Background

As an airplane travels through the air, it must maintain a balance between the forces it produces: weight and thrust, and two aerodynamic forces: lift and drag. Road vehicles move essentially in a two dimensional plane and so have comparatively little contention with weight and lift. Thrust and drag, however, are of utmost importance. Thrust is dictated by the performance of the vehicle’s propulsion system. Drag depends upon a number of factors, some of which can be manipulated by designers in creating an aerodynamic vehicle.

The four aerodynamic forces acting on a ball rolling across a flat surface. Weight and lift are not important in the ball's forward movement, which is due to thrust and is restricted by drag.

Drag can be thought of as aerodynamic friction. Like any friction force, it opposes the motion of a moving body (the thrust) by generating a mechanical force in the opposite direction. The drag force must be matched or overcome by the thrust supplied by the truck’s propulsion system. Although the drag forces on a tractor trailer are generally dwarfed by the thrust, they must be exactly matched during constant speed movement and can be substantial enough to significantly impact the efficiency and performance of the vehicle.

The magnitude of drag is affected by the velocity and inclination of the object to flow, the properties of the air medium and the moving object’s shape and size.

The properties of air that impact the drag forces are its mass, viscosity and compressibility. The initial direction and velocity of any air movement are also impacting factors. These variables are important considerations in any controlled experiment but it would impractical to attempt to control them in everyday situations.

Thus, the most important factors in designing an aerodynamic vehicle are size and shape. Size is related to drag linearly; as size increases, drag increases correspondingly. Shape affects the total drag force through three types of drag: skin friction drag, form drag and induced drag.

Skin friction drag is caused when a layer of fluid around the moving object cannot flow past it and is sheared from the rest of the fluid as it moves with the object.

Form drag is a pushing force created when a pressure differential forms between the front and rear of the object. If air cannot flow over the object, it becomes trapped at the front and is compressed, creating a high pressure zone. Meanwhile, the lack of air flow behind the object creates a pocket of low pressure at the rear of the object. This pressure differential creates a force pushing in opposition to the object’s thrust.

When the fluid (air) cannot flow over the object, it builds up in front of the object and creates a high pressure zone. The build up of fluid in front is paired with a corresponding low pressure zone behind the object and the result is a form drag force pushing backwards on the moving object.

 

Induced drag is created when the air that flows around the object is left in a state of greater turmoil. This greater turmoil requires an increase in the amount of energy in the air. The extra energy is is taken from the moving object's thrust and is called induced drag.

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