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YH-18 / HO5S ( S-52 )

Until 1948 there was not a helicopter specifically designed for military observation in actual production. In existence, however, was the Bell model 47E in flight test at the Bell factory and the one Sikorsky S-52, which had already completed flight test; both of which, it was believed, could easily be converted for military observation use. Sikorsky Aircraft Corporation's two-seat S-52, the first helicopter to have all-metal rotor blades, first flew on 12 February 1947.

The Sikorsky S-52 was a two-place, three-bladed single-rotor-system utility helicopter built completely at Sikorsky's expense and concurrently with the larger and now practically defunct XHJS-1. The 2,100-pound gross weight of the S-52 permitted the aircraft to carry a useful load of approximately 1,000 pounds at a maximum airspeed of 91 knots. The Sikorsky S-52 was the first production helicopter to have all-metal rotor blades. In view of the climatic effects and sand abrasion on wood and fabric rotor blades, this represented a basic improvement in durability and lifetime. During April 1949, the S-52 established a world's speed record at Cleveland, Ohio, of 129.55 miles per hour.

By November 1948 evaluations had been completed and the results sent to the Commandant by Colonel Dyer. Based upon Colonel Dyer's letter, on 24 November 1948 General Cates asked permission of the CNO to change the complement of a Marine observation squadron from its previously authorized eight OY aircraft to four OY and four helicopters. The Commandant stated that all of the helicopters observed and tested as replacements for the OY aircraft, the latest model Bell HTL-3 and the Sikorsky S-52 closely met the Marine Corps' requirements of size, configuration, and gross weight, with the S-52 rated as the most desirable of all models.

Aviation Plan Number 21-49, dated 7 April 1949, outlined the plans for outfitting the VMO squadrons. The plan specified that the HTLs were considered satisfactory for Marine observation requirements and that as the HTL helicopters became available they would replace half the observation aircraft in existing VMO squadrons.

Approximately three months after the 1949 aviation plan appeared, the Commandant submitted to the CNO the Marine Corps ' specific requirements for the desired type of observation helicopter. The letter, dated 1 July 1949, mentioned that HMX-1 had conducted extensive evaluation of helicopters as replacements for the OY type aircraft for VMO squadrons and determined that some specific requirements were necessary if the aircraft were to be suitable for observation work.

Generally, the specifications required that the helicopter carry a useful load of between 800 to 1,000 pounds, have dual controls, be capable of flight at maximum gross weight for a duration of four hours, and carry a pilot, observer, and one additional passenger. The requirements listing the maximum air speed and dimensions were omitted The new observation helicopter requirement stipulated that the aircraft should be capable of operations within allowable center of gravity limits at minimum and maximum loading conditions without having to resort to shifts in ballast, or equipment, to stay within operating center of gravity limits. In relation to size, the aircraft was to be small enough to lend itself to ease of concealment and transportability on a widely varied number of vehicles, and to be able to operate from small areas in the field.

As a result of the favorable flight evaluation of the Sikorsky S-52, BuAer initiated a contract with Sikorsky for the S-52-2, a version of the original S-52 (Navy designation H05S-1). When further modified and later delivered to the Marine Corps, it would be a four-place, 245-horsepower, three-bladed machine with a quadricycle landing gear. Official missions descriptions were listed as observation- liaison, reconnaissance, gunfire adjustment, evacuation of wounded, transportation of personnel, and general utility. As a medical evacuation aircraft, the copilot's seat could be removed and two litter patients carried internally, in addition to the pilot and attendant. An unusual feature, one which would later amount to a great impairment in its use, was that its take-off weight was limited to 2,769 pounds. With a pilot and' observer, and a full fuel load of 222 pounds, the 245-horsepower engine would allow for a skimpy 157 pounds of payload. The delivery date for the first aircraft was scheduled for September 1951. Sikorsky developed a four-seat model and gave it the more powerful Franklin 0-425-1 engine. This model was first accepted by the Marine Corps and received the designation HO5S-1. The HO5S-1 helicopter's first flight was August 1951. It was utilized for observation-liaision purposes. Its missions included reconnaissance, gunfire adjustment, evacuation of wounded, transport of personnel, and general utility with troop units. For use in evacuating wounded, the co-pilot's seat could be relocated and two litter patients carried internally in addition to the pilot and medical attendant.

The HO5S-1 is the Navy version of the Sikorsky Model S-52-2 and the Air Force YH-18.

Design features include self-sealing fuel and oil tanks, both main and auxiliary rotor blades of all-metal construction, anti-coning blade restrainers for shipboard operation, and non-scuffing quadricycle type landing gear.



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