New bug, .38 CD why?

[aerohead]

Originally Posted by 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.

Separation is the LAST thing you want.If anything,you want to do like Ernie and put something down below and back for the air to re-attach to.

[aerohead]

Originally Posted by Rob10_99

I was reading on some old VW electric(youtube I think) about the herard helper, and he said his batteries had quite a bit more charge at the end of the daily trip than they usually have. So it seems that the wing helps a lot with the drag, correct?

Rob,I don't think it helped a lot,but it was measurable,and for something so 'minimalist' it's pretty impressive.

[aerohead]

Originally Posted by ChazInMT

Morgan, You DO NOT want to keep airflow attached. VG’s are used to try and keep flow attached when a rear shape tries to change too quickly for it to remain attached but not quickly enough for it to be a clean break. VG's would be useless here, the slope of the Bugs back is such that flow is remaining attached way too long already.

Understand that when I say you are generating too much lift, it is not all in the straight up direction, a very good portion of the lift is in the backwards direction, which is a huge drag load on your car. This is why you want it to separate at the top of the windshield. If you fear that you are creating a huge area of dead air that is starting at the top of your windshield, you should not worry about it, because this “Dead Air” drag will be there no matter what you do to your car....I hope that makes sense.

Below is a basic illustration of what I’m trying to say. By keeping the air from moving down the rear window, you eliminate the large lift, (Black Arrow) and it’s associated Drag Vector (Red Arrow).



The "Dead Air" drag is about the same as "Aerodynamically Shaped Perfect" drag. As soon as the air on your bug starts going down past the top of the rear window, it is costing you. So VG’s would do no good since the flow is already attached.

Go with a roof spoiler as I suggested earlier, it will yield the best results.

The dead air that forms will be less drag than the lift.

I apologize if I'm getting in out of sequence.
If the air over the backlight were attached,it would be gaining pressure as it moved rearward,actually reducing lift.
With un-attached flow,the wake pressure is at the pressure of the point of flow separation, and since it is acting 'above' the car,it is generating lift.
If you put the back of the Audi TT,with the spoiler onto the Beetle,separation is eliminated and along with it ,lift.
Sportauto-online has a windtunnel shot of the TT and you can compare it to the older Beetle windtunnel shot which is much like the New Beetle would be.
automobilsport.com - your platform for motorsports, photos and lifestyle also has many useful windtunnel photos.

[aerohead]

Originally Posted by RobertSmalls

Old bug, 0.48 Cd why?

Erm, I mean, why did the 1938 VW Beetle have a Cd of 0.48?

The Germans certainly knew how to build a low-drag car. What led to the selection of such high drag bodywork, instead of styling the VW bug after the Schloer pillbug or a streamlined, rear-engined Tatra? They could easily have halved the Cd, and thus cruised the Autobahn with a smaller engine and fuel tank.

Adolf Hitler seems to have thought that Ferdinand Porsche hung the Moon and while Porsche's kdf -wagon had Cd 0.485 it was WAY better than Opel's P4 which was competing for the Volkswagen,at around Cd 0.65,typical for 'real ' cars of the period.
The Beetle would trounce the P4 on the Autobahn in mpg,there's just no way Opel could have competed.

[autoteach]

Originally Posted by Rob10_99

Title pretty much says it all,

For being round and what looks fairly aero, it isn't. Could it be the back wake creating most of the drag?



That doesn't seem like it would have much drag, does it?

cant post images... need 2 more posts

The link states that the rear bumper is where the flow is disrupted, and that causes the lift and severe drag as well.

I know that this picture shows aero extreme for the purpose of downforce, but there are some cues that could be taken from it to lesson the amount of drag.

1. Behind the front wheels you could extend the humb and square it off, venting the wheel wells to reduce drag in the wheel area, and increase cooling and diminish drag from underneath.

2. Extend the rear humps and square off the rear while extending a deck lid from the bottom of the window. Vent rear wheel wells in the process and diffuse the rear.

[Frank Lee]

"Vents" oftentimes INCREASE drag because now you have flow through an un-aero location.

[autoteach]

vents that pull a vacuum on the wheel wells often decrease drag. I am sure that if someone was able to model it in 3D it would show benefits.

[Frank Lee]

You know that the ricer vents on rear bumpers does more harm than good...

[autoteach]

Well, there are reasons that they do what they do. I can find more pictures of these, and there is no downforce gain from venting the wheel well above to the fender or behind it like this example. It has to do with the attached flow along the side and the turbulence that occurs at the wells.

Frank, post a pick of the "ricer vents", I am not sure what you are talking about.

[Frank Lee]