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Originally Posted by COcyclist
Just for clarification, the Cd 0.12 is hypothetical?, based on the research if Chevy had boattailed the 2013 Malibu Eco or did they test a boattail on that car?
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It's beyond hypothetical. And well beyond "theoretical".
A pet peve of mine is the incorrect use of "theoretical" (and I am not suggesting that you have used "hypothetical" incorrectly here). In the very early Aptera days, they announced a Cd of .06 and a mileage of 330 on a diesel straight (non-plug-in) hybrid. They later said this was "theoretical." The problem is it is not "theoretical" in a meaningful sense, because they are not using the
right theories: they are using one convenient one when 10 apply. It is as if one were to say, "a standard Honda Accord can theoretically get 100 mpg (whisper: provided the engine efficiency is 100%)". (The problem is that other relevant theories dictate the engine efficiency will not get remotely close to 100%.) The sun and moon are
theoretically the same size because they subtend similar visual angles. (But, oops, they are different distances away.)
From 3000 miles away (from Aptera land) I was able to say "no, that shape with wheels will equate to .15 or so, but only if they do a good job of attending to all of the details. No, 330 real mpg is not plausible." So I later declared my self a human CFD machine, because I could do better from 3000 miles away than they could do right there in California. In the X prize competition, Aptera got almost 200 MPGe on electricity, making the vehicle about twice as efficient as a Leaf. (This is about as expected, given the Leaf's high weight and mediocre aero.) The Leaf, on gas, would get 30ish mpg (as the Juke does). The Aptera, on gas, would get 65 mpg. (That is significantly different than 330 mpg.) (For perspective: the VLC gets better than 100 mpg on gas, and the VLCE gets about 350 MPGe on electricity.)
There are many reasons sailplanes look the way they do. There are many reasons solar racers look they way they do. It is not coincidence that they look very "clean" at the front, as well as at the rear. It is no coincidence that solar racers have impossibly skinny tires.
.12 [thanks, Neil] is solar race territory. A Malibu cannot look anything like the "eco" does and come remotely close to that, boat tail or not.
Sharp leading edges (parallel to the ground plane) work well, BTW. Rounded leading edges are only required if the angle of attack varies substantially, but in a car it doesn't change significantly relative to the ground plane. Once cars can fly, that will change.
So, if you pull out the stops, you get something that looks like this:
(Photo of Nuna5 from gizmag)
No Hucho template upper curve, no blunt leading edge: just impeccably clean everywhere, not just at the tail.
The Probe 4 is "all wrong" -- sharp at the front, blunt at the rear -- but had very low Cd -- about the same as the Aptera, which "looks" right -- like a teardrop.
The Malibu numbers must be taken with a grain of salt, but represent possibilities for a particular shape going to another closely related one. (it is a particular cleanup of a particular car.) The numbers clearly show that lots can be done at the front of many cars. One can assume that they are exaggerating the aero work, because the fuel efficiency is awful, especially around town where a hybrid should excel. The Malibu is actually a "heavier" hybrid than the Insight (with a 15 kW motor generator vs a 10 kW one) but does far worse than the only slightly smaller new Insight, and tons worse that the Prius (which is mid sized, like the Malibu). The Prius gets essentially double the fuel efficiency of the Malibu on the urban cycle.
It is as if they name it "eco" to disguise the fact that something went wrong in the engineering. "31 combined mpg vs 50 in the Prius. Embarrassing. Let's call it 'eco' to fix it." Certainly, the aero is much worse than the Prius's, because unless they are breaking new ground in just how inefficient engines can be, the expectation would be that the Malibu engine is about 10% less efficient than the Prius (so with comparable aero, you'd expect the highway ratios to be 10% apart: 50 for the Prius, 45 for the Malibu).
I've already said this, but your contention that it all counts is correct. The Mooney 201 and 231 aircraft were famously clean, because of attention to a great many details.