HILLER 1031 'Flying Platform'

Military designation: YHO-1E, VZ-1-UH 'Pawnee'

Powerplant: 2 x 44 hp Nelson H-59

First flight: February 1955

National Advisory Committee for Aeronautics (N.A.C.A.) engineer Charles H. Zimmerman had developed a concept known as the "Flying Shoes". Zimmerman, to the amusement of his engineering peers, proved the theory that rotors on the top (i.e. helicopters) are inherently unstable. Zimmerman theorized a person's natural balancing reflexes would suffice in controlling a small flying machine. Charles coined the term "kinesthetic control," similar to riding a bicycle or balancing a surfboard.

Zimmerman built a flying platform to prove his hypotheses. It consisted of two small target drone engines mounted vertically to the sides of a small, steel-tube truss. A pilot stood on the truss and supported himself by grasping a long, vertical tube. To control the contraption, nicknamed the 'Flying Shoes', the pilot shifted his weight against the pole. The 'Flying Shoes' were unstable because the engines provided unequal thrust but the original concept greatly intrigued Stanley Hiller. In 1948, Zimmerman made a deal to allow Hiller Helicopters to continue developing this idea and the engineer continued to work on other projects at the NACA (National Advisory Committee for Aeronautics, forerunner of the National Aeronautics and Space Administration, or NASA) laboratory in Langley, Virginia.

The US Army showed great interest in Zimmerman's experiments. Nuclear, chemical, and biological weapons proliferated around the world during the Cold War and U. S. Army infantrymen seemed particularly vulnerable to attack. Small, vertical takeoff and landing (VTOL) aircraft, and especially one-man flying platforms, appeared to offer the soldiers an ingenious method of escape. The biggest technical barrier to designing such an aircraft was to make it field-portable, and safe and easy to fly. It was hoped that kinesthetic control would allow a pilot to fly such platforms with little training. Stanley Hiller had founded Hiller Helicopters specifically to develop innovative approaches to helicopter design and he was more than willing to undertake this challenge.

On 17 September 1953, Hiller Helicopters signed a contract with the Office of Naval Research's Naval Sciences Division (ONR), acting as technical direction agent for the Army, for the development of a one-man, twin-engined ducted fan VTOL research vehicle to incorporate Alexander Satin's ducted-fan research with Charles Zimmerman's "kinesthetic" theories. The De Lackner Company received a similar contract direct from the Department of the Army for its 'Aerocycle'. Both aircraft were intended to explore both the practicality of the ducted fan as a propulsion unit for V/STOL aircraft and the potential military value of the flying platform as a tactical reconnaissance and transport vehicle. Engineers from both companies visited the NACA test facilities to view Zimmerman's progress. NACA tests involved pilots "flying" tethered platforms, at first lifted by compressed air, and then by rotors. They demonstrated the technical validity of the concept.

The then-classified project was turned over to the Advanced Research Division (A.R.D.) at Hiller Aircraft, and construction of the prototype Model 1031 'Flying Platform' (later found in the press as the 'Flying Carpet') began in January 1954 in complete secrecy. Unlike De Lackner, Hiller's engineers went back to Zimmerman's original patent application for his 'Flying Shoes'. That patent described the two rotors located inside what the engineer termed "venturi rings." These rings were actually circular airfoils or wings. Someone coined the term 'ducted fan' to describe the venturi ring with propellers inside. The airfoil duct around the fan accelerated airflow into the rotors, in the same way that the airflow accelerates over the curved, upper portion of a conventional aircraft wing. A ducted fan could generate approximately 40 percent more thrust than an unducted propeller of the same diameter. Hiller cured the instability of the original 'Flying Shoes' caused by unequal thrust from the engines by mounting twin counter-rotating rotors co-axially. The duct improved safety during takeoff and landing. The duct also provided additional lift, since there was a horizontal "lip" around its top edge that curved down into the duct. The airflow into the duct resulted in low air pressure above the lip, and the pressure difference between the top and bottom of the lip generated a net upward force, providing as much as 40% of the total lift of the aircraft.

The Model 1031 used 44hp/4000 rpm, four cylinder opposed, two-cycle, Nelson H-59 engines, each directly driving one of the rotors inside the 1.5 m (5 ft) diameter duct. The Nelson engine was the first two-stroke engine certified by the FAA for aircraft use. Utilizing the Bernoulli principle, 40% of the vehicle's lift was generated by air moving over the ducted fan's leading edge. The remaining 60% of lift was generated by thrust from the counter rotating propellers. The pilot stood above the duct, surrounded by a circular handrail and protected by a safety harness. He controlled the engines with a twist-grip throttle and leaned to guide the aircraft. The Model 1031 was delivered to the ONR in September 1954, and given the interim, non-standard Army designation YHO-1E (in which 'E' designated Hiller, Navy-style). The first free flight took place on 27 January 1955, and went in the record books as the first time man had flown a ducted fan vertical take off and landing (VTOL) aircraft. In April, the veil of secrecy was lifted, and as the Hiller designs was better thought out than its competitors, they attracted a good deal of public attention; soon it seemed that all the world wanted a vectored thrust vehicle for their own.

By September 1954, construction was finished on the Model 1031. It consisted of a steel-and-aluminum tube platform fixed atop a fiberglass duct. Hiller's chief test pilot, Philip T. Johnston, made the first test flights. With little to protect the pilot in case of engine failure or loss of control, technicians tethered the flying platform to a high wire suspended between two towers. The Model 1031 proved relatively stable and easy to handle when hovering. In horizontal flight, the vehicle exhibited an automatic, self-righting tendency. This occurred because the leading edge lip of the duct generated more lift than the trailing edge, causing an upward pitching moment. This force worked to balance the pilot's weight as he shifted the platform in the direction he wanted to fly. This effect made the platform almost impossible to topple. Unfortunately, it also limited the forward speed to a mere 26 kph (16 mph) and caused erratic handling in windy conditions. Hiller engineers realized that failure of either engine would result in a catastrophic loss of control. They devised and installed a new transmission that allowed both engines to drive both rotors. Now an engine failure would cause rapid descent but not loss of control. With the new transmission, it was no longer possible to use differential rotor braking to control yaw. Hiller engineers designed a set of four movable vanes and placed them in the fan's downwash to provide a new mechanism for yaw control. The firm designated this improved platform the Model 1031-A.

After test-flying the Model 1031-A, Hiller found that the vehicle could not produce enough thrust to climb out of ground-effect (about 1.5m or 5 feet, the diameter of the platform). Larger rotor blades were the only solution. This change also necessitated a larger duct and platform and the new device was designated the Model 1031-A-1. By this time, the Department of the Army was dissatisfied with De Lackner's progress and Army officials began to take control of Hiller's Navy contract. Army Air Mobility Division and the Office of Naval Research soon shared control of the project. The new platform, now sporting 2.1 m (7 ft) rotors, first flew on November 20, 1957. It could operate successfully out of ground effect, but testing also revealed a new problem. The increased size of the revised platform also lowered the center of gravity so much that kinesthetic control was greatly impaired. Hiller engineers attempted to correct this by further elevating the pilot 's perch above the duct but the total weight of the platform was now so great that precise, stable weight-shift control was not possible.

Hiller solved this new problem by introducing a gyro-stabilization system that used aerodynamic servos similar to those used on the Hiller UH-12 helicopter. Engineers linked the new system to the yaw control vanes and it significantly improved hover stability. An Army infantryman dramatically illustrated these improvements when he aimed and fired his rifle with both hands while the Model 1031-A-1 hovered in free flight. Forward flight remained problematic except in the calmest conditions. The platform experienced erratic oscillations that the gyrostabilizer servos could not dampen. Engineers tried various duct configurations, but those that showed the greatest increase in stability also produced the least amount of leading edge thrust.

While Hiller was addressing these control problems, the Army, favorably impressed by the VZ-1's performance during the initial ONR-managed flight test programme, placed an order in November 1956 for two upgraded examples of the Model 1031-A-1. as the VZ-1-UH 'Pawnee' (sometimes found as VZ-1E), in which 'UH' stood for United Helicopters. The first of these, which made its maiden flight in 1958, was to be used for service testing and operational evaluation. It was similar in design to the ONR-tested one, but differed from the first example in having an eight-foot diameter duct without lower control vanes, and a simplified control pedestal. Army experts insisted that the new platforms should have a third engine as a backup in case of engine failure. Hiller tried to increase the platform's size to accommodate new 2.4 m-(8 ft) diameter rotors. However, the enlarged rotors and duct brought the empty weight of the 7-foot high vehicle up to 180 pounds, which severely affected the pilots' ability to use kinesthetic movements for directional control; in fact, it rendered kinesthetic control nearly impossible.

Hiller attempted to solve these problems in the third 'Pawnee' ordered by the Army (serial 56-6945), by deepening the duct to increase stability and lift performance, and using a more conventional control system. As kinesthetic control became less effective as vehicle power and weight increased, the new version had a seat and conventional helicopter controls, with a stick linked to the control vanes to move the platform in both yaw and pitch. The new configuration was also meant to increase range while reducing noise and downwash. This version first flew in 1959. It also featured a circular landing skid instead of the multipoint wheeled landing gear of the two earlier Pawnees. The VZ-1 handled very well in flight tests. The machine was then modified with longer landing gear legs to increase ground clearance, and eight vanes were mounted underneath the duct to improve flight control.

The revised 'Pawnee' had its merits, and it could fly faster than the original Model 1031, but it did not resolve the control or instability issues. The VZ-1 ultimately proved to be too small, too ungainly, too heavy, too slow, of limited usefulness and practicality for combat service and too mechanically delicate to be of any real value on the battlefield. Further development of the type was therefore suspended, the Army abandoned the program in 1963, and all three types were soon withdrawn from service.

Army and Navy officials transferred the Model 1031-A-1 to Mississippi State University for further ducted fan research after the vehicle failed to meet their requirements. In 1959, the Smithsonian asked that the military to permanently transfer the aircraft to the National Air Museum. The request was vetted through Hiller and in 1960, Mississippi State University shipped the vehicle to Washington, D. C.

Of the six Flying Platforms that were built, the (ONR) vehicle is on exhibit at the Hiller Aviation Museum, and the National Air & Space Museum has a VZ-1 'Pawnee' that is on loan to the Pima Air & Space Museum. Both platforms may be viewed on line at their respective websites (see below). Hiller Helicopters worked on two further platform type projects: the VXT-8 'Coleopter', initially funded by the ONR, then cancelled; the 'Flying Jeep', which didn't make it in the US Army competition; and its civilian version, the 'Aerial Sedan'. None of these ever evolved past the drawing board. The location of the remaining VZ-1 'Pawnees' is unknown. The program provided a wealth of valuable information about VTOL flight in general and the value of the ducted fan in particular, and almost 50 years on, the Hiller flying platform, the closest design to mimic a magic carpet still captures the public imagination, as exemplified by the resurrection of the 'Pawnee' concept in the form of the very similar commercial 'Hummingbird'.

Population:
1 as YOH-1E, later VZ-1 [no serial]
2 as VZ-1-UH [56-6944/6945]
and 3 civil demonstrators

Specs:
Rotor diameter: 2.1 m (7 ft.) each
Platform diameter: 2.5 m (8 ft. 4 in.)
Height: 2.1 m (7 ft)
Empty weight: 167.8 kg (370 lb.)

Crew/passengers: 1

Main sources:
- Hiller's official Flying Platform page
- National Air and Space Museum's VZ-1 page
- article on Flying Platforms at vectorsite.net
- NASM's Zimmerman Flying platforms page
- http://www.faqs.org/docs/air/avplatfm.html
- http://avia.russian.ee/vertigo/hiller_platform-r.html
- "The Sky-High Invention" by John DuBarry