Hiller Flying Platform (Model 1031-A-1)
During the 1950s the U.S. armed forces were desperately seeking ways to improve the mobility of their troops on battlefields that were subject to attack by nuclear, chemical, and biological weapons. Small, one-man helicopters were seen as a possible alternative to large, piston powered helicopters that seemed to be at the limit of their development. The military's greatest technical hurdle was to develop an aircraft that was relatively safe and could easily be flown by a combat infantryman. Stanley Hiller, founder and president of Hiller Helicopters, had built a successful business, founded on innovative approaches to helicopter design, and was more than willing to undertake the challenge. Although the flying platforms that resulted from the company's efforts proved to be an aerodynamic dead end, Hiller did demonstrate the practicality of the ducted fan for more conventional forms of Vertical Takeoff and Landing (VTOL) aircraft.
In the late 1940s, noted aeronautical engineer Charles Zimmerman, who had made a name for himself developing Vought's V-173 "Flying Pancake" (see NASM collection), began to develop a new approach to vertical flight. He hypothesized that if a small horizontal platform, with a person balancing on top as on a bicycle, was lifted upward by a vertical thrust vector, then the pilot's innate kinesthetic responses would stabilize the platform and also allow it to be controlled in pitch in roll. Although the high center of gravity of such a configuration would seem to result in severe instability, Zimmerman's theory proved correct. If the platform began to tilt in one direction, then the pilot would naturally lean in the other direction to remain upright. This natural balancing tendency would place the center of gravity above the thrust axis, which would result in an upward pitching moment that counteracted the toppling action, thus maintaining a stable condition. The pilot could control the aircraft simply by leaning in the desired direction and the platform would begin to tilt and gain forward momentum. As the aircraft was controlled by instinct, a person could fly it with minimal training, making it ideal for use by soldiers in the field.
Zimmerman set out to construct a flying platform to prove his hypotheses. His first effort consisted of two small target drone engines mounted vertically to the sides of a small steel-tube truss. The pilot was to stand on the truss holding an attached pole. Control for the contraption, nicknamed the "Flying Shoes," was accomplished entirely by weight shifting against the pole. The Flying Shoes suffered instability problems because the engines provided unequal thrust. Stanley Hiller heard about this experiment and was greatly intrigued. In 1948, Zimmerman made a deal that allowed Hiller Helicopters to continue development, while he returned to his old aeronautical engineering position at the NACA (National Advisory Committee for Aeronautics, forerunner of NASA) laboratory in Langley, Virginia.
Unfortunately, Hiller's first successful production helicopter, the UH-12, demanded the company's complete attention and the Flying Shoes were quickly set aside. In the meantime Zimmerman convinced his peers of the merits of his theories and with official support, began to develop new experimental kinesthetically controlled flying platforms. The first of these efforts used compressed air channeled through attached fire hoses for thrust, while the second, known as the Whirligig (see NASM collection), used a compressed air-driven propeller on the underside of a lightweight platform to generate thrust. These developments proved to be extremely easy to fly and were stable in flight, although the larger and heavier Whirligig proved to be less stable and more difficult to control than its predecessor.
The military finally began to take notice of Zimmerman's experiments and issued contracts for the construction of prototypes. On September 17, 1953, the Army initiated contracts with De Lackner and Hiller for kinesthetically-controlled flying platforms. As the Office of Naval Research had pre-existing research programs with Hiller, its leadership agreed to manage the program on behalf of the Army. Engineers from both companies then visited the NACA test facilities to view Zimmerman's progress.
De Lackner's approach for their DH-4 Aerocycle (HZ-1) was to have the pilot stand atop a large coaxial rotor system. This arrangement was stable and performed well, but any concept that forced the pilot to stand on the airborne equivalent of a food processor with nothing to prevent inadvertent contact with the rotors was unlikely to generate much enthusiasm. Hiller's engineers went back to Zimmerman's original patent application for his Flying Shoes, in which the rotors were to have been located in what he termed as "venturi rings." These consisted of airfoil profiles formed into a circle. The venturi ring would soon become known as a ducted fan. The ducted fan's airfoil accelerated the airflow into the rotors, increasing thrust to a level approximately 40 percent greater than an unducted propeller of the same diameter. Hiller's solved the Flying Shoes' problem of asymmetric thrust by mounting the counter-rotating propellers co-axially. The Model 1031 used two engines, each directly driving one of the rotors inside the 1.52 m (5 ft) diameter duct.
By September 1954, construction had been completed on the Model 1031, which consisted of a fiberglass duct and steel-tube platform. Initial flights were made by Hiller's chief test pilot, Philip T. Johnston. Because there was little to protect the pilot in the event of an engine failure or loss of control, he was tethered to a high wire suspended between two towers. The Model 1031 proved relatively stable, and easy to handle when hovering. A natural self-correcting tendency in forward flight was noted. This occurred because the forward lip of the duct would generate more lift than the trailing edge causing an upward pitching moment. Unfortunately, while this made the platform almost impossible to topple, it also limited the forward speed to a mere 26 kph (16 mph), and resulted in erratic handling in windy conditions.
Hiller engineers realized that an engine failure on either of the engines would result in a catastrophic loss of control. Thus, a new transmission was installed that allowed the combined power of both engines to power the rotors so that if an engine failure occurred, a rapid descent would occur, but no loss of control. This improved design used the same components as the original platform and was designated as the Model 1031-A. On the Model 1031, differential braking on the rotors had controlled yaw, but with the combined transmission, this was no longer an option, so a pair of movable vanes was placed in the ducted fan's inflow to provide a new mechanism for yaw control.
The Model 1031-A could not operate out of ground effect because of its limited thrust. Hiller engineers determined that loading on the fixed-pitch rotors' was too high and that the only solution was to use longer rotor blades, which necessitated construction of a larger platform, designated the Model 1031-A-1. By this time, the Department of the Army, dissatisfied with De Lackner's progress, had begun to take over control of Hiller's Navy contract. The new platform, with 2.13 m (7 ft) diameter rotors, which first flew on November 20, 1957, was able to operate successfully out of ground effect, but a new problem was encountered. The increased weight of the platform lowered the center of gravity to the level that kinesthetic control was greatly impaired. Hiller engineers attempted to correct this by raising the pilot's platform. However, controllability was still a problem since the total weight of the platform had increased to the stage where weight shift alone could not insure an adequate level of stability or control. The solution to this was the addition of a gyro-stabilization system that used aerodynamic servos similar to those that Hiller had used on the UH-12. The new system, which was linked to four control vanes in the outflow, improved stability in the hover significantly. The most dramatic illustration of the new system occurred when an Army sniper was able to aim and fire his rifle while hovering in free flight, without any thought to aircraft control. However, in forward flight in anything but the calmest conditions, the platform experienced erratic oscillations that the gyrostabilizer could not dampen out. Various duct configurations were tried, but those that showed the greatest increase in stability also produced the least amount of thrust.
While Hiller was just discovering the extent of the Model 1031-A-1's control problems, the Army placed an order for three upgraded models, for its own experimentation. However the Army insisted that the new platforms, designated VZ-1E, should include a third engine as a backup in case of a failure of either of the two main engines. This requirement necessitated construction of a larger platform, with 2.44 m (8 ft) diameter rotors. The extra weight of the additional engine and larger airframe further increased the control difficulties to the point that kinesthetic control was no longer practical. Hiller attempted to remedy the situation by lengthening the ducted fan for greater stability and developing a more conventional control system in which a seated pilot would maintain control with a stick linked to the control vanes that had originally been intended only for yaw. This latest development proved to have a faster forward flight speed than the Model 1031, but did not resolve the control or instability problems. However, the modified VZ-1E did lead to a new line of thinking.
The Hiller engineering team realized that, while they had demonstrated the impracticality of kinesthetically controlled flying platforms in forward flight, the ducted fan idea still held a great deal of promise for more sophisticated VTOL aircraft. Thus, the Office of Naval Research funded the development of a prototype coleopter, or ring-wing VTOL aircraft, consisting of a lengthened ducted fan, which was intended to act as a lifting fuselage when the aircraft pivoted from a vertical to horizontal attitude for high-speed forward flight. A mockup of the coleopter, designated the VXT-8, was constructed, but the design went no further. Control of the coleopter in forward flight would have been extremely difficult. Hiller's final attempt to exploit the merits of the ducted fan was a proposal for an Army "flying jeep" competition. The Hiller design, which was not accepted, consisted of four ducted fans powered by a gas turbine attached to a simple frame. Considering Hiller's experience with ducted fans on the Model 1031, the design may well have revolutionized battlefield transportation if a suitable control system had been developed. By this time, however, turbine helicopters such as Bell's new HU-1 Huey, were overcoming many of the hurdles faced by the piston-engine models, and the flying jeep was abandoned.
Hiller's experimentation with the Model 1031/VZ-1E proved to be of little practical use to the company, which went out of business in 1966. However, the company's efforts did validate the ducted fan, which began to see practical service in hovercraft in the 1960s, and has emerged more recently in several Unmanned Aerial Vehicles (UAVs) prototypes. In spite of its ultimate failure, the flying platform program proved to be a public relations bonanza for Hiller. When photos of the Flying Platform appeared in the media, the public was immediately captivated, perhaps because the platform's lack of visible propulsion seemed to be inspired from the flying saucer frenzy then sweeping the nation. While the apparent successes of the flying platform were widely reported, its inherent aerodynamic flaws were not publicized. Thus, many enthusiasts, even decades later, felt that a viable mode of transport had been unjustly abandoned. As a result, ducted fan platforms have occasionally appeared in the backyards of amateur inventors, who are unaware of the Model 1031's potentially fatal problems. However, the Hiller Flying Platform's vivid demonstrations of the potential of ducted fan technology may yet result in the inspiration for other new approaches to vertical flight.
Rotor Diameter:2.13 m (7 ft) each
Platform Diameter:2.54 m (8 ft 4 in)
Height:2.13 m (7 ft)
Weight:Empty, 167.8 kg (370 lb)
Gross, 251.7 kg (555 lb)
Engine:2 Nelson H-59 two-cycle engines, 40 hp each
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.
