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Old 11-21-2022, 01:18 PM   #1 (permalink)
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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.
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.
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 ].
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.
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.
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.

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Last edited by aerohead; 11-21-2022 at 01:19 PM.. Reason: typo
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