Drag coefficient
 

I was compairing different car's and their Cd. From reading about boat tails and the streamlining template on this forum i expected that most cars that have a clean body with a small angle taper at the back will have a better Cd than cars that are hatchbacks.

The following cars/truck have a suprisingly low Cd.

2008 Chevrolet Tahoe Hybrid Cd. .34
2010 Ford Escape Cd. .29
2005 Honda Odyssey Cd. .30

How can such a huge vehicle that has the opposite of a boat tail be better than most small, fuel efficient cars with a Cd. of about .35? If they can lower the drag on these big SUV's without a boattail, then what's making them better than sports cars from the 90's?

...lookup the Cd for the GM VOLT, it's a notchback with a sub-0.30 Cd value.

Cd is just the coefficient of drag. To calculate the loss, you have to factor in the frontal area, so a Tahoe may have a fairly low Cd, but it's still as big a barn door and costs more fuel to push it through the air.

The trailing vortices from the greenhouses of 3-box designs must take more energy than the random wake behind a squareback. The squareback and the tapering in towards the rear are Kamm's secret to success.

How can
 

*A vehicle with absolutely zero roof camber and zero body sides camber would have a drag minimum of about Cd 0.5.
*By having a small amount of either it's a no-brainer to achieve Cds on the order of 0.29.
*The 2013 Ford Focus hatchback with only 26% of aft-body has Cd 0.29.
*The 2012 Prius V has 25% aft-body for Cd 0.29.
*The Prius II @ 32% is Cd 0.26
*Insight gen-II 33% is Cd 0.28
*Mercedes-Benz Boxfish @ 32.5% = Cd 0.19
*Renault Vesta II @ 28.5% = Cd 0.19
*Chevy Citation IV @ 47.7% = Cd 0.18
*M-B C-111 III @ 100% = Cd 0.178
*AeroCivic @ 50% = Cd 0.17
*1993 GM EV1 LSR (USAC) @ 55% = Cd 0.156
*Shamu (UC Davis ) @ 62% = 0.15
*GM Aero 2002 @ 53.7% = Cd 0.14
*Ford Probe V @ 54.6% = Cd 0.137
*1921,Jaray 'pumpkin seed' @ 73.5% = Cd 0.13
*1987 GM Sunraycer @ 89% = Cd 0.12
You can see that the elongation to the aft-body translates into lower drag as Hucho suggests.
If you compare the Prius V and 2013 Focus to the 'Template' you notice a dead match and the Cds are within 3.5% of predicted values.

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)?

No, because it's the product of frontal area (A) times Cd that counts, so if the frontal areas are different then their aero drag forces will likewise be different.

The smaller the A×Cd product is, the lower the resultant aero drag force is.

Don't confuse Cd with CdA; If CdAs are the same then the aero load is the same.

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.

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.

...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

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.

...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.

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.

if
 

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.

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.. :D

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

My 2 ¢.

I don't think they should be advertising the Cd. of a car becuase it's like saying "a Dodge Ram gets 3.2mpg per 1000 lbs" it doesn't matter untill you apply it to a vehicle size, so unless you know the total weight of a Dodge Ram it's pointless.

Because when they talk about a Prius having a Cd of .25 and a Honda Odessey, they sound like they could compaire but it's like apples and watermellons

Interesting, but lets think about how we MEASURE Cd.

We actually MEASURE Cda. Then we MEASURE Area.

Then we CALCULATE Cd.

I guess Cd is somewhat kind of useful for figuring out what is the best bang for changes, but that is about it.

It basically just comes down to separating the effects of size vs shape--A tells you the size, Cd tells you the effect of the shape, together they tell you the combined effect.

Actually, it's quite common to use gallons/ton-mile as a unit of fuel consumption in the trucking and freight industry. It allows you to tell how much fuel your using per ton of freight moved. If you were only concerned about how many miles/gallon you used, you could just haul less freight, so that's why they use gallons/ton-mile.

One of the easiest ways to improve the Cd of a vehicle is to make it bigger, which natch increases total drag...and I think that's why the manufacturers of big sedans big SUVs etc tout their Cd rather than their CdA. As I recall, there's an example in Hucho showing how bulging a flat roof up reduces the Cd, but no more than the frontal area increases, leaving CdA unchanged.

Jack, you may have that backwards -- if you shrink the vehicle (but don't change the shape) then the Cd will improve, I think. Punching a bigger hole in the air cannot improve the Cd.

I think you're both wrong, Jack & Niel, size doesn't affect Cd at all. That's the whole point. Same shape = same Cd. A 1x1x1 cube in theory has exactly the same Cd as a 2x2x2 cube. The frontal area of the 2x2x2 cube is 4 times larger so the total drag is 4 times larger, but the Cd is the same.


Check out the link below for more info.
https://ecourses.ou.edu/cgi-bin/eboo....1&page=theory
Note how different shapes are given different Cd's, with not mention of thier size.

I have been looking all over to find the cd of my econoline, thanks.

Dave, you may well be right. But, I do know that if one tests a quarter scale model of a car, you have to increase the velocity and/or the air pressure to get an accurate Cd of what that shape would be full size. This has to do with the Reynolds number; but that is as far as my knowledge goes.

So, for a given shape the size would seem to change the Cd? Or, maybe this is just the CdA that is changing, and that affects the calculation of the Cd?

And I know that Cd is a dimensionless ratio comparing the tested shape with a square flat plate broadside to the air flow.

Sure it can, and usually does. If you double the frontal area of a vehicle, but only increase its drag by 1.98, you’ve dropped the Cd by 1%. Cd varies with Reynolds number (in general, as speed and/or size increase, drag increases at a slightly lower rate than the Reynolds number, which is why Reynolds number matters when designing for low drag) and if we’re comparing vehicles at a given speed, the bigger vehicle has the higher Reynolds number.In theory.That link does mention size, though indirectly. It gives the Reynolds number. Right you are. In theory, Cd is a constant regardless of size or speed. Using Reynolds number in drag calculations gets the theoretical results closer to observed results. So in theory you’re right, Neil and Dave, but there’s a difference* between theory and practice, and though Cd by itself is a useful cocktail-napkin-calculation tool, it misses out on some subtleties that show up in wind tunnels (and the real world), such as how increasing size (at automobile-scale size and speed) reduces Cd.

*The difference between theory and practice is: in theory there is no difference, and in practice, there is.

Yes, technically, Cd does depend on Reynold's number, surface roughness, etc, etc., but those effects are pretty small. It's a fairly common assumtion that Cd is constant for a given shape, however. That's why there can be a Cd quoted for a given car. Technically a car will have different Cd's at different Reynold's numbers (and other factors), but often just assumed to be constant because those effects are fairly small.

...except for the Mythbuster idjiots, nobody I know of has "dimpled" their car's body-panels in an effort to reduce its Reynolds Number!

This is why i thind Cd is pointless advertising about fuel economy....Take any car and make it twice the size without increasing the weight... The Cd will be the same yet the original (smaller car) will have less wind resistance and get better fuel economy

If you are comparing two very similarly sized cars, then the one with a lower Cd is likely to get better mileage. More information is better. And lowering the Cd is the best way to improve the efficiency, after improving the drivetrain.

Advertising a car's Cd is an indication that they've at least paid attention to it. Nothing but good can come of manufacturers producing more aerodynamic vehicles, bragging about it, and thus making the public aware of its impact on efficiency.

worthless
 

The Cd of a car would give a potential consumer the same information as the thermal R-Factor for doors,windows,walls,and ceiling in a prospective home purchase,as the inverse of R-Factor is the Heat Transfer Coefficient for the home.A thermal Cd if you will.
The square footage of the home wood give an overall sense of energy efficiency as would the frontal area in a vehicle.
It is quite proper to compare Cds of aircraft,birds,submarines,bullets,fish,radomes,'n such,as they are all subject to the same forces and we can learn from their performance vs shape.