My next car I get I want to do a full belly pan and also I want to add a lower wing to send air up from the bottom of the rear bumper to complete the teardrop aerodynamics. I had first wanted to drill 3 or 400 holes in the middle of the rear bumper allow air to join the flow comming off the top of the car. I'v seen the import crowd add screened duct work to lessen the rear scooping effect of the lower bumper, but then it stuck me, why not add a wing a couple of inches right below the rear bump. Since most of the designs of cars stop abruptly and leave a 3X6 square area back there creating a massive vacuum area, why not fill the void and join the 2 airstreams together?
I'v seen hatch backs and modern VW Beetles have great success with wings straightening the airflow in the back of the car, but there is still allot of area that can form a vaccum. A couple of the cars I'v seen in the garage have had massive boat tail booms installed on them, almost to the point of looking like the tail on a small airplane. All in the name of smoothing everything aerodynamically, making air smoothly move around the body of the car is what were after. A smooth transition on the front of the car and then a gradual joining of the 2 airstreams at the back, the tear drop effect.
The wing doesn't have to be a wing shape, it can be one third of a 4 inch piece of round steel heater ducting, throw in a couple of turnbuckles to adjust the pitch angle and I think this could really improve highway mpg. The scoop wing would grab the lower air flow and aim it up into the void behind the back bumper. This way not having to go to the great expense of constructing a boat tail on the back of your car.
It's a little unclear what it would look like, but the theory behind it seems sound. As long as you stopped the air from being trapped in the bumper you could def. redirect it up and into the back airstream. I like the idea.
Best fill up so far is now 29 MPG!
What I see is 2 airfoils, compound style on top of each other, right on the bottom of your bumper, the lip would be straight, going under the bumper about 3 inches, at the bottom seem of the bumper line it would curve up. It would hang about 3 inches below the bumper line and run the full width of the lower bumper line. The top foil would angle up sharply to fill the void behind the bumper and trunk section. The lower foil would point up at about 45 degrees, this wing would shape the air.
So one wing fills the void, the other shapes the flow. I wish I could attach a drawing, but my computer wants to cut all my drawings in half. Essentially think of a rear spoiler wing, but on the bottom of your bumper, instead of on top of the trunk lid.