Go With The Flow - 1930's Streamlining

Aerodynamic history is a bit of a drag,

Besides creating a distinctive look, the shape of a car's body has a very important functional role. It plays an important part in determining maximum speed, as well as fuel consumption at high speed. This is because maximum speed is achieved when the force transmitted by the tires to the road exactly equals the force created by the resistance of the air.

Of course, it is not just the airflow that absorbs engine power; there are transmission and traction losses to be countered as well. But overcoming drag is a major consideration.

Fortunately for the theorists, there is a formula that links the power and speed to show just how much power is needed. The parameters, which are multiplied together, are:

• air density;
• frontal area;
• drag coefficient;
• and vehicle speed, raised to the third power.

This formula will be discussed more fully in Part 2.

What concerns us particularly here is the drag coefficient, Cd. It varies from about 0.85 for a box-shaped car with square corners to 0.29 for some modern saloons. A good value for a ten-year-old saloon would be about 0.45.

Clearly, the drag coefficient pays an important role in determining the power needed to cruise at a certain speed, as well as the maximum speed. Its value depends on the shape of the body. Size is a separate issue - as you can see, our formula brings in size in the form of ´frontal area'. This is the area of the front end of the vehicle, as viewed from dead ahead.

It has become fashionable for automobile manufacturers to advertise the drag coefficient of their vehicles, especially if they can somehow come up with a very low value. Although there is no doubt that the drag coefficients of production cars have declined steadily ever since the early days, some of the advertised figures have to be viewed with suspicion. We say this not only because different wind tunnels give different answers, because of design differences that we will look into, but also because small differences between various models from the same manufacturer can result in measurably different Cd values. For example, if a number of different models share the same body, then the smallest-engined version with the skinniest tyres and the smallest set of exhaust pipes disturbing the airflow underneath will give a significantly lower reading than the luxury version - but the manufacturer will often claim the same value for all the models.

These three shapes, tested as models, summarise the search for the ideal. The teardrop shape, here typified by Schlor’s profile (CD = 0.19) is not practical, and Everling’s truncation attempt (CD = 0.31) is too severe. Kamm (CD = 0.23) strikes a happy medium. This car was actually built as the K1 in 1938, on a 3.5- litre BMW chassis, and tested on the road, as the photo above shows. The graph compares the performance of the K1 with an SSKL Mercedes-Benz. It shows that (theoretically) above 150kmph the K1 needs only half the power of the SSKL to reach the same speed, although its output limited the maximum speed to 180 kmph.

Shaping a car's body so that less effort is needed to force it through the air has been the dream of many a visionary. The early attempts, based on the dictum ´what looks right, is right' eventually gave way to a scientific study, which effectively means ´what looks right is right, if the right person is looking'. Initially, being able to cruise at respectable speeds with smallish engines on the new fangled expressways was one of the major aims, but today improvement in fuel consumption at all speeds is the driving force behind the research.

A 1921 Rumpler on test in the VW wind tunnel. It recorded a sensational CD value of 0.28!

The first car to get into the headlines with what looks like a streamlined body was the cigar-shaped La Jamais Contente (French for Never Satisfied), with which Camille Jenatzy covered the flying kilometre in 34 seconds in 1899, to set a land speed record of 105.81kmph. Propulsion was by means of an electric motor, so that the body did not have holes for cooling air to enter or an exhaust pipe to exit, ensuring that the car most likely had a favourable drag coefficient.

For the next decade, various crude streamlining efforts were mainly confined to racing cars or land speed record attempts, because many engineers believed that ordinary cars would not benefit from streamlining. This was close to the truth at that time because most cars could not cruise at much over 40kmph. Later, in the early Thirties, when the first autobahns were built, and cruising speeds started to rise, this conservative viewpoint was still quite common. An incident involving German coachbuilder Vetter illustrates this. He built a streamlined body on an Opel chassis, but when he proudly took it to the factory the engineers refused to look at it.

The first signs of any real progress appeared just after WW1, and here we immediately come across an engineering mystery. Edmund Rumpler, the inventor of the swing axle while at Adler in 1903, caused a sensation at the 1921 Berlin Motor Show with an advanced car bearing his own name. The design was based on patents he had taken out in 1919. Most of the components were enclosed in a metal tub that had a closed underbelly. The car was teardrop-shaped when viewed from the top. Its engine was a W-six, with three banks of two cylinders each, and suspension was by leaf springs in front and a swing axle at the rear. It was too advanced for its day, and production lasted only from 1921 to 1925 despite some interest from the Benz company, before amalgamation with Daimler in 1926.

Benz bought some Rumplers to experiment with, installed their own engines, and later produced some very streamlined sports and racing cars incorporating some of Rumpler's ideas. Sadly, the normal Benz production cars did not benefit, and remained unexceptional.

So far the story is not too different from many other attempts to go against the norm, but there is a sequel. In 1979, a Rumpler exhibited at the Deutsche Museum in Munich had its aerodynamic drag measured in Volkswagen's full-scale wind tunnel. The result was a sensational Cd value of 0.28. This not only bettered any car in production in 1979, but is also better than 99.9 per cent of the cars in production right now, 70 years later! There were no wind tunnels capable of taking a full-sized car when the Rumpler was designed, so how did Edmund produce such an advanced shape while using only small models? We simply don't know, but he must rate as one of the first of the gifted automotive aerodynamicists.

Aerodynamics, the study of the way air affects the movement of the bodies, was given new impetus by the invention of the aeroplane. Research organisations were founded just before, during or after the first World War to study airflow. The most active and certainly the most famous aerodynamic laboratories were located in Gottingen, Stuttgart and Berlin.

Many of the results achieved could also be applied to road vehicles, with the result that in the Thirties smooth-flowing shapes became popular, not only for cars, but also for trains and non-moving objects, and many designers hastened to conform. Unfortunately, a great deal of bogus shapes, which were not really aerodynamically correct, saw the light of day. The Chrysler Airflow (copied by Peugeot, Singer and others) belongs to this period. On the other hand, a number of European companies had very active experimental departments, with the result that some outstanding experimental and production cars were produced.

Carozzeria Touring of Milan used these body contour lines to build the streamlined BMW 328 coupe that won the 1939 Le Mans race

Experimental bodywork is classified as pure aerodynamic if the shape can be regarded as an attempt to produce the best possible result, regardless of its practicality, or semi-aerodynamic if practical details such as door-handles, a reasonable ground clearance and sufficient interior space and head room are included.

This difference in drag between the two shapes can be quite striking, and many an engineer became dejected when his beautiful pure shape yielded poor results when practical details were added. One of the problems was that the long tail, demanded by theory, could not be included in a semi-aerodynamic shape, because it wasn't practical, especially in traffic.

However, in 1936 a prominent engineer, Baron Reinhard Koenig-Fachsenfelt, applied for a patent for a cut-off tail, based on experimental results with buses. This application appeared at about the same time as a textbook by Professor Wunibald Kamm, head of the Automotive Research Institute at the Stuttgart Technical College (abbreviated to FKSF in German), describing a similar cut-off, based on a modification of the theory. The Baron was persuaded to sell his patent to the state, and Professor Kamm was asked to develop it further. Initially, models were used, and mention must be made of a third person with the same idea, a Professor Everling of Berlin, whose model was amongst those tested by Kamm. Thus it appears that three people had the same idea at about the same time, but because Kamm was given the funds to develop it, this cut-off became known as a K-tail.

Go with the flow - 1930's Streamlining pt.2

This did not mean that the longer tail died a sudden death. It was still used on many a smooth shape, and was most vigorously advocated by Paul Jaray. He was an engineer whose interest in aerodynamics was kindled when he worked on aircraft construction during World War 1. In 1921 he applied for a patent for a streamlined vehicle with a long tail, which was contested by Edmund Rumpler and another engineer, but the patent was granted in 1926. He started a consulting agency in 1923 and many models and even full-scale cars were built to his designs.

1939 2.5-litre Le Mans Adler

The long tail became known as the J-tail, and Jaray's ideas influenced many designs. But he was seldom given a chance to oversee a complete vehicle, because he was mainly interested in pure shape, and hated any practical compromise. After World War 2, the K-tail gained more support, because it was more practical. Most modern cars, especially hatchbacks and station wagons, use proportions first set out by Kamm.

There is no doubt that the impetus given to the German motor industry by their research organisations placed them in the forefront of pre-war aerodynamic development, and the best way to assess this development is to look at the achievements of the most active companies.

An Adler advert from the late 30's, streamlining is a selling point.

Pride of place must go to Adler, a famous old company that used to manufacture motorcycles, cars and typewriters. Some of our older readers may still remember the beautifully streamlined Adlers that were a feature of the immediate pre-war Le Mans races. The early models had long J-tails, but the tail was drastically shortened for 1939. These cars are claimed to have Cd values of less than 2, which is easy to believe in view of maximum speeds of well over 160kmph achieved with 1.5 and 1.7-litre sidevalve engines. The production Adlers of the last year or two before the war also appear to be very modern and would certainly have had a greater influence if Adler had been one of the major producers. During this time two interesting experimental production Adlers were built for comparative purposes. They were identical except for the tail, which was J-shaped on the one and K-shaped on the other. The drag was identical but the J-shape was chosen because it was considered prettier.

BMW also built a comparatively large number of experimental bodies in the late thirties. At first the company supplied chassis to coach-builders Wendler, who specialised in streamlined bodies for rich customers, mainly to the designs of Baron Koenig-Fachsenfelt.

BMW was so impressed with the results that it soon produced its own series of open and closed versions of the type 328. These designs achieved some fame when a BMW 328 coupe with a J-type rear won the 1940 Mille Miglia and the two-litre class in the 1939 Le Mans 24-hour. One of the open two-seaters is said to have been the inspiration for the design of the XK120 Jaguar. A number of type 332 four-door saloons were tested by Kamm with K-type rear ends, but series production was halted by the war.Daimler-Benz was not nearly as enthusiastic about streamlining on its production cars as on its fabulous racers. On one occasion the coach-builder Erdmann and Rossi, who had built a streamlined body on a Mercedes-Benz 200, was asked to remove the Mercedes-Benz badge. Later, Daimler-Benz relented and supplied four identical type 170 chassis to different engineers, who designed pure streamlined bodies for comparative research purposes.

The VW Beetle was the first mass-produced car to have its body tested extensively in a wind tunnel. Here is one of the first models used, during testing in 1936. Beetle owners will agree that Porsche should have insisted on investigating the effect of sidewinds.

1938 VW38 wooden mock-up

Ferdinand Porsche, recently named as the automotive engineer of the century, started an independent design office in 1930 in Stuttgart and employed Erwin Komenda as stylist and Josef Mickl as aerodynamicist. These two engineers fine-tuned the basic Beetle's body-shape so that by the time it reached production in 1939, the drag coefficient was just below 0.4 at a time when most small cars had values of greater than 0.6. Post-war excesses such as bigger bumpers, headlights and tyres most likely corrupted this value to something nearer 0.46, but the original achievement remains. In 1939 Mickl designed a very smooth VW coupe that is the spiritual forerunner of the modern Porsches. Its Cd value is not known, but it cannot be far from 0.3. Three examples were built and one example of this, the rarest of all Beetles, survived the war and is in the hands of a collector in Germany.

Forunner of the Porsche - The VW 60K10 built for a Berlin-Rome rally, which never took place.

Another significant manufacturer of streamlined cars in the pre-war period was Tatra, whose chief engineer, Hans Ledwinka, early on appreciated the research being done by Jaray. Ledwinka and Jaray collaborated on a series of four-door saloon prototypes featuring a J-type tail, air-cooled rear engine, backbone chassis and swing axle rear suspension, culminating in the Type 87. This model was produced from 1936 to 1950, throughout the war, in small numbers. Its Cd was measured in the VW wind tunnel in 1979 and found to be 0.36, making it the first production four-door saloon with a modern drag coefficient.

Many of the other European companies took a passing interest in streamlining, and consulted Koenig-Fachsenfelt, Kamm or Jaray, and sometimes more than one at the same time. But the major sustained effort must be credited to the above-mentioned companies.
Some pretty semi-streamlined Fiats, Alfa Romeos and Lancias were produced by various Italian coach-builders, but they were mostly based on the designs of the well-known German designers. Peugeot, Renault and Panhard also experimented with some streamlined one-offs. Various weird designs saw the light of day in Europe and the USA, but only the Germans supported a sustained scientific investigation into ways to make the production car more streamlined.

Source:  ltv-vwc.org.uk