Rule of Thumb

[aerohead]

Considering the viability of a specific delta-Cd vs delta-mpg rule-of-thumb, post 1998, I spent some time in my archive.
For those who do not live in the United States, or are too young to remember, in the post-2nd-Edition-Hucho world, the proliferation of SUVs and Full-Size pickup trucks introduced 'passenger' vehicles, in which a single vehicle could constituted the aerodynamic equivalence of six of my modified Honda CRX HF.
Occasionally, I'll see some of the famous examples, Cd 0.70, and Af 38-sq-ft, CdA 26.6 sq-ft, compared to CdA 4.347-sq-ft for the CRX.
This would constitute a market segment in the USA which might fall under the umbrella of these-shoot-from-the-hip, easy, back-of-the-envelope metrics for contemplation.
For me, information is just information. I don't understand the uproar when taken in the context of 'guestimating.'
They come from the people who design the vehicles we own and drive. And the EPA which certifies the vehicles for regulatory compliance. Career scientists and engineers. Hucho used them. Sovran invented some. They're close friends according to Hucho. Where's the foul ?

[freebeard]

Where's the foul ?

Ban-hammered

[aerohead]

I noticed that the thread was closed.
Perhaps we're no longer interested in where the US Constitution came from either.
Maybe we'd be better off without reading and writing altogether. It only seems to lead to trouble.
And think of all those silly meteorologists! What a bunch of losers they must be. Thinking they could actually figure anything out by looking at all those isobars, temperatures, winds, and other worthless stuff. Have you ever gone up in space to see if those weather satellites are actually up there?
Ratios! Who the ---- thought up that ----? The work of the devil I tell you!
When my fund manager invests on Wall Street, they never look at any of those useless statistical trends. Livery stables and kerosene lanterns! That's the future my good lads.
I'd carry on, but I have a speed-swimming lesson coming up. The Little Mermaid had made me a pretty good deal, but SpongeBob SquarePants undercut her price. I figured, he's just as fast. All you need is to look at him!

[redneck]

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free upload pic


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[Vman455]

Originally Posted by aerohead

They come from the people who design the vehicles we own and drive. And the EPA which certifies the vehicles for regulatory compliance. Career scientists and engineers. Hucho used them. Sovran invented some. They're close friends according to Hucho. Where's the foul ?

You've been quite happy, on this forum, to disparage other career scientists and engineers, who also design the vehicles we own and drive, including no less than the (now retired) head of aerodynamics at Porsche. (And if you recall, you were temporarily banned largely as a result of your comments about him). Why do you trust Hucho and Sovran, and not other aerodynamicists? Why do you trust old information, and not modern sources? You'll push back against something like throttle-stop testing, quite vehemently, yet you'll gladly accept a 20-year-old+ method of guessing at drag reductions, when that guessing is worth far less, IMO, and subject to much greater uncertainty than throttle-stop testing, high-speed coast down testing, or direct pressure measurement. Someone comes on this forum and gives away information, for free, on how to test in the real world, and you fight him tooth and nail, for months, when you could be experimenting yourself, trying out his methods, and then having a reasonable and reasoned conversation on their merits and drawbacks. Yet you dismiss them out of hand. You've accused me of not understanding you; explain this to me, because I do not understand why.

I think it's beyond time for us to get off the internet and get out in the real world. I spent an hour after work today testing something on my car; you (general "you"--anyone reading this post) can too.

[aerohead]

Originally Posted by Vman455

You've been quite happy, on this forum, to disparage other career scientists and engineers, who also design the vehicles we own and drive, including no less than the (now retired) head of aerodynamics at Porsche. (And if you recall, you were temporarily banned largely as a result of your comments about him). Why do you trust Hucho and Sovran, and not other aerodynamicists? Why do you trust old information, and not modern sources? You'll push back against something like throttle-stop testing, quite vehemently, yet you'll gladly accept a 20-year-old+ method of guessing at drag reductions, when that guessing is worth far less, IMO, and subject to much greater uncertainty than throttle-stop testing, high-speed coast down testing, or direct pressure measurement. Someone comes on this forum and gives away information, for free, on how to test in the real world, and you fight him tooth and nail, for months, when you could be experimenting yourself, trying out his methods, and then having a reasonable and reasoned conversation on their merits and drawbacks. Yet you dismiss them out of hand. You've accused me of not understanding you; explain this to me, because I do not understand why.

I think it's beyond time for us to get off the internet and get out in the real world. I spent an hour after work today testing something on my car; you (general "you"--anyone reading this post) can too.

That's a very good question, and I'll be happy to take it up Friday when I return. The store will be closing soon and I'd like to devote as much time as necessary to adequately frame a response.
And I'll again, request that we stick to data. Implications of internal experiences aren't proper material for the forum, and will only lead to closure.
Thanks again, and have a great next couple of days.

[AeroMcAeroFace]

Originally Posted by aerohead

Maybe we'd be better off without reading and writing altogether. It only seems to lead to trouble.
And think of all those silly meteorologists! What a bunch of losers they must be. Thinking they could actually figure anything out by looking at all those isobars, temperatures, winds, and other worthless stuff. Have you ever gone up in space to see if those weather satellites are actually up there?
Ratios! Who the ---- thought up that ----? The work of the devil I tell you!
When my fund manager invests on Wall Street, they never look at any of those useless statistical trends.

The point is though, an overpaid fund manager can't predict things even given all the data, only a few percent outperform the market over 20 years. Data is useful, but sometimes you can't draw accurate conclusions from it.

only a few weeks ago you were saying that you might not notice a 30% drop in aero drag due to gearing, yet now you are saying 10% is easily noticeable. I trust modern science that supersedes and is more accurate than something that may have worked 20 years ago with tyres of that era and aerodynamics of that era on the specific and limited set of cars that they tested in that era.

To test your theory I put the numbers in for a 1970 lotus europa, 700kg 0.008 Crr, 1.51m^2 frontal area, 0.29 Cd. At 70mph it gets 49.4USMPG according to the calculator. With a 10% Cd reduction it gets 54.0USMPG. That suggests 10%=10%, maybe rules of thumb don't work?

[Vman455]

Originally Posted by aerohead

That's a very good question, and I'll be happy to take it up Friday when I return. The store will be closing soon and I'd like to devote as much time as necessary to adequately frame a response.
And I'll again, request that we stick to data. Implications of internal experiences aren't proper material for the forum, and will only lead to closure.
Thanks again, and have a great next couple of days.

I'll look forward to your reply, and I appreciate it. But this isn't a question of data; it's one of logic. An analogy:

I play the pedal solo at the beginning of Bach's Toccata, Adagio and Fugue in C (BWV 564) a certain way, using mostly toes and almost no heels. If someone comes along and says, "Hey, you can play that much more easily if you use heels in these several places!" I have a choice in how I respond: I can dismiss that person's ideas out of hand, or I can try them out. If I dismiss them, I will never know if they are indeed better than what I've been doing; but if I try them, I might discover they are better or I might discover they are not and I'm satisfied with the way I was doing things previously. Either way, I'll have learned something.

The question of logic is this: Is it better to engage in the first type of response, or the second? Why?

[RustyLugNut]

From BOTH Aerohead and Julian.

Classic shapes and theories is where we look when we start.

Testing and modifications is where we continue.

What I end up with will be a compromise.

Both approaches have their value.

[aerohead]

Originally Posted by Vman455

You've been quite happy, on this forum, to disparage other career scientists and engineers, who also design the vehicles we own and drive, including no less than the (now retired) head of aerodynamics at Porsche. (And if you recall, you were temporarily banned largely as a result of your comments about him). Why do you trust Hucho and Sovran, and not other aerodynamicists? Why do you trust old information, and not modern sources? You'll push back against something like throttle-stop testing, quite vehemently, yet you'll gladly accept a 20-year-old+ method of guessing at drag reductions, when that guessing is worth far less, IMO, and subject to much greater uncertainty than throttle-stop testing, high-speed coast down testing, or direct pressure measurement. Someone comes on this forum and gives away information, for free, on how to test in the real world, and you fight him tooth and nail, for months, when you could be experimenting yourself, trying out his methods, and then having a reasonable and reasoned conversation on their merits and drawbacks. Yet you dismiss them out of hand. You've accused me of not understanding you; explain this to me, because I do not understand why.

I think it's beyond time for us to get off the internet and get out in the real world. I spent an hour after work today testing something on my car; you (general "you"--anyone reading this post) can too.

1) 'happy' would be a mischaracterization of what I experience when making observations about paid employees of auto manufacturers, of which I have no background as to their education, drag reduction contributions, priorities, etc.. They are 'experts' only upon the word of individuals, of which the ability to discern this qualification has yet to be demonstrated.
2) Dr. Thomas Wolf, as a ,low-paid functionary, subservient to the stylist above him on the corporate food chain, will have to answer for his behavior one day.
3) The 'golden years' of aerodynamic reporting seems to have existed before your modern sources. Perusing the modern literature reveals nothing more than a nibbling around the edges of drag reduction, and for needlessly 'complex' shapes.
Nothing offered by your 'three-amigos' consortium has revealed any new knowledge worthy of excitement. I'm to be wowed by a 2-count drag reduction, leaving 150-counts on the table, which go unmentioned. Really !
4) The moment I looked at 'Throttle-stop testing, I 'smelled' something. And as I re-discovered materials about road testing, reaching back into antiquity, the odor only increased in intensity. I've counted eleven ( 11) unknowns attributed the the T-S methodology. Any claim, as to a 2% resolution, on the face of it, seems quite laughable.
And why is it, that it comes down to me to point it out. Why is this information missing from the preamble. Where were the 'expert panel of experts' ?
5) Have ANY of these 'experts' actually conducted aerodynamic drag reduction / tractive-energy, on-road tests? Where are their bona-fides ?
6) MIRA claimed that, for 1974, there were only eleven ( 11 ) days in England/Wales, when the weather was even good enough to conduct road testing.
7) I published a dedicated thread on 'rule-of-thumb' and their origin. I neither endorse, nor condemn their use. I simply offer them as 'information'.
Caveat emptor.
8) As to the pressure testing, I've yet to understand how that is 'actionable' information. I attempted an analogue, with the house insulation thought experiment. It's challenging to come up with comparisons that might resonate with viewers. I have no formal training as an educator. I'm just basically a technologist. ( Independence Technological Works [ ITworks ]).
9) As to 'not understanding' I'll need something to tickle my memory. Too much water under the bridge.
10) Personally, I've already done enough testing to know where to go with mods. Julian Edgar is completely wrong, scientifically, about 'templates.'
It's my very educated opinion that, to proceed to modification without using them as a 'first approximation' is utter folly. There's too much accumulated evidence, from a broad spectrum of the automotive community about their efficacy to be an outlier 'black swan.'
11) So far, I've yet to witness any peer-reviewed evidence which would quantify their lack of efficacy.
12) And in all the published literature which explains WHY 'templates' were not chosen in automotive design, not one had anything to do with aerodynamics.