18 December 1953: At Bridgeport, Connecticut, Sikorsky chief test pilot Dimitry D. (“Jimmy”) Viner and co-pilot James Edward Chudars made the first flight of the Sikorsky XHR2S-1 (Sikorsky Model S-56). The XHR2S-1 was a prototype assault and heavy-lift helicopter for the United States Navy and Marine Corps. It was later adopted by the U.S. Army as the H-37 Mohave.

The S-56 was a large twin-engine helicopter, following the single main rotor/tail (anti-torque) rotor configuration pioneered by Sikorsky with the Vought-Sikorsky VS-300 in 1939. The helicopter was designed to be flown by two pilots in a cockpit located above the main cabin. The two engines were placed in nacelles outboard of the stub wings which also housed the helicopter’s retractable main landing gear. Two large clam shell cargo doors and loading ramp were placed in the nose. The HR2S-1 incorporated a stability system and an automatic torque compensating tail rotor.

The S-56 series was the largest and fastest helicopter built up to that time, and remains the largest reciprocating engine helicopter ever built.

The S-56 was equipped with a five blade articulated main rotor. This allowed increased lift and higher forward air speed before encountering retreating blade stall than earlier three and four blade systems. A six blade rotor system was tested, which showed further improvements, but was not adopted. The main rotor diameter was initially 68 feet (20.726 meters), but later increased to 72 feet (21.946 meters). The main rotor blades had a chord of 1 foot, 9.5 inches (0.546 meters) and used the symmetrical NACA 0012 airfoil, which was standard with American helicopters up to that time. Later in the program, the blades were lengthened and the chord increased to 1 foot, 11.65 inches (0.601 meters). The airfoil was changed to the NACA 0010.9 airfoil. These changes resulted in increased lift and higher speed. The four blade tail rotor had a diameter of 15 feet (4.572 meters). The individual blades had a chord of 1 foot, 1.5 inches (0.343 meters). As is common with American helicopters, the main rotor system turned counter-clockwise as seen from above. (The advancing blade is on the right.) The tail rotor turned counter-clockwise when viewed from the helicopter’s left side. (The advancing blade is above the axis of rotation.)

With the longer blades installed, the helicopter’s length with rotors turning was 88 feet (26.822 meters). The fuselage had a length of 64 feet, 10.69 inches (19.779 meters), and the height was 17 feet, 2 inches (5.232 meters). The HR2S-1 had an empty weight of 21,502 pounds (9,753 kilograms), and maximum weight (overload) of 31,000 pounds (14,061 kilograms). Its fuel capacity was 1,000 U.S. gallons (3,785 liters) carried in 6 tanks located in the nacelles, wings and fuselage. It could carry 20 fully-equipped troops, or 16 litters. Its maximum cargo capacity was 10,000 pounds (4,536 kilograms).

The HR2S-1 had an automatic main rotor blade folding system, and its tail rotor pylon could be folded alongside the fuselage, reducing the length to 55 feet, 8 inches (16.967 meters) and width to 27 feet, 4 inches (8.331 meters). This allowed the helicopter to use aircraft carrier elevators and reduced storage space on the hangar deck.

Early S-56 models were powered by two air-cooled, supercharged 2,804.461 cubic inch displacement (45.957 liters) Pratt & Whitney Double Wasp R-2800-50 two-row, 18-cylinder radial engines rated at 1,900 horsepower at 2,500 r.p.m. These were upgraded in later models to R-2800-54s. These were direct drive engines with a compression ratio of 6.75:1. The R-2800-54 was rated at 2,100 horsepower at 2,700 r.p.m. to 5,000 feet (1,524 meters) for takeoff; with a normal power rating of 1,900 horsepower at 2,600 r.p.m. to 7,000 feet (2,134 meters). It required 115/145 octane aviation gasoline. Each engine was supplied with 13.3 gallons (50.35 liters) of lubricating oil. The R-2800-54 was 6 feet, 9.00 inches long (2.057 meters), 4 feet, 5.00 inches (1.346 meters) in diameter, and weighed 2,300 pounds (1,043 kilograms).

The helicopter’s engines were installed at an 80° angle to the aircraft center line, with a 12.5° upward angle to align with the main transmission input. The front of the engines faced inboard. According to Sikorsky, this unusual installation resulted in high oil consumption, and because the engines were operated at continuous high r.p.m., the time interval between engine overhauls was reduced from the normal 2,000 hours to just 350 hours.

The production HR2S-1 had a cruise speed of 100 knots (115 miles per hour/185 kilometers per hour), and a maximum speed of 121 knots (139 miles per hour/224 kilometers per hour) at Sea Level. The helicopter’s service ceiling was 13,800 feet (4,206 meters), and its absolute hover ceiling was 5,400 feet (1,646 meters). It had a maximum rate of climb of 1,580 feet per minute (8.03 meters per second) at Sea Level, and a vertical rate of climb 950 feet per minute (4.83 meters per second), also at Sea Level. The combat radius of the HR2S-1 was 100 nautical miles (115 statute miles/185 kilometers) at 100 knots (115 miles per hour/185 kilometers per hour.)

55 HR2S-1s were delivered to the U.S. Marine Corps. The U.S. Army purchased 94 S-56s in the H-37A Mohave configuration. 90 of these were later returned to Sikorsky to be upgraded to H-37Bs. This added the automatic stabilization system of the HR2S-1, changed the variable incidence horizontal stabilizers on both side of the fuselage to a single stabilizer on top of the tail rotor pylon. Engine oil capacity was increased to 30 gallons (113.6 liters) per engine.

A total of 154 S-56s were built between 1953 and 1960.

From 9 through 11 November 1956, a U.S. Marine Corps HR2S-1 flown by Major Roy Lee Anderson, USMC, and Sikorsky test pilot Robert S. Decker at Windsor Locks, Connecticut,  set three Fédération Aéronautique Internationale (FAI) world records for speed and payload:

On 9 November, the helicopter reached an altitude of 3,722 meters (12,211 feet) with a payload of 5,000 kilograms (11,023 pounds).¹ The following day, 10 November, it set a record for the greatest mass carried to a height of 2,000 meters (6562 feet): 6,010 kilograms (13,250 feet).² Then on 11 November, the HR2S-1 reached a speed of 261.91 kilometers per hour (162.74 mph) over a  3-kilometer (1.86 miles) course.³ For these flights, Major Anderson was awarded a third gold star in lieu of a fourth award of the Distinguished Flying Cross.

¹ FAI Record File Number 13129

² FAI Record File Number 13124

³ FAI Record File Number 13098

© 2023, Bryan R. Swopes