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Old 02-04-2010, 12:07 PM   #21 (permalink)
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Ah, that's your Civic being tufted. Thanks for posting those. Do you have anymore pics of that test (or know of any others), particularly further back? I am surprised to see that the folding the mirrors in helped, but I need my mirrors anyway.

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Old 02-04-2010, 01:50 PM   #22 (permalink)
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I'm contemplating what can be done to improve aero at the rear of the car.


The rear undertray extending past the bumper is the best I can come up with for the underside.



Not much can be done with the sides, especially since the rear of the fender curves in so sharply that there is no way to get flow to reattach past the rear wheels (those little vertical strakes are worthless, I realize).

Perhaps I will try wheel covers in the summer, but that will test my fabrication skills quite a bit.


For now I am toying with some sort of coroplast lip at the rear, either from the roofline or trunklid. At the roofline I can keep flow fully attached for another foot or so while reducing the wake profile, but can't extend it further as it would block my sightline from the rearview mirror. Since the flow on the bottom stays attached several inches past the front bumper, the difference between when flow detaches at top and bottom is creating quite a swirl effect, I assume, which is very bad. A lip at the trunklid extending to at least the edge of the undertray can reduce this, but will not prevent flow from separating at the roofline. A full boattail is not an option, as I need my trunk and rear lights, and I'm no basjoos. Here are the two options, with the red dotted line showing the angle from the roofline to the edge of the trunklid lip, which is too steep to keep attached flow between these points, but neither will it break up quite as much as in the wake.



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Old 02-04-2010, 06:50 PM   #23 (permalink)
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Daq,they might be worth a go.Mercedes did this sort of thing before and it made a showing in their wind tunnel.
End-plates would help prevent adulterated flow,in from the sides.They would also give some structural reinforcement.The captured vortex created,would help give you some 'phantom' aft-body.
You WOULD have some 'blind-spot' and you'd have to rely on your mirrors a bit more.
The extra length is always welcome as far as air is concerned.
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Old 02-04-2010, 06:56 PM   #24 (permalink)
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How about a quickie duct-tape and coroplast test of the green line? You could add some vertical supports underneath the outer edges down to the bumper bulge. That and the curvature of the trunk should make it stiff enough.

EDIT: Aerohead beat me to the post but I think we had the same idea about the side pieces. He said it much more eloquently.
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The power needed to push an object through a fluid increases as the cube of the velocity. Mechanical friction increases as the square, so increasing speed requires progressively more power.
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Old 02-05-2010, 12:21 PM   #25 (permalink)
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I agree with (both of) your comments. I am not familiar with the idea of a"phantom" body, but it makes sense if attached flow breaks cleanly. I will perhaps try the trunklid extension first, since it will be much easier to fabricate, and also I can do a before/after tuft test on the rear window to gauge whether its helping. aerohead seemed to be saying that the roof extension is the better option (is that what Mercedes tested?), and I agree if done correctly, but it will be harder to make and I would have to get someone to snap pics from the outside with tuft testing it. Side extensions would definitely add to support and make more effective.
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Old 02-05-2010, 01:58 PM   #26 (permalink)
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Daq, If I understand it correctly, the "phantom body" would be similar to the idea of pick-ups having less drag with the tailgate up. It's not as perfect as an aerocap or full boattail but you still get some of the benefit. With the tailgate up, you have a captured bubble of air that the slipstream slides over. You may not see much difference tuft testing on the rear glass with a trunk extension because of that captured bubble above the trunk but you may have less drag. My suggestion refers to a couple of vertical extensions inside the tail lights at the outside edges of the trunk. The rear end of the extension would match the 7(?) plan taper from the sides of the car and help blend the side airflow in addition to the airflow coming over the roof and the trunk.

You have me excited to try this concept on my hatchback. I have started on a cargo box style form that would attach to my receiver hitch to serve as a "phantom boattail". Hopefully, Aerohead will weigh in if I have this all wrong.
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Old 02-05-2010, 05:16 PM   #27 (permalink)
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Old 02-05-2010, 05:42 PM   #28 (permalink)
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Originally Posted by daqcivic View Post
I agree with (both of) your comments. I am not familiar with the idea of a"phantom" body, but it makes sense if attached flow breaks cleanly. I will perhaps try the trunklid extension first, since it will be much easier to fabricate, and also I can do a before/after tuft test on the rear window to gauge whether its helping. aerohead seemed to be saying that the roof extension is the better option (is that what Mercedes tested?), and I agree if done correctly, but it will be harder to make and I would have to get someone to snap pics from the outside with tuft testing it. Side extensions would definitely add to support and make more effective.
The 'phantom' tail dates to Koenig von Fachsenfeld/Kamm with the 'K-car',or Kamm-back ( but Fachsenfeld deserves equal credit with Kamm).
Kamm considered the 'utility' of a full boat tail car and decided only a 'shortened' version would be practical for parking lots,parallel parking,etc..
So he essentially just started sawing away the tail of his car,as you would with a loaf of home-baked bread.
He discovered that you could remove quite a bit of length without degrading the Cd terrifically,and when examined in the wind tunnel under smoke,the air behind the car skipped over an invisible 'phantom' cone of air which mimiced the original long tail.
Bomber designers noticed the same effect when they chopped a fuselage off to add a gun turret at the rear.
This theme has been re-quantified over the decades with the likes of Walter Korff at Lockheed and Dr. Alberto Morelli at Pininfarina's wind tunnel in Turin,Italy.
The important thing is to fashion everything up to the chop as if you were going to continue to the full tail.
The Aerodynamic Streamlining Template Part-C,2nd image provides a preliminary guide to design from.It's purely amateur,but it's grounded in very good science of road vehicles operating in ground-effect which don't have the benefit of the axisymetrical flow jet pumping action of a free-flying structure.
The template can be used for a roof or side of a car.You'll notice amlost imperceptible curvature at the onset,building into steeper curvature the further back,up to a maximum tangent angle of 22-degrees which can't be obtained in less than one body height length behind the point of max body camber,whether top or sides.
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Old 02-05-2010, 05:43 PM   #29 (permalink)
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Quote:
Originally Posted by COcyclist View Post
Daq, If I understand it correctly, the "phantom body" would be similar to the idea of pick-ups having less drag with the tailgate up. It's not as perfect as an aerocap or full boattail but you still get some of the benefit. With the tailgate up, you have a captured bubble of air that the slipstream slides over.
That tecnique is used on theese trains.

The rubber front capture a big bubble of air, and reduces resistance, and it's actually areodynamicly not that bad, as it actually has an invisible rounded nose made of air.

http://funini.com/train/denmark/imgs/ic3.jpg
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Old 02-06-2010, 05:36 AM   #30 (permalink)
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hows this project goin?

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