Maybe the world of prototype race cars is a parallel one to formula one, because these have a massive technology inside their projects and also is fascinating. In terms of aerodynamics, closed wheels race cars were always more complex than open wheels ones. Prototype aerodynamics evolution not only represents the development of a field inside the motorsport, but actually an area inside the automotive industry.

80’s era

In the 80’s era of Le Mans Prototype (LMP), the aerodynamics was based in a front underwing, the same concept of a closed front-end which was later used in GT racing. In other words, the front-end is a kind of aerodynamic device that works the flow field and its pressure variation near wheel arches. The suction is made at the top and the downforce is generated at the front. Hence, LMP from 80’s were very sensitive to ride height variations. This was the reason behind of some take-offs that occurred with LMP cars.

Late 90’s

In the late 90’s Toyota appeared with the first front-end as an open system. This concept provides for the air a passage through the front splitter that exits the front underwing by the sides of the car. This means an exit boundary condition less dependent on the ride height, because the pressure that acts on the sides of the car is always the same. Hence, if the front ride height (FRH) increases a bit, as occurs in usual movements at the track, air will not act as an airbag below the front underwing and underfloor, because there is also the evacuation at the side pods of the car. This reduces the RH sensitivity. The great improvement of the open front-end was on the performance. Since the car is less dependent on the ground clearance, it becomes more predictable and easy to drive. There is no aero balance transfer at braking and acceleration. Hence, the car is more drivable. Actually, the big improvement of the open front-end was not the concern of safety, instead it was a matter of handling, drivability of the car. In case of GT cars, the sensitivity to RH is twice the same from LMP, but this is not in terms of safety, instead, this is all about performance. Since GT cars are slower, this is not so critical. For LMP cars, the open front-end was a big step in terms of safety and performance, because it makes LMP cars more drivable.

Early 2000’s

In the early 2000’s era a new concept appears, the split bodies. In this one the open front underwing has the air evacuation driven to the car sidewalls as usual, but it leads the air flow to another profile behind the rear wheels. This is a similar concept to the open wheel car, because the front assembly is physically and aerodynamically independent from the rest of the car. The objective is separate the front underwing to the mid-rear body. This results in a significant improvements in downforce and stability. Although LMP became more predictable, these cars had preserved a flat underwing. Consequently the sensitivity to roll and yaw was still present on them. This problem became even more highlighted after the accident that occurred during one of the tests hold by Audi for its R8 LMP car. After this accident, Audi and Technical Working Group (TWG) worked a lot in wind tunnel tests with 1:1 and 40% scale R8 models, respectively. As a result, it was detected that LMP cars with flat underwing motivates quick variations of downforce, that goes from downforce to lifting very fast. This occurs in situations which the car exhibits angles of yaw and roll together. In these situations, the flat plate did not avoid that a lateral airflow lifted the chassis.

The proposal from the studies of the flat underwing applied on LMP cars was a new design of it. Figure 4 illustrates a drawing of the new underwing which is being used today. At this one the yellow surface indicates the flat portion of the underwing, which is reduced. The lateral portion of the underwing has a ramp of 7°. This results in a dihedral underwing cross-section. The objective is to create a suction air flow when the car exhibits a high yaw movement. The result is a behavior similar to a wing profile, which generates suction at the central section. Other limitations are performed on the front diffuser, which obey a minimum height of 50 mm for the mid to 1000 mm portion. In other words, the front underwing is higher from the ground than before. The result is a shorter and taller front. The leading edge of the flow was moved from near the front axle to 400 mm further from it.

One of the last modifications done on LMP is the vertical fin at the mid rear portion of the car. The objective is to improve the lateral stability. The main effect of the vertical fins occurs when the car is at high yaw angles. These induce a straightening effect when the car is under this situation. If the vertical fin has a convex surface, it can also provide a suction effect that helps in the stability, but in most of the applications these are small and long, without suction effect, but with stability improvements.

Early 2010’s

The next improvement occurred in the front under wing on the LMP car from the 2010’s generation. To reduce even more the dependency from RH, it was introduced the slotted front underwing. First it was raised in the central part and slots were introduced. These divide the airflow between the bodywork and the splitter. In this way, the front-end works more as a wing, because the air divided by the slots can go to the body and to the flaps. This last one creates a downforce over the wheel archers, which has the same effect of a wing. As a result, the slots create one or more planes for the flow field. Hence, the front ride height sensitivity is further reduced, because slots create a wing which the reference plane is not the ground, instead is the front-end together with the wing created by the slots.

Similar to GT cars, LMP also exhibits louvers. These have been mandatory since 2014. However, it always was a controversial topic during inspections. Hence, it was defined that louvers are a top cut-out on the front axles, while in the rear axle the cut is at the top and the lateral of wheel archers. The main objective of the louvers is to help the air evacuation.

Figure 8 illustrates the global aerodynamic performance of cars. As can be seen, only the road cars generate lift, some of them generate downforce, but in a very small amount. Actually, road cars are much more complex to design, since they must deal with customer perception of the product and emissions. Two very complex variables. Above road cars there are the high performance road cars, which have more focus on performance, despite the fact these are road legal. For this reason these generate a significant downforce. Above them there are the main racing car categories, GT, open wheels and prototypes. This last one is capable of producing the highest downforce with the lowest drag. This occurs due to the lower dependence of the front wing aerodynamics, which was the case for open wheels race cars. These also produce a very high downforce, but it is reduced since there are exposed wheels which disturbs the airflow for the rear wing. GT cars are basically the race version of high performance road cars, these are based on the front underwing concept. Due to its increased frontal area, there is a significant drag.

The top body of a prototype is completely different from a production derived race car, as can be seen in Figure 9, it looks like a streamlined body, mainly at the upper part of the cockpit and the wheel archers. The objective is to reduce drag since the upper body is not, at least usually, providing downforce. In other words, reduce frontal area. Although there are some trade-offs between style and downforce, because the endurance series has a great appeal for vehicle selling, it is always ensured a style that does not disturbs the aerodynamics. An opposite situation is seen in GT cars, which can be considered a blunt body.

At the rear part of the car there are two features, the rear wing and the compression zones. The first is self-explained. However, in prototype race cars their rear wing has a large aspect ratio to provide a very high efficiency (4 < Eff < 6). In other words, high downforce production at the cost of a small drag. The compression zone refers to the rear bonnet of the car.

Actually, all cars have the tendency to produce drag and some lift in the back. On prototypes there is a pressure rise near the trailing edge of the bonnet (the prototype is midship car) and this works together with the rear diffuser and rear wing. This also helps in the rear wing efficiency. The height of the rear bonnet trailing edge is the key point of the rear aerodynamics, it increases the compression, thus the drag and downforce and improves cooling. However, it also has a negative impact on the suction of the rear wing.

Another characteristics of the rear part of a prototype is the blanks. They are used for blowing (Figure 11 – Lighting yellow blanks). It results in a beneficial effect called, ‘base-bleed’ that improves the drag reduction at the rear part of the car.

Prototypes aerodynamic summary

Figure 13 illustrates the aerodynamic overview of prototype race cars. As can be seen, the highest pressure coefficient (C_P) is obtained at the front underwing. Since its value is negative, it means downforce. The huge gain in downforce is observed at the front, while the flat plate and rear diffuser provides a significant gain with relatively stability. The term device refers to the rear wing, which provides a high downforce, but with some drag. The interesting point about the upper body, is that it holds some parts with C_P = 0, but there are parts producing downforce, as the one due to the front slots, but the wheel arches result in lifting generation.

Conclusion

Figure 14 provides a histogram with a comparison between prototype and GT race cars. It is interesting to observe the differences. In terms of drag, these are basically the same, despite the more efficient rear wing of the prototypes. The huge difference is observed in downforce, which prototypes are capable of producing much more downforce from its underbody (front under wing is included in underbody). The body refers to the bodywork. GT cars have less lift when compared with prototypes, but with more drag than this one. Therefore, it is possible to conclude how prototype race cars are able to extract too much downforce, these are strongly dependent on the underbody. Wings are basically secondary aerodynamic devices.

References

• This articles was based on the lectures notes written by the author during the Industrial Aerodynamics lectures attend in Dallara Academy;
•  Romanelli, R. Pannullo, A., Zanello, M. Endurance WEC. Dalle Gruppo C ai Prototipi ibridi. Ediz. bilingue italiano/inglese. Giorgio Nada Edizione, Ottobre, 2021.