10/31/2007. On February 7, 1972, Sikorsky announced that the company,
under contract to the Army Air Mobility Research and Development
Laboratory, was designing and building a research helicopter, to
study the Advancing Blade Concept (ABC) rotor system. This new system
consisted of two rigid, contra-rotating rotors that took advantage of
the aerodynamic lift of the advancing blades. At high speeds, the
retreating blades were unloaded, the majority of the load was
supported by the advancing blades of both rotors and the usual
penalties of retreating blade stall were eliminated. This system did
not require a wing to supplement the rotor to provide speed and
agility, and also eliminated the need for an anti-torque tail rotor
and its drive system.
Powered by an 1,825 shp Pratt & Whitney PT6T-3B Twin-Pac the first
XH-59A (73-21941) made its maiden flight on July 26, 1973, only to
crash on August 24, being damaged beyond repair. The pictured second
prototype had a number of design changes and a modified control system
installed as a result of the crash investigation, and flew for the
first time on July 21, 1975, as a pure helicopter as shown in the
Another views.
Late 1973 the ABC technology demonstrator became a compound aircraft
when two 3,000 lb (1,361 kg) st Pratt & Whitney J60-P-3A turbojets
were added for forward propulsion. It was flown early 1978 and
evaluated by the US Army, USN and NASA. The aircraft is presently
preserved at the Army Aviation Museum, Fort Rucker, Alabama, USA.
