AUDI R10 – AERODYNAMICS OF A SPORT – PROTOTYPE RACE CAR
Conceiving a Sport-Prototype race car represents, by its peculiar characteristics, the childhood dream of any aerodynamics’ engineer.
In fact, this vehicles involving bodywork allows not only the exploitation of a big area for the generation of downforce, but, also, the maintenance of a linear undisturbed air flow, that surrounds it, in opposition to what happens in the open wheelers racing cars, by influence of the turbulent vortexes originated by the rotating wheels.
In these cars the level of aerodynamic efficiency reached, the relationship between the downforce, generated by the bodywork, and the drag produced by the vehicle, at the same speed, is nearly the double of the efficacy of an open wheeler racing car with a similar engine power.
THE LE MANS SPECIFIC CONDITIONS
Sport-Prototype cars are widely used all over the world, in distinct Championships, running in circuits that differ considerably on mechanical and aerodynamic solicitations.
When these cars run in a “Mickey mouse” type of circuit, with low to medium velocity curves, joint by short straight lines, the needs in downforce have to be obtained by the use of all the kind of wings, disposed on the bodywork, and working in high angles of attack.
In the case of Le Mans, like in any other velocity track, the rear wing, the only real wing that is usually used, works mainly on fine tuning of the front and rear axles aerodynamic loads, and for stability purposes.
In this high velocity circuit, and, as we know, the level of aerodynamic loads created is directly proportional to the square of the velocity, the downforce generated by the bodywork itself, without the rear wing, is enough for the good handling of the car.
In La Sarthe, with it’s so long Hunaudiéres straight line, the high velocities reached with these cars forced the organizing entity, the ACO (Automobile Club de l’Ouest), to conceive and apply a specific Technical Regulation for their 24 Hours race.
And so, these regulations impose the use of a “skid block”, that is, like in open wheeler racing cars, longitudinally applied under the Reference Plan, between the front and rear axles.
Forward to the front axle, in a minimum span of 1 meter, the under floor of the car cannot be under a minimum of 50 mm over the same Reference Plan that extends backwards, under the cockpit.
All the under floor car surface, including the rear diffuser, and the round edges that joint this surface to the lateral faces of the side pods, are well limited by these specific Technical Regulations.
THE AUDI CASE STUDY
For its new challenge, of winning Le Mans with a car powered by a Diesel engine, Audi took the excellent, and well proved, base of the R8 to improve everything that was left to be improved.
Compared with its ancestor, the new car has a chassis with a narrower front section, a new and more sophisticated front part, more complex side pods, that reveals a great work in internal fluid dynamics, and a well designed rear section, apparently more effective.
As we can see in the drawings, made with the photographic documentation that we have on the R10, together with the Technical Regulations of the ACO, the air that flows over and around the new Audi, has to overcome, first, a very complex and interesting front part of the car.
Here we find a first splitter that separates the fast air flow that goes under it, from the one that is stopped against the almost vertical front wheels fender, and against the vertical surface between the front fenders (drawings 2 and 3). A pressure differential will be generated here, with consequent downforce that will be applied over the front axle.
As we can easily see, the leading edge of this splitter is also the leading edge of a camouflaged inverted wing, that extends backwards to overcome the front axle.
On both lateral extremes of this wing, we can find two flux deviators, with little splitters on their inferior borders that channel the air that flows under the wing to the big side pods fences (drawings 5 and 6).
On the trailing edge of this camouflaged wing the air that flows over the wing will blow the air flow that flew under the same wing, accelerating this flux and improving this wings efficacy by the increase of downfoce generation (drawing 5).
On the upper face of the front wing, we can see, internally limited by small red flux deviators, the air intakes for cooling the front brakes.
Just ahead of the front wheels, the under floor surface of the bodywork turns slightly upwards, in order to create a small diffuser, in lateral continuity with the front wing that improves the splitters efficacy, by accelerating the air that flows under that splitter.
In drawing 4 there is a schematic view of the positive effect induced over the air that flows under the front wing, by the rotation of the front wheels. The rotating wheels will aspirate part of this air flow and will drag it to the louvers open in the highest par of the front wheels cover bodywork. Here, the same air flow is, again, aspirated by the very fast and low pressure air that flows over the bodywork, and has been accelerated during the climbing of the front fender.
In the same drawing is explained the way the two dive plates work. Used at Sebring, but not in Le Mans, these two small delta wings generate an additional downforce over the front axle, by the effect of the small vortexes created under their leading edge.
At the front axle level, the air that flows under the camouflaged front wing, will be confronted with another splitter, that is, simultaneously, the leading edge of the Reference Plan.
Here, part of the air flow will be aspirated by the front wing’s diffuser, and will flow over the splitter, in order to be drained out by the louvers opened on the lateral walls of the side pods, or to be deviated to cool the radiators and the turbo air exchangers, positioned in tandem, on both side pods, with the radiators leading (drawings 2 and 5).
The other part of the air will flow under the splitter, under the Reference Plan and the “skid block”, to be accelerated, immediately in front of the rear diffuser, on the throat of the Venturi Tunnel created between the under floor car surface and the track, by the suction effect produced by the rear diffuser over the air that flows in this area (drawings 2 and 6).
We can, also, note that the round edges that joint the under floor with the lateral face of the side pods, will allow the air that flows along the lowest part of this vertical wall, to be suctioned, to the under car, by the rear diffuser, to increase it’s efficiency and the downforce generated and applied over the rear axle of the car.
In drawing 6 we can observe that there are only two very simple flux deviators, on the rear diffuser. Here is another Technical Regulation’s limit that, also, imposes that these flux deviators must be placed in a parallel and symmetric position, between them and the longitudinal axle of the car.
Placed over the trailing edge of the rear diffuser, the exhaust of the gas escape from the engine will boost the air that flows on this aerodynamic device, increasing its velocity and the efficiency of the diffuser.
Article by António Eiras - 27/03/2016
Schematic view of the rotating wheels effect over the air that flows under the front wing (acrylics with aerograph on paper).
At each side of the rear diffuser, and behind the rear wheels, there is a device, imposed by ACO Technical Regulations, that not only avoids any backwards projection of detritus, and of the turbulent air flow from the wheels, over any persecuting car, but, also, by the induced acceleration on the air that flows out from the same rear diffuser, and that is forced to do a 270 degrees angle, in the horizontal plane, improves this device’s efficiency.
On drawing 5 we can see, on the lateral face of the side pod, the out flow of warm air, in red arrows, that cooled the radiators, from small vertical fences. The warm air that flows out from the air exchangers is, in turn, drained from small louvers opened on the beginning of the upper surface of the engine cover.
On this area we find two air intakes in each side pod: the inner one will feed the same side turbo, and the upper, a periscope one, will canalize the cooling air flow to the rear brakes (drawing 2).
In the same drawing we can follow the engine cover curvature, backwards and downwards, progressively, to reach the trailing edge with a Gurney flap, and also, how the air flow, that will cross the rear wing, is aspirated, in a downwash, by the engine, gearbox and rear wheels bodywork cover.
This air flow will form, with the rear wing, a higher effective angle of attack, than the one that would result from the interaction between the rear wing with a non conditioned free air flow.
This higher angle of attack will, naturally, increase the downforce generated by the rear wing, with a small extra drag.
Together with this highly refined aerodynamic package, the R10 has a very effective chassis, a powerful and economic engine, and a reliable gearbox. And so, with this superb Sport-Prototype car, Audi won the 24 Hours of Le Mans, and placed very high the level of efficacy of these vehicles.