[aerohead]

Originally Posted by 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.

Chaz,remember that for low aspect ratios and ground effect,tables of aerodynamic performance for wing sections have absolutely no applicability.Sections operate basically in 2-dimensional flow and under 'flight' conditions.
The other thing about lift,when separation occurs 'above' a car's tail roofline,as in the Lange-type Porsche and pseudo-Jaray Beetle,there will be a vertical force vector component.
Your Audi pictured,with K-form roofline has no separation 'over' the car,only 'behind' it,acting horizontally.It's major lift will occur at the windshield header as in most cars.
Many have recommended chopping the body off at 50% of frontal area do to 'practicality' issues.
That's fine.But it's important to remember that half of the drag remains.And addressing that 50% can get you up to an extra 25% better mpg on the highway.
Some of the claims about 'phantom' tails must be taken within the context in which they were made.Hucho has mis-spoken on the issue,as well as Professor Morelli.I hope to clear that up in a thread I'm working on.
Just remember,Cd 0.12 can be had with the tail.