drag 'Equations'
 

I performed a test on two equations offered in the literature, which were associated with predicting what would happen to the Cd of a streamlined body, if one truncated the aft-body, at any given length.
I chose the 1981 VW-Flow Body ( langheck ) Cd 0.14, & ( kurzheck) Cd 0.15.
Both equations failed to predict values anywhere in the ballpark of the known coefficients.
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The first equation is from Goro Tamai's, The Leading Edge, and is from Sighard F. Hoerner's 1965, AERODYNAMIC DRAG, which turns out to be a modification a second, original, equation ( 3, 14), page 37, AERODYNAMIC DRAG, Hoerner, 1951-Edition.
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Equation-1)
Delta-Cd is approximately = to (0.34, divided by the cube-root of the frontal area-based Cd of the entire body ) multiplied by [( the thickness of the vertical truncation divided by twice the body height ) raised to the 4/3rds power ].
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Equation-2 ( 3, 14 )
Delta- Cd = ( 0.34, divided by the cube-root of the Cd-minimum ) times [ ( the delta chord, divided by the chord ) raised to the 4/3rd power )].
This second equation is from: Ergebnisses AVA G'o'ttingen Volume III, Munich, 1926.
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So far, best I can tell is that, the two equations are valid only for bodies with zero plan-taper, as in a 2D wing section-based solar race cars. Which wasn't mentioned specifically in the book.
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Remaining 'tools' we may guestimate with are from:
* Jeff Howell et al., SAE Paper 2020-01-0673, which commented:
' the drag reduction from an elongated tail varies almost linearly with the reduction in cross section area.'
* ' when vehicles developed for the lowest drag coefficient are evaluated for Cd vs length, the correlation is discerned and expected trend is in fact confirmed.' W.H. Hucho, page-202, 2nd-Ed.
* ' On long vehicles, where more length is available for the rear end length, a larger angle can be used resulting in a higher reduction of the drag.' Hucho, page- 154, 2nd-Ed.
* ' The greatest potential for aerodynamic improvement is at the rear. The longer, narrower and more angular it is- the better the aerodynamic figures.' Dr. Teddy Woll, Mercedes-Benz, 2014.
* ' The perfect aerodynamic shape is a teardrop.' Dr. Teddy Woll, 2014.
* ' Aero is more about the rear of the car than the front.' Freeman Thomas, Director, Strategic Design, Ford Motor Company, Los Angeles Auto Show, 2008.
* ' [T]he optimum shape in terms of drag is a half-body, which forms a complete body of revolution together with its mirror image- produced through reflection from the roadway.' Hucho, page-15, 2nd-Ed.