Drag coefficient

[GRU]

Originally Posted by kach22i

So, in all these threads looking to gain insight on the most aero-efficient cars out there, we should perhaps be listing the CdA, and not the just the Cd?

Yes, I agree to that.

[MetroMPG]

Except it's relatively easy to modify a vehicle to reduce Cd. Reducing frontal area... less easy!

So the emphasis on Cd is warranted, I'd say.

[gone-ot]

...frontal area (A) usually doesn't change with a vehicle, so it's basically a CONSTANT. This is true for most vehicles, but it's often NOT true for hypermiler vehicles where LOTs of body changes are often done in the name of improving aerodynamics, ie: lowering the Cd coefficient, which is the coefficient to frontal area (A), ie: Cd·A

[Diesel_Dave]

There's some value in both Cd and Cd*A. Cd*A is what does ultimately determine the aerodynamic drag force, however, Cd allows comparison of different sized vehicles. For example, it makes no sense whatsoever to compare the Cd*A of a tractor-trailer to that of a small passenger car, however, comparing Cd is a fair comparison.

It also makes sense for scale testing in wind tunnels. Cd stays the same for the scale model and the full-sized version, Cd*A obviously changes (because of A).

[drmiller100]

Originally Posted by Diesel_Dave

There's some value in both Cd and Cd*A. Cd*A is what does ultimately determine the aerodynamic drag force, however, Cd allows comparison of different sized vehicles. For example, it makes no sense whatsoever to compare the Cd*A of a tractor-trailer to that of a small passenger car, however, comparing Cd is a fair comparison.

It also makes sense for scale testing in wind tunnels. Cd stays the same for the scale model and the full-sized version, Cd*A obviously changes (because of A).

what you wrote makes no sense to me.

I don't compare fish to birds. I don't compare tractor trailers to hatchbacks.

Comparing Cd is worthless between the two.

Cda is what matters, because if you combine speed, Cda, and weight, you can KNOW the drag of the car.

Cd is a way to sort of calculate Cda, but it is kind of worthless by itself.

[gone-ot]

Originally Posted by drmiller100

what you wrote makes no sense to me.

I don't compare fish to birds. I don't compare tractor trailers to hatchbacks.

Comparing Cd is worthless between the two.

Cda is what matters, because if you combine speed, Cda, and weight, you can KNOW the drag of the car.

Cd is a way to sort of calculate Cda, but it is kind of worthless by itself.

...hmmm, let me try to explain Cd's "usefulness" like this:

1) Someone has a given vehicle, so it's A (frontal area) is set.

2) But, that someone wants to reduce the vehicles' aero drag at highway speeds but doesn't know *which* mods to do first.

3) So, that someone looks at the various "tweeks" available and their percentage-affect upon "total" Cd in general (ie: effectiveness).

4) Now they can select & use the tweek(s) that produce the most change in Cd, within cost effectiveness, in order of estimated "total" drag reduction (Cd·A).


Two "older" ROAD & TRACK magazine articles on this very subject worth reading are:

• Dennis Simanaitis, "Seeking Light at the End of the Tunnel: My Cx is lower than yours; or is it?" ROAD & TRACK, Aug-1982, pp. 32-35.

• Del Coats, "Aero Estimation, Self-Taught," ROAD & TRACK, Aug-1982, pp. 41, 42 and 44.

[NeilBlanchard]

Coefficient of drag (Cd) is dimensionless and is a way to access the design of a vehicle, and compare that to the design of any other vehicle.

CdA has a dimension -- units of area, and is a way to know the actual drag of a vehicle, and compare that to the drag of any other vehicle.

Reducing either the Cd or the area helps improve the efficiency of the vehicle. But if the Cd is say about 0.33, you have to reduce 3X as much to equal a reduction in Cd.

For example, if a car has ~25 sq ft of frontal area, and the Cd is 0.20, that is a CdA of 5 sq ft. To reduce the CdA down to 4 sq ft, you can either improve the Cd down to 0.16 or reduce the area down to 20 sq ft. The Cd is much easier to improve -- reducing the frontal area by 5 sq ft would require a huge amount of work.

Knowing both the Cd and the CdA are very useful, in my opinion.

[aerohead]

Originally Posted by GRU

I see but i guess my question is, if 2 very different (frontal area) cars with the same Cd were going down the highway at 60mph wouild it take the same amount of energy to push them at that speed (assuming that weight and rolling resistance was the same)?

The VW New Beetle and Ford Econoline van are both rated at Cd 0.38.
The Beetle has about half the frontal area as the Econoline and consequently,half the drag at any given speed up to around 250 mph.And it follows that the VW would require half the energy as the Ford to overcome air drag.

[CigaR007]

Take an airfoil and perform a homothetic transformation by doubling the original size.

The result is a bigger, yet proportionally identical airfoil.

Both these airfoils have the same Cd. Yet their projected frontal area (A) are not the same.

Hence Cd*A ≠ Cd*2A.

I hope I am correct on this one..

[ChazInMT]

CigaR007..I hope I'm correct on this one, you were just itchin for a chance to use the "≠" symbol.

My 2 ¢.