Track aerodynamic measurements
The measurement of aerodynamic data on the racetrack has long been an important factor in developing a vehicle's performance, no more so than in Formula 1. With significant costs and efforts directed towards development in wind tunnels and using computational methods, it’s vital to know that this actually translates to performance at the track. In fact, this is the only aspect that truly matters. Wind tunnel and CFD testing are idealised versions of reality and do have limitations.
It is of course possible to measure performance based purely on lap times with a stopwatch, particularly if the performance gain is sufficiently large enough. This was the case in the distant past, but today the gains can be very small and can be overwhelmed by other factors on track (tyres, weather, etc). Engineers also want to be able to understand the performance in more detail, and a single measurement of whether your vehicle is faster or slower over a lap (or sections of a lap), provides limited information.
To provide further information to engineers, sensors were developed to measure suspension displacement, and knowing the suspension kinematics, compression forces can be calculated acting at the front and rear wheels, and hence the car’s downforce can be known. This data was the first step in increased understanding of the aerodynamic loads the car experiences during a lap. With advancements in computer and sensor technology, this type of data could be measured and transmitted live via wireless communication to the engineers in the pit lane. While this was a big step forward, the measured data was not without issues. The biggest issue being noise in the signal data due to the dynamic nature - such as vibrations due to bouncing on the track, and from the drivetrain. Not such a big issue at high speed, where the compression forces can be very large, but a significant issue at low speed (<150kph) where the noise can overwhelm the force data.
Given the importance of low speed corners to the performance of the car, further measurement tools were implemented to supplement the suspension force data. A surface pressure measurement can be taken using a pressure tap: which is a very small hole flush and perpendicular with the surface, connected via tubing to a pressure transducer. These pressure measurements are an excellent way to compare real car measurements directly to scale wind tunnel and CFD data in the exact same location. Surface pressure measurements were initially added to the car in key areas; for instance, on the surface of the underfloor diffuser, being one of the critical aerodynamic performance areas. Several location points would be added, typically in areas with high pressure gradients.
The evolution and utilisation of surface pressure measurements has accelerated in the past 5 years or so, to become a significant contributor to understanding the aerodynamic performance of the car at the track. Large arrays of these sensors now cover many surfaces of the car - in fact, all of the main areas that contribute towards the aerodynamic performance of the car (such as the underfloor, and the front and rear wings). The quantity and distribution of the pressure measurements is now sufficient to be able to determine the force on the respective surface/component, which when summed can be used to produce the amount of downforce being created at any given time. This measurement process allows much greater accuracy of track data at slower speeds, therefore supplementing the data from forces through the suspension. It is the combination of these data types that are reviewed live during a race in the pit lane, with engineers being able to identify aerodynamic issues and changes to specific areas or components on the car, giving feedback to the driver.
The evolution of this measurement technique has been helped by the advancements in pressure scanners: which can log a huge amount of pressure taps simultaneously, have extremely fast response times, and are now small enough to fit within the most extreme locations on the race car.
There are multiple benefits of such track data. In addition to live track data, it allows correlation with the tools used for development (scale wind tunnel testing and CFD), where data can be directly compared and overlaid. It also provides a wealth of information in areas that are not as well simulated in the wind tunnel/CFD, such as in geometry local to the tyres and their contact with the ground. This has allowed engineers to further their understanding of the aerodynamic performance of the race car, in the environment that is of the most importance.