Hiller VZ-1 / HO-1 Pawnee / YHO-1E Flying Platform

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. The deLackner Aerocycle was an early, 1950s design that used 15 ft counter-rotating coaxial propellers driven by a 25 HP outboard motor. Later versions used a 40 HP motor when the vehicle proved underpowered in early tests. Kinesthetic control limited top speed to alxmt 20 mph.

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 [earlier Army designation: HO-1]. For improvement to produce enough thrust to climb out of ground-effect, it was designed with a larger 8' diameter duct. Hiller's successor was a larger version designed to overcome the design deficiencies of the earlier model. The second platform used a 8 ft diameter ducts with propellers driven by three interconnected engines. The vehicle proved to bt too big, too heavy to control kinesthetically, and had a lower top speed than the VZ-1. The added weight affected the pilot's ability to use kinesthetic control. The unsolved control problems caused it retired in 1959.