Aerodynamics and wind tunnel testing

Aerodynamics is an experimental science whose aim is the study of the relative motions of a solid body and the surrounding air. Its application to the design of a car body constitutes one of the chief lines of the search for energy economy in motor vehicles.
In order to move over flat ground, a car must overcome two forces:
1 Resistance to tyre tread motion, which varies with the coefficient of tyre friction over the ground and with the vehicle’s mass.
2 Aerodynamic resistance, which depends on the shape of the car, on its frontal area, on the density of the air and on the square of the speed.One of the objects of aerodynamic research is to reduce the latter: in other words to design a shape that will, for identical performance, require lower energy production. An aerodynamic or streamlined body allows faster running for the same consumption of energy, or lower consumption for the same speed. Research for the ideal shape is done on reduced-scale models of the vehicle. The models are placed in a wind tunnel, an experimental installation producing wind of a certain quality and fitted with the means for measuring the various forces due to the action of the wind on the model or the vehicle. Moreover, at a given cruising speed, the more streamlined vehicle has more power left available for acceleration: this is a safety factor.

The design of a motor car body must, however,remain compatible with imperatives of production,of overall measurements and of inside spaciousness.It is also a matter of style, for the coachwork must be attractive to the public. This makes it impossible to apply the laws of aerodynamics literally. The evolution of the motor car nevertheless tends towards a gradual reduction in aerodynamic resistance.