New bug, .38 CD why?

[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.

[Dawie]

Some food for thought...

The DKW 1000S and 3=6 cars of the late fifties and early sixties has a similar shape as the beetle. Think cd is 0.40.
Main differences are : at the rear, where the roofline ends, it is less rounded than the beetle, creating a breakaway point.
Also the rear part of the front fenders blend into the bodywork, reducing turbulence there.
Even with it's more upright windscreen and fifties-design,
it had nearly the same cd as the new beetle.

[bnmorgan]

ok then, the original question? why is our cD so "high" for what would, on the surface, appear to be a fairly good aero design?

[RobertSmalls]

Originally Posted by bnmorgan

ok then, the original question? why is our cD so "high" for what would, on the surface, appear to be a fairly good aero design?

Simply because it is, in fact, a very poor aero design. The trailing end falls away too fast and creates a large, low-pressure wake.

Perhaps the Beetle borrows lines from good aerodynamic designs, and then compresses them longitudinally until they're poor.

[aerohead]

Originally Posted by bnmorgan

ok then, the original question? why is our cD so "high" for what would, on the surface, appear to be a fairly good aero design?

What you'll need to do,is find out where tangent angles of 22-degrees occur for the roofline,body sides and fenders.This is where the flow must separate and the base pressure must be that of the point of separation.
If the body were more gently curved,as with the Audi TT,the flow would remain attached to the 'end' of the car and separation there would produce a higher base pressure ( lower delta-P ) and drag would necessarily be lower.
It's all about pressure differentials.

[ChazInMT]

Ya know.....It is rather interesting that the 22° tangent occurs at the top of the rear window.

Hasn't someone around here suggested a roof spoiler in this location to separate flow? Maybe it was Frank Lee....or Patrick....hmm, let me go back and look.

[Patrick]

I know, stick one of these on it!

[Frank Lee]

The spoiler isn't on yet? Dang.

[bnmorgan]

No, I've got too many conflicting things going and #118 just confused me more. I'm still trying to find a decent 3d model or enough measurements to make one, even trying to do a sketch-on-image to get enough for some even marginal CFD.

side note, anyone else having trouble with chernyshenko's site?

[ChazInMT]

I just started a new thread to take some pressure off this one, and my ideas in it apply to things other than Beetles.

Beetle Inspired Aerodynamic Thoughts