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Old 07-26-2012, 11:09 PM   #21 (permalink)
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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

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Old 07-26-2012, 11:19 PM   #22 (permalink)
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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.
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Old 07-27-2012, 09:35 AM   #23 (permalink)
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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.
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Old 07-27-2012, 09:38 AM   #24 (permalink)
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Quote:
Originally Posted by GRU View Post
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.
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.
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Old 07-27-2012, 10:29 AM   #25 (permalink)
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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.
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Old 07-27-2012, 10:33 AM   #26 (permalink)
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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.
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Old 07-27-2012, 12:26 PM   #27 (permalink)
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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.
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Old 07-27-2012, 02:45 PM   #28 (permalink)
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I have been looking all over to find the cd of my econoline, thanks.
Quote:
Originally Posted by aerohead View Post
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.
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Old 07-27-2012, 11:31 PM   #29 (permalink)
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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.
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Old 07-28-2012, 12:00 PM   #30 (permalink)
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Quote:
Originally Posted by NeilBlanchard View Post
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.
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.
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Originally Posted by Diesel_Dave View Post
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.
In theory.
Quote:
Originally Posted by Diesel_Dave View Post
Note how different shapes are given different Cd's, with not mention of thier size.
That link does mention size, though indirectly. It gives the Reynolds number.
Quote:
Originally Posted by NeilBlanchard View Post
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.
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.

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