Many years ago, the site that is now home to Silverstone race track was an airfield and naturally, these were built in flat areas. An obvious consequence of this is that they are very exposed to any prevailing winds. Silverstone is no exception, and an understanding of where the wind is coming from, and how it affects the car, is important when preparing for any event at this circuit because, as Fernando's qualifying lap earlier this year in Barcelona demonstrated, invisible and unpredictable gusts of wind can potentially have a significant impact on the handling of the car.
A Formula 1 racing car is fundamentally a very sophisticated aerodynamic device, and therefore the wind plays an important part in its performance. For many years, racing cars were tested in wind tunnels with the wind blowing directly onto the nose of the car in the straight ahead position. These days, it has been understood that this is only part of the story and testing now takes place with the model at many different attitudes. One of the most important of these is to test the vehicle in yaw. Yaw can be thought of as a rotation of the vehicle about a vertical axis through the centre of the car, so that the wind, instead of blowing straight on over the nose, is coming at the vehicle from a slight angle. When one looks at the aero devices on a racing car, we can see that items such as the wing endplates, while being very effective when the wind is travelling fore and aft, also have the effect of masking the wing slightly if the air is coming from an angle, and thus reducing the amount of downforce generated once the car is turning. If the car encounters a crosswind, this induces what we term 'apparent yaw'.
The downforce generated by a racing car is proportional to the square of its speed through the air. Consequently, if the wind is blowing towards the front of the car, then windspeed is added to wheelspeed to produce additional downforce. If the wind is blowing towards the rear of the car, then it is subtracted from vehicle speed to produce less downforce. It might be thought that the effect of wind which rarely blows at more than 20kph would not have a big effect on a vehicle that can be travelling at 320 kph, but with a racing car you are always trying to operate at the very limit of the performance envelope and hence even relatively small changes can affect what the driver feels. As downforce is proportional to the square of its velocity, a 20kph headwind - which represents a 6% increase in speed at 320kph - will actually see the downforce increased by approximately 13%, and the effect on what the driver feels will be magnified the faster he is travelling, once again because downforce is proportional to the square of the speed.
Of course, if the wind is blowing at any angle, then not only do you have a component of the wind that may be adding to or subtracting from the total downforce, but you also have a degradation in performance due to the apparent yaw effects mentioned earlier.
The main area for understanding these effects is the wind tunnel. The less sensitive you can make the car to abnormal wind conditions, the faster it will be, irrespective of prevailing conditions. In order to understand what the wind has done to the car, we also continuously record the 'apparent wind' the car sees using two methods. On the front of every car, you will see a pitot tube, a device that measures windspeed. The speed it measures is the combination of the true atmospheric wind and the 'apparent wind', which is the airspeed generated by the movement of the car. By comparing the airspeed velocity with the car velocity measured by wheel rotation, we can deduce the windspeed. In addition, the cars sometimes carry a more sophisticated device known as a yaw probe. This works in a similar way as a pitot tube by measuring the dynamic pressure of the wind and comparing the static pressure of the air, but with added sophistication of being able to determine the angle of the wind.
With the wind having such an effect on the performance of the car, we obviously need to be aware of it and do what we can to mitigate its effects. From Silverstone onwards, we will once again be able to time our qualifying runs in order to be on circuit when the wind is at its lowest - a luxury the one-lap system did not afford. However, with the sessions requiring a minimum number of laps to be completed, and being short in length, occasions will undoubtedly arise when we are forced to run irrespective of the prevailing climatic conditions.
However, it is also important for the drivers and the engineers to understand the wind conditions so they can best handle them. For example, T1 at Silverstone - Copse Corner - is one of the fastest corners of the season, taken very nearly flat. A headwind on entry is an asset the driver should exploit in order to take the corner faster, while a tailwind means he needs to exercise extra caution. The complication for the drivers comes when the wind gusts unpredictably - at this point, they must make a split-second judgement of the conditions in order to judge exactly how fast the can take a particular corner.
