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.
Computational Fluid Dynamics (CFD) helps to predict aerodynamic forces. In this example, v
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.
For example, it takes almost five times more power to maintain your car at 100 mph than it
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.
Check out Part 2
Automotive Aerodynamics Part 2
By John McNulty
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