The H-18 was the first Sikorsky helicopter to be procured for service evaluation by the Army Ground Forces, as distinct from the USAAF. Design work on the Sikorsky Model S-52 began in late 1945, and with the first flight in the summer of the following year. The S-52 was first American helicopter equipped with all-metal main and anti-torque rotor blades. It featured a semi-monocoque, pod-and-boom type fuselage, a single 175hp Franklin engine, quadricycle wheeled landing gear, and a fully-enclosed cabin that could seat three. The machine's performance was quite impressive by the standards of the day. In 1949 the Army decided to purchase four examples of the slightly modified Model S-52-2 for evaluation.
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