You knew this day was coming. It was only a matter of time before we would have to add automotive aerodynamics to our long list of technical rants. But this isn't what you think. Although we wince at triple-decker aluminum wings and ultra-wide, overducted body kits, we'd be the last to tell you that such alterations are not important. We don't have a problem with the spirit of these modifications; it's simply the execution that leaves so much to be desired.
But before we can get into that, it's important to understand the fundamentals. Although complicated and steeped in theory and equations, common sense and a basic understanding of the mechanics of airflow go a long way towards providing an understanding of how aerodynamic aids work and how to utilize them.
We all know a little about aerodynamics. Stick your hand out of the window of a moving car and you can immediately feel the effects of airflow-lifting your hand up or pushing it down, in addition to moving it backwards. Place your hand perfectly flat in the wind, parallel to the ground, and you should only feel your hand being tugged backwards a little. That tugging force is drag. As you increase the angle your hand makes to the oncoming wind, your hand is forced upwards. This is lift. Decreasing the angle of your hand from the flat position will cause the wind to force it down, hence the term downforce. In both situations, the effects of drag increase because the surface area exposed to the airflow increases.
As applied to cars, the broad logic of these concepts is pretty easy to understand. Generate enough lift and you can fly, which is about the last thing you want a car to do. Turning lift upside down, however, creates downforce, which race cars use to corner faster. But all of this comes at the cost of drag, just like in the hand experiment. The trick is to find that magic balance of getting the most downforce with the least amount of drag. Doing so requires digging a little deeper into aerodynamic theory.
The point of automotive aerodynamics is to shape a car so that it disturbs the air it moves through as little as possible. Literally, aerodynamics means the study of the forces acting on a body moving through air. In our case, the body happens to be a car and the forces will depend on many factors such as the shape and size of the car, in addition to some of the properties of air.
Engineers measure power in units such as watts (W) or horsepower (hp). Torque is measured in Newton-meters (Nm) or pound-feet (lb-ft). Aerodynamicists determine how well an object slices through the air by using coefficients, which differ from other measurements in that they lack units. This is because unitless coefficients allow for the comparison of cars of different sizes and shapes. For example, a Honda Civic is shorter and has a smaller frontal area than a Mack truck, but by using the coefficient of drag (CD) as a standard of measure, the aerodynamic efficiency between the two can be fairly evaluated. In addition to CD, other commonly used measures are the lift coefficient (CL), and side force coefficient (CY).
By now you might be wondering what all the fuss is all about. The hand-out-the-window example is useful for explaining the concepts of lift and downforce, but doesn't do justice to the negative aspects of poorly designed aerodynamics. Aerodynamic drag hurts performance. It is chiefly responsible for the top-speed limitation on any car since it takes exponentially more power to maintain higher speeds. For example, it takes almost five times more power to maintain your car at 100 mph than it does to drive it at 60 mph. More drag (a higher CD) magnifies this effect, negatively impacting top speed and fuel consumption by requiring the engine to do more work.
There are three primary ways to reduce drag. One is to reduce the frontal or cross-sectional area of the vehicle. Think of a car punching a hole in the atmosphere while in motion. Cars with less frontal area punch smaller holes, thus creating less disturbance. Of course, there are few things you can do to reduce frontal area on a car beyond taking off the side mirrors and running on skinny doughnut tires.
A better way to reduce a car's CD is to make it more streamlined. This happens at the design phase and involves implementing smooth curves on body panels, rounded lights, soft windshield-to-roof transitions, smooth A-pillar-to-side window treatment, and more.
The third method is a modification of streamlining and centers around using a flat or smooth tray under the car to allow air to escape with less drag. Without using an underbody tray, airflow beneath the car is highly turbulent, due to the clutter of mechanical bits like the exhaust system, driveshaft, fuel tank, shift linkage, and suspension. A smooth underbody also reduces lift by allowing the air to move faster and therefore at a lower pressure. The added stability from the reduced lift is why most supercars and race cars now have smooth underbellies. It's the most practical way to improve performance compared to reducing frontal area or streamlining the body.
Like drag, lift becomes a problem for cars at high speeds. Think about airplanes. To fly, airplanes have wings that generate a lifting force equal to or greater than their massive weight. A car body has a shape similar to that of an airplane wing, with a curved upper surface and a relatively flat underside. When moving, air going over the top of the car accelerates, while the air going under stays at approximately the same speed as the car. Given enough speed the car will lift like an airplane.
Bernoulli's principle states that faster air above the car has a lower static pressure than slower air below it, and so the car is literally being pushed upwards. The result is lift, a curse to almost all production cars. For example, the 1995 BMW M3 has a lift coefficient (CL) of about .34, which means that a lifting force of approximately 500 pounds is generated by the body at 100 mph.
Certain high-end sports cars have found ways to eliminate lift. The Ferrari F430 actually generates downforce to the tune of about 300 pounds at 124 mph and 616 pounds at 186 mph. This is due to a reduced cabin height, low ground clearance of the smooth underbody, and very effective diffusers. Surprisingly, the F430 doesn't use add-on downforce producers such as front splitters or rear wings. It's a testament to Ferrari's racing pedigree that they can achieve this level of downforce and still maintain a relatively low CD of .32.
The third aerodynamic factor to contend with is side force, which is similar to lift but acts upon the automobile from the side. Side force is often ignored on street cars, but for F1 and rally cars that experience yaw at high speeds, lift caused by air passing over and under the car from the side becomes a significant factor in handling and stability.
But why is lift so bad? You'd be right in assuming that no car produces such a dramatic amount of lift to be dangerous to drive. But lift does negatively affect performance in two ways. First, lift reduces the load acting on the tires. Since the maximum amount of traction available from each tire is a function of the load acting upon it, a reduced load means less available traction. Lift also causes extra drag called "induced drag," which is a good percentage of the overall drag on the car.
Now that we know a bit about the aerodynamic forces that act on moving vehicles and the coefficients that define them, it is helpful to understand how aerodynamicistis collect and use this information. Aerodynamics is an incredibly complex field. Even with the use of complicated equations, the effects of aerodynamic forces can only be determined for simple scenarios. For something as complex as a car with rolling wheels, vents, and spoilers, all on a moving roadway, aerodynamicists have to rely on two general methods.
One is a computer simulation method called Computational Fluid Dynamics (CFD). With CFD, computer algorithms approximately solve aerodynamic equations for a given car design and airflow velocity. Extracting usable data requires extremely powerful super-computers and accurately digitized three-dimensional car models. Most large car companies and top Formula 1 teams use CFD to understand how air moves around the various parts on the car and improve troublesome areas where the air is not flowing smoothly. Although expensive and time consuming, the CFD method allows for many virtual designs to be explored before time and money is spent building an actual prototype.
The other method is the wind tunnel testing. For this kind of simulation, a one-third to one-half scale model is usually made. Air is then blown over the stationary model, which is positioned on digital force-measuring transducers. These force transducers measure the drag, lift, and side forces acting on the model. These results are used to calculate the drag and lift coefficients that we referred to earlier.
With the data gathered from CFD simulations and wind tunnel testing, engineers sculpt modern production cars with blended sleek curves, gently sloping windshields, and smooth underbodies. As a result, these vehicles possess performance features only dreamed about just a few years ago. Some of the benefits include improved fuel economy, better high-speed handling and acceleration, enhanced airflow to the engine, and higher top speeds.
In the next installment of this series, we'll discuss ways to apply these basic concepts and improve handling using some of the available aftermarket products. We will focus on ways to improve performance by reducing drag and lift and generating usable downforce through the use of wings, splitters, air dams, canards, spoilers, and side skirts.