April 2016

2016 April photoPetróczy-Kármán-┼╜urovec PKZ 2
Austro-Hungarian Army Unmanned/Tethered Observation Helicopter
First flight: April 2, 1918

The Petroczy–Kármán-Zurovec PKZ 2 helicopter is the better-known of the first two attempts to create a tethered observation helicopter, and was largely (though generally not acknowledged) the creation of Zurovec, who singularly received German patent 347,578 on Feb. 12, 1918. The aircraft, unlike the PKZ 1, which had been government-funded, was a private venture of the Hungarian Bank and the firm of Dr. Liptak & Co. AG. Most helicopter histories that deal with the PKZ 2 helicopter attribute it to Kármán, largely due to his later fame as an aeronautical designer in America and the fact that Kármán claimed sole authorship of the achievement in 1919!

The PKZ 2 helicopter was a co-axial design featuring two large, wooden, counter-rotating propellers that were 6 m (20 ft) in diameter, powered by three Gnome rotary engines that spun the rotors to 600 rpm through a common gear-box. The frame of the aircraft was constructed of light metal tubing and rested on a central airbag and three smaller cushions extending downward from tubular out-riggers. The cushions were kept inflated by means of an air pump attached to the rotor drive. Its most unusual feature was the placement of the pilot and observer in a cylindrical open cab above the propellers. It should be noted, however, that there is no evidence that the PKZ 2 ever flew with an observer on board, and while the initial model could hover for an hour, it could not hover safely above 1.2 m (3.9 ft) due to inadequate power.

The Gnome engines were subsequently replaced by three 120 hp Le Rhône engines and flight testing resumed on May 17, 1918, during which altitudes between 10 m and 50 m (32 ft and 160 ft) were recorded as the unmanned aircraft remained in stable hovering flight. It was observed that at higher altitudes the aircraft tended to oscillate — but the aircraft crashed on June 18, 1918, while making exhibition flights, against the designer’s wishes, before assembled senior officers. Due to engine over-heating, the craft began pitching back and forth such that the ground crew holding the tethers lost control. The end result was that the aircraft dropped approximately 2 m (6 ft), seriously damaging the airframe and rotors, and that ended the experiment.

As later observed by BG H. Franklin Gregory, the PKZ 2, “possessed no means of horizontal motivation and could not move about in the air at the pilot’s will. It was a powered kite. And such it deserved little recognition, but . . . this strange craft is to be remembered for two facets of its brief and relatively inconsequential existence — it was able to hover in a stable mode for extended periods of time at lower heights and achieved an altitude of 50 m (32 ft) and it was the first rotary-wing aircraft to be designed to meet a purpose — and that purpose occasioned serious discussion about the potential military uses of rotary-wing aircraft.”

(Text by Bruce Charnov)

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