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Old 04-18-2012, 10:10 PM   #341 (permalink)
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A 4' Kamm Back should get you about 12 percent less drag, and 6 percent better FE on the highway. This is a "top only" estimate, and does not include what might happen if the bottom is completed also. The estimate is based on my SedanKamm that yielded these numbers.

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Old 04-18-2012, 10:57 PM   #342 (permalink)
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you could build that back to the 30% point and the car would be no longer than it is. The challenge is to make it look good and have the hatch function well.
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Old 04-18-2012, 11:10 PM   #343 (permalink)
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Quote:
Originally Posted by cfg83 View Post
Frank and skyking and aerohead -

Thanks for the corrections. I am happy that I can slide the template for my wagon :



CarloSW2
I'm glad we have more clarification on it. Looks to me like you have the top nailed.

Now I'll throw a small wrench in it and say it looks like the template could be split horizontally so that the belly line can be moved forward...
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Old 04-19-2012, 12:02 AM   #344 (permalink)
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Quote:
Originally Posted by cfg83 View Post
aerohead -

Would these two elevations be equally aerodynamic? :



I'd say that the first one is lower drag, and here's why: the shape of the front accelerates the air out and around the high point on the roof, and this creates a localized high pressure, which then help the air "spring" back and it helps maintain attached flow. So, it adds to the atmospheric pressure and this improves the air flow a bit.

If the rear of the shape is most important, then the front is next most important -- it matters less, but it matters. At the very least the second one has more surface drag.
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Old 04-19-2012, 12:31 AM   #345 (permalink)
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Frank -

Quote:
Originally Posted by Frank Lee View Post
I'm glad we have more clarification on it. Looks to me like you have the top nailed.

Now I'll throw a small wrench in it and say it looks like the template could be split horizontally so that the belly line can be moved forward...
- Noooooooooooooooooooooooooooooooo!!!!!!!!!!!!!!!!!

Ok, here it is :



CarloSW2
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Old 04-19-2012, 02:03 AM   #346 (permalink)
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Errr... I don't see the belly line anymore? Ignore the wheel fairings...
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Old 04-19-2012, 03:49 AM   #347 (permalink)
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Belly line on the template is incorrect for a wagon profile as it stands because that will necessitate a 0* angle on the diffuser.
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Old 04-19-2012, 07:10 AM   #348 (permalink)
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Quote:
Originally Posted by skyking View Post
The forward and rear shapes are the same, but the wetted area is greater. It will have a little more drag based on that observation.
But its length/diameter ratio is better, which should be lowering drag again.
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Old 04-19-2012, 07:13 AM   #349 (permalink)
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Quote:
Originally Posted by euromodder View Post
But its length/diameter ratio is better, which should be lowering drag again.
The ratio is not better cuz the template is sposta be the optimum.
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Old 04-19-2012, 08:17 AM   #350 (permalink)
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Quote:
Originally Posted by NeilBlanchard View Post
I'd say that the first one is lower drag, and here's why: the shape of the front accelerates the air out and around the high point on the roof, and this creates a localized high pressure, which then help the air "spring" back and it helps maintain attached flow. So, it adds to the atmospheric pressure and this improves the air flow a bit.

If the rear of the shape is most important, then the front is next most important -- it matters less, but it matters. At the very least the second one has more surface drag.
Yes, this is the reason why boat tailing a train does not improve aerodynamics (well that and a lot of other things...). The ideal shape of the tail will change as the length of the forebody changes. When we are practical about it (on a car, on an airplane) it doesn't matter so much, but when taken to an extreme (a train, some test article in a wind tunnel) we will find that new aft end shapes are needed. It's for exactly the "springieness" reason you callout (actually a high pressure zone that is created that pushes back down). Given enough time, that high pressure zone is absorbed back into the atmosphere, and has nothing to give back to the vehicle. In some cases we will find that there is too much turbulence from things upwind, or there is too thick of a boundary layer built by all that constant diameter, that boat tailing has no effect anymore, regardless of the shape.

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