How to estimate vehicle drag force with minimal data

Often automotive manufacturers will boast about having a low "Cd" number (coefficient of drag). This is useful information but on it's own is insufficient to tell you what the aerodynamic forces on a vehicle will be. In order to simulate aerodynamics it is necessary to know the correct force to apply. Another important consideration is the frontal area of the vehicle, for simple analysis this can be approximated as the product of the vehicle width and height. Air density is also a factor, the standard air density of 1.225kg/m^3 is suitable for many applications but it's worth considering whether this is accurate for your situation. Additionally vehicle speed is especially important, all of this can be seen in the equation below.

Drag Force = 0.5 * Cd * Frontal Area * Air Density * (Speed ^ 2)

Where:
Cd = Coefficient of drag
Frontal Area = Vehicle Width * Vehicle Height
Air Density = density of the air/fluid
Speed = Longitudinal speed through the air/fluid

It is important to ensure that consitent units are used, if using SI units then use speed in m/s and vehicle dimensions in m.

From this equation it can be seen that vehicles with the lowest aerodynamic drag force will often be ones which are narrow and short, not necessarily ones with a low drag coefficient.

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More detailed aerodynamic force representation

The approach detailed above only gives a magnitude of aerodynamic force which should be applied at the height of the aerodynamic centre of pressure for the vehicle. In reality the height of the application of the drag force will change, therefore aerodynamic drag is often represented by a combination of drag force and aerodynamic pitch moment referenced from an individual datum.

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