Petróczy-Kármán-Žurovec

Petróczy, Kármán and Žurovec were Hungarian and Czech engineers who worked on helicopter development immediately before and during World War I. Between them they produced two experimental prototypes, the PKZ-1 and PKZ-2, intended to replace the dangerous hydrogen-filled observation balloons then in use. As such, these craft were tethered on long cables and were not intended to fly freely. After the war other engineers, notably Oszkár von Asboth, further developed the design.

In 1916, the aviator Oberstlt Stephan von Petróczy proposed an electrically driven rotorcraft to replace the dangerously flammable observation balloon. His original concept was for the electric motor to be supplied by a dynamo driven by an internal combustion engine.

Austro-Daimler were at that time developing a lightweight electric motor for aircraft use, but would take several years to develop one able to handle the electrical power required. A major problem was in providing high-quality insulation for the motor windings, which could get very hot in use.

Meanwhile tests on the large propellers then available showed them to be too inefficient and so a research programme into efficient large propellers, for use as rotors, was begun at Fischamend airfield. Dr. Theodore von Kármán was the director of the research group at Fischamend and Ensign Vilém Žurovec was an engineer there.

Model tests showed that proposed designs with a single tether were unstable. Initially four tethers were used to provide stability, but this was later reduced to three.

By 1917 the technology appeared ready and two rotary-wing aircraft, the PKZ-1 and PKZ-2, were built under separate projects. Both types hovered briefly while tethered, though even with the tethers they were barely controllable and required skilful handling of the tethers. At the time these were referred to as schraubenfesselflieger or SFF (propeller-driven captive aircraft). The PKZ designations were applied later in a postwar article by Karman.

A third design, for a small unmanned version powered by a single Gnome rotary piston engine, was constructed in 1918. It was intended to carry meteorological (weather) instruments or radio antennas aloft, but it is not known whether it ever flew.

The PKZ-1 was designed by Karman and Žurovec, and built by MAG in Budapest under Karman's direction. It had four radiating arms with a 3.9 m four-bladed rotor or propeller on top of each, geared in pairs such that each pair spun in the opposite direction. The rotors were driven from a single Austro-Daimler electric motor located centrally, beneath the observer's cockpit. The 195 kg motor produced 190 hp at 6,000 rpm, limited by the heat resistance of the insulation around the windings (In other respects it was capable of producing 250 hp). to A ground-based generator fed direct current (DC) through the tethering cables to the motor. Landing gear comprised four inflated rubber-fabric cushions, one under the end of each arm.

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