Drag penalty for having a shallow rear window angle, and using spoiler to re-attach ?

[Cd]

I was wondering if you guys could post a chart showing the aero penalty for having to use a spoiler / trunk ( boot ) to re-attach the airflow when the rear window angle is not steep enough.

I have seen a couple of cars that had exceptional Cd figures despite having too shallow a curve to the rear window. The cars usually re-attach the air flow at the trunk/boot, or by using a spoiler.

Thanks.

[Cd]

Something like this truck here that had a .315 Cd.


I was surprised to see some - open bed - GM truck studies even dipping into the .20s ( .28 - .29 )

That's a lot of turbulent air flowing back there.

[aerohead]

Originally Posted by Cd

I was wondering if you guys could post a chart showing the aero penalty for having to use a spoiler / trunk ( boot ) to re-attach the airflow when the rear window angle is not steep enough.

I have seen a couple of cars that had exceptional Cd figures despite having too shallow a curve to the rear window. The cars usually re-attach the air flow at the trunk/boot, or by using a spoiler.

Thanks.

I think we'd have to keep this on a case-specific basis.There are just too many variables which affect the flow back there:
*Length of aft-body (upper) as a function of total body length
*Length of the aft-body (sides) as a function of total body length
*Roof contour/type (flat or curved),notchback/fastback/hatchback
*Presence of upper gaps or not
*Trip strips or not
*Greenhouse tumblehome or lack thereof
*Height of boot with respect to body height
*Length of boot with respect to body height and backlight base
*C-pillar plan-view taper or lack of it
*Submerged backlight
*Flush backlight
*Crowned backlight
*Compound backlight
*Presence of C-pillar buttresses and length
*Body side camber/boat-tailing
*Body edge radii
*Diffuser area architecture
*Spoiler type
*Spoiler position
*Spoiler dimensions
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Hucho insists that the 'fastback' car produces the lowest drag,which infers that any notchback will pay some drag penalty.Depending on the car,this may not be the case for all 'mutilated' bodies.
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I gave up on attempting to cover all aft-body contingencies.Here are a few I looked at



Here are some spoiler examples

[aerohead]

I had to remove 15-images from the above post.I'm adding more here

[Cd]

Wow. That was an awesome post.
Thanks Aerohead !

[aerohead]

Originally Posted by Cd

Something like this truck here that had a .315 Cd.


I was surprised to see some - open bed - GM truck studies even dipping into the .20s ( .28 - .29 )

That's a lot of turbulent air flowing back there.

I can think of at least six aero mods that were done to the S-15.
The original half-tonneau was only good for an 8% drag reduction on the C-10 pickup.We don't know for sure what its effect to the GMC was.
We do know that the rear has reattached flow and that it captures a vortex in there.Here's the GMC (bottom)

Here's a link to an article about the 1986 Pontiac Grand Prix 2+2 with its aeroback rear treatment for NASCAR slaying which illustrates one car's different 'attitude' to the air
Homologation Hotness - 1986 Pontiac 2+2 | Hemmings Motor News

[Cd]

So apparently, the drag was actually better on a rear window angle of 18* versus 12* ? ( .270 versus . 275 )

( From the Charger Daytona on the second image section )

Is that due to being just outside the template ?

[aerohead]

Originally Posted by Cd

So apparently, the drag was actually better on a rear window angle of 18* versus 12* ? ( .270 versus . 275 )

( From the Charger Daytona on the second image section )

Is that due to being just outside the template ?

Yes,for only the Daytona Charger we know that the drag minimum was achieved with a backlight angle of 18-degrees.
Chrysler referred to a 12-degree slope as a 'fastback,' but it includes zero curvature,is not incorporated into the cars body,and would cause an 'overshoot' jet,with the downwash actually interfering with the pressure recovery of the ideal contour,degrading the drag.
These notchbacks have the most complex flow of all body types.They do provide nice rear vision,they're lighter,and the trunks are easy to use,hence their popularity in everyday driving.
In Kamm's last official assignment for the Battelle Institute,he discarded the Audi's notchback,and went directly for his Fachsenfeld/Kamm signature K-form roof.
For lower drag,you'd just provide the elongation which Hucho has been pushing since the early 80s.

[gone-ot]

Originally Posted by Cd

So apparently, the drag was actually better on a rear window angle of 18* versus 12* ? ( .270 versus . 275 )

( From the Charger Daytona on the second image section )

Is that due to being just outside the template ?

Look closely at the two pictures and you'll notice there's also a difference in the rooflines. In one the roofline stops before the rear spoiler (18º) while in the other the roofline contines all the way back to the rear spoiler (12º).

[Cd]

Originally Posted by Old Tele man

Look closely at the two pictures and you'll notice there's also a difference in the rooflines. In one the roofline stops before the rear spoiler (18º) while in the other the roofline contines all the way back to the rear spoiler (12º).

I noticed this too. Something else I was wondering about is whether or not the chart is shown with the buttresses drawn as hidden. ( I would have thought they would have the usual dashed lines in that case )

As we all know, the stock '68 - '70 Charger had buttresses on the sides of the rear glass.