest Helicopter" though there were (and have been) other similar competing helicopter designs in the works elsewhere (the Sikorsky X2 demonstrator being one such competitor). All told, the X3 did prove itself a sound concept with excellent agility and impressive speeds during its presentation flights meaning that its application in the real world would have been rather limitless as helicopters go.
The overall design of the X3 was given well-contoured lines from nose-to-tail. As in the EC155, the X3 sported a finely-sharpened nose assembly with the two-person cockpit just aft. A passenger compartment was situated directly behind the cockpit with the twin Rolls-Royce engines seated atop the fuselage. The engine arrangement was used to power the main rotor installation as well as the pair of propeller-based units - as these units cancelled out the torque generated by the spinning main blades, no conventional tail rotor was used (the portside propeller spun at fewer RPMs than the starboard mounting, countering the inherent torque effect from the main rotor). As such, energy was not wasted with a long shaft used to power a tail rotor but, instead, added to the forward speed output. The empennage was smooth and rounded towards the extreme aft-end of the aircraft to which a pair of horizontal fins were affixed. The undercarriage was wheeled for ground-running and wholly retractable to maximize aerodynamics.
Power for the X3 was derived form 2 x Rolls-Royce Turbomeca RTM322 series turboshaft engines, each delivering 2,270 shaft horsepower. These powerplants rotated the five-bladed main rotor atop the fuselage as well as the pair of five-bladed tractor/puller propeller systems fitted to wingstubs along the fuselage sides. Cruising speed could reach an impressive 252 miles-per-hour while a service ceiling of 12,500 feet was ultimately estimated.
Only a single prototype form of the X3 was been completed and used solely for testing. Some sources stated that EADS intended the X3 to be a true production-ready helicopter and not simply a technology demonstrator or "proof of concept" design. In either case, the project was eventually ended with no push for serial production.