The Horizon and Diplomate are relatively high performance retractable undercarriage four seaters which were built in relatively modest numbers.
The initial GY-80 Horizon began life as a privately developed design penned by Yves Gardan, who had also been responsible for a number of other postwar light aircraft designs. Gardan built a prototype as a private development which flew on July 21 1960. Two years later, Sud Aviation (now Aerospatiale) acquired the design and production rights for the GY-80 and placed the type into production. Sud Aviation built three preproduction development aircraft before placing the GY-80 into production in late 1963.
A key aspect to the GY-80 was its simple design and method of construction. The GY-80 was of conventional appearance but slightly unusual construction, with an aluminium skinned steel tubular framework for the cabin area, and monocoque tailcone bolted aft. It featured a retractable tricycle undercarriage, Frise slotted ailerons and Fowler flaps. The flaps are permanently connected to the undercarriage linkage which is wound up and down manually with a handle protruding from the instrument panel. To reduce maintenance costs, the flaps, ailerons and horizontal tail pieces were interchangeable (in theory, but they needed to be modified if they were previously used at any other position). The Horizon was built using car assembly techniques, with just 11 production jigs.
Three versions of the Horizon were built, differing in powerplant and fuel tankage. These were the 110kW (150hp) Lycoming O-320-A powered GY-80-150, the 120kW (160hp) O-320-B2B powered GY-80-160, and the 135kW (180hp) Lycoming O-360-A powered GY-80-180. These were built firstly by Sud Aviation, and then by its newly established light aircraft division, Socata, from 1966.
Socata flew an improved development of the GY-80, initially called the Super Horizon 200, on November 7 1967. This new development featured a stretched cabin and a more powerful 150kW (200hp) fuel injected Lycoming IO-360-C. In production between 1969 and 1974, the Super Horizon was renamed Provence, and then the ST-10 Diplomate, the latter becoming the type's definitive name.
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