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Early drawings for a personal flying platform, produced in 1947 by Charles Zimmerman. via Bill Rose This was driven by bottled gas fed through fire hoses to a two-bladed rotor on the under- side of the platform, with the air exhausting from the rear of the rotor tips. Given the name Whirligig, this platform had a diameter of 10ft (3.05m), a height of 3ft (0.914m), and used a rotor with an overall length of 7ft (2.13m). The prototype was considerably heavier than Jet Board, somewhat less stable and harder to control. The Whirligig made its first flight on 21st October 1953, but the platform was not a great success and the project was aban- doned soon afterwards. Personal Platforms Having secured the rights to develop Zim- merman’s personal air vehicle, Hiller Heli- copters began a lengthy research programme to make the concept into something that would find favour with military customers. The company successfully promoted the idea of aone-man VTOL flying platform to the Pen- tagon and Hiller received a development con- tract on 17th September 1953 from the Office of Naval Research (ONR). This body also acted on behalf of the US Army and both were interested in using a small manned VTOL vehicle for special reconnaissance missions and light transport operations. Once funding had been secured, Hiller started constructing the first prototype during January 1954, which was initially designated YHO-1 by the US Navy and classified as a secret programme. Completed in September 1954, but never issued with a serial number, YHO-1 (which was known within Hiller as Model 1031) made its first free test flight in January 1955 and the platform was then des- ignated VZ-1 by the US Army. Given the less formal name Pawnee (a Native American tribe), VZ-1 consisted of a tubular steel and aluminium framework above a 5ft (1.52m) fibreglass shroud that enclosed two co-axial counter-rotating blades that were used for propulsion. Each of these was powered by a Californian-built Nelson H-59 two-stroke 4,000rpm _ engine, which produced 40hp (29.8kW). The pilot stood upright on top of the plat- form within a railed enclosure and was secured by safety belts. Control of the vehicle was achieved with a motorcycle-style twist throttle, a torque adjuster and by using the method of body sway pioneered by Zimmer- man to determine direction. When it was at rest, four landing legs were used to support result in neutral stability. Consequently all the pilot needed to do was lean in the required direction of flight, which would make the platform tilt and move. This led to the construction of a prototype vehicle based on his initial design called the Flying Shoes. It was a very basic proof-of-con- cept machine comprising two vertically posi- tioned 65hp (48.5kW) four-cylinder, two- stroke engines (normally used for target drones) driving 30in (76.2cm) diameter three-blade rotors. Held together in a simple steel tubular framework and equipped with an upright pole for balance, Flying Shoes was tested on several occasions. It hovered about 12in (30.5m) above the ground but proved unstable due to problems of balance between the two engines, which could not be overcome by existing methods. Zimmerman improved the situation to some degree by fit- ting shrouds to the rotors, which created ducted fans, and this had the effect of boost- ing engine performance. While technical difficulties with engine bal- ance continued, during 1948 this research project came to the attention of Stanley Hiller (the head of Hiller Helicopters). He then pur- chased the rights to the Flying Shoes project and Zimmerman moved on to develop alter- native ideas for small flying platforms. Hiller’s own research programme follows shortly. Zimmerman’s next idea was to build a small personal VTOL device called the Jet Board, which had no engine but relied on thrust from gas bottles connected via two fire hoses. The compact steel-framed device measured 19in (48.3cm) x 29in (73.7cm) and had foot restraints for the pilot. Zimmerman made the first test flight on 2nd February 1951 and the platform proved to be outstandingly stable. Jet Board was never intended to be anything more than a technology demonstra- tor and, although Zimmerman experimented with onboard gas cylinders to facilitate very brief untethered flights, he soon began work on the construction of a third VTOL platform. 134 Secret Projects: Flying Saucer Aircraft