Sunday, October 25, 2009

X2 Coaxial Rotor Helicopter

X2 Coaxial Rotor Helicopter:
Members of the X2 team, photographed for Popular Mechanics on July 28 at the Sikorsky Development Flight Center in West Palm Beach, Fla., where the helicopter is undergoing flight testing. Left to right: Steve Cizewski, Dave Walsh, Ken Arifian, Mark K. Wilson, Kevin Bredenbeck, Steve Weiner, Jim Kagdis. (Photograph by Williams & Hirakawa)

Last year in the town of Horseheads, N.Y., Kevin Bredenbeck became the first pilot to fly Sikorsky Aircraft’s X2 technology demonstrator. The successful test proved that the X2 engineering team had overcome a basic limitation in rotary-wing aero¬dynamics—and had set itself on the path to building the world’s fastest helicopter.

The problem with fast helicopters is a phenomenon called dissymmetry of lift. When a helicopter starts to fly forward, the advancing blade cuts through the air faster, and it generates more lift. At the same time, the retreating blade’s relative velocity and lift decrease. The faster the helicopter goes, the greater the discrepancy. If the speed increases too much, the machine will tend to roll to one side and the retreating blade will stall, generating no upward force at all. The speed limit for any conventional helicopter is about 185 mph. But that wasn’t good enough for Steve Weiner, the X2’s chief engineer: “We wanted to go faster.”

Weiner’s team replaced the single rotor with twin, 26.4-foot-long rotors that spin in opposite directions on the same axis. The rotors both produce dissymmetry of lift, but in countervailing directions. Goodbye, instability. Hello, speed records.

The X2 is the descendant of the XH-59A, a machine with the same stacked rotor configuration that the company built with NASA and the U.S. Army in the 1970s. It was unwieldy, and the project was shelved—but Sikorsky engineers never gave up on it. In recent years, they innovated a new vibration-control system and digital fly-by-wire controls, and added a pusher propeller on the tail to boost speed. (The X2 doesn’t need a tail rotor, which counters a typical helicopter’s tendency to spin in place.)

Sikorsky doesn’t know whether its first clients will be military or commercial or both. For now, the X2 team has set the speed bar high—at 287 mph. “But the physics say we can probably go to 300 knots,” Weiner says. At that speed—nearly 350 mph—a medical transport helicopter could fly 150 miles, pick up a patient and return to a hospital by the time a conventional helicopter simply arrived on the scene. 


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