# Aerodynamic Drag

Aerodynamic drag is the resistance of a vehicle body to motion through the air. A smooth surface has less drag than a rough one.

It may be broken down into three main components:

1. skin friction: this is drag due to the surface texture and area.
2. profile drag: this is drag from the three-dimensional shape of the aircraft/vehicle.
3. service drag: this is drag from air ducted to cooling components.

where

• F = aerodynamic drag force [N]
• Cd = drag coefficient
• A = frontal area [m2]
• ρ = density of fluid [kgm-3]
• v = velocity of object relative to fluid [ms-1]

Note: Re = Reynolds number and this is important when defining the drag coefficient of a body.

The fluid dynamics of a vehicle are really important as the resultant aerodynamic drag is often the dominant energy dissipation factor for a vehicle. Cruising down a motorway and this determines most of the energy used per distance travelled. In aircraft this will be the dominant factor.

### Drag Coefficient

The frontal area of the vehicle or object is as important as the Drag Coefficient, however, we use the drag coefficient as a measure of aerodynamic design. Therefore I thought it would be good to list a number of objects based on their drag coefficient

Airfoil Cd ~ 0.05

Airfoil about to stall Cd ~ 0.15

Car Cd ~ 0.2 to 0.5

• Tesla Model S Cd = 0.208
• Mercedes-Benz CLA 180 Cd = 0.22
• Toyota Prius Cd = 0.26
• Honda Insight Cd = 0.28
• McLaren F1 Cd = 0.32
• Citroën DS Cd = 0.36
• Original Volkswagen Beetle Cd = 0.48

Sphere

• smooth sphere, Re = 106 Cd=0.1
• smooth sphere, Re = 105 Cd=0.47
• rough sphere, Re = 106 Cd=0.48

Bullet (subsonic) Cd ~ 0.295

Light truck Cd ~ 0.35 to 0.45

Formula 1 race car Cd ~ 0.7 to 1.1

Bicycle Cd ~ 0.9 (with a faring Cd ~ 0.6)

Wires and cables Cd ~ 1.0 to 1.3

Person standing Cd ~ 1.0 to 1.3

Circular flat plate Cd = 1.12

Skydiver Cd ~ 1.0 to 1.4

Flat plate perpendicular to flow Cd ~ 1.98 to 2.05

## References

1. Shape Effects on Drag – NASA