Thanks IFMU,
The effort and the people behind the helicopter with extremely-rigid coaxial rotors is neat.
However, for a full appreciation of the effort one must include the earlier work, before Sikorsky called it the 'Advancing Blade Concept'.
"Stanley Hiller flew the first successful American co-axial helicopter, the XH-44, in July 1944, when he was only 19 years old. It also featured the world's first successful all-metal rigid-rotor blades." Hiller X-2-235
Dave