Sikorsky

A year after we printed Sikorsky Aircraft's claim

that its stacked, double-rotor helicopter design would break rotorcraft airspeed records, the company pulled it off. On September 15, Kevin Bredenbeck, Sikorsky's chief pilot, nudged the X2 to 250 knots (287 mph), unofficially setting the world record (it has yet to be certified). The official speed record (as recognized by the Fédération Aéronautique Internationale, the world's air sports organization) was set in 1986 by a modified British military helicopter called the Westland Linx, which flew at 216 knots, or 249 mph.

Last year, we gave Sikorsky a Breakthrough Award for the X2's coaxial design. Now, Jim Kagdis, Sikorsky's manager of advanced programs, explains some of the innovations that have allowed the X2 to shatter speed records.

The X2 is the trimmer, more computerized offspring of the XH-59A, a coaxial aircraft that the company tested in 1973. That prototype had some flaws.

"[The XH-59A] went fast, but the vibration was so high it was unacceptable," Kagdis says. It required two pilots to handle the workload, it was outfitted with four engines and it was loud.

"The efficiencies that things like fly-by-wire technology, advanced prop digital flight controls and digital engine controls, all bring to bear make for a smaller aircraft that's highly integrated with these technologies," Kagdis says. "What two pilots did back then, we can do now with one pilot and one engine fully in control of the aircraft. The XH-59 alpha program kind of accomplished its mission back then, but it was not ready for prime time."

The company put the rotor technology on hiatus for 20 years and hunkered down to develop a host of other innovations it would need to create the X2. Then, in 2005, with a $50-million budget, Sikorsky began to build its prototype. Here's how it did it.

Vibration Reduction System

The principle behind the advanced vibration reduction system is similar to that in noise-canceling headphones—when the system detects vibrations, it counters them with a force that moves in the opposite direction. The detector is linked to an array of force generators in the belly of the X2.

Fly-by-Wire Control

"Traditional helicopters have a stick-to-head, a mechanical system," Kagdis says. "So when the pilot moves the stick, a mechanical system moves the push-pull rods on the rotor system. We have a fly-by-wire system, where the control of the pilot goes into the electronics of the aircraft and then ultimately moves the swash plate or the control rod servos." It's like driving a car with a joystick rather than a steering wheel.

The electronics reduce the helicopter's weight and make it easier to fly. Plus, Kagdis says, "It gives us an electronic backbone to introduce things like autonomy or semi-autonomy to the aircraft."

Rigid Blades

"The blade technology had to be dead-on," Kagdis says. "We couldn't steal blades from another aircraft like we took the landing gear from a Cessna." The X2's blades are rigid, all composite and coaxial. "The rigid rotor system was built to do one thing really, really well, and that's go fast," Kagdis says.

On the X2, coaxial blades are stacked one on top of the other, and they turn in opposite directions. That allows one blade to compensate for the other when the helicopter approaches high speeds. In a normal helicopter, the blade spinning toward the nose of the craft when it's flying forward generates more lift than the blade on the opposite side that's spinning toward the tail. When the difference between the two becomes too great, the helicopter becomes unstable. A coaxial system fixes that.

Pusher Propeller

The pusher propeller in the tail of the X2 is not for stability, it's for speed. "This is a high-performance blade so the aircraft can go really, really fast," Kagdis says. Interestingly, most of the power goes to the pusher prop at high speeds, and very little goes to the main rotor.

"The key message here is that there is a marriage of technologies to the pilot, which allows us to do these four key performance parameters–the speed, the acoustics, the vibration and the lower workload," Kagdis says.

What's Next?

And while it may already be a record-breaker, the prototype is not complete. Still on the to-do list is the addition of more aerodynamic faring around the rotor system. In between the drag-reducing faring around the upper and lower disks, the company's technicians plan to add a sail faring. "With that integration into the aircraft, we believe there are speeds higher than 250 knots that we can reach," Kagdis says. "My guess is, by the end of the year or early next year, we will complete our flight test program."

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