Sikorsky S-69 XH-59

The ABC ( Advancing Blade Concept ) was designed to prove the viability of two counter rotating main rotors on a common axis as a means of achieving higher speeds and better agility. Its simple design eliminates the tail rotor for anti-torque and directional control with the additional advantage of eliminating noise. Rudders in the twin tail are used instead, as in a fixed-wing aircraft. The ABC has been extensively flight tested as a pure helicopter and, with auxiliary propulsion, flown at speeds in excess of 480 km/h