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Air-Craft: Aerodynamics of Audi LMP

At Le Mans, top performances in aerodynamics are particularly valuable. Nowhere else are such high speeds driven with LMP sports cars. Thanks to improved airflow excellent lap times are consistently achieved over and over – despite the opposite effect of the regulations.

A look at Audi’s first prototype and its youngest one is quite revealing, as the differences between the aerodynamic concepts of the two cars are clearly evident. The 1999 Audi R8R with an open cockpit is contrasted by the closed R18 e-tron quattro. And not a single detail resembles another one.

When Audi built an LMP sports prototype for the first time 14 years ago, Fondmetal Technologies was the partner in aerodynamics. In Italy, the engineers tested the air flow on the R8R using a 40-percent scale model. Back then, such models had carbon fiber tires that were fixed in position from the outside. “Today’s state-of-the-art technology is completely different,” explains Axel Löffler, who as Head of Design Chassis/Bodywork was also responsible for aerodynamics for many years before Jan Monchaux assumed responsibility for this function in 2013. “We’ve now reached a model size of 60 percent. Thanks to today’s rubber tires we can create the airflow around the model with a lot more realism. Likewise, a moving floor in the wind tunnel helps us obtain more accurate measurement results. The suspensions of the models have also been fully emulated and are movable today.”

The basic aerodynamic concept of the various evolutions of the LMP race cars from Ingolstadt and Neckarsulm has obviously been subjected to further development. In 1999, the radiators of the engine still lay flat at the front end. The warm exit air escaped from the hood in front of the cockpit opening, partially flowing across the top of the cockpit and to the right and left. To optimize airflow to the rear end, Audi has been integrating the radiators and intercoolers into the side pods in the Audi R8 as of 2000. “This has clearly improved airflow,” says Löffler. “Plus we gained some new freedom of design at the front end. We were able to guide the exit air of the front diffusor with much higher precision.”

Audi R10 TDI

Audi took yet another step with the R15 TDI, which set a new distance record at Le Mans in 2010. “The car’s extremely high nose made it possible for us to guide the air to the underfloor with even less eddying than before. This supports the ground effect, in other words the suction generated by the underfloor,” says the expert.

But improvements are not always achieved. The aerodynamicists repeatedly had to accept limitations. When diesel direct injection was introduced in the Audi R10 TDI in the 2006 season the cooling requirements increased by around 30 percent due to the different combustion process. Furthermore, the Audi R18 e-tron quattro that has been fielded since 2012 has a low-temperature circuit for cooling the hybrid system, which poses an additional challenge. Still, no other Audi LMP sports car has ever been as aerodynamically efficient as the current hybrid sports car.

Existing latitudes are limited by the regulations time and again. For example, when the project was launched in 1999, the rear wing was allowed to fill a maximum volume of 2,000 mm (width) x 400 mm (length) x 150 mm (height). Today, these dimensions have been reduced to 1,600 x 250 x 150 mm. Through a large number of individual solutions, such as the rear wing suspended from the top since the 2009 R15 TDI, Audi has compensated for a major portion of the lost downforce. It allows significantly improved airflow to the wing. For comparison: If the wing supports are installed at the bottom, downforce is significantly reduced. The new mounting principle was subsequently used by many other constructors too.

The specifications for the underfloor were significantly modified as well. As of the Audi R10 TDI (2006), the specifications have been requiring a seven-degree increase of the profile cross-section toward the sides and a wooden board being installed underneath the chassis. Despite such limitations a modern LMP sports car achieves very high levels of downforce. Theoretically, at high speed, it could run on the ceiling of a tunnel without falling down. The aerodynamic loads involved are instructive. The front diffusor, for instance, together with the rear wing generates half of the downforce, while the underfloor including the rear diffusor delivers the other half. This downforce is counteracted by the inevitable lift that is caused by the airflow through the cockpit and over the body. It accounts for around a fourth of the downforce produced.

Audi R18 TDI

“The regulations have since been severely limiting the freedom in aerodynamics,” says Axel Löffler. “In the past, we were able to use the desired aerodynamic configurations of the Audi R8 for fast tracks like Le Mans as well as for slower road courses in the American Le Mans Series with a single body version. Now, the minimal latitude that is allowed forces us to optimize a car for a single requirement. That’s why a long-tail version of the R18 e-tron quattro was created just for Le Mans 2013.”

The long rear end is only the most visible change. The entire aerodynamics of the hybrid sports car has been modified for Le Mans in 2013 to meet the special demands. An example of numbers illustrates how extreme the conditions are: A year ago, Audi factory driver Loïc Duval set the fastest lap in the 24-hour race at La Sarthe, achieving an average speed of 240.289 km/h. Including all the times the car spent at rest during 33 pit stops, the victorious R18 e-tron quattro of Marcel Fässler/André Lotterer/Benoît Tréluyer still achieved an average of 214.468 km/h that was thus clearly above the 200 km/h-mark. There is no other circuit in the FIA World Endurance Championship (WEC) where the cars run as fast as this.

Engineers keep finding ways to improve aerodynamic efficiency – in other words the relationship between downforce and aerodynamic drag. This ratio expresses the degree to which the aerodynamicists have improved the downforce of a race car without an equivalent increase in drag. Since 1999, Audi has improved the aerodynamic efficiency of its LMP sports cars by around 65 percent.

“The lap times reflect the significance of the strides that have been made in aerodynamics,” emphasizes Head of Audi Motorsport Dr. Wolfgang Ullrich. “Of course there are many other influencing factors – the powertrain, the tires, the chassis, the ultra-lightweight design or the distribution of weight. To name just one example for the sake of comparison: In 2006, the fastest race lap at Le Mans was 3m 31.211s. The R10 TDI back then had 12 cylinders, 5.5 liters of displacement and, delivering more than 650 hp, was our most powerful LMP race car. Six years later, the best lap time was 3m 24.189s. Our cars had become more than seven seconds faster. But the V6 TDI engine of the Audi R18 ultra in 2012 was only allowed to have a displacement of 3.7 liters and delivered around 510 hp. A major share of these advances is owed to optimized aerodynamics.”

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