PAT SYMONDS
PUTTING A NUMBER ON HISTORIC AERO FIGURES
Feedback is always welcome but when one of our loyal readers, Ben Halford, asked me to write about the historical development of aerodynamic efficiency, I hadn’t appreciated what a rabbit hole I was about to disappear into.
Firstly we need to establish what we mean by aerodynamic efficiency and how we express it. In this context efficiency would be expressed as the total downforce divided by the total drag. Here comes the first problem. While it’s been possible (albeit difficult) to pull together some numbers for downforce, it’s been much harder to get drag figures – so for the most part we’ll focus on downforce here.
Again we need to define how we will express downforce. It could be expressed as a force but this would then depend on speed. To get round this engineers use a ‘non-dimensional’ number which is generally known as the lift coefficient, or Cl, downforce just being negative lift. This should mean one car could be directly compared with another. Unfortunately this isn’t that easy for two reasons.
Firstly, to arrive at a lift coefficient one needs to equate it to a reference area which, in vehicles, is normally the frontal area. Now the frontal area of a Formula 1 car is just under 1.5 square metres so many teams will use 1.5 as the reference area. Others are more pedantic and will use a more accurate number – say 1.47 – while others still will say it’s only a notional thing, so let’s use one square metre. This means that if we quoted our lift coefficient as, say, 4.5 based on 1.5 square metres it would be 6.75 if based on one square metre.
It gets even worse, though. We like to express the lift coefficient as a single number but of course that number changes depending on the ride height of the car, the steering angle, and the angle of the wind amongst other things. Aerodynamicists get around this by quoting a weighted number with the weighting they apply to each individual condition based on its effect on lap time. So, for example, the drag is much more important when the steering is straight and the ride heights are low – in other words at the end of a straight – than it is when the steering is turned and the ride heights are high, the conditions you would find in a slow corner and where the downforce is more important.
It’s also true that windtunnel testing, which is where these numbers come from, has got significantly more precise since some of the earlier numbers were determined. So too have the methodologies for giving useful weighted values.
So, with these caveats, what does his
