Extreme aerodynamics

“It was clear to us that we had to develop a new car if we wanted to continue our winning ways at Le Mans,” explains Head of Audi Motorsport Dr. Wolfgang Ullrich. “The Audi R15 TDITDISynonymous with muscular torque delivery and outstanding efficiency: TDI. Today, TDI engines are sporty, smooth and economical.TDI is the result. Everything that we learnt with the R8 and R10 TDI, and also from racing in the DTM, plus a couple of innovative ideas have been put into the car.”

The Audi R15 TDI developed in line with the Automobile Club de l’Ouest (ACO) LMP1 regulations is the first “second” generation diesel racing sports car. Apart from the basic concept it does not have much in common with its forerunner, the R10 TDI unveiled at the end of 2005: Both are open topped roadsters with a TDI engine, a five speed gearbox and rear wheel drive. “The step from R10 to R15 is considerably greater than it was from R8 to R10,” emphasizes Dr. Ullrich.

First Audi prototype with “high nose”

It is the first thing you notice about the new Le Mans sports car: The R15 TDI is the first Audi prototype with a “high nose.” This helps to improve airflow through the roadster’s front and rear ends and reveals the basic concept of the “new one.” “The aerodynamics take precedence during the development, since they are also of great significance at Le Mans,” explains Dr. Martin Mühlmeier, Head of Technology at Audi Sport.

In the process, the Audi Sport engineers chose similar routes to the current Audi A4 from the DTM. With help of CFD calculations (Computational Fluid Dynamics) the attached airflow and specifically the airflow through the car was optimized. With the R10 TDI the airflow was directed primarily over the bodywork, with the R15 TDI a part of the airflow is directed through the car.

This allowed Audi Sport to reduce the car’s virtual frontal area. According to the race track, this can be used for less drag or more down force.

Novel rear wing design

The design work around the rear end was particularly challenging. For the 2009 season the regulations stipulate a 40 centimeter narrower rear wing. A part of the down force lost as a result was recovered by the technicians through the extremely flat and flow optimized rear body and a novel rear wing design: The wing mountings are no longer on the underside, which is especially important for generating down force, but on the upper surface.

Also the exhaust tail pipes now emerging upwards just behind the engine have – in addition to being lighter – an aerodynamic effect: They optimize the incident flow to the rear wing.

Furthermore, to optimize the airflow the cooling air exit ducts behind the front wheels were subject to extensive refinement. They are very complex and look like gills on the R15 TDI.

The aerodynamic concept of the front end is also entirely new featuring a high nose with air partly flowing through it. The front suspension cowlings are also even more refined than on the R10 TDI.

“We are convinced that our aerodynamic concept offers a huge advantage over the R10 concept,” says Dr. Martin Mühlmeier.

The R15 project started in autumn 2007 with the engine development and the definition of the concept. The first wind tunnel tests followed at the beginning of 2008. Design work started just before the 24 Hours of Le Mans at the end of the second quarter. Every available resource was channeled into the R15 TDI immediately after Le Mans. The maiden outing followed in December 2008.

The following development targets were given highest priority: to reduce the weight, which was partially due to the diesel engine in the R10 TDI being heavier than a conventional gasoline engine, to decrease the slightly excessive tail-heavy weight distribution also resulting from this, in general – as is normal for every new race car – to improve the performance and to optimize the aerodynamic efficiency. To achieve this the technicians started by targeting the engine, the aerodynamics and weight.