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Old 05-11-2020, 03:39 PM   #5 (permalink)
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Originally Posted by aerohead View Post
Just read this thread.I'm away from my library and do not have access to my materials.
While I don't disagree that there is evidence that there are discrepancies between early data compared to more contemporary data obtained for the same vehicle/model,there do exist modern data which compares favorably with early reporting.
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.

Originally Posted by aerohead View Post
I don't believe that broad stroke generalities best serve this argument,but rather, maintain that any specific vehicle should be treated on a case-specific basis.
The Tatra was incapable of 'low drag' due to its aftbody.
Of thefive 'K-cars' built by Fachsenfeld at FKFS,only the Mercedes-Benz'experimental-chassied,diesel-powered,overdrived,1941-42 K-5,personal personal car of Kamm's was rated as low as Cd 0.23.It's the only 'drop-nosed' K-car.It's the only K-car with full wheel skirts.K-5 maybe the only K-car with Kamm's patented cooling system (which is what he is actually famous for,as the
'Kamm' -back is actually the property of Fachsenfeld,and the reason Hitler wanted Fachsenfeld at FKFS).K-5 had the longest aftbody of all the K-cars,the secret to its low drag.
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.

Originally Posted by aerohead View Post
Airfoil shapes would be relegated to sub-critical Reynolds number solar racers,and NACA (NASA) quanta would be valid.
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.

Originally Posted by aerohead View Post
The really 'low-drag' shapes which could be applied to passenger cars would be based upon Paul Jaray's,1922, streamline half-body,which modern wind tunnels and CFD re-verify,with each generation of university investigation.
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.

Originally Posted by aerohead View Post
In his second edition of Aerodynamics of Road Vehicles,Hucho wrote that any difference in drag,associated with wheel rotation during testing indicated such an insignificant difference in results that it wasn't pursued.General Motors Research Labs had come to the same conclusion from it's testing at Cal Tech in 1953.It would have been considered an absurd proposition in truck and trailer testing,due to wheel environment turbulence.Only open-wheel race cars were deemed worthy of the extra trouble/cost.
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.

Originally Posted by aerohead View Post
Today's F-1,Indycar,and NASCAR' billion-dollar racing programs mandate that 'ALL' areas of aerodynamics be investigated.
One curiosity always hiding in the room is that,given rolling-road wind tunnels,and million-dollar public relations budgets,and the ability to develop 'porous' low-drag wheelcovers,when tasked with setting land speed records,all auto manufacturers choose the totally sealed,convex disc,MOON wheel covers,the lineal descendant of WW-I era aviation; and can be seen on all extant Boeing 727s.
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.
Modifying the Aerodynamics of Your Road Car

A really good book that should be added to the library of everyone working in automotive aerodynamics, as well as those making car aero modifications at home. - Rob Palin, former Tesla aerodynamicist
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