Sikorsky X2 Goes Faster
Sikorsky's unofficial 258 mph run last week of its X2 counter-rotating coaxial rotor technology demonstrator bests a previous record set by a Westland Lynx in 1986, but is still short of the X2's goal. Sikorsky aims to cruise the X2 at 288 mph by late this year. The helicopter itself is far from traditional. It includes a six-blade rear-facing propeller that relieves the main rotors of certain aerodynamic and engineering hurdles created by high-speed rotor-wing flight. Normally, traditional helicopters rely on the main rotor for all forward propulsion and lift. At high forward speeds, the rotor blades meet increasing stresses as they advance and retreat. At some point, the retreating blades can stall or the advancing blades can achieve tip speeds that reduce their efficiency. Those complications, among other things, effectively create a speed barrier for traditional helicopters. The X2 approaches the problem differently.
The X2 transfers forward thrust to the rear-facing propeller, which allows it to slow its main lift-generating rotors (perhaps as much as 20 percent) as forward speed increases. That reduces stresses on the rotor system while still allowing it to produce sufficient lift. The system allows the aircraft to make use of high lift-to-drag ratio rigid blades. Those blades converge in low-drag hub fairings and are integrated with a fly-by-wire system that maintains full rpm control of the main rotor throughout the flight envelope. The X2 is not set to become a production model, but to lay the groundwork for future designs. Sikorsky hopes the technology can be applied to expand the utility of helicopters and their use in the field.