Hey Morgan, I have given up on trying to "correct" anyone. If I wonder WTF with someone, I tend to do the math on the "Thanked vs Posts" part of our profile. If someone has posted 3,500 times and been thanked 200, we're looking at a 6% average for helpfulness. I figure my mission in this forum is to get thanked as many times as possible, otherwise, what are we doing?
I was looking for more information on cutting off the back end of a stream line shape. From my understanding, since a full boat tail is pretty impractical, I have seen where if you chop off the back end of it, you're not compromising the aerodynamics that much.
In other words:
All three of these shapes should have about the same CD.
I just can't seem to find where I've read this. If I'm grossly mistaken on this point, I'd like for someone to say why....with a reasoned explanation FFS.
So in my logic, you want to make the shape of your car come as close as possible to these shapes. The front is not as important. It is all in how the air comes back together after the widest point. If you keep this shape, it will create the least amount of lift and turbulence. If you flatten the top of the shape, the air will tend to leave the back with a lot of turbulence. If you try to steepen the rear angle, it will start to create a lot of "Lift Drag" as I pointed out in my Beetle pic earlier.
This is why I recommend the roof spoiler, you are chopping off the shape of your car aerodynamically while the shape is still desirable.
I think people get hung up way too much on maintaining attached flow down the back of the car to the point where it creates the lift drag. The point here is subtle. And I believe attached flow is easier to maintain than people think. It is counter intuitive that a big ol flat rear end on a vehicle can be OK just so long as the shape in front of the rear is ideally shaped, but this is what I've come to after a ton of research. And it is the only possible explanation for this:
