[aerohead]

[QUOTE=Piotrsko;658768]Would the trailer also need the 4-5% radius on the edges? Does explain why V nosed trailers aren't more efficient.[/QUOTE
* Kamm- Fachsenfeld's research at FKFS indicated a delta- Cd = 16% for a 'softened' upper body vs 'square.'
* According to FIAT's research, on an flat-roofed automobile ( NASA's Ford Econoline ), behind the A-Pillars, there's no change in drag, with, or without the radii.
* If rear downslope is introduced, then there's a drag advantage to having the C-Pillar radii ( 4.37% delta-Cd with a 28.5-degree fastback ).( FIAT )
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* Hucho, page 318, Figure 8.37, Daimler-Benz' aerodynamic concept semitrailer, clearly has all-upper-edge radii.
* Hucho, page-320, Fruehauf's aero concept semi, ditto.
* Hucho, page-321, Ford of Europe, box-truck, ditto.
* Hucho, page-324, Figure 8.46, an 11% radius on a rectangular cylinder, of 3:1 length / diameter ratio, shows Cd 0.20, vs Cd 0.85 without.
* In Hucho's 2nd-Ed, page-332, Figure 8.63, for the motor bus / RV motorhome, they didn't test for longitudinal edge radii, so no info there.
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* General Motors', 1980s, Class-8 semi-trailer 'Optimum' boat-tail is predicated upon the trailer van 'HAVING' upper edge radii.
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Seems like we'd have to say that it's 'conditional', but for my money, I'm going all-in on upper radii.