Sikorsky's long and eventful partnership with Army aviation can accurately be said to have started during the Second World War when the company's XR-4, the first helicopter built for military service, was delivered to the USAAC. Two other Sikorsky designs, the R-5 and R-6, also entered Army Air Force service before VJ Day, and all three types made important contributions both to the American war effort and to the development of military rotary-wing doctrine. However, it is the H-18 that holds the distinction of being 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 the craft made its first flight in the summer of the following year. The first American helicopter to be equipped with all-metal main and anti-torque rotor blades, the S-52 had 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 up to three people. The machine's performance was quite impressive by the standards of the day; indeed, the commercial S-52 set three international speed and altitude records in 1948.
The S-52's performance was certainly a factor in the Army's 1949 decision to purchase four examples of the slightly modified Model S-52-2 for service test and evaluation.
S.Harding "U.S.Army Aircraft since 1947", 1990
The first US helicopter with all-metal rotor blades, the Sikorsky S-52-1 two-seater was first flown on 12 February 1947, powered by a 133kW Franklin engine. In 1948 it established three international helicopter records for speed and altitude and was developed into the S-52-2, a three/four-seater with a 183kW Franklin O-425-1 engine which was ordered by the US Marine Corps as a replacement for the ÍO3S.
D.Donald "The Complete Encyclopedia of World Aircraft", 1997