|Eurocopter’s X3 demonstrator, a compound helicopter, made its maiden flight on September 6 in Istres, France. Eurocopter Photos
In a move that evoked the 1950s, when secret prototypes and proofs-of-concepts abounded, Eurocopter unveiled the X3 (“X cube”) on September 27 in Istres, France. The X3, which is laying the framework for Eurocopter’s Hybrid Helicopter (H3), is a compound helicopter demonstrator that first flew on September 6. The strange-looking aircraft uses components from existing models and features two short wings, each supporting a propeller, and no tail rotor. Targeting high speed and long range at an affordable cost, the company’s main commercial focus seems to be the 20-seat segment for offshore operations.
While Eurocopter has identified a business case for a 220-knot speed, it wants to preserve “the versatility of a helicopter,” as Jean-Michel Billig, executive vice president of research and development, put it. Moreover, “high speed shall not be achieved at any cost,” he said. Hence a concept where the hover capability is retained and “the high-speed cruise efficiency of a turboprop” comes on top.
The X3 was launched in January 2008, recalled Philippe Roesch, vice president of technology and product innovation. At high speed, “rotor rpm is reduced to avoid drag divergence at the tip of the advancing blade; the small wing then partially unloads the rotor, which avoids retreating blade stall,” he explained. The wing and empennage account for 30–40 percent of the lift in cruise, according to flight test engineer Daniel Semioli.
Auxiliary propellers provide propulsive force in forward flight and anti-torque control in hover. In high-speed cruise, the main rotor provides no more propulsive force. It then adopts a “flat” profile. As Semioli pointed out, this significantly cuts vibration and drag. To optimize the aerodynamics of the X3, flaps could be added to the wings and the empennage could be modified.
The demonstrator has conventional flight controls. Above 80 knots, the pilot uses a trim button on the collective lever to increase the propellers’ pitch (and hence the power they receive). Simultaneously, he has to decrease the collective pitch. Changing the power breakdown between the rotor and the propellers could be made more transparent, in future, with fly-by-wire controls.
According to flight test pilot Hervé Jammayrac, the aircraft is easy to fly in hover. Semioli said that, ergonomically, it could be flown by a single pilot. To minimize the development effort, Eurocopter engineers have not built an entirely new airframe. Rather, they have borrowed components from other medium twins. The fuselage comes from the AS365 N3 Dauphin. The five-blade main rotor is that of the EC155. The RTM 322 engines usually power the NH90. The main gearbox has been adapted from that of the EC175. “We added two lateral power outputs towards the propellers’ gearboxes,” Roesch said.
Eurocopter officials believe that, unlike a tiltrotor, such a compound could be certified in an existing category of helicopters. They insist that their formula is much less expensive than a tiltrotor.
The 220-knot speed was found to be the right compromise between time savings and cost. Roesch and his team estimate that costs (especially acquisition) will increase by a maximum 25 percent. On the other side, speed is 50 percent higher. This means that those costs measured by the hour should decrease, in total. As a result, the cost per passenger-mile is expected to be 20 percent lower than that of a conventional helicopter. The flight test program calls for 100 hours to be accumulated in 18 months. By December, the X3 should have flown at 160 knots. The target speed of 220 knots should be attained during the first quarter of 2011. Semioli estimates that a new model prototype could appear six years from now. Hence an entry into service two years later. An application in the 20-seat category would weigh about 13 metric tons—two more than today’s EC225. It would need three times the power of an RTM 322 (a total 8,700 shp). Eurocopter hopes that Rolls-Royce and Turbomeca will be able to offer a more powerful, more fuel efficient RTM 322 derivative in time. Another difference would be the fuselage, which would be much more shaped by aerodynamics, due to the higher speed. (From November 2010 Rotorcraft Report)