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Old 07-22-2018, 12:52 AM   #8 (permalink)
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Air brakes are perpendicular [in use]. The perforations increase turbulence over a solid plate, thus increasing drag.

For the parallel case, I saw a PDF (that DDG can't find today) on using round holes on the front of the sides of cattle trucks, over horizontal rectangular slot the length of the sides (as is the fashion) to prevent animals suffocating. So location and shape/size matter.
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