Drag Coefficient
The drag coefficient quantifies the drag or resistance of an object in a fluid environment.
Any object moving through a fluid experiences drag  the net force in the direction of flow due to pressure and shear stress forces on the surface of the object.
The drag force can be expressed as:
F_{d} = c_{d} 1/2 ρ v^{2} A (1)
where
F_{d} = drag force (N)
c_{d} = drag coefficient
ρ = density of fluid (1.2 kg/m^{3} for air at NTP)
v = flow velocity (m/s)
A = characteristic frontal area of the body (m^{2})
The drag coefficient is a function of several parameters like shape of the body, Reynolds Number for the flow, Froude number, Mach Number and Roughness of the Surface.
The characteristic frontal area  A  depends on the body.
Objects drag coefficients are mostly results of experiments. The drag coefficients for some common bodies are indicated below:
Type of Object  Drag Coefficient  c_{d}   Frontal Area 

Laminar flat plate (Re=106)  0.001  
Dolphin  0.0036  wetted area 
Turbulent flat plate (Re=106)  0.005  
Subsonic Transport Aircraft  0.012  
Supersonic Fighter,M=2.5  0.016  
Streamlined body  0.04  π / 4 d2 
Airplane wing, normal position  0.05  
Sreamlined halfbody  0.09  
Long streamlined body  0.1  
Bicycle  Streamlined Velomobile  0.12  5 ft^{2} (0.47 m^{2}) 
Airplane wing, stalled  0.15  
Modern car like a Tesla model 3 or model Y  0.23  
Toyota Prius, Tesla model S  0.24  frontal area 
Tesla model X  
Sports car, sloping rear  0.2  0.3  frontal area 
Common car like Opel Vectra (class C)  0.29  frontal area 
Hollow semisphere facing stream  0.38  
Bird  0.4  frontal area 
Solid Hemisphere  0.42  π / 4 d2 
Sphere  0.5  
Saloon Car, stepped rear  0.4  0.5  frontal area 
Bike  Drafting behind an other cyclist  0.5  3.9 ft^{2} (0.36 m^{2}) 
Convertible, open top  0.6  0.7  frontal area 
Bus  0.6  0.8  frontal area 
Old Car like a Tford  0.7  0.9  frontal area 
Cube  0.8  s2 
Bike  Racing  0.88  3.9 ft^{2} (0.36 m^{2}) 
Bicycle  0.9  
Tractor Trailed Truck  0.96  frontal area 
Truck  0.8  1.0  frontal area 
Person standing  1.0 – 1.3  
Bike  Upright Commuter  1.1  5.5 ft^{2} (0.51 m^{2}) 
Thin Disk  1.1  π / 4 d2 
Solid Hemisphere flow normal to flat side  1.17  π / 4 d2 
Squared flat plate at 90 deg  1.17  
Wires and cables  1.0  1.3  
Person (upright position)  1.0  1.3  
Hollow semicylinder opposite stream  1.2  
Ski jumper  1.2  1.3  
Hollow semisphere opposite stream  1.42  
Passenger Train  1.8  frontal area 
Motorcycle and rider  1.8  frontal area 
Long flat plate at 90 deg  1.98  
Rectangular box  2.1 
Example  Air Resistance Force acting on a Normal Car
The force required to overcome air resistance for a normal family car with drag coefficient 0.29 and frontal area 2 m^{2} in 90 km/h can be calculated as:
F_{d} = 0.29 1/2 (1.2 kg/m^{3}) ((90 km/h) (1000 m/km) / (3600 s/h))^{2} (2 m^{2})
= 217.5 N
 compare car air resistance with car rolling resistance
The work done to overcome the air resistance in one hour driving (90 km) can be calculated as
W_{d} = (217.5 N) (90 km) (1000 m/km)
= 19575000 (Nm, J)
The power required to overcome the air resistance when driving 90 km/h can be calculated as
P_{d} = (217.5 N) (90 km/h) (1000 m/km) (1/3600 h/s)
= 5436 (Nm/s, J/s, W)
= 5.4 (kW)
Related Topics

Fluid Mechanics
The study of fluids  liquids and gases. Involving velocity, pressure, density and temperature as functions of space and time.
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