Eurocopter, Sikorsky develop high-speed compounds
Eurocopter (Booth No. 4637) and Sikorsky (Booth No. 2737) have successfully demonstrated the feasibility of a high-speed compound helicopter, but using different configurations. Sikorsky’s X2 reached its target of 250 knots last September in West Palm Beach, Fla., and also met vibration and workload targets, according to the U.S. manufacturer. A few days later, Eurocopter unveiled its X3 (X cubed) in Istres, France, with a 220-knot speed goal.
Both the X2 and the X3 are taking advantage of 21st-century technology to develop the relatively old compound concept. Both claim to be less complex than a tiltrotor, and their main applications will likely be military and commercial, respectively.
Sikorsky is exhibiting the X2 technology demonstrator at Heli-Expo. X2 team members, including chief test pilot Kevin Bredenbeck, are prepared to field questions from customers and media, a Sikorsky spokesperson told AIN.
The aircraft is a compound helicopter featuring two counter-rotating coaxial main rotors and a pusher propeller.
The X2’s flight test program is complete. “With all of our key performance parameters met, we will be moving our attention and resources to the [military] S-97 Raider program,” the spokesperson said. There is no longer a plan to fly a full hub fairing on the X2, which was expected to afford an extra 15 to 20 knots.
The 250-knot speed was the program’s main objective since launch in 2005 and was attained in level flight. The X2 even reached 260 knots in a shallow dive. Program officials said they are happy with the aircraft's aerodynamic performance. The vibration level, as hoped, is said to be similar to that of the Black Hawk military transport at its cruise speed of 140 knots. Pilot workload is relatively low thanks to the fly-by-wire control system.
Sikorsky president Jeff Pino announced the construction of two prototype X2 Raiders late in October. The U.S. Army will evaluate the prototypes in armed reconnaissance. The X2 Raider will have a two-pilot cockpit and space for armaments and auxiliary fuel or troops. First flight is planned in 2014. The proposed helicopter model to be developed from the X2 Raider is the S-97.
At last year’s Heli-Expo, a Rolls-Royce official accidentally and publicly alluded to the “Eurocopter X3.” About six months ago, the European company unveiled the X3, designed to cruise at 220 knots–about 50 percent faster than today’s medium twins. The aircraft made its maiden flight on September 6. It was built from a Dauphin helicopter airframe, with two short wings and two propellers in puller configuration. A conventional empennage replaces the tailrotor.
The X3 is doing “surprisingly well,” Eurocopter CEO Lutz Bertling said recently, noting that it reached 180 knots at reduced power in November. The aircraft’s flight envelope was expanded to 12,500 feet and up to 60 degrees in bank. Bertling also said the development simulator the pilots have been flying for two years has been very helpful.
The first test phase is complete and the engineers are now investigating all critical parts. The main gearbox, adapted from the EC175 medium twin, is being upgraded to accommodate full power from the aircraft's two Rolls-Royce RTM322 engines. Fadec software is also being modified.
The helicopter manufacturer believes it has found a sweet spot at a target speed that, although slower than the Sikorsky X2’s 250 knots, will still make the X cube profitable to operate.. The 220-knot optimal speed also helps meet the goal of keeping operating costs low. The design is inherently more efficient and fuel burn (per passenger mile) is said to be similar to that of a conventional helicopter flying at 140 knots. And by spending less time in the air for a given trip, maintenance costs per flight hour will be lower. The bottom line should be a 20-percent cut in per passenger mile costs.
The price premium of a commercial X3 should not exceed 25 percent more than the price of a conventional helicopter equivalent. Hoped-for civil applications for this high-speed, long-range concept include search and rescue, border patrol and commercial passenger transport, especially offshore. Military applications are under consideration, too. An application in the 20-seat category would weigh about 29,000 pounds–4,000 more than today’s EC225. The first application of the X3 concept may be in service in six years from now, Bertling estimated.
A combination of technology developments enables the X3 innovations, flight test engineer Daniel Semioli had explained to AIN last fall. The engines are now powerful enough, without excessive fuel burn. Manufacturers have a better command of materials, both metals and composites. Finally, test equipment, especially telemetry, is more sophisticated. Engineers can thus conduct a test campaign without too much trial and error.
The X3 demonstrator has conventional flight controls. Above 80 knots, the pilot moves a trim button on the collective to increase propeller pitch and, hence, the power they receive. Simultaneously, the pilot has to lower collective pitch. In the future, changing the power relationship between the rotor blades and the propellers could be managed more transparently with fly-by-wire controls.