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The drag coefficient (Cd or Cx) is a number that describes a characteristic amount of aerodynamic drag caused by fluid flow, used in the drag equation. Two objects of the same frontal area moving at the same speed through a fluid will experience a drag force proportional to their Cd numbers. Coefficients for rough unstreamlined objects can be 1 or more, for smooth object much less.

\mathbf{F}_d= - {1 \over 2} \rho \mathbf{v}^2 C_d A    

A Cd equal to 1 would be obtained in a case where all of the fluid approaching the object is brought to rest, building up stagnation pressure over the whole front surface. The top figure shows a flat plate with the fluid coming from the right and stopping at the plate. The graph to the left of it shows equal pressure across the surface. In a real flat plate the fluid must turn around the sides, and full stagnation pressure is found only at the center, dropping off toward the edges as in the lower figure and graph. The Cd of a real flat plate would be less than 1, except that there will be a negative pressure (relative to ambient) on the back surface. The overall Cd of a real square flat plate is often given as 1.17. Flow patterns and therefore Cd for some shapes can change with the Reynolds number and the roughness of the surfaces.

Cd in automobiles

The drag coefficient is a common metric in automotive design, where designers strive to achieve a low coefficient. Minimizing drag is done to improve fuel efficiency at highway speeds, where aerodynamic effects represent a substantial fraction of the energy needed to keep the car moving. Indeed, aerodynamic drag increases with the square of speed. Aerodynamics are also of increasing concern to truck designers, where a lower drag coefficient translates directly into lower fuel costs.

About 60% of the power required to cruise at highway speeds is taken up overcoming air drag, and this increases very quickly at high speed. Therefore, a vehicle with substantially better aerodynamics will be much more fuel efficient.

CdA

While designers pay attention to the overall shape of the automobile, they also bear in mind that reducing the frontal area of the shape helps reduce the drag. The combination of drag coefficient and area is CdA (or CxA), a multiplication of the Cd value by the area.

In aerodynamics, the product of some reference area (such as cross-sectional area, total surface area, or similar) and the drag coefficient is called drag area. In 2003, Car and Driver adapted this metric and adopted it as a more intuitive way to compare the aerodynamic efficiency of various automobiles. Average full-size passenger cars have a drag area of roughly 8.5 ft² (.79 m²). Reported drag area ranges from the 2005 Chevrolet Corvette at 6.1 ft² (.57 m²) to the 2006 Hummer H3 at 16.8 ft² (1.56 m²).

More examples of ACd ft²: *

Drag in sports and racing cars

Reducing drag is also a factor in sports car design, where fuel efficiency is less of a factor, but where low drag helps a car achieve a high top speed. However, there are other important aspects of aerodynamics that affect cars designed for high speed, including racing cars. Notably, it is important to minimize lift, hence increasing downforce, to avoid the car ever becoming airborne. Also it is important to maximize aerodynamic stability: some racing cars have tested well at particular "attack angles", yet performed catastrophically, i.e. flipping over, when hitting a bump or experiencing turbulence from other vehicles (most notably the Mercedes-Benz CLR). For best cornering and racing performance, as required in Formula 1 cars, downforce and stability are crucial and these cars have very high Cd values.

Typical values and examples

The typical modern automobile achieves a drag coefficient of between 0.30 and 0.35. SUVs, with their flatter shapes, typically achieve a Cd of 0.35–0.45. Notably, certain cars can achieve figures of 0.25-0.30, although sometimes designers deliberately increase drag in order to reduce lift.

Some examples of Cd:

Figures given are generally for the basic model. Faster and more luxurious models often have higher drag, thanks to wider tires and extra spoilers.

Image:Worn Brick on Beach near Heysham atomic powerstation.jpg|2.1 - a smooth brick Image:Lotus79.jpg|0.7 to 1.1 - typical values for a Formula 1 car Image:PoliceOfficerOnBike.jpg|0.9 -a typical bicycle plus cyclist Image:Super 7.jpg|0.7 - Caterham Seven Image:220505 scania.jpg|at least 0.6 - a typical truck Image:Hummer H2.jpg|0.57 - Hummer H2, 2003 Image:Citroen2cvtff.jpg|0.51 - Citroën 2CV Image:Dodge Viper GTS.jpg|over 0.5 - Dodge Viper Image:Lamborghini Countach LP500S.jpg|0.42 - Lamborghini Countach, 1974 Image:1971 Triumph Spitfire MKIV.jpg|0.42 - Triumph Spitfire Mk IV, 1971-1980 Image:Fusca estacionado.jpg|0.38 - Volkswagen Beetle Image:91MazdaMiataGreen.jpg|0.38 - Mazda Miata, 1989 Image:Cars-redcapri-blackpool-amoswolfe.jpg|0.374 - Ford Capri Mk III, 1978-1986 Image:Ferrari F50.jpg|0.372 - Ferrari F50, 1996 Image:Talon.jpg|0.36 - Eagle Talon, mid-1990s Image:1974 Citroen DS23 Pallas.jpg|0.36 - Citroën DS, 1955 Image:Ferrari-TESTAROSSA-02.jpg|0.36 - Ferrari Testarossa, 1986 Image:Civic Sedan 2001-05.jpg|0.36 - Honda Civic, 2001 Image:Citrcx1silver.jpg|0.36 - Citroën CX, 1974 (the car was named after the term for drag coefficient) Image:NSU Ro 80.jpg|0.355 - NSU Ro 80, 1967 Image:1985 Ford Sierra 2.0L.JPG|0.34 - Ford Sierra, 1982 Image:Ferrari F40 in IMS parking lot.jpg|0.34 - Ferrari F40, 1987 Image:2006 Chevrolet Corvette convertible.jpg|0.34 - Chevrolet Corvette Z06, 2006 Image:1996 Chevrolet Camaro in Venice, California.jpg|0.338 - Chevrolet Camaro, 1995 Image:1970 Citroen SM.jpg|0.33 - Citroen SM, 1970 Image:Citroen AX red vl.jpg|0.31 - Citroën AX, 1986 Image:Citroen gsa special.png|0.31 - Citroën GS, 1970 Image:1993Vision.jpg|0.31 - Eagle Vision Image:R25white.jpg|0.31 - Renault 25, 1984 Image:Sonett3.jpg|0.31 - Saab Sonett III, 1970 Image:Audi 100 III silver vl.jpg|0.30 - Audi 100, 1983 Image:BMW Series3 black l.jpg|0.30 - BMW E90, 2006 Image:Porsche 911 Gris.jpg|0.30 - Porsche 996, 1997 Image:1950saab92.jpg|0.30 - Saab 92, 1947 Image:1991CRXSi.JPG|0.29 - Honda CRX HF 1988 Image:1985 subaru xt gl-10 rear right.jpg|0.29 - Subaru XT, 1985 Image:Lancia Dedra silver vl.jpg|0.29 - Lancia Dedra, 1990-1998 Image:Lotus-Elite-'60.jpg|0.29 - Lotus Elite, 1958 Image:2005ToyotaCamry.jpg|0.28 - Toyota Camry and sister model Lexus ES, 2005 Image:Porsche911997.jpg|0.28 - Porsche 997, 2004 Image:Renault25ts.jpg|0.28 - Renault 25 TS, 1984 Image:Saab9-3sportsedan.jpg|0.28 - Saab 9-3, 2003 Image:2004 Infiniti G35 Sedan.jpg|0.27 - Infiniti G35, 2002 (0.26 with "aero package") Image:Toyota camry hybrid.jpg|0.27 - Toyota Camry Hybrid, 2007 Image:SC06 2006 Mercedes-Benz S550.jpg|0.26 - Mercedes-Benz W221 S-Class, 2006 Image:Prius2004.JPG|0.26 - Toyota Prius, 2004 Image:Opel Calibra.jpg|0.26 - Vauxhall Calibra, 1989 Image:Honda-Insight-2703.jpg|0.25 - Honda Insight, 1999 Image:Audi A2 L Silber.jpg|0.24 - Audi A2 1.2 TDI, 2001 Image:Tatra T 77a.jpg|0.212 - Tatra T77 a, 1935 Image:GM EV.jpg|0.195 - General Motors EV1, 1996 Image:Dodge Intrepid ESX 1.jpg|0.19 - Dodge Intrepid ESX Concept , 1995

Cd in aircraft

Some examples of Cd *:
  • 0.027 - Cessna 172/182
  • 0.027 - Cessna 310
  • 0.022 - Learjet 24
  • 0.048 - F-104 Starfighter
  • 0.021 - F-4 Phantom II (subsonic)
  • 0.044 - F-4 Phantom II (supersonic)
  • 0.031 - Boeing 747
  • 0.095 - X-15

Cd in other shapes

Some examples of Cd *
  • 2.1 - a smooth brick
  • 0.9 - a typical bicycle plus cyclist
  • 0.4 - rough sphere (Re = 10&sup6)
  • 0.1 - smooth sphere (Re = 10&sup6)
  • 0.001 - laminar flat plate (Re = 10&sup6)
  • 0.005 - turbulent flat plate (Re = 10&sup6)
  • 1.0-1.3 - man (upright position)
  • 1.0-1.1 - skier
  • 1.0-1.3 - wires and cables
  • 1.3-1.5 - Empire State Building
  • 1.8-2.0 - Eiffel Tower

See also

External links

Aerodynamics | Aerospace engineering | Dimensionless numbers

Součinitel odporu | Strömungswiderstandskoeffizient | coefficiente di resistenza aerodinamica | Resistencia aerodinámica

 

This article is licensed under the GNU Free Documentation License. It uses material from the "Drag coefficient".

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