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George, instead of a rectangular spoiler, it looks like half of an ellipse. My thinking was that by tapering the edges, I could keep most of the downforce, and reduce drag. It turns out that the rectangular spoiler tests better in both respects.
Here's the kicker. I ran several tests using different spoiler widths, expecting to find a minima, and an obvious winner. Boy was I surprised. All of these tests ran spoilers that were 2" above the trunk deck, and varying widths:
60" wide
Cd .305
Fz 203 N
56" wide
Cd .304
Fz 231 N
50" wide
Cd .304
Fz 187 N
45" wide
Cd .302
Fz 208 N
40" wide
Cd .304
Fz 223 N
The 45" wide spoiler doesn't run away with it, and it has the lowest Cd and low lift. The vortices spilling off the rear are much less energetic than stock. Next I will vary the height, and curvature of the spoiler to see if there's any improvement to be had. I wonder if raking the spoiler benefits anything.
Note that the increase in downforce is all on the rear, and that there's only 208 N of lift on the entire car. Yes, the pitching moment is skewed, but it's only 45 lb, and I'm not racing.
Aero is so senstive to small changes. 45" ISN'T the magic number for the real car...2" likely isn't the ideal height. I'll have to do tuft testing to locate the edges of the spoiler. This CFD is just giving me (hopefully) a better sense of cause and effect. It seems that I want to keep it really close to the streamlines.
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