Quote:
Originally Posted by CFECO
When the bed edge is square that air cannot make a smooth flow up to the cap area, which probably causes the vortices and the less than ideal gains from the tapered cap.
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This would appear to be a common mistake that many (including me) have made, in failing to note that air on the sides needs to move in the same general direction as air coming over the top. At speed, my version 2 bedcap makes pretty large counterrotating vortices on either side of the tailgate, and I am certain it's due to the fact that the side air is more-or-less going in a straight line, while the air coming from the top is moving downward. This is a good recipe for fuel-economy-robbing vortex generation.
I saw evidence supporting this when I re-read the Naval Post-Grad school paper on the Dodge Ram bedcap. The author's bedcap sloped downward, but evened out at the end. From the data in his report, I calculated a gain of about 15% from his aerocap alone, which is better than the 10% I found with my aerocap.
I think it should be emphasized that, for the last foot or so of the vehicle in question, all of the surfaces should ideally match each other with regard to slant. If the sides taper in just a little at the end, then the top should also. If the sides do not taper in at all at the end, then the top should also not taper at all. Version 3 of my bedcap will incorporate this.