Winds of change
Taking curves at faster speeds, while consuming less fuel in the process—aerodynamics play no small role here. The new aero-acoustic wind tunnel at the Weissach Development Center is helping
Three hundred km/h. That is a magical figure, above which is a realm inhabited only by super sports cars and race cars. An airplane would take off at this speed. But not the 911
That is a challenge, because even the smallest modifications to the car’s contours can affect both the drag and the downforces. Therefore, every new model version has to be tested in the wind tunnel. “It is primarily a matter of precision,” explains test rig director Dr. Hauke Stumpf, who together with his team is in charge of the new wind tunnel that was opened in the spring of 2015. “We have to be able to measure forces of a single newton.” One newton is the weight force that a mass of approximately 100 grams brings to the scale. This force is essentially nothing when compared to a car that weighs about one and a half tons, moving at 300 km/h, in addition to the mutually reinforcing effect of vehicle weight and aerodynamic downforces.
The previous wind tunnel in Weissach, which was built in the mid-1980s, was already very good. But “very good” is never good enough for
The heart of the new wind tunnel thus consists of its belt system and scale. A special feature of the Weissach tunnel is that it offers test engineers a choice between two different belt systems. One system has five steel belts: one runs below each of the four wheels and a larger one down the middle below the vehicle floor. “This enables us to make especially precise measurements,” says Stumpf.
The other is the one-belt system, which, as its name suggests, consists of a single steel belt that runs below the test object. It reflects real conditions more closely, but yields measurements that are somewhat less precise.
Another crucial benefit of the belt systems is that they allow the engineers to measure the forces that the wind exerts on the car. Linked with a precision scale that stands on its own foundation, the belt system can measure minimal changes in the wind forces on the car or their distribution among the wheels. This measurement in turn enables the engineers to calculate the drag and the up- or downforces on the front and rear axles.
The wind is generated by a huge fan with carbon vanes and a diameter of about eight meters. It has a peak output of around seven megawatts, or 9,300 horsepower, generated by an electric motor the size of a small bus. The wind tunnel is not terribly loud, however, and at 200 km/h is considerably quieter than its predecessor. Consequently, certain acoustic measurements can now be done in Weissach which used to be sent to external service providers. Of interest here is not so much the absolute noise level, but some detailed detective work. For example, how does a certain mirror shape or a new door seal affect the sound pattern? Several hundred microphones are set up on the car in the test section, allowing a computer to generate a three-dimensional representation of the sound propagation from the car.
Many of the measurements, such as those to determine drag, will also be done later on freeways at the usual European speed of 130 km/h. However, it is important to be able to generate considerably higher wind speeds, in order to test the structural strength, for example, or to contribute to developments in racing. One of
The new wind tunnel in Weissach is not set apart, but rather is integrated into an ensemble of buildings that includes design studios, workshops, and aerodynamics development facilities. Deliberately so, as Stumpf confirms: “Spatial proximity enables the aerodynamics developers to test design studies very quickly under the strictest secrecy conditions.” At
After the board gave the green light for the wind tunnel, it took about four years before it went into operation. That’s a long time, but, considering the construction requirements, not so long. Almost all of its components are custom-made, and there are only a few suppliers worldwide. Not least of all, the tunnel itself had to undergo precision tests and careful adjustments. “A wind tunnel needs to be prepared for its job in the same way a musical instrument has to be tuned for a concert,” explains Stumpf. For instance, a team of experts spent weeks checking the distribution of the air flow in the test section, or “plenum,” which has a cross-section of 22 square meters. In order for the results from this space to be viable, any fluctuations in the wind speed have to be less than one percent. “We’re well below that,” says Stumpf.
This is a lot of effort “just” for air. But it’s worth it. The mission of the wind tunnel specialists will be accomplished when the next model to hit the roads consumes a little less fuel, when a
By Johannes Winterhagen
Photos by Rafael Krötz