The Helikopter Technik Wagner, formed by Josef Wagner, designed a series of helicopters with coaxial rotors during the 1960s. The earliest test models were fairly rudimentary machines built to test the concept, and one design was a roadable machine titled the Rotocar III. The first production-standard prototype to fly in July 1965 was the Wagner Sky-Trac 1 (D-HAJE), which still had a frame structure but was fitted with a 260hp Franklin 6AS-335-B engine, and had a sliding canopy forming an enclosed cabin for the single pilot. A second machine was built (D-HARB) which was fitted with a pontoon landing gear and a longer boom with a V-tail. The subsequent three-seat Sky-Trac 3 (D-HAJI) had a more complete cabin structure and was fitted with spray bars and under-slung tanks for agricultural chemicals. Wagner also built a prototype (D-HAGU) of the Aerocar roadable helicopter, powered by a 260hp Franklin 6AS-335-B engine, which had a complete body shell with twin fins and four wheels linked to an automobile drive system for its ground transport role. It was first flown in 1965. It was subsequently rebuilt with a Turbomeca Oredon turbine engine.
After extensive development work had been carried out, the Wagner designs were passed to a new company, Helikopter Technik Munchen (HTM) in 1971. HTM abandoned the Aerocar, but built a new prototype of the FJ-Sky-Trac (D-HHTM) in tandem two-seat configuration, as a utility helicopter with a 260hp Lycoming IO-540 engine. This still had the coaxial rotor system but was extensively redesigned to accommodate the new engine and cockpit layout. HTM then went on to build two examples of the HTM Skyrider (D-HTMS and D-HHTF) which reverted to the configuration of the Sky-Trac with a four-seat cabin and a fully enclosed streamlined structure. The Skyrider was an attractive machine with a skid undercarriage and V-tail, which commenced flight testing on 21 February 1974. Unfortunately, HTM was forced to abandon further development in 1975 through lack of further funding.
R.Simpson "Airlife's Helicopter and Rotorcraft", 1998
Prototype four-seat helicopter derived from the Sky-Trac, with virtually the same structure and two contra-rotating two-blade rotors, but a new fiber-glass fuselage with room for three passengers on a bench seat behind the pilot.
G.Apostolo "The Illustrated Encyclopedia of Helicopters", 1984
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fred w piasecki, e-mail, 17.04.2020 Chrobak