Quote:
Originally Posted by Blacktree
The rear section of many vehicles is sloped downward. Any lift acting on that surface will have a force vector perpendicular to the surface. Since the surface is sloped to the rear, the force vector will be angled to the rear as well. In other words, that lift also creates drag.
That said, when designing a drag spoiler you also need to consider the rear wake. For example, if you make the angle of attack too aggressive, you'll increase the size of the wake. And that will create more drag.
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I would just like to interject the observation that, it's not necessarily the 'size' of the wake, but the base pressure within it that will determine the drag.
There's going to be a 'sweet spot' for every vehicle spoiler. Size and geometry.
The worst-case is a slope that will create a vortex-pair, which induces a downwash-induced small wake , but coincident vortex drag, creating enormous total drag.
In the 1st-gen VW Golf/ Rabbit, the Cd could vary from 0.34, to 0.437, just by altering the rear slope angle.