The Eurocopter X³

The Eurocopter X³ (X-Cubed) is an experimental compound helicopter under development by Eurocopter. It is intended to fly at speeds of over 220 knots (250 mph; 410 km/h).
The X3 demonstrator is based on a Eurocopter EC155 helicopter with the addition of short span wings each fitted with a tractor propeller. The tractor propellers are gear driven from the two main turboshaft engines which also drive the five-bladed main rotor. The helicopter is designed to prove the concept of a high-speed helicopter which depends on the slowing down of the rotor speed to avoid drag from the advancing blade tip, and to avoid retreating blade stall by unloading the rotor while a small wingis intended to provide up to 80% lift instead. 
Conventional helicopters use tail rotors to counter the torque effect of the main rotor. The starboard propeller of the X3 has a higher RPM than the port to counter the torque effect.

General characteristics

• Crew: 2
• Powerplant: 2 × Rolls-Royce Turbomeca RTM322 turboshaft engines, 1,693 kW (2,270 hp) each
• Propellers: 2-bladed five bladed tractor propellers gear driven off main engines.

Performance

• Maximum speed: 430 km/h; 267 mph (232 kn) at 2,500 m (8,202 ft)
• Cruising speed: 407 km/h (253 mph; 220 kn) 
• Service ceiling: 3,810 m (12,500 ft)
• Rate of climb: 25 m/s (5,000 ft/min)