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Old 01-15-2008, 02:41 PM   #1 (permalink)
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Video: tuft testing (rear) 1993 Ford Mustang notchback

This is the style of car:




Note the turbulence over the entire rear window & most of the trunk lid.

The sharp transition & steep angle of the rear glass causes flow to separate at the trailing edge of the roof, with likely little to no reattachment on the trailing end of the trunk lid due to its low height & short length.

This design creates a large turbulent wake.

Foxbody Mustang Aero: Tuft Test @ 70mph (Stock)




Foxbody Mustang Aero: Tuft Test @ 50mph (Stock)



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Old 01-15-2008, 02:47 PM   #2 (permalink)
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It's probably a safe guess that the owner doesn't care a lot about fuel economy.

But the poor rear aero of this design limits the top speed, and hurts acceleration at higher speeds.
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Old 01-15-2008, 02:55 PM   #3 (permalink)
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I would assume then that something similar to what is pictured below would benefit the aerodynamics in this instance? By raising up the trunk (possibly a lot) and reattaching airflow?




Is there a generally agreed upon relief angle (rear window angle off the roof) which should not be passed as far as aero is concerned? I've heard 15-20° is generally good. Obviously emulating the tear drop is optimal.

I should really just go and buy a book on aerodynamics and/or fluid mechanics... Any recommendations there?
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Old 01-15-2008, 03:03 PM   #4 (permalink)
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Quote:
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I would assume then that something similar to what is pictured below would benefit the aerodynamics in this instance? By raising up the trunk (possibly a lot) and reattaching airflow?
Except remember the angle of that particular appendage is designed to generate more downforce than reduce Cd.

If it were level, or even angled slightly down (retaining a sharp cutoff), then there's more of a chance flow would reattach and separate cleanly, leaving a smaller wake.

Ford actually added something similar to the hatchback version of the car:



They could have done the equivalent to the notchback too.
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Old 01-15-2008, 03:23 PM   #5 (permalink)
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Quote:
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Is there a generally agreed upon relief angle (rear window angle off the roof) which should not be passed as far as aero is concerned?
I've read the initial transition shouldn't exceed approx. 10 degrees. You can exceed the angle, after the initial 10 degrees, if it's done gradually.

As for overall rear angles, I can't find the reference at the moment. But there is definite agreement that up to a certain overall (average?) angle is OK, then there's a zone past that where drag increases significantly, and beyond that, it's reduced again. I'll have to dig it up.

Books: you could look for Hucho or R H Barnard.
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Old 01-15-2008, 03:30 PM   #6 (permalink)
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Have you read either of these books?
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Old 01-15-2008, 03:50 PM   #7 (permalink)
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I have Barnard's book. But from what I hear, the Hucho one is better (good luck finding it at less than $140).

Re: rear hatch angle, Barnard says: Drag is reduced with increasing angle up to the 10-15 degrees range (no exact figure is given). As you go beyond that, it begins increasing again and peaks around 30 degrees.

About a 30 degree angle creates more drag than any other angle. It's actually worse to have a 30 degree slope than to simply chop the vehicle off at that point.

Beyond approx. 30 degrees flow separates at the roof, and the drag is like that of a squareback.
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Old 01-15-2008, 03:52 PM   #8 (permalink)
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Anybody want to split on a copy of Hucho's book? We could arrange joint custody. :P
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Old 06-24-2008, 10:09 AM   #9 (permalink)
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Anybody want to split on a copy of Hucho's book? We could arrange joint custody. :P
I'll throw in $20 (US).
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Old 06-24-2008, 10:12 AM   #10 (permalink)
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I already got my copy! Also, it's a fat, heavy book. Shipping a shared copy all around the continent probably wouldn't make sense.

I'd highly recommend getting a copy if you're interested in the subject. You won't regret spending the money. And you'll spend at least as much time in the book as you would in a movie theater for an equivalent amount of money

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