Automotive aerodynamics
WHEN I was studying engineering in Cape Town during the early Seventies, one of the topics covered was aerodynamics which came under the umbrella of fluid dynamics. Comprising of equations for drag, Reynolds numbers and boundary layers, these were all theories and maths, which I didn’t find very interesting.
But that changed a decade later when I was designing my first complete car, the XJ41, and took a quarter-scale model to the Motor Industry Research Association’s (MIRA) wind tunnel at nearby Nuneaton to test the shape.
Aerodynamics had always been about streamlining but by the time we were doing XJ41, drag and CD (drag coefficient) was becoming the focus of attention.
However, when we got into the wind tunnel, I discovered CD was actually the lowest priority. When I asked why, I was told we have to start with cooling; if there’s not enough cooling, the engine soon overheats and you don’t have a serious production car. So cooling was the first thing we tested for and it didn’t all come from the grille. The most effective cooling – as I was to later discover with the XJ220 – was from the air dam which is why most cars have lower vents. The image above shows an XJ220 in a wind tunnel with sensors for measuring air velocity ratios through the radiator.
The second most important aerodynamic factor was lift, which can affect dynamic stability. Too much lift results in less tyre grip with the road, which affects handling. So, one of the key aerodynamic mantras was to have more rear lift (CLR) than front lift (CLF), so as you go faster, the car pushes itself nose-down. Unlike the XJ-S which had more front lift, meaning the car is less stable at high speed.
The normal way to make the rear lift greater than the front lift is to use a chopped-off high tail, (usually called Kamm named after the German designer who discovered the phenomenon in the Thirties) or spoilers on the boot lid. This is why I changed my original design on the XJ41 from a round tail to a squarer cut off.
TOO MUCH LIFT RESULTS IN LESS TYRE GRIP WITH THE ROAD
As an aerodynamicist, the man behind the C, D and E-Types, Malcom Sayer, was interested in drag but not lift and therefore his cars didn’t feature wings or air dams which allowed the air to flow both under and over the cars. A
