Sikorsky S-52
The Sikorsky S-52 is a utility helicopter developed by Sikorsky Aircraft in the late 1940s. It was used by the U.S. Navy, Marines, and Coast Guard. The S-52 was the first US helicopter with all-metal rotor blades. A two-seater, it was developed into the four-seat S-52-2. It was designated "HO5S-1" by the U.S. Navy and Marine Corps; "HO5S-1G" by the Coast Guard; and "YH-18A" by the Army. Design and development. Sikorsky Aircraft began designing the S-52 in late 1945. The prototype S-52, first flown in 1947, was a two-seater and used a  178 hp (133 kW) Franklin air-cooled flat-six piston engine. The two-seat version was modified into the S-52-2, a four-seat helicopter using a 245 hp (183 kW) Franklin O-425-1 air-cooled flat-six. It had a semi-monocoque fuselage of pod-and-boom arrangement with a large bubble-like front greenhouse, a three-blade rotor, and quadricycle fixed landing gear. The production S-52-3 (HO5S-1) incorporated a downward sloping (anhedral) v-tail stabilizer. It also had sliding doors on the right forward and left rear sides, and a vertically split front bubble, allowing the left  half to swing open in a clamshell fashion. The engine was placed at the aft end of the cabin and was canted forward 30 degrees to couple with the clutch and transmission. The pilot-in-command occupied the right front seat. The first American helicopter to have all-metal rotor blades, the prototype set several speed and height records in 1948, including 129.6 mph (204.2 km/h) on a 3 km (2 mi) course, 122.75 mph (197.54 km/h) on a 1 km (1,100 yd) circuit, and an absolute height of 21,220 ft (6,468 m). It was capable of hover  out of ground effect at 5,900 ft (1,798 m) or 9,200 ft (2,804 m) in ground effect. The S-52 was the first helicopter to be flown in a loop, as flown by Harold E. Thompson on 19 May 1949. The S-52 also served as the basis of the turbine-powered S-59, which as the XH-39, competed for and lost the contract that produced the Bell UH-1 Iroquois. This aircraft differed in having a four-bladed rotor (against the S-52's three) and retractable tricycle gear. Operational history. The Navy operated the aircraft as a utility type, and it was used  by the Marines for observation and scouting in Korea, where the HO3S proved more popular. Four S-52s were evaluated by the United States Army for utility use in 1950, as the YH-18A, but not purchased in quantity. Many of the former military HO5S-1 units were demilitarized, rebuilt, and licensed as civil aircraft by Orlando Helicopters, who acquired the parts inventory from Sikorsky.