Ideal car rear roofline angle and shape for low drag?

[aerohead]

Originally Posted by Cd

Phil, I was aware that the Morelli form would not fully adapt to the forebody shape of my Civic.
I was just curious about the roofline taper.
It swoops down steeply, and then corrects back to the template farter downwind.
I ask, because I have seen steep rooflines like this on other low drag cars that were not Morelli in shape.
( That's why I mentioned cars like the Tesla, the Lucid, and new Mercedes EV in some of my last questions to you )
I know that the airflow at the floor level of the car would not follow the Morelli form, as well as the sides.

I think a lot of non math types like me would love to see some "thermodynamic energy balance protocol " figures for other vehicles on this site.
For example, Darin's "Flea" with the boattail, and that Prius that is on this site " ChamPrius" with that full boattail.

- Thanks

1) I'd mention that the 4-wheel Morelli body has twice the drag as the AST ( Cd 0.16 vs Cd 0.08. And that's with a very long, thin, tail, and knife-edge separation line.
2) The guy who 'invented' the S-Curve body shape used by Morelli ( Richard Von Mises ) advocated for 'streamlined bodies of revolution' as the 'solution' for low-drag cars, nothing else. Just like Hucho. Just like Dr. Teddy Woll.
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3) The 'Kamm-back', 2012, Coventry University-modified 1999 Audi A2, for it's 'length', has lower drag than Tesla, Lucid, M-B EQXX, EV1, Morelli's CNR-PF, M-B EQS, Lightyear One, Loremo, and M-B C-111 III.
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4) If you're bound and determined to 'ruin' the CIVIC's roofline with the steep slope, your friend is doing the reflex-cambered tail extension.
Your going to wreck the supervelocity deceleration profile and it's pressure recovery.
An extension which reaches out to the AST will provide the reacceleration, reattachment, and pressure recovery ( like GM used on the 1993 Impact/EV1 'Yellow-Ferret' 183-mph LSR car ( Cd 0.137 ).
Goro Tamai refers to the phenomenon as 'mending' the flow.
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5) If you have a 'blueprint' of your car, and some drafting instruments, you can 'build the modifications on paper ( VSAERO, AUTOCAD, QUADPAN, CATIA if you have access ).
The 'larger' the drawing, the higher resolution you'll have for scaling measurements ( not an issue with software ).
Engineers would use from: Differential Geometry Applied to Curve and Surface Design, Volume 1 & 2, Anthony W. Nutbourne & Ralph R. Martin, Wiley, 1988, $ 115.

[aerohead]

Originally Posted by Phase

my understanding is you want a mellow enough tail that gets smaller and smaller while maintaining flow attachment, try to reduce the wake size as much as possible, then detach the flow as fast as possible with hard edges. using small lip spoilers can help recover pressure too when it flips off the trailing edge

look at the mercedez eqxx , the light year zero, and and even a little bit of the vw xl1 car. they all have tear drop rears that then curve up/flatten out for a few inches at the rear for pressure recovery. the original honda insight did not have the mini rear spoiler shapes. it just dropped off. im assuming thats because engineers didnt know as much back at the time of development, and why its drag cd is 0.25 even with a flat bottom and rear fender skirts

By the time they designed the Insight, HONDA R&D had already produced Cd 0.10 solar race cars, so they 'CLEARLY' understand low drag.
The Cd 0.25, measured at Honda has never been demonstrated to be 'incorrect'.
International, inter-tunnel, Cd variability is a 'ground-proven' fact.
And it doesn't matter in the grand scheme of things.
The Cd 0.30, as measured at A2 can be simply intuited in the context of a full-scale wind tunnel blockage-ratio calibration model.
As far as I'm concerned, it remains Cd 0.25. It's all 'conditional' , 'contextual.'