file.newsgroup.cars.103530 Maven / Gradle / Ivy
From: [email protected] (Albion H. Bowers)
Subject: Re: Drag CoefficientsVx?s?
In a previous article, [email protected] (Gary W. Mahan) says:
>Could someone explain how to make sense of drag coefficients (i.e Cd) mentioned in magazines. I understand that lower numbers signify better aerodynamics but
>what does this mean in the real world. Is there a way to calculate new top speeds(assuming the car is not rev limited at top speed) or mileage benefits if a identical car had the Cd reduced from .34 to .33.
It's pretty complex, and Cd isn't the whole story either. Cd for cars is
usually calculated based on the frontal area of the car. So a large car
with a good Cd could get the same drag force as a smaller car with a
poorer Cd.
To calculate drag use this formula:
D = 1/2 * rho * v^2 * Cd * S
Where D is the drag force (lbs), rho is the local air density (slugs/ft^3),
V is the velocity (ft/s), and S is the frontal area (ft^2). Note that the
pieces called 1/2 * rho * v^2 are sometimes called qbar or dynamic pressure
(a fancy aero term for air pressure or force).
Note that power is:
P = F * v
Where P is power (lbf-ft/s), F is the force, drag in this case (lbf) and v
is velocity (ft/s).
Note that if you put the whole equation into one (by substituting D for
force) you get a velocity _cubed_ term. That's why huge increases in power
result in little increases in speed. Ditto for decreases in Cd.
So if you have a 100 mph car and reduce Cd from .34 to .33, your new top
speed is: (sound of trumpet fanfare)
101 mph
Sorry to dissappoint.
--
Al Bowers DOD #900 Alfa Ducati Hobie Kottke 'blad Iaido NASA
"Well goodness sakes...don't you know that girls can't play guitar?"
-Mary Chapin-Carpenter