Sikorsky flew its prototype counterrotating helicopter yesterday (press release). Typical helicopters, however expensive, are limited to about 165 knots of cruise speed. When the helicopter is moving faster than that, the “retreating blade” is not getting enough airflow, i.e., the blade is going backward about as fast as the helicopter is going forward. This results in a loss of lift on half of the disk. With two rotor systems rotating in opposite directions you still get retreating blade stall but it happens to both rotors at the same time and on opposite sides of the helicopter. Instead of the helicopter pitching and rolling it should just keep flying. The goal with this style of helicopter is to achieve cruise speeds closer to 250 knots, albeit probably at Sikorsky prices starting at $20 million.
How new is this idea? The U.S. military tried this around 1970 and gave up due to uncontrollable vibrations. The Russians built and flew some helicopters like this, also around 1970, but never went into large scale production.
What makes it practical today when it wasn’t practical in 1970? Better computer systems that can run active vibration dampening (like noise-canceling headsets but for vibration).