Quote:
Originally Posted by JulianEdgar
I don't want to overly labour the point, but this thread is a good indication of what has gone utterly wrong with this forum (and I am referring only to the aero section; I don't read anything else here).
1. I link to a current, free and excellent paper that is relevant to anyone aerodynamically modifying their cars - no matter its shape.
2. The article has the potential to correct a lot of errors often made here over the years (eg that fastback shapes have low lift - they don't).
2. Some good debate ensues - including picking up a mistake I made in citing a particular diagram. (Absolutely fine.)
3. But Aerohead then enters.
(1) Aerohead disagrees with what the paper calls a 'fastback', despite the shapes in the paper all being based on widely agreed current definitions - in fact the standardised DrivAer models used in much current aero literature.
(2) Aerohead sees it as a good opportunity to hark back to his favorite hobbyhorse, the aero shapes of the 1930s. About as relevant are these shapes as, well, the air/fuel ratios Ricardo was using in his test engines in, um, the 1930s. Aerohead's point has almost nothing to do with modern car shapes.
(3) When challenged, Aerohead asks for justification. Current references are cited, but since Aerohead doesn't read any current technical references, that doesn't help.
(4) Aerohead starts to become more and more bizarre in the statements he makes. He writes things that have absolutely no justification, yet alone any foundation in evidence, and that are in fact disprovable by a 2-minute web search. (Like that a Porsche Macan has separated flow on its roof. Just so wrong that it's honestly literally laughable - like, early this morning, when I read that, I actually laughed out aloud.)
(5) I now await the entry to the thread of:
- Freebeard (He will say: "A Type III Volkswagen of 1962, if equipped with a Coanda rear duct, has a drag coefficient dramatically different to a Type 1.")
- Vekke (He will say: "Do you have no politeness; how can you be so rude to someone who has helped so many people, and anyway, turbulent boundary layers behave differently.")
- California98Civic (Very quiet recently, perhap as he realises how much Aerohead says is completely wrong, will say: "I believe in the template and I think it is completely right for everything.")
- and then, and I won't name them, people who genuinely want to learn, and perhaps who didn't understand much of the paper, and who will take their cues from the misleading subsequent posts and so, thinking that obscure language equals the route to understanding, will ask stuff like "So if the template has a turbulent boundary layer and is influenced by vorticity, will base pressure be mutilated by Koenig-Fachsenfeld's 1941 theory?"
And so a clear, free ( and as correct as we can find right now) tech paper on car aero will be subjugated to the steam roller of BS that is (largely) this forum, and knowledge here for amateur car aero modifiers will be set back yet another increment. |
1) For EPA new car vehicle certification purposes, a vehicle is not recognized as a 'fastback', unless its roofline is an un-interrupted profile.
2) The roof profile in the paper would represent a ' Sportback', in the contemporary corporate lexicon.
3) 'Fastback' can denote a pseudo-Jaray form, which is a styling adulteration of a 'Kamm-back', or, Jaray-form of a specific minimum Vergungsungverhaltnis percentage, as defined in the FKFS drag tables.
4) The ' lang-heck' Kamm-back ( 'template') presents the most widely-spaced streamlines, lowest velocity flow, consequently highest static pressure, and lowest lift available ( pressure acting over vehicles exit architecture will be 80% to 90% of local station pressure ).
5) Shapes of the 30s can be seen with the VW XL1, Lamborghini Sesto Elemento, Cambridge University's 2013 CUER, 2015 Delft University ECORUNNER-V, 2016 M-B IAA, 2021 Chevrolet Corvette C8 Stingray, and 2021 McLAREN Speedtail, as a few recent examples. I've listed others elsewhere.
6) As to 'challenges', my experience so far, is that no one can convey, in their own tongue, the spirit of the cited material, which leaves me with no confirmation that the material was presented without filtering, or elements lost in translation. For example: if one cannot link the significance between local velocity and Dynamic Pressure, there's little opportunity for the advancement of knowledge.
7) A large portion of the extrapolated roofline of the Cd 0.37, Porsche Macan is 'missing,' while Mitsubishi went to the trouble of extending a similar roofline as far as the rear bumper on their Cd 0.27, Mirage 'G' model. There exists a high degree of certainty that had Porsche followed Mitsubishi's example, the Macan would exhibit far less drag.
Brett Herndon's aero cover and boat-tail provided Cd 0.30 'Cybertruck' aerodynamics for the Ford F-150, using nothing other than tried-and true 'templates' of the 1930s. Ford Motor Company was impressed enough to pay for the wind tunnel testing. Delta Cd 0.098.
8) Turbulence takes on the pressure which exists at the separation line.
9) If the separation line is closer to windshield header ( suction peak ), streamlines at the separation line are more closely spaced.
10) 'Closely-spaced streamlines mean high velocity and consequently low static pressure.' Hucho, page- 2, 2nd-Edition.
11) Low pressure = lift ( by definition )
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* Had Porsche simply extended the roof of the Macan all the way to the actual rear of the vehicle ( Kamm-back), streamlines would have been more widely-spaced, of lower velocity, consequently higher static pressure, and lower lift. The Macan exhibits ' lift-due-to-separation.' Just as with Cayenne, Panamera, Cayman, Taycan, etc..