Tom, your testing of the model indicated you indeed had ground effect (GE).
But as Pork Pie suggested, the distribution (or location) of this down-force
on the vehicle is important, and I doubt your "pick-it-up" test would tell
you the exact location of the effect. I am assuming you want the down-force
on the read end. Additionally, I have some confusion with both your
description and Pork Pie's.
So back to some basics. Mr. Bernoulli says high velocity equals LOW pressure
and LOW velocity equals HIGH pressure. The location that is the narrowest
has the highest velocity and therefore greatest down-force. Your
description inferred that at the mid-point of the car the narrow area opened
up into a large, car-wide plenum, from the mid-point back. That plenum would
become a HIGH pressure area, in exactly the wrong place if you are trying to
counteract rear-end lift. I agree with Pork Pie's concern about the wide
area in the front. That would be a high pressure (low velocity) area.
However, he suggest gradually increasing the width towards the rear from the
narrowest area. That means there would be a gradual INCREASE in pressure
(the velocity slows down) as the width becomes wider towards the rear. That
may not be the effect you are looking for. It is a fine balance exactly
where you want the most down-force. Another way of looking at it would be
similar to moving the center of pressure forward or backward to suit vehicle
needs. Without knowing all the details, it looks like you are creating a
high pressure under the rear half of the car and I would be very concerned.
-Elon
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