In the 1980s, Sikorsky had a test aircraft that they called the ABC (Advancing Blade Concept) helicopter. It used a coaxial rotor, and offloaded the retreating blades to eliminate retreating blade stall (as a helicopter goes faster, the blade going backwards goes more slowly relative to the air stream).

Sikorsky unveiled its successor, the X2 test aircraft at Heli-Expo:

It’s certainly a lot cleaner and more developed than its predecessor.

It also promises performance close to that of a tilt-rotor with the low speed handling and auto rotation of a helicopter.