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Old 10-11-2020, 03:49 PM   #71 (permalink)
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Quote:
Originally Posted by jakobnev View Post
0.45 and 0.25 are pretty much the extremes.
Not when you're using a rule of thumb developed in the 1960s.

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Old 10-12-2020, 03:38 PM   #72 (permalink)
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a way

Quote:
Originally Posted by CigaR007 View Post
Is there a way to roughly calculate or derive the reduction of Cd based on preliminary increase (percentage) in fuel economy, especially in the context of highway cruising at a specific speed ?
Yes there is.
1) under observed conditions a coast-down test would be conducted, at SAE test weight ( ballasted by 300-pounds )
2) from the coast-down, the road load would be broken down into its constituent rolling-resistance and aerodynamic components.
3) from this, the R-R rolling force coefficient would be derived based upon mass and polar moments of inertia of all rotating components.
4) As well as the coefficient of aerodynamic drag based upon the known frontal projected area.
5) With the mechanical efficiency of the entire powertrain known, the brake horsepower requirement would fall out.
6) and with a known baseline for that speed, normalized to standard SAE conditions, the brake specific fuel consumption ( BSFC ) can be ascertained.
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7) If, only 'shape' is altered, without introducing additional mass ( the 300-pound ballast can be altered to maintain 300-pounds, then when re-tested for fuel economy under normalized conditions, any change observed must be attributable to a delta- Cd.
NOTE: a constant BSFC is assumed!
CAUTION! streamlining can move the engine load lower, and into a less efficient regime on the BSFC map of the engine, losing potentially up to 30% of a streamlining benefit unless the gearing is altered to restore the original load.
In the past, and due to the complications of road testing, only loose rules of thumbs were contrived to allow the observance of trends.
If you saw a 5% improvement in mpg at a constant 55-mph, there was a statistical probability that you'd just experienced a 10% drag reduction.
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Another coast-down test would provide additional information.
Wind tunnel testing would provide the information directly.
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With today's electronic fuel injection, electronic data is provided in real time to the CPU, providing the pulse-width modulation of the injectors, running at a constant fuel rail pressure, and highly accurate flow rate values, against a backdrop of other sensor data. All meteorological excursions are accounted for. Your on-board display or Scan-Gauge can tell you a lot.
You just need to know your car intimately before proceeding.
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Old 10-14-2020, 10:52 PM   #73 (permalink)
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Lower fascia air curtain

It has been raining for the past few days so I decided to start building a tentative wheel air curtain.



The lower fascia foam plugs were already made. The plan is allow the incoming high pressure air to enter the naca duct and redirect it to the wheel well.


Here is the progress thus far :








The "curtain" will be created by a narrow 5" long vertical duct with 3/4" openings, as shown below.


Last edited by CigaR007; 10-14-2020 at 11:07 PM..
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Old 10-15-2020, 12:23 PM   #74 (permalink)
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What is that air curtain piece from? Is the front opening a naca duct?
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Old 10-15-2020, 01:32 PM   #75 (permalink)
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Quote:
Originally Posted by M_a_t_t View Post
What is that air curtain piece from? Is the front opening a naca duct?
The air curtain piece is a defrost duct. The air flow is divergent, which is something I might have to tweak eventually. We will see from the results of the wool tuft testing.

The front opening is indeed a NACA duct with a 2-1/2 inch fitting.

Last edited by CigaR007; 10-15-2020 at 01:34 PM.. Reason: Typo
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Old 10-21-2020, 02:14 PM   #76 (permalink)
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Fount of rule of thumb

I believe that the origin of the 10-to-5 rule can be attributed to Kent B. Kelly, and H. Holcombe, General Motors Corporation, SAE Paper No. 649A, June, 1963.
' a 10% reduction in aerodynamic drag makes a 5% reduction in fuel consumption possible at 55 mph, and a 6% reduction possible at 70 mph.'
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Curiously enough, forty-six years later, in his Fall, 2009, Masters Thesis, Feysal Ahmed Adem, California State University, recorded a bona fide 4.997% fuel economy improvement, for a 10% drag reduction, in his Dodge Ram 1500 pickup truck with 3-D, 12-degree aerocap.
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Even more weird, was the 40.7% rear lift reduction from this 'fastback', modification, compared to the 17.5% rear lift reduction Texas Tech indicated for a 'notchback', full-tonneau cover.
Isn't it interesting?

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