Quote:
Originally Posted by aerohead
brucepick,it does function exactly like the raised tailgate.
If the trailing edge is close to the template the spoiler captures the low-pressure vortex and isolates that low pressure in front of it,preventing it from communicating that pressure to the wake behind which would otherwise lower the base pressure,increasing the delta-P across the car,increasing drag.
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When the outer flow does separate at the trailing edge of the spoiler, it has decelerated to a lower velocity/higher pressure ( that evil Daniel Bournoulli ) leaving the base pressure of the wake higher,the delta-P lower,an concomitant lower drag.
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If you tilt the spoiler up at 30-degrees from the back of the trunklid( boot),it will hit the 'Template' sooner,and it can be 'shorter'.
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If you were to also extend the C-pillars back along the 'Template' line as I did with the CRX,you can tailor the vortex even finer with this buttress,and in so doing,foster even better side flow towards the rear by,eliminating a potential breeding ground for longitudinal attached-vortices on each side.
You'll have a bit of new blind-spot,but you can compensate for that with your mirrors.
Anyone with a 1st-gen Insight would be familiar with this.
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If you can't reconcile yourself to the 'Template' try to set 22-degrees as your maximum local tangent angle.Air cannot follow an angle steeper than this,and typically,you need to have close to a body height worth of length in progressive curvature before you can get to 22-degrees.
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It took a while but I'm making plans now. Hope will be in the driveway cutting and gluing tomorrow, based on what aerohead wrote above.
My plan is for a spoiler that's also a false raised trunk lid, starting at a line about half way up the rear glass, going to a level meeting the template, as far back as the rear bumper. I plan to also extend the C-pillars rearward, and taper the extensions inwards slightly.
The C-pillar extensions will use mount brackets that will be silicone-glued to the window glass, or will attach those brackets with heavy duty double stick foam tape. That should get me out of gluing, duct-taping or screwing to the sheet metal.
A question to aerohead:
Re. the max height of the C-pillar extension panels.
Do you prefer Option 1 or 2 below?
Option 1: Start the forward end of the vertical panels at the full height of the C-pillar, measured at the point where the extension panel begins. It would taper down to the rear, ultimately to the level of where the spoiler/false trunk lid meets the template curve. This leaves a sort of chamber between the two extended side panels, above the spoiler.
Option 2: Have the side panels only go as high as the level of the spoiler/false trunk lid, wherever that turns out to be. Something like 6" above the trunk lid. So, no chamber between them, at least not above the horizontal spoiler panel.
Visibility: one more "feature" to the design:
The forward edge of the spoiler at the rear glass, will be at a height so that the whole spoiler should appear only as a horizontal line when viewed via the rear view mirror. So I hope to keep good rear visibility that way. Likely this will mean the forward edge will be raised a few inches vs. its rear edge. However I won't know exactly where it will end up until I put it together and look in the mirror.
Aerohead, you already mentioned that I'll get a bit of a blind spot from the side panels. I'm considering eventually making them from Lexan, but they will be coroplast or even cardboard for starters.