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The Sikorsky Model XH-39 (S-59) was the Army's first turbine-powered helicopter, and was for a time the world's fastest rotorcraft. In late 1953 the Army awarded Sikorsky a contract for the experimental conversion of two existing H-18 helicopters from piston to turbine power. Sikorsky allotted the two craft the company designation S-59, and began the conversion work in early 1954. The Sikorsky aircraft was built using a modified YH-18A airframe, derived from the unsuccessful postwar S-52 project.

The four-seat H-39 retained the H-18's basic pod-and-boom layout. The H-39 was powered by a single 400shp XT51-T-3 Artouste II turbine engine, which drove a new, fully-articulated, four-bladed main rotor. Other changes included a modified tail rotor, strengthening of the fuselage, incorporation of retractable wheeled landing gear, and the addition of updated electronics.

On 26 August 1954 Army Warrant Officer Billy Wester flew the craft to a new world helicopter speed record of 251kph; less than two months later Wester and his H-39 set a new world helicopter altitude record by reaching 7474m.

The H-39 was rejected by the US Army, despite its speed and the advantage of retractable landing gear, as being old-fashioned in design and unsuitable for further development. Instead, the Army chose the Bell XH-40, which soon gained fame as the UH-1 Huey.

Nevertheless the H-39 provided the Army with much valuable information on the capabilities of turbine-powered helicopters, and the success of its three-year evaluation was instrumental in bringing Army aviation into the turbine age.

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