Sikorsky S-69 ABC / XH-59
In late 1971 the Army Air Mobility Research and Development Laboratory awarded Sikorsky a contract for the development of a single-engine research helicopter prototype designed specifically to flight test the company's Advancing Blade Concept (ABC) rotor system. This new system consisted of two rigid, contra-rotating rotors which made use of the aerodynamic lift of the advancing blades. The XH-59A's ABC system consisted of two three-bladed, coaxial, contra-rotating rigid rotors, both of which were driven by the craft's single 1825shp PT6T-3 Turbo Twin Pac engine. During high-speed flight only the advancing blades of each rotor generated lift; this off-loaded the retreating blades and thereby eliminated the aerodynamic restrictions caused by blade-stall and the high mach number effect of the advancing blade tip. This, in turn, produced greater stability and manoeuvrability while eliminating the need for either a supplementary lift-generating wing or an anti-torque tail rotor. The XH-59A's streamlined fuselage more closely resembled that of a conventional airplane than a helicopter, having a cantilever tail unit with twin endplate rudders, side-by-side seating for the two crewmen, and fully retractable tricycle landing gear.
First using scale models for wind tunnel tests at the Ames NASA research center, and then the real aircraft, the resultant Model S-69, which was allotted the military designation XH-59A and the serial number 73-21942 (c/n 69-002), made its first flight on 26 July 1973. This was extensively flight tested as a pure helicopter and, with auxiliary propulsion, flown at speeds in excess of 480 km/h / 300 kt and altitudes of more than 25,000 ft. This prototype was lost in an accident a month after the first flight.
The first prototype was written off and the cockpit was used in the Paris air-show to demonstrate a sighting system for LHX.
Following an enquiry, design modifications were requested, plus improvements to the control system. Tests were resumed in July 1975 with the second prototype incorporating several significant control system modifications. This second machine (73-21941 c/n 69-001) flew for the first time in 1975, completing the pure helicopter portion of the program, and in 1977 was converted into a compound rotorcraft through the installation of two 1350kg J60-P-3A turbojet engines. The modified machine was jointly evaluated by the Army, Navy, and NASA at NASA's Moffet Field, California beginning in 1978, and was later able to reach and maintain speeds in excess of 515kph in level flight.
In 1982 the plan was to develop this aircraft into a new XH-59B configuration with advanced rotors, new power plant, and a ducted pusher propeller at the tail. This approach was seen as a possible solution to the Army's search for a new light attack helicopter (LHX), and further funding was recommended. The S-69/XH-59 program was abandoned, however to pursue the XH15.
The existing XH-59A aircraft was officially transferred to the Army museum at Fort Rucker Alabama following the 1981 end of joint Army/Navy participation in the tri-partite flight test program.