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Old 11-10-2022, 05:36 PM   #1 (permalink)
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Ideal car rear roofline angle and shape for low drag?

What is ideal rear roofline(rear window and trunk) angle and shape(straight line, curve ..etc) for lower drag? Keep in mind if you increase angle, you reduce butt area / wake, but induce low pressure at rear window/trunk , if you decrease angle you increase butt/wake but increase pressure at rear window/trunk.

What car aerodynamics theory says about this?

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Old 11-10-2022, 06:02 PM   #2 (permalink)
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'ideal'

1) 'The perfect aerodynamic shape is a teardrop.' Dr. Teddy Woll, Daimler-Benz, 2014.
2) the lowest drag teardrop has a fineness ratio of 2.5:1 and Cd 0.04.
3) the optimum shape for a subsonic, low drag vehicle, is a streamline half-body, based on the 2.5:1 streamline body of revolution, with Cd 0.08.
4) Adding wheels increases drag to Cd 0.13.
5) Wheel fairings can reduce wheel drag by 70%.
6) For an amateur, this would make a natural first approximation.
7) Dr. Wolf Heinrich Hucho recommended that car companies also use this as their beginning point for a Cd 0.09 automobile.
All present low-drag concept cars happen to fit the ASTs. Including Think Flight's boat-tailed 2022 Subaru Impreza Wagon, of est. Cd 0.1764 ( configuration #4).
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The Aerodynamic Streamlining Templates are all based upon the 2.5:1 streamline body of revolution in ground reflection with the road surface.
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I recommend that you study all of them and read every word.
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Old 11-10-2022, 06:20 PM   #3 (permalink)
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Quote:
Originally Posted by aerohead View Post
1) 'The perfect aerodynamic shape is a teardrop.' Dr. Teddy Woll, Daimler-Benz, 2014.
2) the lowest drag teardrop has a fineness ratio of 2.5:1 and Cd 0.04.
3) the optimum shape for a subsonic, low drag vehicle, is a streamline half-body, based on the 2.5:1 streamline body of revolution, with Cd 0.08.
4) Adding wheels increases drag to Cd 0.13.
5) Wheel fairings can reduce wheel drag by 70%.
6) For an amateur, this would make a natural first approximation.
7) Dr. Wolf Heinrich Hucho recommended that car companies also use this as their beginning point for a Cd 0.09 automobile.
All present low-drag concept cars happen to fit the ASTs. Including Think Flight's boat-tailed 2022 Subaru Impreza Wagon, of est. Cd 0.1764 ( configuration #4).
--------------------------------------------------------------------------------------
The Aerodynamic Streamlining Templates are all based upon the 2.5:1 streamline body of revolution in ground reflection with the road surface.
--------------------------------------------------------------------------------------
I recommend that you study all of them and read every word.
What about this, how would you read this 2 graphs?
Why Cd has negativ and positive numbers at first graph?


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Old 11-10-2022, 06:22 PM   #4 (permalink)
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also from Dr. Woll

' A long and tapering rear is still good for the vehicle's aerodynamics. That simply depends on the negative pressure that is generated there.'
Dr. Teddy Woll, Mercedes-Benz Group, 2018 ( M-B's new Cd 0.22 A-Class )
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Old 11-10-2022, 06:35 PM   #5 (permalink)
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graphs

Quote:
Originally Posted by AEOLUS View Post
What about this, how would you read this 2 graphs?
Why Cd has negativ and positive numbers at first graph?


These are for simplistic, primitive, prismatic test models, of which no current productions share any similarity.
On the upper table, for 0.09%, and 0.18% aft-body lengths, you're witnessing an initial drag reduction, then introduction of increasing vortex drag, until vortex-burst occurs, and wake takes on a squareback wake.
The lower tables show all the test permutations of Giugiaro's (sp?) original Golf of 1975. Again, drag drops, then vortex drag builds until 'burst.'
Lesson: a 'large' high-pressure wake is preferred to a 'small' low-pressure wake.
Vortex-drag is BAD!!!!!!!!!!!!!!!!! Really bad!
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Old 11-10-2022, 07:09 PM   #6 (permalink)
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Quote:
Originally Posted by aerohead View Post
These are for simplistic, primitive, prismatic test models, of which no current productions share any similarity.
On the upper table, for 0.09%, and 0.18% aft-body lengths, you're witnessing an initial drag reduction, then introduction of increasing vortex drag, until vortex-burst occurs, and wake takes on a squareback wake.
The lower tables show all the test permutations of Giugiaro's (sp?) original Golf of 1975. Again, drag drops, then vortex drag builds until 'burst.'
Lesson: a 'large' high-pressure wake is preferred to a 'small' low-pressure wake.
Vortex-drag is BAD!!!!!!!!!!!!!!!!! Really bad!
What is delta Cd at first graph,why delta?
And why negative numbers?
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Old 11-10-2022, 07:18 PM   #7 (permalink)
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my understanding is you want a mellow enough tail that gets smaller and smaller while maintaining flow attachment, try to reduce the wake size as much as possible, then detach the flow as fast as possible with hard edges. using small lip spoilers can help recover pressure too when it flips off the trailing edge

look at the mercedez eqxx , the light year zero, and and even a little bit of the vw xl1 car. they all have tear drop rears that then curve up/flatten out for a few inches at the rear for pressure recovery. the original honda insight did not have the mini rear spoiler shapes. it just dropped off. im assuming thats because engineers didnt know as much back at the time of development, and why its drag cd is 0.25 even with a flat bottom and rear fender skirts
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Old 11-10-2022, 07:19 PM   #8 (permalink)
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Quote:
Originally Posted by aerohead View Post
1)
All present low-drag concept cars happen to fit the ASTs. Including Think Flight's boat-tailed 2022 Subaru Impreza Wagon, of est. Cd 0.1764 ( configuration #4).
--------------------------------------------------------------------------------------
The Aerodynamic Streamlining Templates are all based upon the 2.5:1 streamline body of revolution in ground reflection with the road surface.
--------------------------------------------------------------------------------------
I recommend that you study all of them and read every word.
Whoa ! I am just now seeing this. I just posted a similar question Aerohead. Sorry 'bout that

Since you bring up ThinkFlight's design, you might notice that it falls out of the Template curve. ( Have a look at his second video )
This got my attention, because despite having too steep a curve at the rear, the design had great attached flow all the way to the tip of the tail, and even along the centerline.
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Old 11-10-2022, 09:31 PM   #9 (permalink)
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Quote:
What about this, how would you read this 2 graphs?
Why Cd has negativ and positive numbers at first graph?
...
What is delta Cd at first graph,why delta?
And why negative numbers?
In the first graph what is charted is a ratio of tail length over overall length. The numbers go negative because it is charting the delta, or difference for five test cases between included angle and length ratio.

This is all mooted by compound curve surfaces.
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Old 11-11-2022, 11:07 AM   #10 (permalink)
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Convention. Up is positive, below would be negative from a theoretical datum. Bigger negative means more and sharper deviation aka drop.

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