[aerohead]

1) In Hucho's 2nd Edition, he used ' Equivalent roughness' in his first graphic presentation, Figure 4.80, page 166, and only qualitatively.
2) 'Sweet spots' occurred with all actual production vehicles investigated and reported with graphical drag data.
3) Twelve different configurations of four different airdams were presented for the Brazilian, Volkswagen do Brasil 1600X. A blueprint with dimensions would allow any interested party to reproduce the dimensional interplay leading to the twelve Cds, 1973.
4) Same for Max Schenkel's test vehicle, 1977.
5) Ditto: H.J. Emmelmann's OPEL Corsa,1982.
6) Ditto: Feysal Ahmed Adem's pickup truck, 2009.
7) Ditto: USN Lieutenant Nathan A. William's 1997, Dodge RAM, 1500 Pickup, 2003.
8) Ditto: Don Sherman's Ford Pinto, March 1974
9) Ditto: Don Sherman's Datsun 240-Z, May 1974
10) Ditto: Dennis Simanaitis' Volkswagen Scirocco, August, 1982
11) Ditto: ITworks Project CRX, USFRA, Bonneville Int'l Speedway, 1990.
12) Ditto: ITworks Project CRX/ CAR and DRIVER, Chrysler Proving Grounds, 1991.
13) Ditto: ITworks Project SPIRIT-I, DARKO, 2014
14) Ditto: ITworks Project SPIRIT-I, USFRA, Bonneville Int'l Speedway, 2014.
15) Ditto: ITworks Project SPIRIT-II, DARKO, 2017.
16) Ditto: Subaru Engineering Division, FUJI Heavy Industries, Ltd, SAE Paper 860216
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17) The airdam investigations would note airdam drag, Underbody drag, total drag, lift, volumetric flow through the cooling system.
18) Since pressure drag dominates the aerodynamic drag, and the forward stagnation point pressure is the static pressure, the differential between the nose of the F-150, and the average mean base pressure behind the tailgate/valance/rear bumper would be of great interest.
19) I'm comfortable 'owning' the comment about frontal area, as a first-principle precaution. Hucho's Figure 4.17, page 123, image (D) bottom of the table drives issue home.
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20) As to 'not shielding', other messengers, including R.G.S. White, at MIRA, took the actual complexity of underbody structures, and their impact on drag, such that he created a five-tiered rating system, comparing what one could perceive as the degree of aerodynamic torture chamber they presented to the oncoming flow, from best-to-worst.
There are structures which, un-shielded, can separate the flow to such a degree that, the entire underbody becomes turbulent, with no possibility of harvesting any of it's kinetic energy for pressure recovery.
The 1969 Chevrolet Camaro Z/28 was so nasty that it created 375-pounds of front lift at 115-mph. Adding the optional airdam cut that by 150-pounds.
The 'shielded' chassis components were now 'drafting' in the wake of the Cd 1.19 airdam.
In the absence of an airdam, belly pans and a diffuser knocked 70-counts off the drag of the Audi 100-III.
21) As to your quanta, I don't have any confidence in your testing methodology. For reasons mentioned elsewhere.