Difference between Cd and Cda?

[Hasbro]

Does Cd consider more factors? My 04 Civic Si has a Cda of .33 but would Cd be different and why?

[SteveX82]

CdA actually considers more factors, as it is Cd (drag coefficient) multiplied by A (the frontal area of the object in question). The value you found has to be Cd...it's far too small to be the CdA value of a vehicle that size unless it has a Cd value of <0.15 (which it doesn't )

For example: Car A has great aerodynamics (Cd=0.25), but a very tall roofline and, consequentially, a large frontal area of 3.0m^2. Car B is less slippery (Cd=0.35), but is smaller overall and has a frontal area of only 2.0m^2. At any speed, Car B will cause less aerodynamic drag and waste less energy overcoming it.

Just in case you're interested, the formula for aerodynamic drag is:

1/2 * rho * velocity^2 * Cd * A where rho is air density (which changes with temperature, altitude, etc)

[thornburg]

Originally Posted by SteveX82

1/2 * rho * velocity^3 * Cd * A where rho is air density (which changes with temperature, altitude, etc)

I thought it was v^2, not v^3 ?

[SteveX82]

Originally Posted by thornburg

I thought it was v^2, not v^3 ?

You're right....I was thinking of the power consumed by the aero drag. The appropriate change has been made! Thanks for noticing

[Lug_Nut]

Think of CdA as Cd times A. It's a coefficient times an area. If the car were doubled in size the coefficient would not change, but the area would.
A jet aircraft has a lower Cd than a motorcycle, but the much smaller area of the bike gives it lower total air drag or CdA.