beetle kammback

[skyking]

Like a little 3 sided box below the bumper?

[freebeard]

Yes, it could be the visible lip of a difusser.

[aerohead]

Originally Posted by skyking

I see that but I am hard pressed to believe it stays attached that far aft. My car (that car) has a Cd of .38.
I'd think it would be coming apart sooner than that, maybe halfway between rear window edge and side door edge.

The longitudinal vortices will tend to hold the centerline flow attached down the contour,so the photo is a bit deceiving.
Your contour hits 22-23 degrees at the top molding of the backlight and this is where the boundary layer will be compromised.
The SMART Car sticks closer to the 'Template' and scores Cd 0.38 on a much shorter car.

Koenig-Kamm,at the FKFS was able to achieve Cd 0.14 on a Cd 0.20 car just by lofting the roofline up to the 'Template,as well the same for the sides.


NOTE: Kamm DID include a diffuser,something the other car's body lacked.

[skyking]

fixing the fender/tail light area is a bigger project and change than I want to try. Phil, do you think the small box cavity effect of those 4~5" sides will help out?
Am I right in reducing that side taper to get the flow attached?
I know at some point the flow down the sides is going to fill in across the back, it is what happens on all of the poor shapes we are dealing with.

[freebeard]

I feel your pain. The problem is, if you don't deal with the fenders, it will negate the bottom half of a Kamm- or boat-tail. You could add a trailer plug as a promise to yourself to do v2.0.



Volvo station wagon tail lights?

[aerohead]

Originally Posted by skyking

fixing the fender/tail light area is a bigger project and change than I want to try. Phil, do you think the small box cavity effect of those 4~5" sides will help out?
Am I right in reducing that side taper to get the flow attached?
I know at some point the flow down the sides is going to fill in across the back, it is what happens on all of the poor shapes we are dealing with.

Yes,it should help.Kamm and Koenig-Fachsenfeld was doing this to pseudo-fastback cars at the FKFS.
Here is an example of a mid-thirties Adler which they actually aero-modded the K-form rear onto this bulbous body.The 'dot-dash-dot' line depicts both elevation and plan-view mods.

PS
Carl Breer also tried to blend the rear fenders of the 1934 Chrysler DeSoto Airflow experimental test mule.
As depicted,these mods reduced drag from Cd 0.51,to Cd 0.244.


Here is a schematic of the J-form wind tunnel model tested at the FKFS.The shorter truncations have remarkably higher drag than the K-form truncations (which follow the ideal flow pathway).
The last two (longest length) bodies have the same drag as the K-form.

[aerohead]

Here's a comparison from Kamm's FKFS:
They had a crude Lay model with slab sides which had Cd 0.20 at this length.

Then they 'added' about 10% body width to 'bulge' out the sides,longitudinally and transversely,and the drag came in at Cd 0.16 with this fish-like shaping.


Running the tail all the way out to 100% of their 'template' netted Cd 0.12 when the nose and edges were softened.

[freebeard]

0.12<0.16<0.20. So you can get to 50% of the full boat tail effect without adding length?

I need a body kit for my Superbeetle that bulges out the door and rear quarter panels to full fender width. It would hide the external hinges and the drip rail down the front of the door. 0.40—>0.32?

[kafer65]

How would you be able to open the door if you didn't move the door hinge out with the kit? If you left the hinge inboard then you'd have some deep wedges at the pivot point. Graft a 356 side with pillars and all. That would do it You know your a ecomodder when....

[skyking]

deep wedges covered with a fabric fairing that gets tight with the closed door.
Thanks Phil. Looking down my beetle, I see that bulge. It will always be a Jekyll and Hyde car. I can make it civilized up top but oh those fenders