horlick said: the only way to still employ the counter rotating rotors (so as to negate the need of a counter torque tail rotor) is to use the tandem configuration with the rotors interleave as in the Chinook.
Actually, no. Sikorsky's coaxial rotor is an ABC (Advancing Blade Concept) rotor system. That means only the advancing blade makes lift. The retreating blade makes no lift. The problem with high speed in any conventional rotor system is that the retreating blade will stall at high airframe airspeed. By unloading the retreating blade it cannot stall. But that means you have to have two advancing blades on a common center, hence a coaxial configuration. If the two advancing blades are not coaxial (as in a tandem configuration) the lift will not be symmetrical about one axis making control impossible.
Picture a Chinook with the forward rotor generating lift only on its left side and the aft rotor generating lift only on its right side. To bank right the forward rotor would generate more lift (more lift on left side) and the aft rotor less lift (less lift on right side). But this would not only generate a roll moment to the right, but also a nose up pitch moment. Rolling left would generate a nose down pitch moment. And changing pitch would result in a roll. And because the rotor centers are nowhere near the CG, rolling would also generate a yaw moment. So all three control axes would be coupled to each other. It would be uncontrollable.