Sikorsky S-69

The Sikorsky S-69 was an experimental compound co-axial helicopter developed as the demonstrator of the Advancing Blade Concept (ABC) under US Army and NASA funding.

Design and development
Also known by the military designation XH-59, the S-69 was demonstrator for the Advancing Blade Concept (ABC). This Advancing Blade Concept system consisted of two rigid, contra-rotating rotors which made use of the aerodynamic lift of the advancing blades. At high speeds, the retreating blades were offloaded, as most of the load was supported by the advancing blades of both rotors and the penalty due to stall of the retreating blade was thus eliminated. This system did not require a wing to be fitted for high speeds and to improve maneuverability, and also eliminated the need for an anti-torque rotor at the tail. Forward thrust was provided by two turbojets, which allowed the main rotor to only be required to provide lift. It was found to have good hover stability against crosswind and tailwind. With jets installed, it lacked power to hover out of ground effect and used short take-off and landing for safety reasons.

The first S-69 built (73-21941) first flew on July 26, 1973. However, it was badly damaged in a crash on August 24, 1973. The airframe was then converted into a wind tunnel test article, which was tested in the NASA Ames Research Center 40x80 feet full-scale wind tunnel in 1979. A second airframe was completed (73-21942) which first flew on July 21, 1975. After initial testing as a pure helicopter, the auxiliary turbojets were added in March 1977. As a helicopter, the XH-59A demonstrated a maximum level speed of 184 mph, but with the auxiliary turbojets, it demonstrated a maximum level speed of 238 kn and eventually a speed of 303 mph. At 180 knots level flight, it could enter a 1.4g bank turn with the rotor in autorotation, increasing rotor rpm. Airframe stress prevented rotor speed reduction and thus full flight envelope expansion.

The 106-hour test program for the XH-59A ended in 1981. In 1982 it was proposed that the XH-59A be converted to the XH-59B configuration with advanced rotors, new powerplant (two GE T700s), and a ducted pusher propeller at the tail. This proposed program did not proceed as Sikorsky refused to share costs. The XH-59A had high levels of vibration.

Airframe 73-21941 is in storage at the NASA Ames Research Center and 73-21942 is on display at the Army Aviation Museum, Fort Rucker, Alabama.