Wake area 'cardboard-photographic-calculus'

[aerohead]

For those nuts enough to desire elongating their vehicle, the following is a very 'old-school' technique for quantifying their vehicle's OEM wake area, in anticipation of creating the 'scaled-down' wake footprints, needed to form the new tail's bulkheads.
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Background:
Before CAD and AUTOCAD, complex-shaped areas could be found by accurately weighing a 'photograph' of a shape, and comparing that weight with weight of a reference area of the same photographic material.
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Cardboard is manufactured with remarkably 'tight' quality-control standards, and it's unit density ( mass/weight ), per unit area ( square-foot/square-meter ) is remarkably uniform as well, as with photographic prints.
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The idea is to, create a cardboard pattern of your car's wake, by transferring dimensions directly off the car and onto carboard.
1) An accurate, perfectly-square 'rectangle' of cardboard, equal to the net width and height of the wake is first cut.
2) Then you need to find something like a high-school, or junior-college laboratory, which has a scientific-quality electronic, load-cell type scale, with an accuracy in fractions of a gram, and weigh your cardboard rectangle.
3) This is absolutely critical to the project!
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4) Then, transfer your 'shape' onto the cardboard and cut off the margins as accurately as you possibly can ( you're leaving only the 'shadow' of the wake ).
5) Now, take your pattern back to the 'lab' and re-weigh it.
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6) You knew the original area of the uncut rectangle, and you knew it's weight.
7) Divide the weight of the 'cut' cardboard, by the weight of the 'whole' cardboard.
8) This is the 'fraction' of the original rectangular area remaining.
9) Multiply the original area of the rectangle by this fraction to arrive at your OEM wake area.
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If we accept the linear relationship between wake area and drag, then any desired Cd can be computed simply as a function of reduced wake area.
If you want to cut your Cd in half, reduce the wake area by 50% ( assuming that your contour is streamlined enough to maintain attached flow all the way to the new separation line, top, sides, and bottom ).
( anyone with 'Aerodynamik des Kraftfahrzeugs' will see the genesis of this relationship through the researches of Walter Lay, Baron Reinhard-von Fachsenfeld, and Dr. Wunibald I. Kamm ).

[freebeard]

2) Then you need to find something like a high-school, or junior-college laboratory, which has a scientific-quality electronic, load-cell type scale, with an accuracy in fractions of a gram, and weigh your cardboard rectangle.

Else one gram accuracy and use 10 cut-outs.

( anyone with 'Aerodynamik des Kraftfahrzeugs' will see the genesis of this relationship through the researches of Walter Lay, Baron Reinhard-von Fachsenfeld, and Dr. Wunibald I. Kamm ).

I thought there was a guy named W. A. Mair

www.cambridge.org: Reduction of Base Drag by Boat-Tailed Afterbodies in Low-Speed Flow

[aerohead]

Mair didn't come onto the radar screen until 1969.

[aerohead]

I'd recommend more resolution. We need all the accuracy we can get.
I had access to one at Palmdale Junior College, when I was doing the overhaul and Big-Bore kit for Ghia #1.
And at ABARIS in Reno, for the advanced composites course.
They were around $80 to buy. If you're not going to use more than a couple of times, it's better to 'borrow.'

[freebeard]

I've got a Sargent-Welch double balance beam like this one:


https://www.etsy.com/listing/1213301...-scientific-co

The bottom bar is 10 grams by tenths, then there is the pointer at the to for interpolation but it only has seven subdivisiions. ???

An 'herbalist' gave it to me when they went digital.

[Piwoslaw]

I remember once wondering about the wake reduction of my previous P307's Kammback.
I took a picture (from a long distance, to reduce perspective angles), then traced the outline of the old and new rear edges, and used GIMP or other to count the pixels of each.

Similarly I estimated the FA reduction of folding the side mirror.

[aerohead]

Nice scale!
As long as you could balance the cardboard section on the one surface, without introducing any force, you'd be able to get a very accurate weight.
Since there's no 'motion' involved with the strain-gauge load cell scale, it would require less gymnastics to hold each piece.