Quote:
Originally Posted by j-c-c
IMO, a rear spoiler is supposed to "spoil" air flow, and therefore increasing the local air pressure upon a surface up stream, to intentionally increase DF. Increased drag is thought to be an acceptable trade off in most cases.
A flat added extension (trunk?) should, again IMO, maybe better be called a "smoother (or divider?)", which might reduce drag. If a "smoother", just by forming a hard boundary between a higher pressure area and a lower pressure area in space, will also increase potential DF, with little drag increase, if any.
Doesn't seem to me that a typical rear spoiler often reduces drag, leading to better mpg.
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The spoiler is there to spoil lift.The lift is a function of fast,low-pressure turbulence,created by separated flow over a moment arm created by position over the body.Kamei created the first commercial airdam and spoiler for the BMW 2000,in 1982.The rear spoiler created a flow reattachment at the tip of the 23-degree slope-to-spoiler top,thus capturing a vortex,preventing the separation line pressure from telegraphing all the way to the end of the boot.Without the rear spoiler,low pressure would make it behind the car,lowering the base pressure,and increasing pressure drag.At this 'sweet-spot',the spoiler both kills rear lift,while also reducing drag.