New bug, .38 CD why?

[ChazInMT]

Patrick, your graph proves my point! Thanks for that. If you can't put 2 & 2 together, quit trying to make it look like I can't. You're amazing. Keep living the dream my man, I'll stay here in reality land. I could go point by point again on the misconceptions you have posted yet again, but I've already done that, so I'm done.

COCyclist, Amazing post, I wish I could give you 100 Thanks!!!! I really really like this:



But of course in Patricks dream land, the blunted shape on the bottom left has a +0,099 Cd change, it must just be a typo because there is such a huge flatspot on that shape, it could not possibly be within 6% of the ideal shape, stupid Germans.

[Patrick]

Originally Posted by ChazInMT

Patrick, your graph proves my point! Thanks for that. If you can't put 2 & 2 together, quit trying to make it look like I can't. You're amazing. Keep living the dream my man, I'll stay here in reality land. I could go point by point again on the misconceptions you have posted yet again, but I've already done that, so I'm done.

COCyclist, Amazing post, I wish I could give you 100 Thanks!!!! I really really like this:



But of course in Patricks dream land, the blunted shape on the bottom left has a +0,099 Cd change, it must just be a typo because there is such a huge flatspot on that shape, it could not possibly be within 6% of the ideal shape, stupid Germans.

Chaz, you need to take a course in aerodynamics. If you think my graph proves your point, you either don't know how to read it or you truly are misguided.

By the way, what were the Cd values on the tuft-tested VW, with and without the roof spoiler?

[Frank Lee]

This is more fun than a barrel of monkeys!

Carrion boys

[Patrick]

Originally Posted by Frank Lee

This is more fun than a barrel of monkeys!

Carrion boys

Feel free to chime in anytime.

[aerohead]

The pressure portion of the profile drag is governed by the pressure differential between the front and rear of the vehicle.
The base pressure behind the vehicle is governed by the separation point.
The only way to 'increase'the pressure and thereby lower the drag,is to move the separation point back,ideally to the very back,eliminating separation completely,then you're left with only skin friction.
Induced-drag can be virtually eliminated thanks to Dr.Morelli's "banana-car" of 1978.

[bnmorgan]

$#!+ now i'm confused worse than i was when i didn't know anything.

[ChazInMT]

OK Morgan, let's be logical about this.

This diagram shows us that if we cut off the back end of a very aerodynamic shape, we still gain most of the good stuff from it. The negative effect on our Cd from optimum is only 6%, and we haven't had to add much.



So in my mind, I see that we don’t need to have a full boat tail, and a relatively large flat area is not an aerodynamic death sentence.

I think the really difficult thing to grasp is dynamically induced drag. It is difficult to imagine this because so often, people think "Attached Air Flow" is the absolute Holy Grail. Attached air flow is good when our vehicles approximate these shapes.



And Again with these shapes, we all agreed that the Cd does not suffer terribly when we lose the back end. Obviously longer is better, but short is not horrible.

Here is another example of a vehicle with a huge back end and a Cd of .25, The Audi A2 3L. Do I need to go on?



Now if you go to this article here:

Porsche 993 & New Beetle Aero Testing

It is the basis for this thread. They say the same thing I've been saying about the large amount of lift, and incidentally they refer to the upwards and rearward drag in the article that I have mentioned also. You'll really dig this article, too bad they didn't get a hold of a turbo V Dub to test. It appears as though the whale tail treatment is a bit of a drag though, I suspect it is because it is too far down the glass and the lift drag is still present in a big way.

Now, the logical conclusion is that on a vehicle with a gently sloping rear end which reaches a steep angle, with attached flow, (Your venerable V Dub) the rear end is going to act like an airplane wing at a high angle of attack. And if we refer to this chart, which I found somewhere:



We see that the drag on a wing increases from .02 at a 0° angle of attack, to .22 at 20°, that is a 1,100% increase in drag. So if you could somehow reverse this 1,100% increase in drag, don't you think it would be worth it???

The back end of a VW is essentially a wing at its maximum angle of attack. By spoiling the air, and bringing the shape of your car more in line with the aerodynamic ideal, you end up with something closer in shape to an Audi A2.



Contrary to what some would try and make you believe.

This



and this



Does Not Equal This




All it takes, is a roof spoiler. It ain't ideal, but it's the biggest bang for your buck.

Induced Drag.....Just say No.

[bnmorgan]

Ok. Looks relatively straight forward. I hate to ask, but is there some formula based size/angle/radius that I should look for, or should i "wing it" (sorry). If there is, the math is going to go over my head somewhere after i run out of fingers and toes. I can fab something if i know where to start, and I can likely draw it in SolidWorks to share with the group. I just don't know yet what to draw.

[Patrick]

Frank, I see why you gave up.

bnmorgan, I recommend a wing similar to Ernie's. But if you decide to go with a roof spoiler, please let us know how much it improves (or decreases) your mileage.

[Frank Lee]

Yes, it will be much more entertaining to watch the roof spoiler build/test/oh-so-awesome results process... after that, perhaps I'll chime in with why it's a fail.