Mystery Ship: May 2019

Mystery Ship: May 2019

By Jon Guttman
3/1/2019 • Aviation History Magazine

The Sikorsky S-72X RSRA (rotor systems research aircraft)

When a helicopter suffers engine failure, a skilled pilot can bring it down by autorotation. But what happens if a rotor or some component of the rotor system fails? That was apparently the question Sikorsky sought to address with its experimental S-72X RSRA (rotor systems research aircraft), a project jointly begun by NASA and the U.S. Army in December 1970.  

Installing an S-61 rotor and gearbox in a modified S-67 airframe, Sikorsky also built the S-72X with conventional wings that had a span of 45 feet. Its compound flight system was powered by two General Electric T58-GE-5 turbofans driving the four-blade main rotor and two TF34-GE-404A auxiliary turbofans for conventional propulsion.

Two S-72Xs were built, the first flying on October 12, 1976. While capable of vertical takeoff and landing, the machine could also stop its main rotor in midair and be conventionally flown, propelled and controlled by a five-blade tail rotor. And if anything went wrong with the main rotor? That’s when its crew emergency extraction system came into play. Explosive bolts would sever the main rotor blades from the airframe, allowing the pilot to fly with the conventional wings at a cruising speed of 160 mph up to a maximum speed not to exceed 230 mph. If the situation turned terminally dire, escape panels could be blown off the roof and the crew would eject from the cabin.

Sikorsky delivered its two prototypes to NASA on February 11, 1979. In 1981 NASA and the Army solicited proposals for an appropriate main rotor, candidates coming from Sikorsky’s own UH-60A, Hughes’ YAH-64A and Boeing-Vertol’s YUH-61A or Model 347 (from its CH-47). That test program did not proceed, but in late 1983 Sikorsky got a contract to use one of its S-72s as a testbed for the X-wing rotor system. The testbed was rolled out in 1986, but never flew before the whole project was canceled in 1988.

Both S-72X RSRAs still exist. Airframe serial no. 73-21941 is at NASA’s Ames Research Center, while 73-21942 is at the Army Aviation Museum at Fort Rucker, Ala.

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