Quote:
Originally Posted by daqcivic
Whoa, aerohead, thanks for the history lesson! Very interesting, and helpful for getting a grasp on what I'm trying to do. This rooftop extension will not be anything close to as long as a full boattail or even kammback, so there will not be that much curvature to speak of, though I will try to approximate the template as far as the profile of the roofline allows, which will mean the extension will have a very shallow angle.
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I just found this quote from you:
"As far as the drag reduction goes,you can't cheat on the curvature,and your drag reduction will be an arithmetic function of the new,smaller wake area the extension creates as compared to the vehicles original wake.--------EXAMPLE: your extension allows for a 10% wake area reduction,that translates to a 10% drag reduction.Simple huh? . . . So using,say the template,you can engineer your extension on paper and know the results before you ever begin buying materials."
Does that mean my extension will only reduce drag to the degree that it reduces the area (mainly height) of the wake? That would make it seem pretty pointless unless it extended quite far--enough so that it could curve down significantly according to the template.
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From what I've read on the EV1 and others of similar designs, as the body tapers in from plan (?) view, the taper in profile can be greater than if there was no side taper. That would mean the sides of a rooftop extension could curve in slightly and therefore downward and at a greater angle than the middle (if it has side supports) correct? Or is the boatttail template meant to serve for side taper also?
BTW, do you think I can improve the rear undertray in any way?
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It was Kamm who narrowed the body concurrent with the drop in roofline,providing a progressive reduction of the body cross-section such that when he made his chop,it was at a point where the wake area was 50% of frontal area.
Kamm makes no provision for 'cheating' on the roof in light of the plan-taper.
On a notch-back car it looks like you can cheat the backlight angle when the pillar buttresses are rounded to allow flow carefully in from the sides.Then the whole soup recovers over the trunklid,finally separating at the rear edge.
For a constant,progressive-curve roof it appears that you must stay close to the template until you hit 22-degrees.That angle has been demonstrated to be the maximum for attached flow.
Stay as close to the template line as best you can.Those shallow curves will guarantee good flow.You could always add to it in the future if that struck you.
Reducing the wake area is the goal.
The VW Rabbit photos in the FLOW-IMAGES will show you what the air wants to do.Give it a look.