Turbulence - Friend or Foe? |Here's a lot of Aero info, not spread in several threads

[sonyhome]

Ok, with some ABS plastic, that was a quick prototype... I did a quick drive... I seem to be at 30MPG at 72MPH with cruise control over a short drive, that's pretty good, the # looks higher rather than lower than normal (no A-B-A test).EDIT Did another 2x 25miles trip and got 27MPG this time at 73MPH cruise ctrl... That's not very good.

I put some pics here of the spoiler and shots of the geometry/shape of the car.

Aero

Here's 3 figures hand drawn. I suspect I probably did not quite understand all the car aero so maybe my description is wrong and so plz correct if you see stupidities.

1- Ideal shape, the car pushes the air out evenly and it regroups behind cleanly. Symmetry avoids lift drag.

2- CR-V SUV shape. The desing creates lift, the air travels more on top than bottom. Increasing lift also increases drag (?). Furthermore sharp tailgate angle forces undercarriage and roof air to rejoin at sharp angle, colliding while decompressing, causing turbulence drag.

3- Spoiler extends roof, detaches air cleanly forming a virtual boat tail with less angle so less lift and less associated drag. Undercarriage and roof airflows meet without colliding as much, less turbulent flow means less energy spent in eddies, less drag. This does create an air hole depression behind the SUV.



Airdam
If I were to make one, I'd increase the surface area by 15% or more:
Ground to bumper 15", ground to bottom of car 9", width of car 67" (tires 7" wide each), height 60". That's about 3100sqin frontal surface vs3600sqin with a full air dam.

Spoiler
I made the spoiler to follow the roofline, slopes down a little, and the ABS is flexible so it should sump by 1" or so with wind pressure. Is it still riding too high?



underbody diffuser
Behind the rear left wheel I've got a air hole, that should be fixed. Notice the tape job I did to seal holes around the bumper. Tape joins metal parts to plastic parts. Near the mud flap I didn't finish the tape job.

Incidentaly a friend took me to the BMW dealer and I looked at SUV undertrays. It's all covered & flat, surfaces just have some ridges parallele to airflow.