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In early 1950s, the NACA engineers proposed a concept that placing the rotors on a
bottom of the aircraft, a pilot could steer it by shifting his weight, called
"Kinesthetic" control. In 1953, the Office of Naval Research (ONR)
awarded Hiller Helicopter a contract for the development of a VTOL
research-flying platform as a tactical reconnaissance and transport aircraft.
Hiller used two engines, each driving one of the rotors inside the 5' diameter
duct and the aluminum tube platform fixed atop the duct. The 1st prototype was
given the Navy designation YHO-1E. After a year of flight tests, the U.S. Army was
also interested in its performance, and ordered a modified vehicle for service
testing and operational evaluation. This 2nd prototype was re-designated VZ-1 in
1956. For improvement to produce enough thrust to climb out of ground-effect, it
was designed with a larger 8' diameter duct. However, the added weight affected
the pilot's ability to use kinesthetic control. The unsolved control problems
caused it retired in 1959.
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