Fastest Helicopter: Sikorsky Aircraft’s X2

Kevin Bredenbeck explains the Advancing Blade Concept of coaxial rotors

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This segment is part of the IEEE Spectrum series “Fastest on Earth.”

Susan Hassler: The official speed record for helicopters was set in 1986, at 249 miles an hour. But the unofficial record belongs to a new type of helicopter, the X2, from Sikorsky Aircraft. In 2010, it exceeded 300 miles an hour. Now retired from flight, the X2 is headed to the Smithsonian National Air and Space Museum. David Schneider checked it out in Fort Lauderdale, Fla.

David Schneider: Of all possible modes of transportation, helicopters have got to be the coolest. They can lift off from anywhere, hover around with the agility of a bumblebee, and then zoom off to faraway places—well, sort of. The zooming part is, in fact, a bit of a challenge for most helicopters, which can’t go very fast by aircraft standards. About 180 knots—say, 200 miles per hour—is very tops for most. Steve Weiner, director of engineering sciences at Sikorsky Aircraft, explains why that is and why Sikorsky’s new experimental helicopter, the “X2 demonstrator,” may be a game changer.

Steve Weiner: On a regular helicopter with a single rotor, as the rotor blade is spinning around, on one side of the helicopter it’s moving in the same direction that the helicopter is flying. We call that the “advancing side.” On the other side of the helicopter, as that blade comes around, it’s moving opposite to the direction that you’re flying. So that’s the “retreating side.” That blade sees an airflow that’s a lot less than the one on the other side.

David Schneider: As a result, it can’t produce much lift. To compensate, the pilot can adjust the “pitch”—or angle of attack—of the retreating blade so that it takes a bigger bite of air, which gives some more lift. But he can only do that up to a point.

Steve Weiner: Eventually that pitch gets so high that that blade stalls: It can’t provide any more lift. So you can’t go faster than that speed, because if you do, the blade on the other side, the advancing side blade, which is still producing lift, will actually roll the helicopter over. So in order to keep yourself flying straight and level, you have to slow down.

David Schneider: Sikorsky has long been exploring a way around that problem, which is to have two sets of “coaxial” rotor blades—meaning they rotate around the same axis, one on top of the other, but in opposite directions. Loss of lift on a retreating blade is then no big deal, because another blade on that same side is advancing and can provide lift. Kevin Bredenbeck, Sikorsky’s chief test pilot, explains the origins of this Advancing Blade Concept.

Kevin Bredenbeck: It was in the early ’70s and early ’80s that we had a program that was looking at the dual-coax rigid-rotor system for the first time. You know, four engines on that helicopter. We didn’t have the advanced technologies for engine controls. We didn’t have fly-by-wire. Took two pilots to fly the aircraft. Vibration levels were very high. We didn’t have the composite technologies needed to make some advanced airfoils. But we had the engineering, the science, and the physics that said a helicopter can go fast.

David Schneider: Sikorsky tested the Advancing Blade Concept with an early prototype, the XH59A, which had two main rotors, one stacked on top of the other, but no tail rotor, because you didn’t need one to counteract the main blades’ rotation. To go fast, it used two jet engines attached to the middle of the fuselage on either side.

Steve Weiner: The XH59A had managed to get to 238 knots in the early ’80s. Didn’t do it very efficiently. Had vibration problems, as Kevin said before. But we knew that the basic technology could get us into that speed range.

David Schneider: Aeronautical engineering improved quite a bit in the following decades, enough to prompt Sikorsky’s engineers to dust off the Advancing Blade Concept in 2004.

Kevin Bredenbeck: I remember the day my boss came to me and said, “Go see this fellow Steve Weiner.” [He] basically said, “You two guys, put your heads together. You’re going to be able to pull this off. I know you can.”

Steve Weiner: So then it became, What would really make sense? We really felt that if you set that 250-knot goal—you would probably be designing the aircraft for 275 or 280—we thought that was a good place to be.

David Schneider: Within three years, the Sikorsky team was in the air with its new X2 demonstrator—a coaxial rotor machine that looks very much like the XH59A. But it has a sleeker, almost dolphin-like, profile and uses an airplane prop on the back to push it forward rather than the fuel-hogging jet engines of the XH59A. In September 2010, the X2 demonstrator achieved the team’s 250-knot goal and more: 262 knots—that’s almost 302 miles an hour. Here’s how it went from the cockpit:

Flight audio: That’s pretty much level. We’ll do a little bit of a dive out to maybe like 260 or so. All right. Lower the nose. There’s 259, 260. Yep, 262 and bringing the nose up, bringing the prop back at the same time. Wow, that was nice.

Steve Weiner: This is really the first practical high-speed helicopter—you know, the fact that it does it efficiently and that it doesn’t lose any of the other capabilities that helicopters have, in fact, it enhances several of them, is really the real difference. I’m very optimistic about the future of this technology.

David Schneider: Sikorsky’s S-97 Raider, an eight-seater now in development, should be the first production model in that future world of truly high-speed helicopters. In Fort Lauderdale, Fla., I’m David Schneider.

Photo: Sikorsky Aircraft Co.
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