Big Van Kammback/template help

[mwilliamshs]

That best for its length tail, L, is 42.5" and that's half my vehicle's overall height, H, 85". (makes sense since D = 2H and L = 1/4 D). This is darn close to the proportion of trailer tails on 18 wheelers (tails vary a bit by brand) since they're the width of one door and doors are half the width of the trailer and the overall width is just barely less than the height (couple inches / 2 doors = inch or so less on each side).

Makes me think .25 L/D might have some significance as the sweet spot for cod reduction/tail length on boxy, bluff bodies.

http://www.palmerleasing.com/images/53_closed_van.png

http://www.part20.eu/en/applications/trailer/

Since that's my overall height from the ground, not just lowest point to highest, I think 42.5" is probably actually longer than would be proportional to a semi van trailer. If my back bumper is 10" above ground, half that is 5", and .25D shrinks to 37.5". Better go measure...

Rear bumper is 19.5" from ground. Half is 8.25, 42.5 - 8.25 = 34.25" for L. Dang now I want to lower the van...NOT happening

[mwilliamshs]

Originally Posted by aerohead

Here are some additional images to muddy the waters...

What's the difference in the bottom two images? What makes the last one better by .048?

[kach22i]

Lots of great documentation in this thread, just would like to add one of my favorite images.

http://ecomodder.com/forum/showthrea...-11732-69.html

[freebeard]

What's the difference in the bottom two images? What makes the last one better by .048?

The gray shading suggests plan taper. It's the difference between E and F in that previous chart.

Edit: I just noticed the previous question.

No idea what all ^that^ means.

Van is 85" tall from ground to rooftop.

What's the width? I was taking it for 85". That ground reflection thing doesn't respect the way air works. Frontal area doesn't include the air that moves under the car. So it should be 85 less average ground clearance.

That Vanagon's hatch won't open much. Do you have double doors in the back?

FWIW, here's a model I made for a van I used to have.

[mwilliamshs]

Width is 80" overall. Body itself averages about 70".

Yes, all full-size Fords have "barn doors" in the back. I plan to have the tail installed to either swing apart in halves (one left, one right) or on a hitch stinger that swings entirely to either side like a bike rack.

[freebeard]

The template has a width twice it's height and tapers to a point. A square body gets two tapers to the top's one (plus the little help from the bottom). So the side tapers will meet at a vertical line.

One thing you could do would be flat plates as part of a box cavity that hinge on the same line as the doors. That line is probably 6-8" in from the side. The plates might be shorter or longer than the doors and the doors could open partially before they push open the aero add-ons. The top could stay fixed and the bottom rear bellypan could incorporate a step.

[mwilliamshs]

Originally Posted by freebeard

The template has a width twice it's height and tapers to a point. A square body gets two tapers to the top's one (plus the little help from the bottom). So the side tapers will meet at a vertical line...

The zeppelin template, right?

I'm treating things like semi box trailer (aka van) and think what they do on those must be darn good cuz they keep doing it and of the half dozen or so companies doing the tails they all share all but the smallest of details (likely just differing for patent reasons).

The 2 side tapers : 1 top taper and meeting at a vertical line stuff blew right by me...got a pic?

[freebeard]

My own case is the VW Beetle body. I like it because the body begins it's plan taper at the door edge, rather than the rear axle line. I have established by physical measurements what is shown in the rendering—the side taper meets 3 feet behind the rear valance. The top taper meets the ground at a distance equal to the wheelbase. The fender-skirts are a different matter.

Here's another picture with a truncated boattail and a high-tailed stinger:


The big 18-wheelers are an edge case because the Fineness Ratio, or length of the body, is so great all the streamlines are straight front to back. On shorter vehicles air is creeping up the sides with two counterrotating vortexes springing over the edge and onto the roof. If the sides taper and the roof is flat, the air rolls off the top onto the sides.

[aerohead]

Originally Posted by mwilliamshs

What's the difference in the bottom two images? What makes the last one better by .048?

With the bottom image,the sides are angled inwards @ 10-degrees.

[mwilliamshs]

Originally Posted by aerohead

With the bottom image,the sides are angled inwards @ 10-degrees.

Thanks. Is 10° ideal? Is 15° better or worse? I was thinking 15° but one is as easy as the other at this point