Aerodynamics

It’s a development process that never ends: Engineers work tirelessly to constantly redefine the aerodynamic performance of the Porsche 919 Hybrid. For the 2017 season, the prototype was given a full aerodynamic design facelift. The enhancements were fuelled not only by a drive to improve performance, but also to boost safety: The race regulations are intended to prevent prototypes from reaching irrepressible, ever-increasing speeds while ensuring that the vehicles stay on the track in the event of incidents such as spins.

Each and every minor detail that enlarges or shrinks the front cross-section of the vehicle, which affects the air flow, has a direct impact on lap times. But for Porsche, the lessons learnt on the track today help to optimise air flow in the road vehicles of tomorrow – for lower fuel consumption and even better performance.

Take the car door as an example: The regulations specify a minimum size for the door, which only weighs around three kilograms. The driver must be able to exit via the door within seven seconds. A quick-release system, which allows the door to be released from its hinges, is mandatory for emergencies. The driver door also functions as a support for a headrest structure, which is made from shape memory polymer and covered in aramid fibre composite material. To survive a heavy impact from the driver’s helmet, the door frame is subjected to a 700-kilogram cross-directional load, and must remain undamaged and undistorted after the test. A further key factor in aerodynamics: During the race, a low-pressure area builds up to the side of the cockpit, generating up to 60 kilograms of force pulling the door outwards. The frame – made of fibre composite with highly modulated carbon fibres – must be rigid enough for this pressure not to affect the aerodynamics; the door pane itself is made of polycarbonate at least two millimetres thick.

The wheel arches are a further example: At Silverstone, the aperture on the front wheel arch is subtly and smoothly chamfered. At Le Mans, it is tapered in towards the tyres, while on the Nürburgring, it still has a protruding lip. These kinds of finely tuned details are the result of intensive aerodynamic analysis. This is a process with a clear objective in mind – but it is also a process that will never end. There is always room for improvement.

Detail modifications are permitted to a certain extent for each and every track, although from 2017, the regulations limit the use of complete aerodynamics packages to two per season. The design of these packages is orientated towards the specific requirements of Le Mans, with its long straights, on the one hand, and the remainder of the season with the eight more compact tracks on the other. These tracks require greater aerodynamic downforce; the lower drag has less of an impact on the top speed. The two car bodies differ by up to 80 per cent – yet the variations remain largely invisible to the untrained eye.

On the Le Mans prototype 919 Hybrid, a team of more than 20 aerodynamics engineers works to balance the two sides of the same racing coin: downforce and drag. But what causes downforce? It could be something as simple as a steeply angled front or rear wing profile. If the air flow under the wing profile is faster than the air flow above it, low pressure is created under the profile. This pressure difference generates downforce, which pushes the vehicle down towards the road surface. However, gains in downforce always come at a cost – namely larger air flow contact surfaces. Higher drag will, in turn, bring down the top speed.

And wing profiles are only part of the picture. Every square millimetre of the carbon fibre body of the prototype, every air intake and outlet and every finely finished edge is designed with aerodynamic efficiency in mind. The majority of the details that affect aerodynamics are invisible to the casual observer, hidden under or within the vehicle. The air flows around the entire vehicle and through the vehicle body are complex and interdependent factors, and are exposed to a whole range of driving situations – straight stretches or bends, braking phases, side winds, slipstreams or even vortices created intentionally by driving close together. Due to the conflicting requirements arising in ever-changing driving situations within the same race, it is not feasible to optimise every single detail for every eventuality.

In various stages, the engineers work to determine where the details should be fine-tuned to prioritise downforce, and where it would be more beneficial to reduce drag. Where available, data from previous years plays a key role in this process, and the track profile – its layout, topography, asphalt quality and predicted temperatures – is also important.

Before any body elements for the Porsche 919 Hybrid are developed and constructed as models, CFD (Computational Fluid Dynamics) systems are used to simulate the relationship and interplay between the computed parts. The next step is to construct a model – the Porsche engineers test a 60-per cent model in the wind tunnel of the Williams Formula 1 team in the British town of Grove in Oxfordshire. Only once these tests are complete can components be produced and tested in actual size. Without this rapid prototyping process, parts production would be far too costly and time-consuming. And now the race team at Weissach has yet another asset available: The new 1:1 scale wind tunnel in the Porsche Development Centre, which enables the engineers to test the car in actual size. Whether they are developing series-production cars or racing cars, this technology fosters even closer collaboration between engineers in the discipline of aerodynamics.