“Something like 10 years back, maybe more, Sikorsky
realized that some of our customers’ requirements, both commercial and military, were going to start calling for greater range and speed. And we’ve always had an internal interest in improving the noise level issue and maneuver- ability,” Van Buiten says. In attempting to address these issues, he says, “We looked at a lot of concepts and just gravitated toward this concept.” Forty years ago, Sikorsky studied an advancing blade
concept, which relies on two sets of rigid rotor blades mounted coaxially, one on top of the other, to counteract the inherent helicopter problem of the retreating blade, which moves in the opposite direction of the vehicle’s flight path, losing lift and speed. In flight tests in the late 1970s and early 1980s, Sikorsky’s XH-59A test vehicle—an advancing blade demon- strator augmented by two jet engines mounted on its sides—hit speeds of more than 300 mph in shallow dives. In straight and level flight, it topped out above 250 mph. Tose are speeds no conventional helicopter can hope to achieve because of the retreating blade effect. “Because it had a rotor, it could do everything a helicopter
could do, but [the XH] doubled the speed,” Van Buiten explains.
However, the XH-59A was too heavy, too fuel inefficient,
and too expensive to operate back in those days. It also was more than a handful for two pilots to fly, in part because of significant vibrations, an inherent characteristic of coaxial helicopter designs. Beyond that, no one in the military at that time seemed to have an appetite for taking on such a difficult and risky new technology. In the mid-2000s, Sikorsky returned to the concept again
with the development of the X2, this time pairing the advanc- ing blade technology with an advanced pusher propeller called a propulsar. Te X2 demonstrator also featured fly- by-wire flight control technology, lighter composite materials and a lot less steel and aluminum, and more advanced vibration control systems that took advantage of computer processors and software that had not been invented back in the XH-59A’s days. Te propulsar installed in the tail—which also was not present in the XH-59A—proved capable of pushing the aircraft forward for prolonged periods at speeds approaching 300 mph, roughly twice the top speed of a conventional helicopter. In 2013, two years after the X2’s retirement, the first S-97
Raider began its test-flight regime. While its speed perfor- mance and many other characteristics were excellent, it too
“Because it had a rotor, it could do everything a helicopter could do, but [the XH] doubled the speed.”
WINTER 2019 ROTOR 33
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