Experimental Compound Helicopter Prototype
Two Model S-69 aircraft were built by Sikorsky for the United States Army and NASA to validate various components of a compound helicopter design.
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The road to the Sikorsky S-97 "Raider" compound helicopter program of today was through such developments as the experimentally-minded Model "S-69". For decades, rotary-wing engineers have attempted to exact every ounce of speed and performance from helicopter systems only to be limited by traditional engine and blade arrangements. The compound helicopter was born to provide the same systems with the benefits of a revised main blade arrangement or the addition of a tail thruster/propulsor unit to give the airframe jet-like performance while retaining the Vertical Take-Off and Landing (VTOL) capability inherent in helicopters. The modern-day S-97 embodies this as did the S-69 of the early-1970s.
The S-69 was financed by both the United States Army as well as NASA through the "Advancing Blade Concept" (or "ABC") program. Because of military involvement in the program, the Model S-69 was also recognized under the military designation of "XH-59" and two flyable specimens were ultimately built for the program with the first unit flying on July 26th, 1973.
The aircraft was given a twin-main-rotor mast arrangement in which one main rotor system sat above the other ("co-axial contra-rotating"). No anti-torque rotor was fitted at the tail as the two contra-rotating main blade units countered the naturally occurring torque force. The rotors, of rigid design, sat atop the slender fuselage with two outboard-positioned engine nacelles positioned at each fuselage side. The cockpit sat over the nose in the usual way and featured side-by-side seating for the flight crew of two. The tail unit incorporated split vertical fins set atop upward-cranked horizontal planes. The undercarriage was retractable.
Structural dimensions of the aircraft included an overall length of 40.8 feet, a diameter (with main rotor) of 36 feet, and a height of 13.2 feet. Loaded weight was 12,500lb.
Three total engines were installed in the S-69: 2 x Pratt & Whitney J60-P-3A turbojets of 3,000lb thrust each and 1 x Pratt and Whitney Canada (PWC) PT6T-3 "Turbo Twin-Pac" turboshaft engine of 1,825 horsepower output. Performance specs showcased a maximum speed of 322 miles-per-hour with cruising capable of near-125 mile-per-hour speeds. The service ceiling reached 15,000 feet and the rate-of-climb was 1,200 feet-per-minute.
The first prototype lasted airborne only until August 24th, 1973 when it crashed during a test flight. This example was rebuilt to serve in wind tunnel testing from 1979 onward. The second prototype followed into the air on July 21st, 1975 sans the outboard jet engines. Once successfully passing its early testing phase, the aircraft received its complete powerplant arrangement during March of 1977. This airframe recorded a maximum speed of 300+ miles-per-hour in a dive.
Active testing of the S-69 ran into 1981 by which point the XH-59 was retired from further work after accruing some 106 hours in the air. It was found to burn through fuel quite quickly and the airframe vibrated violently in certain flight envelopes. It was planned that the XH-59 prototype be converted with new GE T700 turboshaft engines, a ducted "pusher" propeller unit at the tail, and all-new main rotors as the "XH-59B" but Sikorsky was not interested in funding the venture, even partially. The work on the S-69 was not all for loss for the company's "X2", flying on August 27th, 2008, carried on the tradition of the Sikorsky experimental compound helicopter - which eventually led to the modern-day S-97 "Raider" concept under consideration by the United States Army.