The Sikorsky S-51 (aka: H-5,R-5, HO3S-1, VS-327) was a general purpose commercial helicopter manufactured by the Sikorsky helicopter company and designed by Igor Sikorsky.
Compared to the R-4 the R-5 had a longer fuselage, larger rotor diameter, tandem pilot seating and a more powerful engine. The R-5 first took flight in 1944.
The SR-51 variant had an even greater rotor diameter and better carrying capacity than previous models. It first took flight in 1946, the year following official production for this helicopter family.
The S-51 has four seats for a pilot and three passengers. Its fuselage consists of three main sections that are bolted together. Passengers and crew access the cabin through separate sliding doors.
Hydraulic controls were not included initially for the S-51, but later became a standard feature. Greatly improving the pilot’s experience.
The S-51 is powered by a single nine cylinder Pratt & Whitney R-984-B4 Wasp Junior engine with 450 kilowatts of power. Both the main- and tail- rotors of the S-51 are made of laminated wood.
Surprisingly for such an early helicopter there were a number of optional equipment choices. These included first aid kits, heaters and fans, landing lights, crop spraying kits, litters for two medical casualties, a hoist kit, a hook for underslung loads and mail bag transportation equipment.
Indeed, the S-51 was the helicopter that proved the feasibility of rotorcraft as an airmail delivery platform. The S-51 delivered airmail until 1955 when it was replaced by the S-55.
More than 300 R-5/S-51 series helicopters were built during production from 1944-1951.