GO WITH THE AIR-FLOW, THE AERODYNAMICS OF A ZEPPELIN…

Today I indulge in the history of the aerodynamics of cars and windtunnel testing. You may wonder what the Zeppelin has to do with this? I will explain…

The Hungarian engineer Paul Jaray was the man who presented numerous designs for streamlined car bodies. Initially he designed seaplanes, but later he focused on streamlining airships. The LZ-120 Bodensee was one of his airship designs.

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The detailed series of experiments in the LZ’s windtunnel were also a start of something new, the use of aerodynamics in car design. Jaray studied the effects of wind direction and airflow over an airship at ground level. This way he also developed his theories about streamlining motorcars. With his streamlining theories Jaray founded in 1927 the Stromlinien Karosserie Gesellschaft. But of the many car manufacturers to whom he presented his designs for streamlined bodies Tatra was the only one which adopted Jaray streamlining into their car production.

Jaray also designed his own car bodies, starting with a 1923 Ley. His own 1933 car was built on a Mercedes-Benz chassis with a body by Huber and Bruehwiler of Luzern in Switzerland. Later he also did designs for Chrysler, Mercedes-Benz, Maybach, Apollo, Dixi, Audi, Adler, Jawa, Ford and Steyr. Jaray was author of a large number of technical patents relating to streamlining, air compressors for railway, and devices for handling gases in silencers.

Aerodynamics was a new science at the time of the Interbellum. The old Austro-Hungarian Empire led in this field with specialized engineers. What have the following names in common – Edmund Rumpler, Ferdinand Porsche (mentioned in my previous Mercedes 500K article), Hans Ledwinka and of course Paul Jaray? What they have in common is that their work had a great impact on the shape of the automobile!

To make it possible to study the airflow around an object the righttp://crankhandleblog.comht gear was needed, like a simulator that could act like a bird. Benjamin Robins introduced the whirling arm. George Cayley, known as the father of aerodynamics, did extensive research with his self-built whirling arm. This led him to identify, define and separate the aerodynamic forces of weight, lift, drag and thrust. But the whirling arm had snags. Much changed with the invention of the windtunnel!

In 1871 the world’s first windtunnel was funded by the Aeronautical Society of Great Britain, at that time known as wind channel. This was a 12 feet long and 18 inches square tunnel, generating a 40 mph wind by a steam-driven fan. After the American Wright brothers had noticed the advancements in Britain, they decided to build their own wind channel and they owned their place in history with an even better device. They experimented with at least 200 different wings in their wind channel. Meanwhile, in Britain at the Farnborough air field, several more windtunnels were built, including the majestic 24-foot version. This one was built for aircraft research, but in later years also racing Jaguars, amongst others, were tested here. Prior to WW2 virtually all windtunnel research was done using scale models, as there were few tunnels big enough for a full-size vehicle.

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One of the earliest and possibly the very first windtunnel tested car was Edmund Rumpler’s Tropfen-Auto (Teardrop car). After having worked for the Nesselsdorfer Wagon Works (later Tatra) and the Adler-Werke, Rumpler set up his own design consultancy in Berlin. The Tropfen-Auto was Rumpler’s definition of the ideal aerodynamic shape, a falling drop of liquid…

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But what Rumpler didn’t realize was that the shape of a falling drop is not the same as immediately before falling, when it has a pointed tail. Once falling the surface tension pulls the tail in and so the drop becomes more flat at the back and more elongated. The radical Tropfen-Auto didn’t reflect this effect. Beside, its design was so radical that it wasn’t very suitable for the general public and most of them ended up as Berliner cabs.

Although the Tropfen-Auto wasn’t a success in itself, there was an important legacy to this weird design, because the Mercedes-Benz Tropfenwagen (Grand Prix car of 1923) was built on a chassis of a Rumpler.

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Time now to give Jarays ideas some more attention, because he was the founder of the ‘ideal’ shape for a truly streamlined car. This he achieved by cutting a Zeppelin horizontally through its cetre line, to produce a horizontal aerofoil. His first complete car bodies were fitted to chassis from Ley, Audi and Dixi. But like the doomed Tropfen-Auto the public wasn’t ready for such a revolutionary car. Consumer resistahttp://crankhandleblog.comnce seemed a lot more difficult to
deal with than wind resistance! Next to that the car looked quite cartoonish with its high chassis and high roof.

Jaray spent much of the 1930s trying to sell his ideas and chasing companies for infringing his patents. A host of manufacturers built one-offs of his designs but without commercial succes. The companies Adler and Tatra did commit to his work, but it was not easy for them to work with the uncompromising Jaray. The body had to be built exactly his way or not at all. This resulted in a series of look-alikes, which lost much of the brand identity of their manufacturers.

Later on Paul Jaray was invited to design the body for the radical rear-engined tour de force that Hans Ledwinka from Tatra had in mind. The T77 of 1934 and the improved T87 of 1936 were the first scientifically streamlined cars that made it into series production.

The Chrysler Airflow, launched that same year, was also quite radical compared with contemporary American cars, but it was far less rigorous in its application of aerodynamic principles than the Tatra.

The aerodynamic efficiency of an object is expressed by a factor, the Cd value.

http://crankhandleblog.comIn the past there have been some extravagant claims made for the aerodynamic efficiency of cars and of the Tatra in particular. Figures of 0.212 for the Tatra T77 and 0.24 for the T87 are often quoted, making them, even by today’s standards, phenomenally slippery shapes. In fact these figures are far too good to be true. These ‘low’ Cd figures were derived from one-to-five scale models, which were known to produce far better results than full-size cars with their turbulence-inducing open seams, window frames, bumpers and wheel wells.

No full-size Tatras were tested until 1979 when Volkswagen decided to put some classic shapes through their tunnel, out of curiosity. The results astonished their engineers. The 1923 Rumpler Tropfen-Auto achieved an astonishing low Cd of 0.28, significantly lower than the T87, which yielded a Cd of 0.36. The T87 would have had a lower figure if Ledwinka had adopted the curved windscreen that Jaray had specified and that featured on the original windtunnel model. The majority of cars in the 1930s would have a Cd well above 0.5. At its launch, the Chysler Airflow (the DeSoto version), had a Cd of 0.546, which barely supports its name.

http://crankhandleblog.comFurther experiments which involved deleting appendages, adding an undertray and extending the tail and nose, lowered the high Cd of the Chrysler to 0.316 and adding a curved false screen could further reduce the Cd to a creditable 0.238. But by this time the shape of the car resembled that of a stretched but ugly Tatra. However meanwhile the Chrysler Airflow had already succumbed to US consumer apathy and was no more.

Jaray, eventually tired of chasing patent pirates, turned his mind to other areas of research, in radio, television and aviation.

Written by Rosemarijn Atalante Veenenbos

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