Sikorsky S-69

The Sikorsky S-69 (XH-59) stands as an American experimental co-axial compound helicopter created by Sikorsky Aircraft to showcase the Advancing Blade Concept (ABC) under the auspices of the United States Army and NASA. In the early 1970s, Sikorsky received a contract from the Army Air Mobility Research and Development Laboratory, later part of the Army Research Laboratory, to develop the first prototype of the S-69 as an ABC demonstrator.

While operating as a helicopter, the XH-59A demonstrated a top speed of 156 knots, which significantly increased to 238 knots with the added turbojets, reaching a maximum speed of 263 knots in a shallow dive.

Sikorsky later pursued the Advancing Blade Concept with subsequent helicopters, such as the Sikorsky X2 and Sikorsky S-97 Raider, developed from 2007 onward. The ABC system featured contra-rotating rotors that utilized aerodynamic lift, eliminating the need for a tail rotor and providing forward thrust through turbojets.