Old cars tested in modern wind tunnels

[JulianEdgar]

I have a copy of the German magazine Motor Klassik that features a bunch of old cars tested in the Volkswagen wind tunnel. The data shows very clearly how optimistic pretty well all the Cd figures are that were quoted at the time of the cars' release. (Cw = Cd.)





Kamm K3 low drag research vehicle (Cd = 0.37):


BMW Wendler (Cd = 0.44):


Tatra 87 (Cd = 0.36):


All wonderful cars for their time, but obviously all with pretty bad Cd values in any modern context. I simply don't believe any old drag figures - instead, I look at the rated engine power and the top speed. For example, the Panhard Dynavia did 81 mph on 28hp, and if we accept these figures, it certainly had low drag.

[aerohead]

It would be informational to retest all the vehicles under contemporary conditions and examine the difference.

[Vman455]

I happened to come across this Volkswagen press release from 2015 this afternoon. It mentions the testing of the Sagitta in 2013 after it was discovered still intact at a car show:

For the V2 Sagitta, a cw value of 0.217 and a drag area of 2.10 m² were measured – outstanding values, although they can be outperformed impressively by aerodynamically optimized modern vehicles such as the Volkswagen XL1: a cw value of 0.189 and a drag area of 1.50 m² were measured in this case.

(Note that the release wasn't translated well; "drag area" means "frontal area").

Further, they tested it in various configurations:

In the course of tests on the V2 Sagitta, the aerodynamic effects of individual features such as the wheel spats were investigated. If they are removed, the cw value rises to 0.252. If the windscreen wipers are removed or a small rear spoiler is installed, the cw value is reduced to 0.216. The cw value of 0.160 mentioned in contemporary magazines at the beginning of the 1950s was a product of a very vivid imagination.

[JulianEdgar]

But Vman455, I don't understand!

How can the V2 Sagitta possibly be low in drag? It doesn't match The Template in any way at all!

How could they have made such a major error?

(Gorgeous car by the way - and a brilliant drag figure.)

[jakobnev]

How can the V2 Sagitta possibly be low in drag? It doesn't match The Template in any way at all!

Isn't the coefficient 70% higher than the template?

[JulianEdgar]

Originally Posted by jakobnev

Isn't the coefficient 70% higher than the template?

Where is the car that matches the template and has the drag coefficient you are quoting?

It has never actually existed... or didn't you realise that?

[jakobnev]

Well that's a mentally ill/deliberate misinterpretation of what I said.

What I was trying to say is, that they would have done a lot better if they had used the template as a sort of... well template, the steepness of that rear is utter malarkey.

[AeroMcAeroFace]

Is there any indication as to why they were wrong? Were they lying? Or was it measurement error or calculation error? I remember reading that the Lotus europa with its claimed 0.29 drag coefficient was with aerodynamic tweaks not offered on the car. Is there any difference in how the calculations were done then? Other than lack of digital systems.

[freebeard]

Modern wind tunnels don't agree today. Zeppelin tunnels didn't require a rolling roadway.

For example, the Panhard Dynavia did 81 mph on 28hp, and if we accept these figures, it certainly had low drag.

IIRC correctly, the Volkhart-Sagitta did 90mph with 25hp [if we accept these figures]. It's nice that JulianEdgar is coming round to the V-S V2 as the missing link between the Zeppelin era and the Porsche Taycan.

T'would be interesting to see what further he has to say.

[AeroMcAeroFace]

But why don't they agree? Barometric pressure? Or are the other variables not able to be controlled well enough?

I don't know how wind tunnel calculations work. But it seems to me that the same car should produce the same results given the same conditions, given the same car either the conditions are different or the measurements are off. Presumably positioning the car fractions of a degree off head on might make a difference.

Am I overthinking this or is it just random variation?