Sikorsky S-69

The Sikorsky S-69, with the military designation XH-59, 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. Its two rigid contra-rotating rotors sat 30 inches apart and used the lift of the advancing blades, and forward thrust came from two auxiliary turbojets. The first airframe flew on July 26, 1973 and was badly damaged in a crash that August. A second airframe first flew on July 21, 1975 and reached 238 knots in level flight with the turbojets fitted. The 106-hour test program ended in 1981, and the concept later informed the Sikorsky X2 and S-97 Raider. One airframe is displayed at the Army Aviation Museum at Fort Rucker.