Soon to hit 250 MPH?
Sikorsky is looking at taking the coaxial rotor compound helicopter to more than 250 miles per hour soon:
Sikorsky expects to exceed 250 kt. with its X2 Technology coaxial-rotor compound helicopter demonstrator early in the year, resetting speed expectations for rotorcraft that have been stuck at around 150 kt. for decades.
A modern reinterpretation of Sikorsky’s XH-59A Advancing Blade Concept (ABC) testbed, which reached 238 kt. in the 1970s, the X2 combines fly-by-wire control, integrated engine/rotor/propulsor system, variable-speed rotor, high-lift/drag rigid blades and active vibration control to realize the speed potential of a coaxial rotor while retaining the hover agility of a helicopter.
The ability to auto-rotate in an emergency is a big plus too.
It works by offloading the retreating blade, and so preventing retreating blade stall, which occurs when the relative speed of the blade versus the air at higher speeds falls.
The issue has always been the complexity of the flight controls, the X59A required two pilots, and vibrations, which appear to be satisfactorily addressed through active digital flight control system on the X2.
I’ve always favored it over the tilt rotor concept, it appears to be both simpler and more redundant, and the rotors are not a compromise between lift and propulsion that they are in something like the V-22.