At the end of the Second World War the helicopter slowly began to prove its capabilities both in the military and the civilian field.
Before the standard configuration with the main and the anti-torque rotor was almost universally adopted by the helicopter industry (nowadays the major part of the helicopters has this configuration), several inventors around the world had the idea to install pulsejets or ramjets on the tip of the blades to spin the rotors of their prototypes. This solution in fact appeared simpler and less expensive.
The long list of the companies that tested this solution also comprises the Swiss company Contraves which built a single-seat prototype with an unorthodox control system during the 1950s. This helicopter for a long time remained wrapped in a curtain of mystery.
The operating principle of all jet engines is essentially the same: the increase of the pressure inside the combustion chamber forces the exhaust gases in one direction. This reaction provides a thrust in the opposite direction in accordance with Newton's third law of motion which reads: "For every action force there is an equal, but opposite, reaction force".
Jet engines like ramjets or pulsejets are very simple and of lightweight designs, consisting of a long tube in which compressed air passes dynamically without the need of a compressor. There are no rotating parts, a considerable advantage over conventional piston or turboshaft engines.
In pulsejet the first ignition is made by injecting compressed air (by means of a compressor or an air cylinder) into the air intake. Then the air-fuel mixture is ignited by a spark plug. Special valves prevent backflow through the air intake therefore the exhaust gases exit only to the rear through the exhaust tube (see diagram).
The inertia of the traveling exhaust gas causes a low pressure in the combustion chamber. This pressure is less than the inlet pressure (upstream of the one-way valve), and so the induction phase of the cycle begins with very short intervals (also known as pulsations).
Contrary to ramjet, pulsejets are capable to run (and therefore to produce a thrust) statically. However due to the low compression ratio they produce a limited specific thrust. On the other hand a ramjet has an exceptional thrust-to-weight ratio, but to run efficiently, very high speeds must be reached.
The pulsejet was developed from the 1930s on and found one of its first uses on the sadly famous flying bombs V-1 launched by the Germans against British targets during World War II which were powered by the Argus AS 014 pulsoejet.
The use of jet engines in the rotary-wing field
It was mainly between 1945 and the beginning of the 1960s that several pioneers around the world experimented with the use of pulsejets and ramjets on helicopters.
Unlike a conventional helicopter, these models from a mechanical point of view were simpler. Since there were no moving parts they were simpler to build and easier to maintain. Tip jets have the advantage of placing no torque on the fuselage. There is no need for a heavy transmission nor a tail rotor which means greater safety both on the ground and in the air. Moreover the helicopter is lighter, less complex and its purchase cost more economical.
Despite these apparently remarkable advantages manufacturers after a testing phase lasting about 15 years abandoned their experiments with jet rotors.
Common problems were high fuel consumption, reduced range, and the excessive noise emissions.
The fuels used were usually gasoline or kerosene. Some models such as for example the Kellett KH-15 Stable Mabel (1954) or the Rotor-Craft RH-1 "Pinwheel" (1954), were powered by hydrogen peroxide.
In this type of engine the thrust is obtained from the energy developed by a violent chemical reaction that raises the propellant's energy content. The thermal energy produced exits in the form of water vapor by through nozzles mounted on the tip of the blades.
In the case of hydrogen peroxide, the main problem was not the noise but the hazard. If at low concentrations the hydrogen peroxide can be used as a disinfectant, in high concentrations the inhalation of toxic vapors irritates the respiratory tract. However Moreover in contact with the skin, it causes deep burns and if ingested it can be lethal.
Hot or cold jet rotors?
There is another technical differentiation in the classification of jet rotors. When the rotor is driven by a ramjet or pulsejet it is referred to as a hot tip-jet rotor.
On the other hand when the rotor is driven by the air produced by a compressor driven by a piston engine or a gas turbine it is referred to as a cold tip-jet rotor. In this case, the air coming out of the nozzles on the tip of the blades is also hot, but certainly not comparable to that of the exhaust gases.
Among the first pioneers who experienced this configuration was Austrian engineer Friedrich von Doblhoff (1916-2000) who along with his team built in Vienna at the Wiener Neustädter Flugzeugwerke four prototypes. The WNF 342V1 made its first test flight during spring 1943. Three more prototypes followed, the last of which (WNF 342V4) was sent to the United States at the end of WW2 to be tested by the US armed forces.
In the USA Doblhoff resumed his experiments and became chief engineer of the helicopter program on behalf of Mc Donnell Aircraft Corporation.
One of the few helicopters manufactured in series where the rotor was driven by compressed-air jets was the French SO 1221 Djinn (1955) powered by a Turbomeca Palouste 4 compressed air generator.
Among the experimental helicopters with a “cold jet rotor” there were the Hughes XH-17 (1952), Kaman K-17 (1958), Monte-Copter Model 15 Triphibian (1960) and Dornier DO-32 (1962).
The list of the “hot jet rotors” comprises the prototypes Mc Donnell YH-20 "Little Henry" (1947), Hiller YH-32 Hornet (1950), American Helicopter XA-6 Buck Private (1951), the Dutch NHI Kolibrie (also produced in a limited series between 1958-1959) or the Hagiwara JHX-4 (1958).
A Swiss prototype
In Switzerland the idea of using pulsejets to spin the rotor of a helicopter captured the interest of Contraves.
This company founded in Erlenbach (ZH) in 1936 was taken over in the 1940s and 1950s by the industrialist Emil Georg Bührle, who later also became owner of the Werkzeugmaschinenfabrik Oerlikon (later Oerlikon-Bührle Holding AG).
Starting in 1955, this company began the development of a single-seater prototype. The few information today available are probably those collected by the rotary-wing historian Peter Wernli (1943-2016) who spent a long time searching for historical and technical information on this aircraft still partially wrapped in an aurea of mystery.
In 1998, Wernli contacted engineer Rudolf Heller who directed the development program and who kindly accepted to fill in a special questionnaire which was used to prepare this article.
It is likely that this document is the only one left, handed by a person directly involved in the development of this prototype conceived for military uses (reconnaissance and liasion) by engineer Slenzel (who later moved to the United States).
Almost all the components were manufactured by Heller's technical team.
Like other models of that period (e.g. Hiller XROE-1 Rotorcycle), the aircraft was conceived to be quickly folded and transported with a jeep.
The intention of the designer was that no flight experience was required to fly this model, but Heller himself was skeptical even if, as we will see, the control system was very simple and unique.
The helicopter (which apparently never received an official designation) had a 6 meter twin-blade rotor driven by two pulsejets developed by Contraves. The engines had a diameter of about 80 mm and produced a thrust of about 15 kg each. The rotor turned with a speed of about 600 rpm.
The pulsejets were mounted on a bar with a wing profile disposed perpendicularly to the blades at a distance of about 1.50 m from the rotor hub.
The fuel was pumped to the pulsejets by means of a double piston pump and was contained in a 30 liters fuel tank which served as support for the pilot’s seat.
By using two levers the pilot controlled the fuel flow and consequently the power. The consumption was estimated at about 60 liters/hour.
The metal blades had a chord of about 12 cm and were equipped with hydraulic shock absorbers. They were connected to the rotor hub by means of joints.
The rotor hub was mounted on the top of a 7-shaped metal structure. Inside there was a compressed air tank used to start the pulsejets. On this pylon that supported the rotor, the were also mounted some essential controls and instruments: one that measured the rotor speed, a pressure gauge indicating air pressure in the tank, a double fuel flow indicator, an indicator fuel reserve, a valve that closed the fuel supply, and an electric switch.
The control system
The helicopter had an unconventional control system based on the simple displacement of the center of gravity.
Among the pioneers who came up with this idea was aviation engineer Charles Horton Zimmermann (1908-1996) who in the 1930s came to the conclusion that the normal balancing of a man was sufficient to control a small aircraft.
His theories were finally confirmed when the Hiller VZ-1 Pawnee flying platform was built in the early 1950s. This concept christened by Zimmermann "kinesthetic control" was then taken over by several inventors such as Eugene Michael Gluhareff who built the MEG-3 flying platform (1959) or William International for the William X-Jet (1969). Just a quick search in the Internet will confirm you that this idea continues to fascinate modern inventors all over the world.
On the Contraves prototype, the pilot's seat was mounted above the tank and could slide back and forth. By moving his weight (and therefore the center of gravity), the pilot, who was secured with a safety harness fixed to a tube in front of his chest, could decide in which direction to move. By moving his body left or right he obtained the lateral control.
By sliding the handle up and down with his right arm the pilot controlled the blade incidence (collective pitch lever). By twisting the handle the pilot controlled the mobile stabilizer made of aluminum sheet. The support tube of the stabilizer was fastened to the support of the rotor hub.
Like the first version of the American helicopter Hoppi-Copter Inc. (1945), it was originally intended that the helicopter should be "worn" by the pilot by means of a harness. This really unusual solution was discarded in favor of a conventional landing gear on skis with a track of about 100 cm.
The empty weight of the aircraft was about 30 kg, while the maximum take-off weight was estimated at 140 kg.
According to the memories of Rudolf Heller, flight testing (or at least part of them) were carried out in the countryside near Delémont. The test-pilot Janotti (unfortunately his name is not known), in the meantime passed away, was charged with the test flights during which the helicopter for security reasons was restrained with safety cables. It seems that during one of these flights the aircraft was damaged.
The idea of modifying the prototype and power it with a conventional gasoline engine was discarded and the project definitively abandoned.
Among the people who worked on this prototype were engineer Heinz Hoz who made the calculations on behalf of the Swiss military Air Forces - Military technical division.
In a letter dated August 11, 1971 sent to the Hubschrauber Museum in Bückeburg (Germany), Contraves wrote that the prototype was dismantled at the beginning of that same year while the technical documentation was probably destroyed or lost in the following years.
When Peter Wernli asked engineer Heller why Contraves abandoned the further development of this prototype, the latter wrote that the noise of the pulsejets was really deafening and their thrust insufficient.
It is a shame that this helicopter is nowadays not displayed in a museum to testify the inventiveness of the (few) Swiss rotary-wing pioneers.
I hope that this article along with the few black and white pictures found will dust off the old memories on this prototype.
Anyone possessing additional information is kindly invited to contact HAB in order to complete the “puzzle” of information about this helicopter “made in Switzerland”.
Did you know that...
The Latin name contra aves means literally "against birds". The factory Contraves started its activities in the mid 1930s to develop anti-aircraft weapons.