Sikorsky S-72 (X-Wing)
Experimental Compound Helicopter Prototype
The Sikorsky S-72 experimental helicopter of the late-Cold War period was used to test a compound helicopter design - two flyable prototypes were completed before the end.
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First flown on October 12th, 1976 the "S-72" was an experimental, high-speed compound helicopter design by Sikorsky Aircraft. The design, born through the Rotor Systems Research Aircraft (RSRA) project, stemmed from a U.S. Army / NASA venture (with work handled by Sikorsky) seeking to collect high-speed data on helicopter rotor characteristics by way of a combination powerplant arrangement. The end-product involved the Sikorsky S-67 helicopter framework coupled with the Sikorsky S-61 main rotor. Beyond the turboshaft engines used to drive the rotor assemblies, additional power was to be had from a pair of General Electric TF34 turbofans situated along the fuselage sides. A low-set, straight monoplane was fitted under the aircraft while the tail rotor (facing portside) was retained. Just two of the aircraft were completed for testing.
The S-72 carried a crew of two or three and had a length of 70.6 feet with a wingspan of 62 feet and a height of 14.5 feet. Empty weight was 21,700lb and the Maximum Take-Off Weight (MTOW) reached 26,050lb. The rotors were powered by a pair of General Electric T58-GE-5 turboshaft engines of 1,400 horsepower each and the turbofan installations were 2 x General Electric TF34-GE-400A engines of 9,400lb thrust. Performance specs included a maximum speed of 230 miles per hour with a cruise speed of 160 miles per hour.
The original S-72 form could be flown with or without the main rotor unit and, due to the uniqueness of the S-72's design, an equally-unique crew ejection system was developed for the compound helicopter to increase survivability of the test pilots. The process involved complete jettison of the main rotor blades via controlled explosives and rockets ejected the crew positions out and away from the fuselage. After a first-flight in 1976 and testing thereafter, NASA received one of the aircraft in February of 1979. Beyond data collection, this model did not progress beyond its experimental design.
An offshoot of the S-72 became the so-called "X-Wing" aircraft which used a rigid main rotor unit (shaped as an "X" over the fuselage) which could be paused by the operator during flight. The aircraft could therefore take-off vertically like a traditional helicopter and then engage forward, high-speed travel with the available turbofan engines, combining the best elements of both aircraft types. Lockheed handled the concept work under DARPA and attention then turned to Sikorsky to modify one of the existing RSRA prototypes for the tests. This product appeared in completed form during 1986 but was not flown for the program was given up for good in 1988.