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Old 06-10-2009, 09:46 PM   #1 (permalink)
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Spoiler based on kamm-back dimensions?

Hello -

Back in this thread I was given feedback on what a proper Kamm-Back might look like for my car :

http://ecomodder.com/forum/showthrea...html#post41152 - Post #16


This led me to this design, which was deliberately limited in length by the distance to the back bumper :




I haven't done any serious work on this because I want to maintain rear-view visibilty, third brakelight visibility, the complexity of the gently curved roof shape, and the risk of a "parachute" effect if the top got loose.

Then I noticed this from the 1970's Pinto mods :

$11 worth of mods plus new tires - Car and Driver improves MPG by 25%

Quote:
Mod #3: Smoother tail (rear spoiler).
The aerodynamically ideal shape at the rear of a vehicle is a gradual taper that helps minimize the size of the turbulent wake left behind. But the slope of the Pinto’s rear window was too steep. By adding a six inch spoiler, they effectively changed the angle of air flow between the end of the roof and the back of the car. The result: a 7% MPG improvement.
This made me wonder if I could compromise and create a simple flat spoiler that would emulate the endpoint of the kammback :




From an aero POV, do you think this would help? I am thinking that this is similar to Ernie Roger's new-Beetle Wing Spoiler.

IMO the positives are easier construction, good rear visibility because only a "single flat line" of coroplast is in the way, third brakelight visibility, and (hopefully) less risk of a parachute effect. The only bad thing is that I would need to disable the rear-window wiper (one fuse), unless I made a gap between spoiler and the rear-window (Ernie did this for other reasons).

CarloSW2

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Old 06-10-2009, 10:00 PM   #2 (permalink)
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FWIW, I don't think that strategy will do as much good on a wagonback as it does on the Pinto hatchback (Edit: and Beetle). My reasoning is, if the Pinto spoiler really does work and promote attached airflow all down the hatch, it would successfully have reduced the wake area at the rear of the car by a large amount... half? But on your wagon you have proposed extending the spoiler up to such an extent that even if you do achieve attached airflow on the top of the back window (or the mimicry thereof) it looks like it wouldn't reduce the area that forms a turbulent wake very much. Now if that spoiler went horizontally out the back for, say, 3', you might have something.

Last edited by Frank Lee; 06-11-2009 at 03:26 AM..
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Old 06-10-2009, 10:14 PM   #3 (permalink)
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Why do people use those aerofoil overlays? The shape of anything you add to your car should be based on the local airflow conditions.
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Old 06-10-2009, 10:15 PM   #4 (permalink)
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I was wondering about spoilers like that one on the Pinto... It seems like they'd trap a puddle of air, and force the air to flow above that, at a shallower angle. I guess that really does happen.

Your angles are much steeper, but since having the liftgate closed on a pickup is more aerodynamic than leaving it open, I'd think your design would help too.
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Old 06-11-2009, 12:12 AM   #5 (permalink)
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Frank & Co -

Thanks for the feedback. Three feet would be a good experiment, but I think I'd have trouble when it's parked (someone would thunk into it, probably a pedestrian on their cellphone). Hrmmm. Maybe a three-footer that was retractable?

I was hoping that I was "seeing" a good aero shape, but maybe it just isn't there.

Because it's a lot easier to do, I may just try it for grins. I could reuse some of the "infrastructure" for future-someday-c-through kamm-back.

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Old 06-11-2009, 12:41 AM   #6 (permalink)
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I think you'd see some gain- say, 10% vs the- say, 50% on the Pinto (turbulent wake reduction, not necessarily Cd reduction or fe increase). My understanding of it is, you can boattail quite nicely, and do things with spoilers re: that airfoil template, but wherever that bodywork ends, you still end up with a turbulent wake. Aerohead has a description of that phenomenon somewhere here.

Yours is kinda the difference between a regular minivan and an Aztek.
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Old 06-11-2009, 06:01 AM   #7 (permalink)
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i think the spoiler on the pinto mailny works because it reduces the vortices that come off the C pillars... back in those days a lot of hatchbacks had C pillar angles similar to the pino.
this angle is to steep to maintain attached flow as is pretty obvious, but the airflow comming off the sides will form a large vortex that starts at the top and curves inwards. the spoiler intercepts these vortices and breaks them up.

I've seen this sort of bodie style refered to as a "morel bodie"

it's for this reason a lot of hatchbacks and fastbacks today still maintain a semi "trunk lid"
but todays hatchbacks have mostly a more steep angle; and more boattailing at the rear wich reduces these vortices
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Old 06-11-2009, 10:33 AM   #8 (permalink)
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Hi,


I think something close to the first example would be the best -- it doesn't have to be as extended as you show it to be a benefit.
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Old 06-13-2009, 04:33 PM   #9 (permalink)
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wonder

Quote:
Originally Posted by cfg83 View Post
Hello -

Back in this thread I was given feedback on what a proper Kamm-Back might look like for my car :

http://ecomodder.com/forum/showthrea...html#post41152 - Post #16


This led me to this design, which was deliberately limited in length by the distance to the back bumper :




I haven't done any serious work on this because I want to maintain rear-view visibilty, third brakelight visibility, the complexity of the gently curved roof shape, and the risk of a "parachute" effect if the top got loose.

Then I noticed this from the 1970's Pinto mods :

$11 worth of mods plus new tires - Car and Driver improves MPG by 25%



This made me wonder if I could compromise and create a simple flat spoiler that would emulate the endpoint of the kammback :




From an aero POV, do you think this would help? I am thinking that this is similar to Ernie Roger's new-Beetle Wing Spoiler.

IMO the positives are easier construction, good rear visibility because only a "single flat line" of coroplast is in the way, third brakelight visibility, and (hopefully) less risk of a parachute effect. The only bad thing is that I would need to disable the rear-window wiper (one fuse), unless I made a gap between spoiler and the rear-window (Ernie did this for other reasons).

CarloSW2
I think that as long as the rear edge falls along the template line,that the air will re-attach,forming a locked-vortex above,and the outer flow will skip over it.It won't be as effective as the ideal Kamm roofline but you'll have good rear vision and it will be relatively easy to fab.All the air below it will be in the turbulent wake.A second wing below,and also extending to the template line would create a second locked vortex and even lower drag.Ford proved this with the 1980s Merkur XR4T1 which sports their patented bi-wing spoiler.
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Old 06-13-2009, 04:51 PM   #10 (permalink)
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overlay

Quote:
Originally Posted by winkosmosis View Post
Why do people use those aerofoil overlays? The shape of anything you add to your car should be based on the local airflow conditions.
The overlay is based on ideal aero flow.The ( aprox 2.5:1 ) teardrop form depicted has the lowest drag recorded for a three-dimensional body of revolution in ground-effect.Mair was able to cheat it a bit,but nowhere else has empirical research demonstrated a form with lower profile drag.We choose this for the template because it's a no-brainer.If your vehicle has attached flow up to its location of frontal area,if scaled properly scaled and placed under the template,any modification based on this architecture,both over and around the vehicle,will be guaranteed to provide separation-free flow for any length,up to its full length.------------ Anything above or below the line will have higher drag.In free air this form has Cd0.04.On the ground this form has Cd0.08.When you add really skinny tires to this form you get Cd0.10,the theoretical minimum for a vehicle in ground-effect.Some wing-sections have lower profile drag but they suffer from higher skin friction at thickness ratios above 3.84:1 and wing section drag is considered 2-dimensional,not 3-dimensional.

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