Quote:
Originally Posted by brucepick
Aerohead -
All other things being equal - which they never are of course,
would a spoiler angled upward 30º generate more downforce than a level one ending at the same point? And would that downforce carry a drag penalty?
Measured angles:
I measured some of the angles on the '96-00 Civic coupe using a print of the drawing that's been posted in these threads, and an old fashioned protractor. Also used a somewhat sketchy straightedge to establish the base lines, so absolute precision is not guaranteed!
The rear glass angle is about 20-22º downward, vs. horizontal. So, its local angle vs. the immediate prior roof is going to be less than 22º because the roof has already begun to curve downward at that point. This may explain the pretty good attached flow that I believe 98CaliformiaCivic found on the rear glass and trunk lid.
- - - tuft video
- - - damp tufts still #1
- - - damp tufts still #2
I measured the angle from roof to trunk lid edge, based from a point on roof slightly rear of the small side window's forward edge line. This is not the roof peak but is (to my eye) where it departs visibly from the template curve. I got 14º.
I measured from top edge of rear glass to trunk lid edge. I found about 16º there.
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I've seen recommendations for rear down angles of 14º from roof edge to trunk rear edge, and aerohead's recent suggestion to keep local angles at less than 22º. Based on those, it looks like the Civic coupe's stock back end is already in pretty good shape. However the trunk edge isn't anywhere near the template line. A "false trunk lid" to meet the template curve would be some 7"-9" higher than the original trunk, even if the false trunk lid is extended back a few inches to be even with the rear bumper. If not extended rearward, the false lid would need to be higher if it is to meet the template.
Aerohead, what do you think? Would there be gain in a false trunk lid if its sides are formed by extending the C-pillars rearward on their own existing angle? Or have the designers already worked the angles to the point where they've minimized the wake cross section - and a raised lid would only increase that cross section? |
brucepick,there's a gray area with respect to your question.I have no data on direct comparisons between 'flat' spoilers and 'kicker' type spoilers ending at the same point.I don't like to guess.
Any spoiler,which terminated
'before' the 'Template'would permit comingling of the low pressure separated flow and general wake.
Spoilers terminating at the 'Template' would prevent this comingling,and wake pressure would be less affected.
The 'kicker' type spoiler would envelope a larger bird bath of vorticity than the flat spoiler and should have the higher drag.
Since the 'kicker' type has vertical architecture that the flat doesn't,it may
embody a vertical force vector impossible for the flat spoiler due to the lower base pressure acting upon it due to the higher vorticity in front of it.
Local tangent angles should be a function of the distance behind the point of max roof camber as per the 'Template'.This is why I created the 'Template',to get people away from talking about 'angles'.
Angles must be mentioned in the context of where they occur along the body structure.
Ideal 'angles' can be 'read' off the 'Template' for any position on the body.
If angles are 'hypo-'Template' the car will suffer increased drag due to separation.
If angles are 'hyper-'Template' the car will suffer increased drag due to increased skin-friction.
If you'll look at Mair's boat-tail graphic in the Phil Knox's Aerodynamic Photos Albums you'll see that even though 22-degrees will support attached boundary layer,it cannot be used until around one car height distance downstream of the point of max. roof camber.