Help me understand. A new Sikorsky counter-rotating rotor design is revolutionary (no pun intended) because they could use stiff flat rotors that could be placed closer together, reducing drag and allowing faster forward speeds.
Jan 2011 Popular Science p. 54
If I understand correctly, a fast forward speed begins to stall the part of the rotor that's retreating as the craft's forward airspeed starts to match its speed. To compensate, rotors had to be wider and hence more flexible to generate lift despite this effect. In the new design, they actually feather the blades during the retreating phase of each revolution, allowing the opposite rotors lift to balance lift. The blade can therefore be shorter, narrower, stiffer, and closer to the other rotor for less forward drag.
"Every conventional helicopter has a maximum speed at which it can no longer produce sufficient laterally balanced lift and maintain controllability on the rotor due to retreating blade stall." - http://www.sharc.co.uk/html/case_sikor.htm
"The goal was to test the premise that rigid counter-rotating main rotors, where the advancing blade on each side produces lift while both retreated blades are feathered, could be used reduce drag and tip velocities to allow for cruise speeds well above the norm for helicopters."
- http://www.flightglobal.com/articles/2007/06/12/214533/x2-marks-the-spot-for-radical-rotor-designs.html
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