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.
aerodynamic drag equation

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

zero angle of attack

Airfoil Cd ~ 0.05

wing about to stall

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
aerodynamics of sphere

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

aerodynamics circular flat plate

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

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