Sikorsky's X2 Technology

It’s as much evolution as revolution
Ken Pole
July 09, 2007
By Ken Pole
By Ken Pole

208-sikorA new initiative by Sikorsky Aircraft Corporation evokes the pioneering efforts by the company’s founder nearly a century ago and is expected to yield a significant breakthrough: production helicopters capable of sustaining at least 250 knots in level flight. That would compare with the 150-170 knots possible with most other helicopters even today.

Although Sikorsky’s fly-by-wire X2 Technology was unveiled only last June, preliminary design was already complete and parts fabrication had begun for an X2 Technology Demonstrator (X2TD) that the company plans to fly at its Schweizer Aircraft subsidiary this year. It will feature closely-stacked counter-rotating coaxial main rotors with a lowdrag profile, augmented by a pusher propellor, all powered by a Honeywell/Rolls-Royce LHTEC T800-LHT-801 turboshaft.

Sikorsky is drawing on decades of experience with programs such as the XH-59A Advancing Blade Concept (ABC) Demonstrator, a 1970-80s project with coaxial rotors and two J-60 turbojets. The X2TD is expected to address the XH-59A’s shortcomings with advances in rotor blades, transmissions, active vibration controls, flight control laws, etc. Sikorsky also is using experience from its Cypher unmanned aerial vehicle, an FBW ‘doughnut’ with shrouded coaxial rotors, and the Boeing- Sikorsky RAH-66 Comanche stealth attack helicopter, which featured a five-bladed bearingless main rotor, shrouded tail rotor and an advanced transmission.

Development of an X-2 fleet will be as much evolution as revolution. A Schweizer 333 surrogate first flown in early November at the main Schweizer plant in Elmira, NY, is being used for initial testing of the triply-redundant system. Also, while FBW technology could concentrate the cyclic, collective and pedals on a single side-arm controller (SAC), the X2TD will feature a conventional collective stick, an SAC cyclic, and pedals for yaw control. It will be able to incorporate the latter into the SAC but initial flights will employ the pedals until pilots are A comfortable with having the unusual yaw control capability. Yaw is induced in a coaxial-rotor helicopter by increasing blade pitch in one rotor and decreasing it in the other, resulting in a differential torque which could be augmented by a rudder at higher speeds.

“We’ll be flying people that are not necessarily familiar with the side-arm control and fly-bywire,” explained Mark Hammond, program manager for the X2 Technology at Sikorky’s headquarters in Stratford, Connecticut. “I’d like to emphasize that the aircraft is just a demonstrator, not a product.”

The concept of a single control isn’t new. The Comanche initially was designed that way but fell afoul of “the human factors interface,” as Hammond put it. “They didn’t necessarily care for how the collective was situated on the side-arm controller, so we went back to our conventional collective stick, separate from the side-arm controller, on the Comanche.”

Peter Grant, program manager for Sikorsky Advanced Programs, added that final configuration would evolve as Sikorsky’s test pilots put time on the X2TD. “As Mark suggested, you balance what you need to control the aircraft with how comfortable the pilots who are actually flying it are.” He pointed out that when the General Dynamics F-16 Falcon arrived on the scene, it had an SAC that went through “some significant evolution – not so much from an engineering perspective of how those controls are translated to the fly-by-wire system, but from an ergonomic perspective that made it comfortable for pilots.”

Sikorsky foresees X2 Technology being used in all weight classes. The sleek X2TD should top out at about 6,000 pounds but Sikorsky has two US Army Applied Aviation Technology Directorate (AATD) contracts for conceptual design of heavy-lifters. One would be a super-heavy-lift coaxial platform with a cruising speed of 165 knots, the other a high-speed super-heavy-lift configuration capable of cruising at 245 knots. The AATD foresees no fewer than eight variations of the latter to identify the impact of changes in payload, range, environmental conditions and shipboard compatibility on aircraft size, performance, operational suitability, cost, schedule and development risk.

Major design differences in X2 aircraft include the rotor blades and hubs. The high-speed configuration will use a rigid advanced composites rotor but the blade profile is different from what is found in a conventional helicopter. Sikorsky isn’t saying much beyond that until it rolls out the X2TD. “What we learn with the Demonstrator will help us to design this technology into other aircraft,” Grant said. “What does that mean for conventional helicopters? Our backlog of conventional helicopters today speaks volumes about the long-term viability of singlemain- rotor helicopters, but X2 Technology provides or satisfies a new desire for some customers.”

Comparisons may be invidious but they’re unavoidable. How would X2 aircraft stack up against the 250-knot Bell Boeing V-22 Osprey tiltrotor? Customers evidently want more flexibility and speed from their helicopters and if hover efficiency is a metric for comparison, Grant said matter-of-factly that Sikorsky’s next-generation aircraft will be “much more” efficient than a tiltrotor. At the other end of this spectrum is the Lockheed-Martin F-35 Joint Strike Fighter, a short takeoff/vertical landing variant of which has been chosen by the US Marine Corps and the Royal Navy. While it is capable of supersonic speeds, it’s perhaps the least-efficient way to hover.

“Those different aircraft configurations really speak to different missions, different utility, different customer sets,” Grant said. “Because of the greater hovering efficiency of X2 Technology, it is first and foremost in our minds a helicopter. It is not being designed around a turboprop mission, for example, and because of that greater hovering efficiency, we see there being therefore greater flexibility in how the aircraft can be used. And frankly that is the success of helicopters in the aircraft market today, because of their difference from airplanes. They’re able to hover efficiently for long periods of time but if you want to go very long distances very fast you’re going to get on an airplane.”

Then there’s manoeuvrability. Grant said there is no question Sikorsky’s new machine will outperform its current and previous platforms in that regard. He cited the company’s experience with the XH59A Advancing Blade Concept demonstrator in collaboration with the US Army, Air Force and Navy as well as the National Aeronautics and Space Administration in the late 1970s. Its coaxial rigid rotors enabled pilots to pull up to 2.5Gs and it topped out at more than 260 knots. It had excellent manoeuvrability but Sikorsky was precluded from putting it into production because other technologies such as FBW simply weren’t developed enough. “The combination of the two rotors and differential controls between them can give you some great manoeuvrability,” he said. “Plus, of course, you have much more power installed in the aircraft for speed and you can use that power, again, in ways that you wouldn’t in a conventional helicopter.”

Counter-rotating coaxial rotors in and of themselves are old hat. Igor Sikorsky’s first helicopter, the S-1, which he built in Kiev in June 1909, had two-bladed coaxial rotors – but meagre engine output meant that it couldn’t lift its own weight. His S-2 a year later used three-bladed rotors and while it could get off the ground, it couldn’t carry a passenger. It also vibrated violently because of its wooden frame, so Sikorsky turned to fixed-wing development and then emigrated to the US. However, a year before the S-1, Emile Berliner and John Newton Williams experimented with the concept in the US, Williams designing a coaxial machine powered by two of Berliner’s rotary engines. It is said to have lifted a gross 277 kg. Also about that time, a professor and students at Moscow University were believed to have built a coaxial helicopter and Jacob Christian Ellehammer of Denmark designed one he evidently flew on several short hops.

Several others had varying degrees of success with coaxial rotors but it was the famed Soviet designer N.I. Kamov who really applied the concept usefully, beginning in 1947. The ensuing decades saw his eponymous company develop and build an extensive range of civil and military coaxial helicopters whose compact dimensions and high thrust-to-weight ratio suited them well for Soviet naval deployment. By the late 1970s, composites and other technologies came together in the Ka-50 Black Shark (NATO codename Hokum) attack helicopter flown by the Red Army.

Once in the US, Sikorsky eventually turned his attention back to helicopters, but with a conventional configuration. It wasn’t until the 1990s that the company returned to coaxial technology with the Cypher unmanned aerial vehicle, which had counter-rotating rotors within a doughnut-shaped shroud. That experience, coupled with transmission technology advances on the Comanche, development of active vibration control, and the advancing blade concept from the XH-59A convinced the company that the time was ripe for the next step in rotary aviation. Asked what Canadian markets he foresaw for X2-equipped aircraft, Grant said Sikorsky had opted to be “very non-specific” about production aircraft. “We first have to demonstrate that it works, that all of these technologies coming together in a platform systemintegrated approach can achieve the goals we’ve set for ourselves with this technology. We’ll have to see what evolves.”

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