AERODYNAMICS

McLaren prides itself on producing extremely high performance vehicles which apply the lessons learnt in racing to road cars. This means that unlike most road cars McLaren aims to produce cars with substantial levels of downforce without excessive drag. It is essential that the downforce is distributed in a balanced manner with the centre of pressure located behind the centre of gravity. The control of the centre of pressure is particularly difficult during braking/acceleration as the car pitches with the effects of weight transfer. McLaren are unique in the adoption of an airbrake which reduces the migration of the centre of pressure under braking. The algorithm for this device receives inputs of both vehicle speed and brake pressure, before deciding whether to deploy. At the extreme deployment angle large amounts of both drag and downforce are produced. The downforce is biased towards the rear and hence counteracts the effects of both front weight transfer and forward centre of pressure migration. The extra drag is a useful by-product in helping to slow the vehicle.

New car development involves the production of several scale wind tunnel models. The models are tested at a Formula One rolling road wind tunnel which enables accurate simulation of under body air flow. The vehicle ride height can be modified during tunnel runs so that any modifications are assessed through the full suite of anticipated vehicle attitudes. In some instances a design which gives good downforce values may be rejected because the downforce value varies too widely as the pitch of the vehicle changes. Internal air flow modelling is another important feature of the wind tunnel model design. By careful selection of porous media, it is possible to get an accurate representation of vehicle radiators etc. This then enables early estimates of a vehicles cooling capability.

Tunnel work is enhanced by the use of computational fluid dynamics (CFD); this enables the mechanisms of drag and downforce to be more fully understood and hence promotes innovative design solutions. The vehicle is placed in a virtual wind tunnel with graphical outputs of air flow vectors and total pressure plots (amongst other options). The flow visualisation possible with these techniques, especially in the area of diffuser development is simply not achievable with traditional tunnel testing on its own.

The results of all this work are verified on the test track. Top speed runs validate the predicted drag coefficient, while high speed handling and the measurement of forces through the vehicles spring platforms, enable downforce predictions to be checked. By careful integration of the styling and development process, McLaren operate a concurrent approach to achieving its aesthetic and technical targets.

THE AIRBRAKE

Moveable aerodynamic devices have been banned in Formula 1 since 1969. No such regulations exist for road cars, meaning McLaren is able to exploit all the innovation that would otherwise appear on its Formula 1 car on its road cars. The airbrake is one such example.

The McLaren F1 featured such a “Brake and Balance ‘Foil” to increase base-suction at the rear to generate additional downforce and more importantly, control the aerodynamic centre of pressure. Well established as a McLaren feature, the SLR also features a similar device.

In the SLR, if the driver steps heavily on the brake pedal, the rear spoiler rises to an angle of 65 degrees, boosting the braking effect by controlling the centre of pressure, thus maintaining optimum aerodynamic balance and stability at the high body pitch angles that 1g braking produces.