The Sikorsky announcement at the 2005 AHS Forum that the company will revisit the Advancing Blade Concept in the form of the X2 has made it timely to review the project that Sikorsky previously did with this configuration. For those who don't know, the idea is to eliminate retreating blade stall by operating this blade at a low angle of attack. On a single rotor this would produce an unacceptable unbalanced rolling moment, but if it is paired with another rotor doing the same thing but the other way,' a total roll balance can be achieved. The result is a coaxial helicopter. To take advantage of the high dynamic pressure on the advancing side in producing high thrust, the rotors must be very stiff so they don't flap; thus the rotors are made like propellers.
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