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R&D: X2 Melds Past for the Future

By James T. McKenna | April 1, 2008
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Developers of the X2 Technology Demonstrator are drawing on components and systems from other aircraft to expedite prototyping of the high-speed aircraft.

The Sikorsky Aircraft development team has been striving to achieve a first flight of the coaxial main-rotor helicopter, a milestone delayed (due to a variety of factors) by more than a year from the original target of late 2006. Nonetheless, Sikorsky executives elected to interrupt that effort to bring its sole prototype to Heli-Expo 2008 in Houston in February.

Program leaders gave Rotor & Wing editors a sneak peek at the flight test aircraft before unveiling it to intrigued Heli-Expo attendees. They reviewed the pedigree of the prototype’s components and updated us on the development work. Their update made clear that much remains to be done before the demonstrator could be cleared for first flight. Sikorsky’s latest target for that milestone going into Heli-Expo was March 31. "We’ve got work to do," said Peter Grant, Sikorsky’s advanced programs manager.


Sikorsky executives chose to trade development time for the opportunity to assess commercial operators’ interest in X2-based aircraft and plumb the uses they might make of such aircraft. The demonstration program aims to field an aircraft that can cruise at 250 kt with low vibration while retaining a traditional helicopter’s capabilities for hover, confined-area, rough-field, low-speed maneuver, and nap-of-the-earth operations, and autorotation.

"We’re really looking forward to gauging the reaction from the audience," Grant said. That will help "guide our direction as we conceptualize future products." Sikorsky will pursue the same conversations with potential military customers, he said. That will help it decide whether the launch X2 product will be a commercial or military one.

X2 team members were nervous about transporting the lone demonstrator from Sikorsky’s rapid-prototyping center of excellence at its Schweizer Aircraft subsidiary to the exhibit halls in Houston. They secured internal systems as much as possible, they said, to prevent damage during transport. They also kept a wary eye on forklifts racing by the aft propulsor and rigid main-rotor blade tips and cherry-pickers moving up and down as exhibits rose around the prototype. It remained shrouded until Sikorsky President Jeff Pino unveiled it on Feb. 24, Heli-Expo’s first day. It survived the show unharmed.

"We’re helping the marketing guys expose the aircraft," said Kevin Bredenbeck, Sikorsky’s director of test and evaluation and chief test pilot, of the value of bringing the prototype to the show.

"A picture is one thing," he said. "You come up and look at the aircraft and it’s the real thing. The blades that are on that aircraft, those are the blades."

The demonstrator had accumulated about 4 hr and 22 starts in its current configuration prior to the trip to Houston, team leaders said. Other engine starts and ground runs had been done last year. They were focused on interfaces with the LHTEC T800-LHT-801 engine, while the most recent ones have focused on power-train interfaces.

Sikorsky’s plan calls for about 10 hr of "bare-head" ground runs, without the main-rotor blades installed, followed by about 65 hr with those blades on. Then the aircraft would have to be run on the ground for about 10 hr with no changes to its flight-control or integrated-power system software before it could be cleared for the first flight.

The team was about halfway through bare-head runs before Heli-Expo. "We’re probably a couple of weeks away from actually getting the blades on the aircraft and turning the blades" for test purposes, Bredenbeck said.

The aircraft at Heli-Expo was the real one, though it hadn’t yet run with all its components. "This is how you know the aircraft is real," said Steve Chisarik, Sikorsky’s X2 program manager, pointing to the engine exhaust. "There’s carbon on the pipe."

The prototype is an amalgam of other aircraft’s components. Its main wheels come from the Sikorsky S-76 and the pitch bearings from the CH-53E. It uses seats and an oil cooler blower from the S-92. The aft propulsor supplied by Aero Composites is based on a racing propeller for a fixed-wing aircraft. The engine was developed by the LHTEC partnership of Rolls-Royce and Honeywell for the cancelled Sikorsky/Boeing RAH-66 Comanche program for the U.S. Army.

"For the things that are not pertinent to the X2 technology itself, we use existing parts," said Steve Weiner, Sikorsky’s chief engineer for the X2.

Most importantly, Bredenbeck said, X2 is building on the research done for the XH-59A Advancing Blade Concept aircraft Sikorsky developed in the 1970s for the Army, U.S. Navy, and NASA. "The math and the physics are done" as a result of that work, he said.

While that aircraft demonstrated a conventional helicopter could fly at high speeds, it was hobbled by extreme vibration at those speeds. Sikorsky officials are confident they can overcome that problem with Sikorsky’s fly-by-wire system and an active vibration control system from Moog.

One objective is to simplify management of the aircraft in flight. While the XH-59A used a variety of analog gauges and controls, "we got it down to two controls," Bredenbeck said: a conventional collective developed for the X2 and a sidestick controller for cyclic control that also is borrowed from Comanche. Conventional pedals can be used to control turns in a hover or the pilot can use the beeper switch on the sidestick.

The X2 has two "dumb" displays. "Everything that you need to do from an advancing-blade concept point of view is sitting there," Bredenbeck said.

The main rotor blades are a blend of Sikorsky-proprietary airfoils, Grant said. Engineers calculate that, with the X2 flying at 265 kt, the retreating blade "is going to see upwards of 80 percent of its flow reversed," he said. "So that led to the need for double-ended airfoils close to the hub as part of our scheme for reducing drag due to the rotor hub."

The ultra-rigid blades were designed by Sikorsky based on XH-59A data and manufactured by Eagle Aviation Technologies in Hampton, Va.

Roughly 90 percent of the X2’s power is expected to go to the aft propulsor at cruise speeds, "when the main rotor is pretty close to autorototation," Weiner said.

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