Decimal places

[aerohead]

I peeked over at Cal Tech.
For pi, they're using 15-decimal place accuracy in celestial mechanics.
I won't freak over using 0.00238 for (rho)

[JulianEdgar]

Originally Posted by aerohead

Speculation?
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Here's an example of a reverse-engineering challenge involving Tesla's Model S
* Julian Edgar reports that the Model S has a 8% cooling-drag system.
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The Tesla Model S has been reported with:
Cd 0.26
Cd 0.247
Cd 0.24
Cd 0.225
Cd 0.208
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SAE 2012-01-0178 provides us with a frontal area of 2.4-meters-squared ( 25.8333 sq-ft )
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This gives us a range for the Model S drag factor, from CdA 6.7166 sq-ft ( 0.6239 meters-squared ), to, 5.3733 sq-ft ( 0.49919 meters-square )
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This range offers a spectrum of at least five-different cooling-system drags.
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Since Julian Edgar has not provided any specificity as to what ' 8% ' actually means, ' 8%' remains an unknown quantity to any of the 136,000 EcoModder.com members who might wish to 'engineer' cooling systems.
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I'm often uncertain what underlies the motivation for some of Julian Edgar's posts. Certainly he must understand that members actually have a desire to 'engineer.'
And since the 'signal-to-noise ratio' of some mods complicates the ability to even discern an actual 'change' in drag, the need of highest specificity cannot be overemphasized.

Good example of Aerohead deliberately trying to create confusion.

The reference I used was the third post in my thread that introduced the topic:

Tesla Model S -

Palin, R., Johnston, V., Johnson, S., D'Hooge, A. et al., The Aerodynamic Development of the Tesla Model S - Part 1: Overview, SAE Technical Paper 2012-01-0177, 2012

I even asked members to read the paper and check my calculation for themselves!

[AeroMcAeroFace]

I can't find your third thread, but for everyone else, selected numbers are.

Total intake block cD change = -0.020

Baseline with 19" wheels = 0.249
19" aero wheels = 0.223
21" style wheels = 0.257

Depending on options specified, such as wheels, it is impossible to say 8% is the same for every option. But this test was done on the baseline model with 19" wheels and that, with all intakes blocked, reduced drag by 8%.

Would the same 0.02 change be found with the other wheels? Who knows? would the same 0.02 change be found on the myriad of numbers listed by aerohead, again we don't know.

Good example of Aerohead deliberately trying to create confusion.

Unfortunately true, I think it is a superiority issue, and good points/arguments get drowned out by the never ending gish gallop of aerohead.

Since Julian Edgar has not provided any specificity as to what ' 8% ' actually means, ' 8%' remains an unknown quantity to any of the 136,000 EcoModder.com members who might wish to 'engineer' cooling systems.

Reading the referenced paper generally helps here.

[JulianEdgar]

Originally Posted by AeroMcAeroFace

Unfortunately true, I think it is a superiority issue, and good points/arguments get drowned out by the never ending gish gallop of aerohead.

Gish gallop - "technique in which a debater attempts to overwhelm an opponent by excessive number of arguments, without regard for the accuracy or strength of those arguments."

I'd never heard that term before - it's certainly apt for Aerohead.

But around here, we'd simply say: "Trying to blind with BS".

[Taylor95]

Unfortunately I see problems with significant figures all the time. For example, nutrition students I evaluate sometimes like to estimate calorie requirements to the tenth or even one hundredth. It is pointless to do that because I would say most people would have trouble reporting within 100 calories of their actual intake.

The number of decimal places used should always correspond to practicality of being so specific as well as the sensitivity of the data collection tools. I would argue that it is pointless reporting FE averages to the tenth. There is too much variation in fuel pump cutoffs and odometer accuracy for the tenth decimal place to matter in FE averages. Also the more data points you have the less individial variation matters.

However, if you can measure FE in a somewhat controlled environment, then I can see justifying going to the tenth decimal place.

[JulianEdgar]

Originally Posted by Taylor95

Unfortunately I see problems with significant figures all the time. For example, nutrition students I evaluate sometimes like to estimate calorie requirements to the tenth or even one hundredth. It is pointless to do that because I would say most people would have trouble reporting within 100 calories of their actual intake.

The number of decimal places used should always correspond to practicality of being so specific as well as the sensitivity of the data collection tools. I would argue that it is pointless reporting FE averages to the tenth. There is too much variation in fuel pump cutoffs and odometer accuracy for the tenth decimal place to matter in FE averages. Also the more data points you have the less individial variation matters.

However, if you can measure FE in a somewhat controlled environment, then I can see justifying going to the tenth decimal place.

Yeah, I am amazed that's its even a topic for debate - shows how much some people here lose the wood for the trees.

At minimum, it certainly shows how little contact some people here have with any science, physics, etc. Or even, just car modification generally?

[freebeard]

Yeah, I am amazed that's its even a topic for debate

It wasn't until the Original Poster started.

[JulianEdgar]

Originally Posted by freebeard

It wasn't until the Original Poster started.

Debate, not discussion.

[aerohead]

Originally Posted by JulianEdgar

Good example of Aerohead deliberately trying to create confusion.

The reference I used was the third post in my thread that introduced the topic:

Tesla Model S -

Palin, R., Johnston, V., Johnson, S., D'Hooge, A. et al., The Aerodynamic Development of the Tesla Model S - Part 1: Overview, SAE Technical Paper 2012-01-0177, 2012

I even asked members to read the paper and check my calculation for themselves!

1) you make a comment without caveats ( those can take up to 28-days later )
2) it can be days before I return to the computer.
3) what I post is predicated upon your original post before leaving for the interim period.
4) in the meantime, you may finally make the qualifying remarks on a later page, which were originally absent.
5) as long as you 'post first' and 'think later' this sort of thing is going to continue.
6) slowing way down, and providing all conditions and caveats from the get-go, would be a real improvement in communicating your themes.

[aerohead]

* Just for giggles, I ran a set of aerodynamic road load calculations for a Cd 0.247 Tesla Model S at top speed ( 155-mph ). In U.S. units.
* In one, I used whatever was in the computer to complete the string.
* In the other, I truncated the values throughout, to two significant figures, regardless of what the computer 'said.'
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* Between the two, the truncated figures resulted in a 6.5% deviation in power, 152.27- hp vs 162.2-hp actual.
* 'Truncated' drag coefficient = Cd 0.2309 vs Cd 0.247 actual ( delta- 16 counts)
* 'Truncated' per-mile energy: 732.9 Wh /mi vs 780.6 Wh / mi actual.
* With zero changes to the car, numerical truncation leads to an implied 16-count drag deficit.
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It appears that truncation leads to an uncertainty which exceeds the spectra of some aero. modifications we might choose to investigate.