Sikorsky S-69
The Sikorsky S-69 is an American experimental compound helicopter developed by Sikorsky Aircraft as the demonstrator of the co-axial Advancing Blade Concept with United States Army and NASA funding.
Development
In late 1971, the Army Air Mobility Research and Development Laboratory, which later became a part of the Army Research Laboratory, awarded Sikorsky a contract for the development of the first prototype. The S-69 was the demonstrator for the Advancing Blade Concept.The first S-69 built first flew on July 26, 1973. However, it was badly damaged in a low-speed crash on August 24, 1973 due to unexpected rotor forces and insufficient control systems. The airframe was then converted into a wind tunnel testbed, which was tested in the NASA Ames Research Center 40x80 feet full-scale wind tunnel in 1979. A second airframe was completed which first flew on July 21, 1975. After initial testing as a pure helicopter, two auxiliary turbojets were added in March 1977. As a helicopter, the XH-59A demonstrated a maximum level speed of, but with the auxiliary turbojets, it demonstrated a maximum level speed of and eventually a speed of in a shallow dive. At level flight, it could enter a 1.4 g bank turn with the rotor in autorotation, increasing rotor rpm. Airframe stress prevented rotor speed reduction and thus full flight envelope expansion. The XH-59A had high levels of vibration and fuel consumption.
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 powerplants, and a ducted pusher propeller at the tail. This proposed program did not proceed as Sikorsky refused to pay a share of the costs. Sikorsky and its partners funded the development of the next helicopters using the Advancing Blade Concept, the Sikorsky X2 and Sikorsky S-97 Raider, from 2007.