Racing drivers, engineers, journalists – in the mid-1930s, streamlining and the potential of aerodynamics in modern automobile construction was on everyone’s lips. Having successfully applied this know-how in racing cars and record-attempt vehicles, Mercedes-Benz gradually transferred it to its series-production models, the 540 K streamlined saloon of 1938 marking one of the milestones in this aerodynamic evolution.
Cutting through air.
As far as aerodynamics is concerned, the 540 K streamlined saloon can rightly be regarded as an exceptional vehicle. Never before had a 540 K been developed so systematically according to the findings of aerodynamics. It was not just the raindrop shape of the body that reduced the air resistance. An equally great effect was attributable to the numerous design details implemented by Hermann Ahrens and his team. These included integrated front headlamps, recessed door handles, elongated, low rear end and almost complete underbody panelling. As on the Silver Arrows, even the traditional Mercedes star was painted on in the interests of aerodynamics.
The totality of these measures produced a sensational result: the drag coefficient (Cd value) of the streamlined saloon was just 0.36, over 35 percent lower than the value for a standard-production 540 K Coupé of the time. The result was all the more impressive for the fact that this was not a record-attempt or experimental vehicle, but a luxury car built for motorway cruising – the perfect synthesis of style and aerodynamics.
The wind tunnel in Untertürkheim – an installation with history.
Building on experiences gleaned from aeronautical engineering, aerodynamics played a growing role at the beginning of the 1920s in the worlds of technology and vehicle manufacturing: in the ensuing two decades, countless aerodynamically optimised prototypes and race cars attracted a great deal of public attention. Edmund Rumpler’s “Tropfenwagen” (“teardrop car”), which debuted in September 1921 at the first Motor Show to be held in Berlin since the First World War, was a real sensation. It was the catalyst for the development of other aerodynamically optimised vehicles. These were built by such renowned researchers and engineers as August Everling, Wunibald Kamm, Reinhard Freiherr von Koenig-Fachsenfeld, Paul Jaray, and Karl Schlör.
The emergence of the wind tunnel at the Mercedes-Benz plant in Untertürkheim is closely linked with this trend. It goes back to Wunibald Kamm’s activity at the Technische Hochschule (TH) Stuttgart (technical university).
The graduated engineer Dr.-Ing. Wunibald Kamm worked in vehicle development at Daimler-Motoren-Gesellschaft (DMG) from 1922 to 1925. Following stints at the Schwäbische Hüttenwerke (SHW) in Aalen and the Deutsche Versuchsanstalt für Luftfahrt (DVL) in Berlin-Adlershof, he became Professor and Chair of the Technische Hochschule Stuttgart and on 15 July 1930 founded the Forschungsinstitut für Kraftfahrwesen und Flugmotoren an der Technischen Hochschule Stuttgart (FKFS) as a non-profit foundation at the technical university, in which the state of Württemberg with the Württemberg Ministry of Culture and Ministry of Economics, the City of Stuttgart, the Verband Württembergischer Metallindustrieller, the Reichsverband der Automobilindustrie, and subsequently the Reichsverkehrs- und das Reichsluftfahrtministerium (RLM) were involved.
In the period that followed, the names and statutes underwent some changes: FKFS came to stand for Forschungsinstitut für Kraftfahrwesen und Fahrzeugmotoren Stuttgart, with the word “Flugzeugmotoren” (aeroplane engines) being replaced by “Fahrzeugmotoren” (vehicle engines). Today the institute is an independent foundation under public law in Stuttgart and is partnered with the University of Stuttgart through contracts; it receives no public funding.
It all began in 1934 with a small wind tunnel
When it was founded in 1930, the institute was located in front of the Untertürkheim plant owned by the then Daimler-Benz AG when approached from the direction of Bad Cannstatt. The plant and the institute were separated at the time by a railway embankment which no longer exists today. In the tax year 1934, a small wind tunnel with a jet diameter of 70 centimetres was installed to conduct tests on vehicle models. In addition, starting in 1935, aerodynamic resistance tests began to be carried out in the small wind tunnel and with coast-down tests using normal-sized vehicles. In the year that followed, a water-flow tunnel was built for aerodynamic resistance measurements on vehicle models.
A wind tunnel for full-scale vehicles came into existence between 1939 and 1943. Kamm recognised that with model wind tunnels, the full complexity of vehicle aerodynamics could not be measured and that research facilities for full-size vehicles was indispensable. The wind tunnel created by Kamm was a system based on the “Göttingen design” with horizontal air ducting, one closed measurement section and one measurement section three-quarters open, and 125 metres long. The air jet cross-section was 32.6 square metres and the measurement section was 10 metres long.
Although the institute was destroyed in 1944 and 1945 by air raids on the Untertürkheim plant, the wind tunnel survived, though heavily damaged. Immediately following the liberation by the French, the institute’s new director, Prof. Paul Riekert, saved the wind tunnel, which was actually slated to be dismantled, by filling it so full of vehicles and instruments that he was able to present it to the Occupation Authority as a warehouse for the institute.
By 1950, the test site at the technical university was a focus of deliberations for Dr. Wilhelm Haspel, Managing Director of the then Daimler-Benz AG. During a meeting of the Board of Management on 10 May 1950, he voiced his opinion that if any property purchases were made, the test site at the technical university should be considered first and foremost, since it already housed test beds and the wind tunnel.
Property and wind tunnel under Daimler-Benz ownership
In 1954, the company acquired from the City of Stuttgart lots on the other side of the railway embankment, on which Mercedes-Benz commercial-vehicle testing is located today, and also where the FKFS wind tunnel is found. At the same time, the FKFS was now again able to take over operations of the wind tunnel after the Second World War. From 1960 onwards, vehicle development at Mercedes-Benz began using more and more of the wind tunnel’s capacity, increasing to 40 per cent of capacity in 1970.
The wind tunnel was then acquired by Daimler-Benz AG in 1970. The FKFS retained administrative authority until 1974. The company then began a renovation project costing 8 million Deutsche mark and the modernisation of the complete installation, which took nearly two years to complete. In April 1976, the Engineering Department at Mercedes-Benz began operating the installation, now with four times more capacity.
As part of the capacity increase, a wind tunnel weighbridge, a multi-point pressure system, and a roller-type performance test rig were installed. Built into a rotating base plate, the wind tunnel weighbridge replaced the suspended balance used previously, with which the vehicle was suspended by four steel cables in an H-shaped frame, and the four steel cables were suspended by the lifting scales in the turntable. With the underfloor weighbridge now installed, the vehicle was placed on four contact plates which were fitted level in the floor of the rotating measuring plate. The contact plates were located on four individual lift scales which were used to measure the lift forces. The rotating measuring plate made it possible to simulate and measure the effects of crosswind.
In the mid-1980s, the wind tunnel was again modernised with a view to speeding up procedures and attaining more precisemeasurements with the wind tunnel. Thus for example a traversing system was fitted, which enabled quick and precise contour measurement, flow measurement, and front surface area measurement. For the contour measurement, which until then had been carried out using templates, photos, and measurement with measuring sticks, the contour was now measured for greater precision in coordinates with a so-called optocator (an optical-electronic diode-light laser indicator). Instead of using manually positioned probes, the flow measurement now took place with probes which automatically moved along a measuring point grid. Whereas before the measurement of the front surface area took four hours to complete with a laser procedure, it now only required between half an hour and one hour thanks to edge tracking. A further advantage was the capability of measuring full-scale commercial vehicles.
Alongside aerodynamic studies, the wind tunnel could also be used for other jobs, such as contamination measurements and measurements of the wiping precision of windscreen wipers in the high-speed range, temperature measurements at the radiator, and brakes and tests of the passenger-compartment ventilation.
Measurement installations and technical data for the wind tunnel at the Mercedes-Benz plant in Untertürkheim:
- Wind tunnel weighbridge
- Multi-point pressure system
- Roller-type performance test rig
- Traversing system
- Data acquisition system
- Length of the measurement section: 10 metres
- Length of the tunnel axis: 125 metres
- Air jet cross-section: 32.6 square metres
- Maximum flow cross-section: 120 square metres
- Maximum blowing speed: 250 km/h
- Maximum power input: 5,000 kW
- Blower diameter: 8.5 metres
- Turntable diameter: 12 metres
- Swivelling range of the turntable: 180 degrees
- Maximum axle load: 10 tonnes
- Contraction ratio prechamber/air jet: 3.6
Automotive aerodynamics were, quite literally, up in the air in the 1930s.
Aerodynamics was a subject that was heavily preoccupying the automotive industry and thus also the companies Daimler-Motoren-Gesellschaft and Benz & Cie during the early decades of the 20th century. Triggered by speed records and motor racing, wind resistance had become a consideration in normal passenger car manufacturing as early as in the 1920s – and even more so in the 1930s – at Mercedes-Benz in the form of the rear-engined models 130 (W 23), 150 (W 30) and 170 H (W 28), and subsequently also in other vehicles across an extensive model portfolio. The pinnacle of this development process was marked by the 540 K, the streamliner built in 1938 whose aerodynamic aluminium body more or less predestined it for use in high-speed tests and for which purpose it was also used by tyre manufacturer Dunlop.
Benz & Cie had gained their own purposive experience with aerodynamics over the 1909/1910 period with the 200 hp racing car that became known from 1911 on as the “Blitzen-Benz”, or “Lightning Benz”, which had set several world records. It was followed in the early 1920s by the “Teardrop” car, so called because of its characteristic design. This was also the world’s first mid-engined racing car.
In the early 1920s, Daimler-Motoren-Gesellschaft undertook aerodynamic studies of racing cars intended for use in Indianapolis. In 1932 Reinhard von Koenig-Fachsenfeld caused quite a stir with a streamlined body for the Mercedes-Benz SSKL, in which Manfred von Brauchitsch promptly won the Avus race ahead of Rudolf Caracciola.
This was also the era of trailblazing designs by various pioneers of aerodynamics such as Edmund Rumpler, Paul Jaray, Wunibald Kamm, Freiherr Reinhard von Koenig-Fachsenfeld and Karl Schlör von Westhofen-Dirmstein, all of whom were primarily concerned with passenger cars. Jaray had a streamlined body built for a Mercedes-Benz 200 (W 21) in 1934, which attracted a lot of attention at the time. We know of a 170 V (W 136) with aerodynamically styled special body by Kamm, and also of a 170 H (W 28) by Schlör.
Several vehicles illustrate the efforts undertaken by Daimler-Benz to reduce wind resistance. The design of the rear-engined 130, 150 and 170 H models, for example, was influenced by aerodynamic considerations. This can be seen particularly clearly in the lines of the model 150 sports saloon that attracted so much attention and ultimately proved successful in the “2000 km through Germany” long-distance race of July 1934. A 500 K Coupé that took part in the same event bears witness to the efforts to achieve low wind resistance in the luxury car segment as well. With its curved windscreen and gently sloping rear section, this one-off vehicle produced in the special vehicle production unit at Sindelfingen under Hermann Ahrens is strongly reminiscent of the 500 K “Autobahn Courier” that had made its debut in March 1934 at the International Motor and Motorcycle Show (IAMA) in Berlin. This two-door coupé set an early milestone in terms of progressive and innovative design. Further vehicles derived from it would go on to be created in the special vehicle production unit.
The years 1935 and 1936 brought similarly designed four-door versions based on the volume-produced models 200 (W 21), 290 (W 18) and 320 (W 142). A characteristic feature of them all is that from a conventional front-end design the roof line curves back and downward in a gentle arc to merge seamlessly into the rear section. In addition, the rear wheel arches are fully enclosed.
The rapidly expanding autobahn network brought new challenges for the vehicle manufacturers and the associated accessories industry. A particularly important part in solving the problem of how to sustain high cruising speeds on the new autobahns was played by the tyre industry. The task was to develop tyres for high-performance vehicles that by this time might weigh as much as three tonnes. And it was for this reason that the tyre industry began increasingly to look around for test vehicles that might be up to this development challenge. In Germany, the Mercedes-Benz 540 K was one of the few vehicles that was capable of meeting this challenge. Fitted with the streamlined body of 1938, with which the Cd value of 0.57 for a classic coupé had been drastically reduced to a remarkable 0.36, cruising speeds of 165 to 170 km/h and with supercharger a top speed of 185 km/h could be achieved – at that time on public roads most extraordinary figures.
In their quest for favourable aerodynamics the engineers also recognised that the brand emblem standing proud on the upright Mercedes-Benz radiator represented something of a challenge. One possible solution was suggested by a 500 K streamlined model designed for a customer in the Dutch East Indies (from 1949 Indonesia) in 1935 by Hermann Ahrens’ special vehicle production unit.
The enclosed Mercedes-Benz pointed radiator, as also the substitution of free-standing headlamps with headlamps that were integrated into the wings, were ideas that were as a consequence quickly implemented on the 540 K Streamliner that the Dunlop works would go on to use as a vehicle for testing tyres from 1938 on. For this vehicle and its proposed use in high-speed situations, the special vehicle production unit built a streamlined body in sheet aluminium that was designed in accordance with the very latest insights into aerodynamic performance.
The market for used vehicles also saw streamlined bodies gaining in popularity, with the result that work continued until 1942 on bodies of this type based around the proposed successor to the 170 V. This work was abandoned due to the war situation.
When it came to vehicles for long-distance races such as the “2000 km through Germany” in 1934 or the planned long-distance Berlin–Rome run, originally set for 1938, the idea of the streamlined body played a similarly important role. After all, here too, the objective was to achieve the highest possible speed and the lowest possible fuel consumption, in other words to improve overall efficiency. However, it was established at a board meeting in April 1938 that not every detail of the proposed streamliner would reach the requisite standard in time to meet the deadline, with the outcome that three conventionally designed 5.4-litre supercharged models should be deployed instead. But then Berlin-Rome was postponed until 1939 and then ultimately cancelled completely due to the outbreak of the Second World War.
What remains is an impressive car featuring pioneering technology – a milestone in automotive engineering.