beetle kammback

[aerohead]

found this online,it gives a sense of where the problem area exists

[skyking]

Originally Posted by freebeard

I see mention of a trailer in the 'tapeee' thread. What trailer did you get?

'three curves' The top and two sides?

'sharp inside corners' I guess I'd need to see a picture. I don't know what other old posts you've found, so here's a comparison of [half-] circular, squircular and square cross-sections.



The square section has the worst cross-wind performance.

Yes the top and two sides. Look at that section you have there and move it back behind the beetle 18". I will make a rounded front that transitions to that section, to take advantage of the height, and not stick out at the fender shadows.
Then it will morph on into a template-like shape, so those three will become the one shape. The blend will be fun to do without "growing" the frontal area much.

[skyking]

Originally Posted by aerohead

found this online,it gives a sense of where the problem area exists

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.

[freebeard]

You're looking at a two dimensional shadow of the 3D reality. The roof drops off too fast and the air along the sides rises and rolls over that sharp edge and the inward/downward rotation reattaches the flow.

The template is a 50% truncation. With geodesic domes based on the icosadehron popular truncations are 3/8ths and 5/8ths. 3/8ths has maximum floor area for volume and high aerodynamic lift. 5/8ths has more volume for floor area; but (allowing for the difference in cross-section, or not?) is less susceptible to lift. Above and below the equator balance each other out.

[skyking]

Sorry I missed your trailer question. It is a converted tent trailer that I will totally remodel, using basically the data plate. The rest will be new/improved.

[skyking]

I started mocking up the lower part of the kamm, and the starting point. That blue tape in the first picture is viewed from close to parallel to the ground get an idea of things.
The stick is normal to direction of travel in pictures 2 and 3.
I think following that body line represented by the blue tape would be a surefire recipe for separation, a box cavity that is more normal to travel would be better. I can hang it out about 4" before it gets into my view for checking for traffic over my right shoulder, which is a safety consideration for me.

[skyking]

First pic shows the basic line moved aft per Aerohead's suggestion that I was forward of separation point. Of course the final product would have a curve there.
Next few the basic cardboard is on, shows that relationship between kamm and sunroof, etc.
Back is too square for a curvaliscious car like the beetle.
Added a curved tail, with view from behind too.
Final has the Colani curve tacked on. I think it will move aft and there will be more kamm there at the notch.
The bottom end can go aft farther without impeding views. I could also add a shelf wing down there out of sight if I can make it work.
None of it was visible in the mirror, only when I turned to look back and to the right, and it could go a little more aft as you move up the trailing edge.

[freebeard]

Looks good to me. And thanks for mentioning Colani.

Here's what I did:



Even if it's only a 4" wide strip, producing the whole line will insure that it visually lies on the smoothest contour. Art not aerodynamics.

What do you plan on doing about the lower area? The pictures really don't show what happens around the tail light. Do you plan a one-off or a mold for reproduction?

[skyking]

That looks great. Air does appreciate art done right.
Tail lights and rear bumper lines will be left alone and unobstructed. I think I will wireform the trailing edge with 3/16" steel and use 12 Ga sheet metal tabs screwed to the hatch sides to mount it. I have some thin hard poly for testing. I will place that 1/8" inside the final dimensions and use it to lay my foam core on. I think I can mask my car and lay up the final product mounted. Remove, glass the inside, epoxy to wire and then figure out how to cut it so the hatch works.
If I make it nice I could pull a mold any time.

[freebeard]

The rear bumper rounds under doesn't it ? Maybe just an small angled lip in the form of a difusser? It could be the same width as the Kammback to tie them together visually, even though the bumper separates them.

Later, down the road.