Aerodynamics is such a vast subject that it would take many, many¬†Simple Techs to explain it fully. But in the¬†simplest of terms, aero refers to air and¬†dynamics, refers to its flow. Aerodynamics,¬†then, is the science of how air flows, in¬†our case, around objects in its path, like¬†a car or a motorcycle. In this context,¬†automotive engineers worry specifically¬†about two interactions between air and¬†our car/motorcycle, drag and downforce.¬†When a car starts moving, the¬†movement causes friction between the¬†surface of the car and air. This resists¬†motion and is called skin friction and is¬†one component of drag. Drag depends¬†on many things but topping the list of the¬†big contributors is the shape of the car,¬†specifically its frontal area. The larger the¬†area, the greater the drag ‚Äď this component¬†is called ‚Äėform drag‚Äô. That‚Äôs why buses aren‚Äôt¬†very fast and aircraft are usually pointy at¬†the front. The density and velocity of the¬†air also play a role and there‚Äôs a constant¬†called drag coefficient which we will come¬†back to. The point is, drag hinders motion.¬†So when you are suffering drag, you have¬†to burn more fuel to overcome it. It gets¬†worse. Air has a unique tendency of acting¬†like a liquid at very high velocities. This¬†behaviour increases drag exponentially as¬†speeds rise. It is the reason why very high¬†speed driving consumes an inordinate¬†amount of fuel.¬†Enter the study of aerodynamics. An¬†aerodynamics engineer‚Äôs job is to take¬†a car design and find tweaks that allow¬†drag to be reduced as far as possible. The¬†general silhouette of all cars today, in fact,¬†is a function of their need to be as drag¬†free as possible.
The reduction of drag begins in a¬†wind tunnel, which is a closed-off space¬†with a large fan that the engineers can¬†control to generate specific wind velocities¬†around a real-size car or a to-scale model.¬†What the wind tunnel helps measure¬†is the drag coefficient, a number that¬†engineers spend vast amounts of effort¬†to reduce. In essence, the bigger the drag¬†coefficient (written as Cd – coefficient of¬†drag), the more resistance that vehicle will¬†encounter.
Typically, a current day production car¬†usually has a Cd between 0.30 and 0.35.¬†The slickest cars like the Jaguar XE or a¬†Mercedes B-Class can drop that to 0.26¬†and still more aerodynamically efficient¬†are cars like the Toyota Prius and a few¬†Mercedes-Benz cars that manage 0.25 or¬†lower. But while you would think that¬†making supercars slippery is why they¬†spend so much time in the wind tunnel¬†but they‚Äôre in the hunt for downforce.¬†Downforce is the exact opposite of¬†lift, which is what aircraft use to fly and¬†stay up in the air. In effect, the wind¬†tunnel time of a supercar is used to refine¬†aerodynamic forms which cut drag and¬†also create a downward force that pushes¬†the car down harder on to its wheels as¬†speeds rise. This downward push generates¬†more grip that fast cars can use, for¬†example, to go around a corner even faster.¬†One way to understand drag and¬†downforce is that if you were to take a¬†family sedan and drive it in a vacuum, it¬†would report a higher top speed as well¬†as better economy. A supercar in the¬†same situation would do that as well but¬†be unable to corner at speeds it would¬†manage normally.¬†Wind tunnels, however, are very¬†expensive. The cheap solution increasingly¬†is to use computational fluid dynamics or¬†CFD to predict (very accurately in most¬†cases) how wind and a car‚Äôs shape would¬†interact. Next month, we‚Äôll dig deeper into¬†downforce and what it has come to mean¬†in the world of motor racing.