Sikorsky S-56 / H-37 Mojave / HR2S
Sikorsky originally developed the Model S-56 twin-engined heavy lift helicopter in response to a 1950 Marine Corps requirement for an assault transport able to carry twenty-three fully equipped troops.
A twin-engine solution was chosen, and to save cabin space, it was decided to house the two 1417kW Pratt & Whitney R-2800-50 Double Wasp engines (1566kW R-280054s on late production aircraft) radial engines in outboard nacelles, from which two drive shafts linked up directly with the reduction gear assembly which drove the big five-blade metal rotor. The S-56 was Sikorsky's first twin-engined helicopter, although the traditional single main rotor layout was retained, this being a 5-blade unit designed to be able to sustain the aircraft in flight with one blade shot away. The 53.80 cu.m cargo bay had a hoist capable of lifting a one tonne load andclamshell nose-opening doors. The main landing gear wheels retracted, but the tailwheel was fixed. The H-37's tailboom sloped downward toward the tailwheel and ended in a sharply upswept vertical tail unit carrying a four-bladed anti-torque rotor.
In May 1951 the Navy ordered four XHR2S-1 prototypes for USMC evaluation, and the first of these made its maiden flight on 18 December 1953.
In 1954 the Army borrowed one of these preproduction machines, designated it the YH-37, and subjected it to operational and maintenance evaluations before returning it to the Marines. On the basis of the large helicopter's excellent showing during the Army evaluation, Sikorsky was in late 1954 awarded a contract for nine production H-37A Mojaves. The first of these reached Fort Rucker during the summer of 1956, at about the same time the HR2S-1 naval variant was entering regular Marine squadron service, deliveries to Marine Corps Squadron HMX-1 starting in July 1956. The Army subsequently placed orders for a further 85 H-37As, and all ninety-four aircraft were delivered by June of 1960. US Army H-37A Mojave helicopters went into service, initially with 4th Medium Helicopter Transportation Company, in February 1958.
For several years the S-56 was the western world's largest and fastest military helicopter, and held two height-with-payload records from 1956-59. 1959 saw the first overseas H-37 deployment, by the Army to Germany.
Production of the S-56 ended in May 1960, but Sikorsky were engaged until the end of 1962 in converting all but four of the H-37A's to H-37B (later CH-37B) standard. Improvements in this version included the installation of Lear auto-stabilisation equipment, crash-resistant fuel cells and modified nose doors, and the ability to load and unload while the helicopter was hovering. The Navy and Marine S-56's became CH-37C's under the 1962 designation system. Some later production S-56's had 2100hp R-2800-54 engines.
Some of the 60 HR2S-1's ordered by the Marine Corps were converted into radar patrol craft (military designation HR2S-1W), with a bulbous dielectric radome and AN/APS-20E search radar under the nose and additional crew members for radar picket duties. Records indicate that the Army also evaluated one of the Navy's two radar-equipped HR2S-1W airborne early warning (AEW) aircraft. This machine (BuNo 141646) retained the AEW variant's large chin-mounted radome and AN/APS-20E search radar, and was operated in Army markings and two-tone 'Arctic' paint scheme.
In June 1963 four CH-37Bs were temporarily deployed to Vietnam to assist in the recovery of downed U.S. aircraft. By the following December the Mojaves had recovered an estimated $7.5 million worth of equipment, most of which was sling-lifted out of enemy-dominated areas virtually inaccessible by any other means. That the CH-37 did not see more extensive service in Vietnam is primarily the result of its replacement in the Army inventory by the turbine-powered Sikorsky CH-54 Tarhe, a machine that weighed slightly less than the CH-37 but which could carry nearly four times as many troops or five times as much cargo.
The last CH-37 was withdrawn from Army service in the late 1960s.
The S-56's rotor and transmission systems were utilised in the development of the abortive Westland Westminster and Sikorsky's own S-60 and S-64 crane helicopters, but hopes of selling the S-56 on the commercial market were not realised, due mainly to the high operating costs of a piston-engined machine of this size, and a proposal to fit Lycoming T55 gas turbines was not adopted.
A standard S-56 was ordered by Okanagan Helicopters Ltd. of Canada, for delivery in 1956-57.
In all, 150 S-56s were built; a prototype, 55 for the USMC and 94 for the Army.
Engines: Two P&W R-2800-C5 Double Wasp, 2100 hp
Engines: 2x 1,900 h.p. Pratt & Whitney R-2800-50
Rotors: 5-blade Main; 4-blade tail.
Rotor diameter: 90 ft.
Weight: 28,500 lb.
Max. speed: over 150 m.p.h.
Range: 200 miles at 120 m.p.h.
Seats: 2 crew and 26 passengers.