This article was originally published on the September 1986 issue of Monthly Design, a major Korean design magazine.
It was a little over 30 years, so I revised and supplemented the article partially to fit today’s context. I hope this is helpful for those who design automobiles.
When Carl Benz first invented automobile in 1886 and even when Henry Ford started mass production of automobile in 1908, the carmakers didn’t seem to have been concerned about the impact of the wind. One of the proofs is that most of the car bodies were boxy, and windshield glasses were usually vertical. As automobiles became faster and the stability of the car became a major issue, and when airplanes that cannot ignore the impact of wind emerged, aerodynamics became important in design and engineering of automobile automatically.
According to Chuck Jordan, a former General Motors Vice President of Design, designing cars that meets not only aesthetic beautiful, but also more aerodynamic is a meaningful task only good designers can perform.
The force of the air that affects a moving car is divided into three kinds – drag (Cd – Drag Coefficient), lift (Cl – Lift Coefficient), and side force (Cs – Side Force Coefficient). Drag is basically the force that will prevent cars from moving forward. When people mention aerodynamics, it usually refers to drag because it directly affects the speed and fuel economy of a car. Lift is the force that lifts cars from the ground, and side force is simply the force pushing cars from the side.
The longitudinal direction of a car is called X axis, the axis that is horizontally to the ground and perpendicular to the X axis is the Y axis, and the axis perpendicular to the ground is the Z axis. And the force to rotate the car around the Y axis is a pitching moment (Cpm), a force that tries to rotate the car around the X axis is a rolling moment (Crm), and a force to rotate a car around the Z axis is a yawing moment (Cym). These moments and the aforementioned drag, lift, and side force are six impacts of wind to the vehicle.
Before dealing with the automotive aerodynamics, another thing to know about is the center of a car. There are two different kinds of centers in a car. One is the aerodynamic center and the other is the center of moments. The center of the moment is the weight center of a vehicle that’s constant as long as the weight of the car remains unchanged. But the aerodynamic center varies from time to time depending not only on the shape of the body but also on the speed, direction, etc. of the wind.
All objects are surrounded by air and the air around this object can be regarded as a thin film of air. As the object moves, the film of this air moves accordingly. As the object moves faster the film of air gradually deforms, and if it exceeds a certain limit, it falls off the surface of the object. This is called a separation phenomenon or flow separation and the point where the separation occurs is called separation point.
The vortex occurs just behind the flow separation point. Vortices are a major cause of drag, especially in the front and rear corners of the roof panel, especially near the front end of the car body and cowl (where the engine hood and windshield glass meet).
The most influential force in the air that interferes with the movement of a car is drag. Of course, the drag also increases when the speed increases. However, depending on the style of the body, the drag can be significantly reduced. The drag of two objects moving at the same speed under the same atmospheric pressure is compared with the drag coefficient (Cd) of the object multiplied by the front projected area (A) of the object. Theoretically, the smaller drag is desirable. But it is not always the best to make it streamlined with less resistance because there are many other important considerations in automobile design such as passenger space, manufacturability, road conditions, cost, and many others. Reducing the air drag can improve gas mileage, increase the speed, secure the ride stability, reduce the wind noise, and prevent the wiper from floating.
The drag on a moving car can be classified into three types: pressure drag, friction drag, and induce drag. The largest part of this is the pressure drag, which is closely related to the generation of vortex. The pressure drag can significantly be reduced by streamlining the shape of the body.
Lift helps airplanes to takes off from the ground, but lift is rather disturbing in case of cars. In other words, when a vehicle experiences force of air lifting it from ground, the friction with the ground becomes less and thus the driving stability is decreased, the gas mileage is worsened, and tire would wear more. While the lift essential for takeoff of an airplane is positive (+) lift, negative (-) lift is beneficial for automobiles. The reason why the wing of the airplane can generate lift is because the shape of the upper part of the wing is convex, so the flow of air through the upper part is accelerated and make the air pressure there lower. According to Bernoulli’s law, the wing moves towards where the air pressure is lower.
Since the shape of a typical automobile is generally similar to that of an airplane wing, the airflow above the automobile becomes faster than the bottom, and thus the lift force is generated. Racing cars are affected by this lift significantly as they are much lighter and faster than the typical cars. So air spoilers such as inverted airplane wings have been developed for race cars and used in performance automobiles too.
4. Side force
If there is no wind or the wind matches the direction of the moving automobile, there won’t be any side force. When the wind blows from the side, the effect of the wind is more serious than you might think. This lateral wind is closely related to the driving stability of the vehicle. When a car is moving, drag and lift aren’t easy to feel. But if you drive on a windy day, you can easily feel that the car swings from side to side. In fact, when the vehicle is moving out of the tunnel, it may be shaken or even overturned due to a strong side wind. In the case of vans with a high center of gravity and flatter side shape, the impact of side wind is not ignorable.
It is important to know how to reduce the influence of the transverse wind by making body ideally reacts to the side force. It is also considered to widen the tread (distance between the centers of the left and right tires) or lowering the height of the vehicle in order to fight against the side force.
Note: Some of the illustrations are from the Aerodynamics special issue of Car Styling
[continued on part 2]