Rear Spoilers vs 'template' contour

[aerohead]

Originally Posted by JulianEdgar

I don't like it when people spread misinformation. In more than 30 years of writing about car modification, I've seen too many people waste their time and hard-earned money following bad advice.

Following a template when modifying car aero is a classic example - the equivalent of saying that an AFR of 12.5:1 always gives best power.

Why do you presume to have a command of discernment with respect to this topic? Does your book collection serve merely as name-dropping bait, or do you actually intend to read them one day for content?
Perhaps the physician who delivered you dropped you on the tile floor of the delivery room. Shame!

[aerohead]

Originally Posted by Vman455

To be fair, aerohead also repeatedly posts lists of cars like this; this is the second such thread in just a few weeks. If you're going to criticize someone for saying the same thing again and again, give credit where credit is due.

Would you please provide an example(s) from which I may ponder inaccuracies?

[aerohead]

Originally Posted by JulianEdgar

Aerohead continually posts information on car aerodynamics that is incorrect and misleading. While I can be bothered, and to prevent others being mislead, I will continue posting information that shows how his posts are incorrect.

If that concerns you, I suggest you take it up with Aerohead - he is the person continually posting material that is incorrect and misleading. Or are you happy to see falsehoods being disseminated?

I wait with baited breath for you to scientifically argue any of your premises.
I've provided chapter and verse, straight out of the mouths of the experts.
Your folk knowledge and prejudice against actual fluid mechanics appears to have become an impenetrable barrier between your mind and reality.
You come off as a complete huckster, using every opportunity to advance book sales, at any cost to fact.
You're not a peer. I'd like you to stop pretending.

[aerohead]

Originally Posted by JulianEdgar

Again, take that up with Aerohead not me. In fact my post used the premise 'a pic is worth a 1000 words' and had very few words (115 in fact) in it. It's Aerohead who writes reams of stuff - one reason I think he has got away with spreading rubbish here for so long... it just overwhelms people.

And as I said to California98Civic: are you happy to see falsehoods being disseminated?

Complicated truth, versus the simple lie.
PLEASE overwhelm us with a DEEP-DIVE into fluid mechanics. I've waited sooooooooooooooooo long!

[aerohead]

Originally Posted by JulianEdgar

This is one reason that I keep posting. You have been sucked in: you are working on the fallacious notion that the template = the lowest drag shape for a road vehicle.

That is simply not so:



I guess when the accepted - but incorrect - wisdom has been in place for so long, it's hard to see the wood for the trees.

Or do you really think the Honda Insight designers made some sort of concession to styling on the angle of the rear hatch? That really they wanted a larger wake to match the template - but stylists said no?

* All three shown would fail, on skin friction, compared to the template. They'd fail on usable interior volume. Center of gravity. Approach, break-over, and departure angles would have to be considered.
* You'll find it structurally impossible to defend your thesis about the template. The fluid mechanics leading to it'd genesis are watertight. Unimpeachable.
* Decades of dimensional analysis were involved in arriving at the template. If you'd had the courtesy to actually read the threads you would have known that long ago. It wasn't the result of some half-baked brain ejaculation as you're fond of burdening us with.
* Go ahead. Pick it apart. Let's see what you're made of.

[aerohead]

Originally Posted by JulianEdgar

Sorry, you've lost me.

You don't need an expensive wind tunnel to see that the template is baloney - just do some tuft testing. Zero cost, easy. I don't have a monopoly on testing - do some for yourself.

Furthermore, if the real world, low-drag car shapes developed in a wind tunnel differ substantially from the template, isn't that even more evidence that the template is wrong?

No, A, B and C don't represent what we will drive, just as the template doesn't either. (If it were to meet frontal crash requirements and have minimum head room, I'd suggest a car based on the template would be about 7.4m long. That's about 24 feet.) I ran the solar car pics because you appeared to believe that the template was the lowest drag shape for a road vehicle, and it isn't.

My only axe to grind is against incorrect information being uncritically disseminated.

Baloney to the tune of Cd 0.1201.

[aerohead]

Originally Posted by JulianEdgar

Yes I do say 'uncritically'. Ancient, long threads are irrelevant to people coming here and seeing bad advice. Errors need to be called out when they are stated - not 'thanked' as they often are. And the more the errors are repeated, the stronger should be the pushback.

It's just one of those strange beliefs I have - people modifying cars shouldn't be given bad advice. Odd I know...

I mean, you only need to look at the title of this thread - absolute misleading rubbish, that I have seen given here as direct advice to someone wanting guidance on a rear spoiler design.

The template is simply a derivative of everything aerodynamicists implicate for low drag design. It's provided for information. Like a 1.5-radius, street-ell in plumbing or ductwork.
If it's cutting into your credibility as an aerodynamic 'GURU' I make no apologies. It is simply what it is.
There's no way you can logically attack it. Your ad hominem attacks on me won't undermine it's physics. You have no scientific tools at your disposal with which to attack. You're a bad loser. Grow a pair.

[freebeard]

A new record. Seven post in 1/2 hour.

[aerohead]

Originally Posted by JulianEdgar

It's not 'trumpet blowing' to say I have written a book on the topic. It's a statement of reality. If I have already written all that I know about the topic in 100,000+ words and 400+ pics, why would I waste my time doing it all over again?

I do the videos only to promote the books. No more, no less. In fact, at the moment I have stopped doing them because Covid has depressed book sales so much it's not worth my time doing the videos.

I honestly don't understand the philosophy that poor advice is better than none. Why not instead aim for good advice? And why this idea that information that is clearly and demonstrably wrong should not be challenged? I've seen that also in other discussion groups and it strikes me as ludicrous: that because we shouldn't rock the boat, we shouldn't call out stuff that people write which is completely wrong. Not just a bit deceptive, but outright wrong.

I've never said that 'the template' is not a low drag shape: I'd imagine it is. But the way that idea has been extrapolated to purport to give guidance to the height of rear spoilers, to guide the shape of car extensions, to be used as some kind of benchmark when judging the aero of existing cars - all are just rubbish. But it gets worse, because the template has then (apparently) fed into Aerohead's weird theory that flow will not stay attached if the shape curves downwards more quickly than the template - and in turn that has led (apparently) to his completely wrong theory on how lift occurs on modern cars.

It's a skyscraper built on a base of sand, and it has resulted in massive misunderstandings that can be seen across almost all aero topics on this group.

Only to the uneducated would it appear a weird theory. Your extremely qualified as uneducated in the face of data and information,repeatedly thrown at your feet. Graduate and post -Doc level material.
We're casting pearls before swine.

[aerohead]

Originally Posted by hat_man

So what you are saying is that if the advice is deemed as flawed it shouldn't be followed? And if it is followed, then it has no advantage? This is what I mean by something is better than nothing. A "flawed" template is better than no template at all, no?

Should this guy be shot down because his shape didn't meet your idea or Aerohead's idea of the template? I'm sure it could be improved on and needs much refining. People here at EM would share their opinions and knowledge. I have a feeling your only advice would be to buy your book.

I think the "template" we have recognized at EM was designed to be a "smidgeon more conservative" rather than a shape that is "right on the ragged edge of flow seperation."

I wish I could find the drawing of AST-II. It might be more to your liking as I believe it was a bit steeper than AST-I. I also believe someone here stated that "The AST-II is the second-most aggressive profile and fits standard rooflines with rapid descending contours. The AST-I fits more conservative contours."

According to a guy name Hucho, the most aggressive profile was by some other guy named Buchheim. I think that Hucho guy wrote a book also. Too bad he isn't around anymore. I'd bet he'd share his knowledge here in the Wiki section.

I have seven templates that I use for the dimensional analysis. AST-I I'll use on a Mercedes-Benz GLC, due to it's 'slow' contour.
I'll use the AST-II on the Mercedes-Benz GLC 'Coupe', as it's a 'faster' contour.
Between the seven, one can get a sense of whether the carmaker is following any extant profile. It's all informational.
Both are derived from actual mirror-images of streamline bodies of revolution of L/D = 2.5, which produce Cd 0.04 in free air, and no more than Cd 0.09 as a half-body. The original is from NASA, AVA, and DVL, and is presented in Figure 5.13, page 69 of Hoerner's 'AERODYNAMIC DRAG', which also appears in Hucho's Table 2.1, Page 61, 2nd-Edition.
* Adding wheels gives Cd 0.14.
* Adding the 2.8-degree diffuser yields Cd 0.12.
* Adding the Goro Tamai full wheel fairing package nets lower drag.( Hucho says that Cd 0.09 was achievable as of 1986 ).
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The template is a 'known quantity.' Hucho talks about it all throughout his 2nd Edition. According to him, it's the only path to really low drag. He refers to it as the 'optimum', and in the context of a real passenger car. Again, I'm just the messenger.
I'm just the messenger.