Sikorsky S-56 heavy-lift helicopter

The Sikorsky S-56 was a large heavy-lift helicopter developed in the early 1950s for the US military. Specifically a USMC request for a heavy-lift / troop transport helicopter. The first prototypes flew in 1953. Upon its arrival, it was the largest helicopter outside of the Soviet Union. It was also the first twin-engined helicopter built by Sikorsky. Those twin engines also helped give the S-56 its distinctive appearance, as they were mounted externally in pods either side of the fuselage. The Sikorsky S-56 remains the largest piston-engined helicopter ever built.

The unconventional exterior location of the engines meant that the interior of the helicopter was more spacious and versatile. It could carry 26 fully equipped troops, 24 stretchers or up to 3 lightweight Jeeps. Access to the cargo area was through a pair of clamshell doors at the front of the helicopter just below the cockpit. There were additional doors towards the rear of the fuselage, while the pilot and co-pilot accessed the cockpit via a fold-away ladder. The helicopter also featured 2,000 lb capacity electric winch which was mounted on a rail system along the top of the cargo bay. A viewing window in the floor of the helicopter allowed external loads to be viewed by the loadmaster.

By 1956 the first examples were delivered to the United States Marine Corps who had ordered 60 aircraft under the designation HR2S. Shortly after, the US Army started receiving the first of the 90 helicopters they’d ordered. These were given the army designation CH-37A Mojave. The helicopter came in several different variants throughout its lifespan. In 1962 the USMC HR2S helicopters were renamed CH-37C. While at the same time the Army version was given an avionics upgrade and retitled the CH-37B. The US Navy also ordered two examples specially equipped with a bulbous early warning radar system mounted onto the chin of the aircraft.

The Sikorsky S-56 had a fairly short operational lifespan, roughly 15 years. Primarily because its piston engine design was rapidly becoming outdated as more modern helicopters were significantly more powerful and reliable thanks to the advances in turbine powered engines. It was no match for the Soviet MiL Mi-10, and the S-56′s replacement, for example, could carry nearly five times the cargo weight, or nearly four times as many troops despite being a lighter aircraft.

The Sikorsky S-56 did make its mark in a small way however during the Vietnam war. Four of the Army’s helicopters were sent to retrieve downed aircraft from enemy held territory. In just over six months they managed to recover roughly $7.5 million worth of equipment which would otherwise have been lost or fallen into enemy hands. In addition eight S-56′s from the Marine Corps provided general transport, flying 1,500 missions and carrying over 31,000 passengers during the war without a single incident.

The last Sikorsky S-56 was eventually retired by National Guard Units in 1969. However many of the aircraft carried on flying in civilian guise for many more years, operated by Keystone Helicopters.