Quote:
Originally Posted by wmjinman
But doesn't anything more than the slightest upslope on the back of the boattail (like 2.5 degrees) INCREASE drag because it tends to cause a partial vacuum between it and the ground, which tries to pull the car backwards into it?
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That probably depends on how much air is being diverted under the car compared to over and around the sides. Some of the displaced air will be diverted underneath, somewhat increasing the airspeed compared to freestream air, so it will have some slowing down it needs to do as it leaves the end of the car. If it does need to slow down some, providing a small upslope on the bottom of the kammback should be beneficial and you shouldn't need to worry about increasing the vacuum (that's already there) for small upward angles from the bottom.
My assumption (whether it's correct, or not...?) is that much more of the air is deflected around the sides and especially over the top of the vehicle, so that air is going much faster and requires much more deceleration behind the vehicle compared to the air underneath the car. Therefore guiding the air from the top and side to keep flow as smooth as possible will provide a more noticeable benefit than from the bottom.