The S-72 RSRA was an experimental hybrid helicopter developed by Sikorsky of America. It first took flight in 1976.
The S-72 was a highly experimental attempt to combine fixed wing and rotorcraft features into one hybrid aircraft. It came about as a result of the Rotor Systems Research Aircraft (RSRA) project and was a collaboration between Sikorsky, NASA and the US Army. Essentially it provided a way for rotor properties to be tested in-flight.
The S-72 uses a highly-modified S-67 airframe. In addition, it used the main rotor from the S-61 as well as the roller gearbox from that craft. It was possible to fit the S-72 with two TF34 turbofans and wings. In fact, it could fly as a fixed-wing craft with no rotor at all!
Amazingly, the S-72 had an emergency crew extraction system. Explosive bolts sever the main rotor and egress panels are also blown off the roof. The crew then left the craft with the use of rockets.
The S-72 was a pure research craft that filled an important void in aircraft design research. It provided a safe way to test rotor performance in real-world conditions. Something that a combination of mathematical analysis and wind tunnel testing could not do.
One of the most famous test-bed roles of the S-72 is the X-Wing circulation control rotor concept. This rotor system could also act as a fixed wing when locked in place and it used a novel virtual wing surface made from ported compressed air to alter lift.
Ultimately, only one RSRA was made and the project proceeded no further.