Quote:
Originally Posted by brucepick
I appreciate your effort to inform us here. But once again, I've discovered how little I know on this topic.
Are you saying that even the best optimized spoiler can't achieve much improvement in drag? Do you think that also holds true for cars like the Civic coupes that have a fairly gentle rear glass slope?
Beyond that concern, I have to admit two other confusions, right from the start.
I'm guessing that backlight angle is compared with a horizontal line continuing rearward from top of rear glass? So a 10º angle is a very gentle downslope, and a 0º angle would be a rear glass that is horizontal?
And pretty much the same question for the % aft-body length item. I'm guessing that refers to the dimension from base of rear glass (or from top glass edge?? to the rear bumper edge, straight back to a point even with the rear bumper. Am I close? That dimension as a percentage of what??
I'm sorry. A little knowledge is a dangerous thing. I thought I was going to build a rear spoiler, but maybe I'd be better off addressing other possibilities.
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brucepick,I'll try and address your questions in order.
* the best 'idealized' spoiler could have significant potential for drag reduction,although due to the turbulence associated with the separated flow above the spoiler,it could never achieve the drag reduction of the K-form 'Template' roofline, which is devoid of such turbulence.
*Turbulence cannot be converted to useful pressure regain,only heat,during viscous attrition as the eddies are eventually worn down by calmer surrounding air.It's a second law of thermodynamics thing we strive to avoid.
* If the CIVIC backlight is steeper than the 'Template' it must have separation.The spoiler is a palliative to help with re-attachment and restoration of laminar flow of the outer flow field 'jumping' over the created locked-vortex of the spoiler.
* On a hatchback type car the backlight and its pillars define the departure angle/(curvature if any) for the aft-body roofline.This would be the case for the the Golf/Rabbit,where there exists no trunk( boot).And it is measured from the horizontal downwards.
* On a notchback coupe or sedan,the line described by a straight-edge laid across from the roof rear center-line to the trailing edge of the trunklid (boot) center-line, defines the aft-body departure angle.( an example would be the 1968 Dodge Daytona Charger.Its backlight is 22-degrees,but when you factor in the trunklid,the angle relaxes to only 18-degrees,an angle W.E.Lay used to achieve Cd 0.12 cars of 'impractical' length.
* The percentage of aft-body is taken from the 'Aerodynamic Streamlining Template Part-C' and is defined within that thread.The 'Template ' will show you the maximum aft-body 'angle' the boundary layer can tolerate without separation.
I recommend that members ballast their cars with 300-pounds( a couple of friends) in the front seat, when this determination is made so as to reflect the inclination of a 'loaded' car when actually moving.