December 2015

2015 DecemberSikorsky S-56
Developed for the US Marine Corps as HR2S-1/CH-37C “the Deuce” and US Army as CH-37A/B Mojave
First flight: December 18, 1953 (XHR2S-1)

The Sikorsky S-56 helicopter was designed in response to a U.S. Navy Specification dated September 15, 1950 for a Class HR (Assault) Helicopter. Sikorsky submitted proposals on December 31, 1950 for two designs – a basic helicopter identified as XHRS-A, and a compound helicopter with folding wings identified as XHRS-B. Both models were powered by two Pratt & Whitney R-2800 reciprocating engines; however they were each also offered with Allison XT-38-A-2 turbine engines. The U.S. Navy chose the model with the least technical risk and in early 1951 provided a Contract (Experimental Order) for the XHRS-A helicopter, giving it the military designation XHR2S-1.

The S-56 helicopters featured a 68 foot (20.7 m) – later increased to 72 foot (22 m) – five-bladed fully articulated main rotor and a four-bladed tail rotor powered by the Pratt and Whitney Double Wasp R-2800-50 with 1,900 hp (1,415 kW) – later R-2800-54 with 2,100 hp (1,565 kW) – 18 cylinder, twin row, air cooled radial engine.

The S-56 was a giant step forward in helicopter design. It was four times larger than the S-58 model and comparable in size to the Douglas DC-3 twin engine transport. At the time of its introduction, it was the largest and fastest helicopter in the western world. It was also Sikorsky’s first multi-engine, retractable main landing gear helicopter and is the largest piston-engine helicopter ever built. It was also the first Sikorsky helicopter with automatic main rotor blade folding (CH-37C) and an auxiliary power unit (APU), which provided electrical power for the cargo winch to allow self-loading.

In 1956, a Marine HR2S-1 helicopter set three world records: a speed record without payload of 141.4 kt (261.8 km/hr); an altitude record of 12,100 ft (3688.1 m) with a 11,023 lb (5000 kg) load; and a load carrying record of 13,227 lb (6000 kg) to 6,561 feet (2000 m).

(Photos and references, Sikorsky Archives; text by Jacques Virasak)

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