In the 1950s, the U.S. armed forces were searching for ways to enhance troop mobility in battlefields prone to nuclear, chemical, and biological attacks. One-man helicopters seemed like a promising alternative to the limited piston-powered helicopters. Stanley Hiller, the founder of Hiller Helicopters, took on the challenge and set out to develop a safe and easy-to-fly aircraft for combat infantrymen. Although the resulting flying platforms turned out to be an aerodynamic dead end, Hiller successfully demonstrated the practicality of the ducted fan for conventional Vertical Takeoff and Landing (VTOL) aircraft.
The Flying Shoes and Zimmerman’s Theory
Aeronautical engineer Charles Zimmerman, known for his work on Vought’s V-173 “Flying Pancake,” began developing a new approach to vertical flight in the late 1940s. He theorized that a small horizontal platform with a person balancing on top, similar to a bicycle, could be lifted by vertical thrust. The pilot’s innate kinesthetic responses would naturally stabilize the platform and allow for pitch and roll control. Despite the high center of gravity, Zimmerman’s theory proved correct, making it ideal for use by soldiers with minimal training.
Hiller Continues Development
Stanley Hiller, intrigued by Zimmerman’s experiments, obtained a deal in 1948 to continue development of the concept. Meanwhile, Zimmerman returned to his position at the NACA laboratory in Langley, Virginia. Although Hiller initially put the project aside to focus on the UH-12 helicopter, Zimmerman convinced his peers of the theory’s merits and continued his work.
From Fire Hoses to the Whirligig
Zimmerman’s early experiments involved using compressed air and fire hoses to generate thrust. The second experiment, called the Whirligig, utilized a compressed air-driven propeller on a lightweight platform. Both designs proved stable and easy to fly, with the Whirligig being the more challenging of the two.
Military Interest and Development
The military finally took notice of Zimmerman’s experiments and awarded contracts to De Lackner and Hiller for the development of kinesthetically-controlled flying platforms. Hiller’s engineers revisited Zimmerman’s original patent application, which proposed using ducted fans instead of open rotors. They solved the issue of asymmetric thrust by mounting the counter-rotating propellers coaxially. The resulting Model 1031 demonstrated stability and ease of handling during hovering, but its forward speed was limited, and wind conditions affected its performance.
Improvements and Control Challenges
To address the limitations of the Model 1031, Hiller introduced a transmission that allowed both engines to power the rotors, ensuring control even in the event of an engine failure. However, control in windy conditions and forward flight remained problematic. Hiller engineers attempted various duct configurations, but increased stability resulted in reduced thrust. Despite these challenges, the Model 1031-A-1 with a larger platform and longer rotor blades proved successful in ground effect operations.
Army Contracts and Control Difficulties
The Army ordered three upgraded models, VZ-1E, requiring a third engine for backup. The larger airframe and additional weight intensified control difficulties, rendering kinesthetic control impractical. Hiller implemented a more conventional control system, but instability issues persisted. The attempt to remedy the situation led to further insights into ducted fan technology.
Beyond the Flying Platform
Realizing the limitations of kinesthetically-controlled platforms in forward flight, Hiller pursued the development of a ring-wing VTOL aircraft, referred to as a coleopter. This prototype had a lengthened ducted fan that served as a lifting fuselage during the transition from vertical to horizontal flight. However, control challenges in forward flight led to the abandonment of this design. Hiller also proposed a flying jeep concept, but it was not accepted.
Legacy and Future Prospects
Although Hiller’s experiments with the flying platform program ultimately proved impractical, they validated the potential of ducted fan technology. Ducted fans found applications in hovercraft and have emerged more recently in unmanned aerial vehicle (UAV) prototypes. While the flying platform program generated significant public interest, its inherent aerodynamic flaws were not widely publicized. Nonetheless, the program’s contributions have inspired new approaches to vertical flight.
References and Further Reading:
Gill, Wilbur J. Report No. ARD-236: Summary Report – Airborne Personnel Platform. Palo Alto, CA.: Hiller Aircraft Corporation, 1959.
Spencer, Jay P. Vertical Challenge: The Hiller Aircraft Story. Seattle: University of Washington Press, 1998.