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Thanks for the respond, Aerohead.
The 18degree was a reference which I gathered from some of the Hucho's article and seems that the rear angle (whole rear end portion) somewhere between 15-18degree seems to be the maximum / critical angle before major drag increase (if the angle is made shallower)...
What I try to understand is that can a spoiler be used at the end rear end shape to reduce drag? Ex. if the rear end shape maximum angle is e.g. 18degree and one installs a spoiler at the end in a way that airflow over the spoiler is "attached", impact to wake is minimal but air flow pressure on front of the spoiler (in larger area) would be changed from negative to positive pressure. If these conditions would be met in a same vehicle, would that lead to reduced drag? Or is this too general description?
E.g. if the rear end angle is made, lets say maximum of 22degree (reduced wake area), so the rear end shape exceeds the critical angle.... But installing the spoiler in away that the tip of the spoiler would still form "artificial" angle of 18degrees or less (from tip of the spoiler to earlier point of the shape, tangent point), and the flow would remain still attached over the spoiler, flow pressure would be changed from negative to positive - would this lead overall reduction of drag?
Ultimately I try to understand that if "wake area" is bad and that would be minimized by creating the rear end shape which has shallower angle than e.g. 18degree, would
(any) spoiler shape / angle / size be able to recover drag penalty caused by the angle which exceeds so called "critical angle" (somewhere between 15-18degrees)...
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