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Old 05-11-2020, 04:59 PM   #6 (permalink)
aerohead
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Quote:
Originally Posted by JulianEdgar View Post
I did not say that all new data disagrees with old, I said that old data should be treated with suspicion when the current evidence suggests that our understanding has changed.



I am quoting data from Stromlinienautos in Deutschland (Kieselbach), P 84 for K1 - K4 and Aerodynamics of Road Vehicles (Hucho) (P 21) for K5.

As is typical with cars from the 1920s and 1930s, when tested in a modern wind tunnel, they all recorded much higher drag figures.

For example, you mention K5 as having a long afterbody as 'secret for its low drag'. It had a measured Cd in a modern wind tunnel of 0.37! So much for its secret. As I already mentioned, K3 was also measured in a modern wind tunnel at 0.37.

They were fascinating for the time, but to bring up these old cars as if they have anything to teach us today is just absurd.



Yes, I agree - so to use data relating to NACA aerofoils for shaping normal road cars is treading on very thin ice indeed. One obvious discrepancy is the thickness of the boundary layer on the rear parts of real road cars versus an aerofoil.



Yes Jaray was a very good aerodynamicist, and refined his craft working on the aero of Zeppelin airships. But his pure shapes are a long way from practical road cars.



These views are now completely outdated for road cars. Any aerodynamics textbook published over the last few decades covers the major increase in drag associated with testing vehicles with rotating wheels. It's why all car manufacturers now use 5-belt wind tunnels with rotating wheels.



What have Boeing 727s, Land Speed Records, F-1, NASCAR and Indycar got to do with road car wheel design for lowest drag? The most recent tech research shows that, for road cars, fully enclosed wheels are not always best for low drag. I've already cited those papers here in another thread. It's an excellent example of why not keeping up with the literature means falling into the trap of "it's always been like this" - when, maybe, it isn't any more.
*The suspicion issue has been reported on for decades.
*Since,as of Hucho's earlier writing,and specifically to good agreement of drag coefficients for specific cars tested in modern tunnels,compared to original literature,only a fair assessment would be derived from those very same shapes being tested with rotating wheels and the variability published.

*Kieselbach may have it wrong.It may have been
Ludvigsen who reported on the confusion of K-car nomenclature.
*The Landsberg (sp?) Castle car tested by VW is not the K-5.It's Cd 0.37,in light of its mutilated belly pan,would not be germane to the K-5.
*The Schl'o'rwagen,at full-scale,if I remember correctly,returned a Cd very close to that of early reporting at Gottingen,compared to the VW tunnel,with static wheel testing.
* Hucho has corroborated Hoerner's numbers.
* Horner has corroborated NASA's (NACA) numbers.
*Which all support Jaray's Zeppelin numbers.
*Jaray's small car returned lower drag in more modern testing than originally reported by Klemperer.
*Modern testing of 'typical cars' of old is not germane to modern testing of 'specials'.We need specificity.
*As to the K-5's 'secret,' the entire premise of much of the FKFS research dealt with body length vs drag.'Verhungungsverhaltnisse'.They corroborated Walter Lay's research,who's research corroborated Jaray's,along with Elliott Reid at Stanford,and others.
*I would want to tread very lightly when implying that the older research has nothing to offer us.I smell confirmation-bias,and that has no place in science.
*Aircraft,by design,are separation free at 'flight' conditions,and are all ruled by surface friction drag,of shock-wave drag depending on Mach#.It's really not germane to road vehicles,of which drag is fundamentally a function of flow separation.
*'Practical' is not a scientific metric.It cannot be quantified.It's very subjective and only an arbitrary invention of the human mind.
*If truly low drag is the topic,then body length is the arbiter.It's not negotiable.Physics as it is.
*And again,FKFS gave us the template for active aerodynamics in the mid-1930s which would address the 'practical' length argument.
*If outdated,can you explain why contemporary record vehicles continue to borrow from near-century old technology?
*I have your book.Your sources haven't proven anything,nor made a case against full-coverage,convex wheelcovers as an all-emcompassing universal absolute.What your messengers have published is highly contextual and dismisses an environment in which even lower drag can be attained if automakers weren't enslaved to the Paris dressmaker.
*Ignoring brake cooling,if porous wheels have superior low drag characteristics compared to full-coverage,convex discs,why don't we see them when land speed records,or fuel economy records are set?
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