Today's work was to stiffen up the center panel with three supports. Hopefully this will give better test results next time I get to do coastdown tests.
Vertical support is coroplast with 2.7 mm thick lauan plywood strip inside. Cross support is 1.25 inch wide lauan plywood strip held in by duct tape. Longitudinal suport is 1"x1/8" aluminum stock held in place by duct tape.
Here's how it looks in three quarter view from the rear:
In the pic above, you can see that the left side panel continues the body line almost perfectly straight rearward. Possibly it bulges outward a bit. My intention was to taper that line inward slightly, in both side panels. They have a good inward lean (tumblehome) but the whole panel needs to taper towards center a bit. This is one aspect I'll put some attention on the next time I do some panel reshaping, hopefully after I do some coastdown testing on this setup.
Driving it before installing these supports - but with the D-shaped bent coroplast center support that is visible in an earlier photo
(see here)- I saw the center panel flexing considerably at highway speed. It bowed up a good 2 or 2.5 inches at the rear, and a matching amount down at the mid point. Plus, the rear edge was fluttering up and down with the air flow. My original design intention of being able to see out the rear view mirror was completely snafu'd by the flexing center panel.
Not to mention that the distorting and vibrating center panel must have wreaked havoc with the intended aerodynamic design.
The three supports work together to give a reasonably stiff panel, though it still does vibrate some at highway speeds. I didn't get to do any formal tests on this version yet but the "butt dyno" at least felt decent.
And, here's a shot of the view out the rear view mirror from the driver's position. The thin curved line across the middle is the spoiler center panel. You can see the vertical support, behind the center brake light housing. This nearly uninterrupted view is one of my design targets. It's a large spoiler, and you STILL can see out the back. Not bad, if I say so myself. The blurred blob at lower left is my hand holding my little Kodak.
Considering that it still has noticeable vibration which would need additional stiffeners if I build the final in coroplast - I'm seriously considering building the final in lauan plywood. That's the 2.7 mm. (just under 1/8") plywood I used for the vertical support and the crosswise stiffener today. It is outdoor rated but there can be voids and also glue voids between the layers. It's very easy to work; it cuts with a "box cutter" knife or a fine saw. Can be sanded, primed and painted. I know someone is going to say to do it in fiberglass but I don't have any fiberglass skills and at this point I don't want to learn a whole new skill area.