AERODYNAMIC RESISTANCE
Aerodynamic resistance is a resistive force that can have significant impacts on vehicle performance. At high speeds, where this component of resistance can become overwhelming, proper vehicle aerodynamic design is essential. Aerodynamic efficiency has long been given primary consideration in the design of racing and sports cars. More recently, concerns over fuel efficiency and overall vehicle performance have resulted in more efficient aerodynamic designs in common passenger cars, although not necessarily in pickup trucks or sport utility vehicles where the need for cargo space often limits what can be done with aerodynamic efficiency.
Aerodynamic resistance originates from a number of sources. The primary source (typically accounting for over 85% of total aerodynamic resistance) is the turbulent flow of air around the vehicle body. This turbulence is a function of the shape of the vehicle, particularly the rear portion, which has been shown to be a major source of air turbulence. To a much lesser extent (on the order of 12% of total aerodynamic resistance), the friction of the air passing over the body of the vehicle contributes to resistance. Finally, approximately 3% of the total aerodynamic resistance can be attributed to air flow through vehicle components such as radiators and air vents.
Based on these sources, the equation for determining aerodynamic resistance is
ρ= 2
2a D fR C A V (2.3)
where
Ra = aerodynamic resistance in lb,
ρ = air density in slugs/ft3,
CD = coefficient of drag (unitless), Af = frontal area of the vehicle (projected area of the vehicle in the direction of
travel) in ft2, and V = speed of the vehicle in ft/s.
