Quote:
Originally Posted by tomi_k
In context of the this thread - Assuming e.g that rear window angle is 18degree at the maximum (lets say a fastback type rear end shape) and flow over the rear window is attached without a spoiler (not a wing). With a spoiler and still assuming that flow over the rear window and the spoiler is attached, can the spoiler reduce the drag caused by 18degree rear window?
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* It's very contextual.
* On a simple prismatic body, with simple angles and no curvature, optimum backlight angle is a function of the percentage of the length of the aft-body, compared to the overall length of the vehicle, expressed as a ratio.
* On complex bodies, where curvature is used, the optimum backlight angle will also be a function of the percentage of aft-body.
* Some cars, designed for maximum interior useful volume, won't have any appreciable backlight angle at all, they'll just be vertical.
* In order to anticipate some hypothetical, we really need to specify a few more details of the body proportions.
* It's best to just pick a specific vehicle, and one in which blueprints are available, depicting all surfaces and proportions as they appear in reality.
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* And we need to be very careful how we use the term 'attached flow'.
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A few specific vehicles:
* Volkswagen's 2013 XL1, Cd 0.189, rear downslope angle = 17-degrees maximum.
* GM's 1996 EV1 rear slope maximum = 20-degrees, then reflexed to 18-degrees, and continued to reflex as low as 14-degrees at the trailing edge.
* GM's 1993 Impact land speed record car's boat tail extension, relaxed the rear reflex to only 10-degrees at the trailing edge, for Cd 0.137.
* Ford's 1985 Cd 0.137, Probe-V concept had a maximum 11.5-degrees.
* GM's 1987 Cd 0.089 Sunraycer rear slope angle never exceeded 17.5-degrees.
* HONDA's Cd 0.10, 1993 Dream solar racer's maximum rear slope was 11-degrees.
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I'll bring Rolf Buchheim's prismatic drag table next Wednesday. It may contain some data for an 18-degree backlight.