The Petroczy-Karman-Zurovec PKZ 2 helicopter, despite its name, was invented by Wilhelm Zurovec for which he alone received German patent No. 347,578, dated 12 February 1918. Unlike the PKZ 1, which was government funded, the PKZ 2 was privately financed by the Hungarian Bank and the firm of Dr. Liptak & Co AG, a large iron foundry and steel fabricator located near Budapest which established an experimental section under Zurovec's direction in late 1917. The PKZ 2 has been amply described in post-war and contemporary literature, yet in virtually all accounts the design is incorrectly attributed to Karman who, in 1919 by claiming sole authorship and totally neglecting to mention the true inventor, Wilhelm Zurovec, deviously reaped the lion's share of the honors (documented by Zurovec correspondence in the Karman Archive). It is a great pity that Zurovec has never received the full measure of recognition that he surely deserves as one of the outstanding pioneers of helicopter development. In the design of the PKZ 2 helicopter, Zurovec incorporated the concept of using counter-rotating rotors to cancel torque effects. Each rotor, with a diameter of 6 meters, represented an ingenious combination of old-world craftsmanship with modern applied engineering. Power was supplied by three 100hp Gnome rotary engines, that drove the rotors at 600 rpm through a common gearbox. The light, tubular airframe, easily disassembled for transport, rested on a patented central air-bag cushion (1 meter diameter) and three smaller cushions mounted on the outriggers. The cushions were kept inflated by an air pump attached to the rotor drive. Three tethering cables affixed to the outriggers ran through pulleys anchored in the ground and were controlled by separate, electric winches. The total weight of the PKZ 2 with fuel for one hour, but without observer and machine gun, was approximately 1200kg. Two forms of parachute rescue were planned. One was to carry the observer and aircraft safely to earth, and the other was a cannon-launched parachute that would clear the rotors and lift the observer to safety. The PKZ 2 was test flown for the first time at the Liptak factory on 2 April 1918. After several flights, one lasting up to one hour, tests were suspended on 5 April because engine power was insufficient to permit safe hovering above 1.2m height. The Gnome engines were replaced by three 120hp Le Rhones. Flight testing continued from 17 to 21 May, during which flights between 10 and 50 meters altitude were recorded. During periods of calm weather and smooth engine operation, the excess lift measured at ground level was 150 to 200kg. As the PKZ 2 climbed, the loss of ground effect and the increase in tethering cable weight steadily reduced the excess lift. Yet, as long as excess lift sufficed to maintain proper cable tension, the PKZ 2 remained in stable hovering flight. At higher altitudes, the excess lift became marginal, causing the machine to slowly oscillate with increasing amplitude. Provided the tethering cables were retracted at about 1.5m/s, the oscillations to cease in about 15 to 20 seconds. On 10 June 1918, Zurovec was called upon to demonstrate the PKZ 2 for high ranking military authorities. Although the rotary engines had recently been overhauled, their operation the day before was erratic, and Zurovec, fearing the worst, was reluctant to proceed. But what could a mere Leutnant do against the weight of official brass eager to see the wonder machine fly? Taking advantage of the zero wind conditions, the PKZ 2 with the observer's basket in place took-off at 5:40 and twice rose to 7-8 meters height, showing "considerable rocking motion." The basket was removed and the PKZ 2 took off again at 6:07 in a wind of 6-7 meters per second, climbing to a height of 12 meters. Due to overheating, the engine power dropped off and the helicopter began to pitch with increasing frequency until the tether-winch crew could no longer control the machine. The PKZ 2 crashed from two meters height, severely damaging the airframe and splintering the rotors. In a careful review of progress to date, Uzelac, realizing that the technical problems were too complex to resolve quickly, cancelled the project on 21 June 1918. Zurovec, Karman, and Liptak were directed to compile a record that would be a basis for further investigation when peace returned. Zurovec, refusing to accept defeat, devised a method to water-cool the rotary engines! He reported on 1 September that the PKZ 2 helicopter would be ready for further testing or 1 November 1918. By then it was too late. Peter Grosz "Austro-Hungarian Army Aircraft of World War I", 2002 Powered by three 90kW airplane engines, this captive observation helicopter was the creation of Stefan Petroczy, a lieutenant in the Austrian Army during World War I. At first the machine failed to perform, but young professor Theodore von Karman — who later emigrated and became a leading American aerodynamicist — joined the effort, and eventually the aircraft flew to a height of over 45m. Supported by two massive wooden propellers turning in opposite directions, the Petroczy-Von Karman marvel was intended to lift a pilot, observer, and fuel for an hour's flight. During flight the machine was anchored to the ground by outrigger cables. There is no record that it was developed beyond the experimental stage. C.Gablehouse "Helicopters and Autogiros", 1969 The PKZ 1 was followed by the 1400kg PKZ 2, primarily developed by Zurovec. The triangular structure used three 100hp Gnome rotary engines powering two counter-rotating propellers. The engines were soon replaced with 120hp Le Rhone engines and flight tests resumed that May. The vehicle was flown over 30 times, eventually with a second observer, and reportedly achieved a tethered height of 50m and an endurance of 30 minutes. A crash during a military demonstration ended the project and the conclusion of the war terminated further study. The PKZ 1 and PKZ 2 demonstrated that the thrust to weight deficiency could be overcome and that useful vertical flight could be attained. Since they were tethered, however, they did not in any way attempt to address the controllability challenges. Comments XSSSafety , e-mail , 15.08.2016 reply