XH-59A Advancing Blade Concept (ABC) / S-69

NASA and DOD developed several rotary-wing-based aircraft that used powered lift technology. These included the XH-59A, advancing blade concept aircraft during the 1970s, the JVX or tilt-rotor aircraft, and the RSRA/X-wing aircraft. These aircraft had the common ability to take off and land vertically like a helicopter, but in flight, they used a variety of technologies to operate as conventional fixed-wing aircraft.

One way to eliminate the problem of stalling and reverse rotor blade flow that limits the forward speed of a helicopter is to use two counterrotating rotors. The Advancing Blade is that half of the rotor disc in which the rotation of the blade is moving in the same direction as the movement of the helicopter. If the helicopter is moving forward, the advancing blade will be the right half of the rotor disc; if moving backward, it will be in the left half; if moving sideward to the left, it will be in the forward half; and if moving sideward to the right, it will be in the rear half. Rotor blades flap to equalize lift between the advancing blade half and retreating blade half of the rotor disc. Dissymmetry is created by forward movement of the helicopter. When the helicopter is moving forward, the speed of the advancing blade is the sum of the indicated airspeed of the helicopter plus the rotational speed of the blade.

The Advancing Blade Concept (ABC) uses two rigid counterrotating rotors in a coaxial arrangement to provide advancing blades on both sides of the aircraft. This makes use of the high dynamic pressure on the advancing side of the rotors at high forward speed, virtually ignoring the low dynamic pressure on the retreating side, while still keeping the rotor system in roll trim. Theoretically, such a rotor system will maintain its lift potential as speed increases. The XH-59A was designed to investigate this theory.

This is the principle behind the XH-59A, built by Sikorsky Aircraft [S-69] in a jointly funded Army, NASA, and now, Navy program. The XH-59A program started in 1971; by 1973, two XH-59As were ready for flight-testing.

The XH-59A Advancing Blade Concept demonstrator wa powered by the P&W-Canada PT6T-3 Turbo Twin-Pac and two P&W J60 turbojets. It had two stacked, contrarotating 36' rotors. As a pure helicopter, it achieved 184 mph level flight speed, 221 mph in a dive; with auxiliary propulsion, it achieved 274mph (>322mph) in level flight. However, one aircraft was lost during a hard landing.

Flight-testing of the XH-59A as a pure rotary-wing craft had been completed by 1977. The XH-59A's extreme agility and maneuverability were even more than had been expected. This maneuverability comes about because of the very stiff rotor blades used on the XH-59A, making the ABC an ideal candidate for a combat aircraft. The stiffer blades are also more rugged and thus more likely to survive encounters with tree limbs and hits from small arms fire.

In 1978, the high-speed test program was started with the addition of two auxiliary 3000-pound thrust jet engines mounted on the fuselage. The YH-59A had two additional P&W J60-P-3As, and two aircraft were built. The first flew on 26 July 1973, but crashed on 24 August 1973. The second aircraft flew on 21 July 1975, and in March 1977 the two J60s were installed for forward flight.

The XH-59B with PT6T-3 and shrouded fan was proposed, but never built. It was to be converted with two T700-GE-700s, contrarotating rotors, and a tail-mounted pusher prop. This project, which was designated XH-59B, was not proceeded with.

Advancing Blade Concept was a contender for the RAH-66, but the transmission weight penalties were too high and so the trade study decided that the advantages were outweighed by the weight disadvantage.

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