The S-56, a very complex and advanced design helicopter, had incorporated into it many new features, such as 'clamshell' nose loading doors, similar to the Bristol 170 fixed-wing freighter, and retractable main landing gear. It was also the first helicopter with a hydraulic main rotor blade fold and pylon fold system, automatic stabilization equipment (ASE), and the first twin-engine rotorcraft in U.S. military service.

The airframe of the S-56 was an all-metal design consisting of the fuselage, main gearbox cowling, stub-wings, engine nacelles, horizontal stabiliser and tail rotor pylon. The nacelles also accommodated the helicopter's fully retractable main landing gear. The two-cell, 400-gallon capacity fuel tanks for each engine were located in the aft portion of the engine nacelles and in the adjacent stub-wing sections. This innovative arrangement made the 30-foot long cargo cabin clear for loading and unloading of troops or cargo up to a 105 mm howitzer and trailer.

The lower portion of the nose of the fuselage contained hydraulically operated 'clamshell' type cargo doors and a loading ramp, enabling vehicles to be driven into the cabin. The pilot's compartment was located in the upper portion of the nose of the helicopter and was accessed internally by a folding ladder or externally by steps on the outside of the forward fuselage.

In contrast to its rather advanced design features, the S-56 was powered by archaic Pratt & Whitney Double Wasp R-2800 piston engines. This was primarily due to the fact that turbine engines were not readily available for helicopter application. The R-2800 was an eighteen-cylinder, double-row radial engine equipped with a single-stage, single-speed supercharger. It was used in many aircraft, including the famous Douglas DC-6 fixed-wing transport and even in World War II veterans like the Corsair and Thunderbolt.

The design of the S-56 helicopter called for the engines to be mounted in separate nacelles at the outboard end of each stub-wing, with driveshafts pointing inboard at an angle of approximately 80 degrees and upward at an angle of approximately 12.5 degrees. A hydro-mechanical clutch was splined to the output of each engine and a driveshaft connected the clutch to the main gearbox in the upper centre of the fuselage. The main gearbox transferred power to the intermediate and tail rotor gearboxes and provided the correct gear reduction for the main and tail rotors.

The R-2800 radials powered the S-56 quite well and were very responsive to the throttles, but due to the inertia of this massive helicopter, pilots had to fly with much anticipation. It was said that maneuvers near the ground were indeed thrilling until the pilot mastered the twist grip throttle.

The rotor system of the S-56 consisted of a single five-blade seventy-two foot diameter main rotor and a four-blade, fifteen foot diameter anti-torque tail rotor. The massive, fully articulated main rotor head assembly weighed more than 2000 pounds. The main rotor hub consisted of two forged steel plates arranged in a manner that incorporated the blade hinge assemblies and the hydraulic blade dampers. This rotor head arrangement is typical for most Sikorsky helicopters and was later adapted to the S-64 'Skycrane' as well as into the Deuce's successor, the CH-53 'Sea Stallion'.

The five all-metal main rotor blades of the S-56 were completely manufactured from aluminum alloy and weighed about three hundred pounds each. All Marine Deuces were equipped with a power main rotor blade and pylon folding system. This brand new system was very complicated, susceptible to many problems, and difficult to maintain. It was, although, highly appreciated by those who have had to manually fold rotor blades that were more than 30 feet long at night in the wind and rain on a rolling, pitching, slippery aircraft carrier flight deck.

Range (max. payload)

145 miles