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The first successful rotorcraft (1) - PKZ-1

Author : 🕔19.02.2010 📕35.085
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Helicopters are a common sight in today's skies, but the path to them was much more complicated than in the case of airplanes. It is gratifying to note that the development of the first powered aircraft capable of steady hovering involved a number of men who were born in what would become Czechoslovakia.

Observation of the enemy is one of the basic conditions for an informed estimate of his intentions. From time immemorial, one of the primary objectives of the war effort has been to gain control of heights from which one could see the necessary distance. The development of air navigation enabled soldiers to carry a "hill" with them.

Long before the Wright brothers first took to the skies in an aircraft powered by an internal combustion engine, air navigation existed with flying apparatus using different principles of flight. Their common disadvantage was a certain "awkwardness", numerous operators and dependence on the vagaries of the weather.


The first flying observatories

The first step toward allowing soldiers to look over the nearest hill was the feat of the Montgolfier brothers, who launched the first manned hot air balloon in Paris on 21 November 1783. Ten days later, the first manned gas-filled balloon, designed by Professor Jacques Charles and the Roberts brothers, took off. By the end of the decade, France had its first airborne force capable of deploying in the field and launching a balloon with an observer in the basket.

Another means of seeing farther than a man standing on the ground could was tethered kites. This dusty technology, originating in ancient China, boomed in the second half of the 19th century. This was the time when steam engines were developed or deployed more extensively at sea. Ships powered by a steam engine could tow the kite at a steady speed, which was ideal for its use.

One of the pioneers of tethered kites was Samuel Franklin Cody. Pictured here on horseback with his distinctive beard, he emulated the "image" of his famous namesake William Frederick Cody, known to us as "Buffalo Bill"

The airframe consisted of a set of canvas-covered panels that were much more stowable and, more importantly, much safer on board than the "explosive hydrogen beam". The development of photography and wireless telegraphy provided a further impetus. Tethered kites were used to carry aerials, photographic equipment and observers.

A little-known fact is the widespread deployment of both devices during the American Civil War. The balloons were used in light winds. When it "blew", they were replaced by tethered kites. But the development of the internal combustion engine brought other possibilities. Heavier-than-air aircraft, with both fixed and rotating airframes.

Balon Intrepid US Signal Corps, the "crates" on the left are gas developers. As you can see the balloons required extensive logistics

The aircraft (I am intentionally using a historical term) that will be discussed saw the light of day through the efforts of a trio of men and several other engineers. The designation PKZ is an abbreviation of the surnames Petróczi, Kármán and Žurovec. Each of the three men has contributed to the development, but only the last of them, who is the author of the technical solution, has had his ideas patented.

Major Stephan Petróczy von Petrócz was born in 1874 in the village of Granč-Petrovce near Spišská Nová Ves. In 1902, as a member of the Luftschifferabteilung (Aircraft Troop), he passed the balloon pilot test. He graduated from the Military Technical Academy in Vienna and became a member of the Austro-Hungarian Air Force in 1910. On 25 February 1916, he was given command of the Fliegeroffiziersschule (Flosch - Air Officers School), garrisoned in Wiener Neustadt (Vienna New Town).

The problems with the stability of the balloon in the wind were solved by August von Parseval, by attaching a stabilizer. Because of its shape, German balloons were given the derisive name Parseval's Heifer

Major Petróczy knew the advantages and disadvantages of both aeroplanes and tethered balloons. He was equally familiar with the difficulties faced by the designers of the first helicopters. There were two major problems - stability and controllability. With the knowledge of flight mechanics and engineering technology available at the time, they were virtually unsolvable.

The biggest problems were caused by the difference in lift on the advancing and receding rotor blade. In forward flight, the speed of the advancing blade adds to the forward speed of the helicopter and the lift increases on that side of the rotor. The helicopter tends to roll over on its back about the longitudinal axis. This problem has been solved by mounting the blade on a lift linkage allowing the blade to be swept around the horizontal axis. It is important to note that wind gusts have exactly the same effect while the helicopter is hovering.

American Observation Dragon. The kites were alternated by tethered balloons in stronger winds. They were also used by the Navy because they were more practical for service on ships. They were more compact, did not threaten the ship with hydrogen explosions, and provided a stable platform when towed at a steady speed

The second problem was the steerability of the helicopter. Designing a functional rotor head was a complicated mathematical and mechanical problem. The rotor head was heavy. Add to this a powerplant with little specific power, and it is not surprising that the construction of a successful and fully controllable helicopter stretched into the Second World War.

Realistic Goals - The Foundation of Success

Major Petróczy's idea was a static tethered platform that could carry an observer and his technical equipment to a sufficient height. The tether was to solve stability and control problems. The platform was simply to rise up and hang at the required height. Thanks to this initial consideration, Petroci's machine dispensed with the complex rotor head, the entire control system and auxiliary drives. It must be said that Petróczy's idea was not entirely original, but he did develop a specific practical application. He called the whole device the Schraubenfesselfieger (SFF tethered rotor aircraft).

Design of a possible solution for a helicopter platform Ing. Kármán


In early 1916, Major Petróczy consulted the feasibility of his idea with experienced engineers, including the chief designer of the Austro-Daimler factory, Ferdinand Porsche, a native of Vratislavice nad Nisou. They confirmed to him that it was realistic to build such a machine. Encouraged by the positive opinion, Petróczi submitted his proposal to the Luftfahrtruppe (Aviation Department) on 28 April 1916 and asked for permission to build a prototype.

The platform was to be carried into the air by two counter-rotating rotors, powered by a 300 hp electric motor. A lightweight aluminium power cable was to supply power from a petrol generator on the ground. Leaving the generator on the ground solved the issue of the weight of the propulsion system, and the operating time was not limited by the fuel supply on board. The platform was to be anchored on three cables to stabilize it, providing for changes in height and for the platform to be lowered to the ground.

One of the conditions for success was sufficiently efficient large diameter propellers, which have been the subject of much research

The advantages of the platform over the balloon were obvious. The platform in the air presented a small target, both to enemy observers and to fighter aircraft. It eliminated ground-based hydrogen evolution equipment, compressors, and the tedious filling of the balloon body. The danger of explosion was much less due to the absence of a hydrogen body with a volume of hundreds of cubic meters. The reduction in size allowed a reduction in the number of ground personnel required for transport and launch. Also, the transition from the march to the standby position and the actual launch of the platform could be much faster. Petróczy had also expected less sensitivity to the vagaries of the weather, but in this case it remained a wish.

The War Ministry approved the project on 5 May of the same year and commissioned the construction to Österrichische Flugzeugfarbrik AG (ÖFFAG). A special lightweight electric motor was to be built by Austro-Daimler. The engine was developed by engineers Zádník and Köhler. Thanks to the extensive use of aluminium and high-grade materials, the weight was kept to 250 kg, an admirable result. However, the engine did not deliver the expected performance and the windings overheated.

Patent Ing. Žurovec filed in the USA - corresponds to the solution used on PKZ-1

Fliegerarsenal (Flars - Air Force Arsenal) issued a rather demanding specification, but the funding released fell far short of the required technical requirements. Nevertheless, Director Ockermüller showed his willingness to co-finance the development of the SFF from ÖFFAG funds. The SFF was to have an unladen weight of 960 kg and a payload of 240 kg, which would be sufficient for one observer, a machine gun with ammunition, a camera and a telephone. It was expected to have a range of 500-1000 m and operate up to a wind speed of 8 m/s.

The design was entrusted to Ing. Karel Balabán. Taking into account the demanding requirements and minimal experience with rotorcraft, he proposed to first test the rotor system, proceed with the construction of models and only then embark on the construction of a full-scale machine. After a series of experiments with rotating wings, he decided on a pair of large twin-bladed counter-rotating propellers. This was to eliminate gyroscopic moment and reduce sensitivity to wind gusts.

First difficulties

The propellers that were available in Austria and Germany did not even come close to the expected diameter and efficiency. The best propellers, manufactured by Öserreichisch-Ungarische Propellerwerke, had an efficiency coefficient of 0.61. Ing. Balabán needed an efficiency higher than 90%. He became so interested in the problem of propellers that he went to the propeller testing laboratory in Fishamend, where he and Oszkar Asboth solved theoretical and practical problems connected with the construction of large-diameter propellers. With Balabán's departure, work on the SFF was abandoned by the ÖFFAG factory, which continued to be intensively involved in the production of aircraft for the front.

The largest of the models on which the PKZ-1 concept was tested. The compressed air hoses can be seen on the sides

Asboth and Balaban tested a total of nearly 1500 propellers, and the entire development stretched to the end of 1917. Finally, using the finest mahogany wood, they were able to build a propeller up to 8 m in diameter. However, for such a large propeller, the tips of the blades reached a circumferential speed close to the speed of sound (0.74 M). For the problems involved, the knowledge of aerodynamics at that time was not sufficient. Ing. Balabán therefore proposed to use propellers with a diameter of 6 meters, which at 600 rpm and an efficiency coefficient better than 0.9 should give a thrust of 11.77 kN.

After the departure of Ing. Balabán, the management of the SFF project was entrusted to Dr. Ing. Theodor von Kármán, who mainly dealt with the theoretical part of the project. The practical design matters were the responsibility of Ing. Vilém Žurovec, a native of the now defunct village of Harty u Petřvald. As there were no propellers of the required diameter available yet and generally higher efficiency coefficients were achieved with smaller diameter propellers, both designers considered using 10-12 smaller rotors arranged in a circle.

They built several small models, powered by rubber bundles, but the conclusions from the tests were inconclusive. The result was only to limit the number of rotors to 4 in a row, as a higher number resulted in a large weight deficit of complicated propulsion. To verify the theoretical assumptions, they made a larger model with a mass of 35 kg.

Another Zurovec solution protected by a US patent. This design was not implemented

To power the model, Ing. Žurovec engine powered by compressed air. At a pressure of 4.9 MPa, it gave 6 hp at 2400 rpm and an empty weight of 4 kg. The compressed air was supplied through a hose from an external source. Around 50 flights were made in the airship hangar at Fischamend between June and July 1917.

The model verified Petróczy's assumption that the SFF could be stabilized by clamping. Using three or four tether ropes and sufficient tension (excess thrust of the propulsion unit), stability was considered satisfactory. Modifications to the model and verification of the tethering system required another 50 flights made by the spring of 1918.

Order from the Arsenal

In the summer of 1917, Petróczy was appointed commander of the Flars, allowing him to order the construction of a full-size SFF on behalf of the Arsenal on 21 August 1917. The Magyar Általános Gépgyár (MÁG) engineering plant in Budapest was commissioned to build the aircraft. At that time, the unofficial designation of the platform PKZ-1 was experienced.

PKZ-1 consisted of a truss structure, welded from steel tubes, forming a triangular cross-section beam. An electric motor was placed in the middle of the beam and an observer's nacelle was built above it. The electric motor, with an estimated output of 250 hp, was supplied by Austro-Daimler.


The PKZ-1 tethered platform with four propellers. The horizontal drive shafts are clearly visible.

There were difficulties with the development of the motor because the aluminium windings, used to save weight, did not have the conductivity of copper and were constantly overheating. Among other reasons, the motor only produced 190 horsepower. Supplies of other components were also delayed, so that the PKZ-1 was not completed until late March or early April 1918, although the original plan was to complete assembly in October 1917.

The engine, via angular gearboxes, drove four 3.9 m diameter propellers via long shafts. The diameter of the propellers was later increased to 4.2 m. The whole structure had a mass of 650 kg and rested on four inflatable bags, used to absorb the shock during landing. The tether ropes were attached to three arms at an angle of 120° to each other.

PKZ-1 platform. In the middle you can see the gondola for observers

Tests of the PKZ-1 were conducted at the Fischamend airship base, initially on short tether lines of only half a metre. The PKZ-1 lifted off the ground when the propellers were at 700 rpm and showed a reserve of thrust. On the second attempt, there were three men in the gondola and the PKZ-1 climbed to the half-metre height allowed by the tether ropes without any problems. Unfortunately, on the fourth attempt, the motor windings burned out and were not repaired by the end of the war. That was the end of the PKZ-1 experiments.

Grosz Peter: PKZ-2 WWI Austro-Hungarian Helicopter, Windsock minidatafile 02, ISBN 0948414731
Periodicals:L+K, Air Enthusiast


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Author : 🕔19.02.2010 📕35.085