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Old 05-02-2020, 04:36 AM   #1 (permalink)
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Beware old and new aero data

Can I put forward an idea?

A preamble: I love historical context. There is nothing in any area of car aerodynamics of the past that I am not interested in. Show me old aero cars and data - and I am always interested. That's a major reason I went and saw the Tropfenwagen in two museums in Germany, and admire the Tatras, Citroens and Chrysler Airflow so much.

But you need to acknowledge that old car aero data comes with some very major shortcomings. Primarily, they are in the measuring and testing methodologies that were then used.

Take the wonderful Tatra T87. It had a drag coefficient, when recently measured full-size in the highly-respected Volkswagen wind tunnel, of 0.36. Nothing special, eh? However, models of the car were being measured at the time of the cars development at CD = 0.24!

Or what about the ground-breaking pre-WWII research vehicles, created by the German FKFS institution under the leadership of Wunibald Kamm? The models were dubbed the K1, K2 and K3. One reference states that K1 had a CD of 0.23 (and a model measured 0.196), K2 apparently had a CD of 0.23 but K3 had the much higher CD of 0.37! So what happened to K3? after all, it looked much the same as the two earlier cars. Well, K3 was tested full-size in the Volkswagen wind tunnel in 1979 In fact, its very likely that K1 and K2 had drag coefficients that were similar to K3. Good at the time - but absolutely nothing to try to take lessons from today.

So old info - based on measurements of models, or done in wind tunnels without moving floors or moving wheels - is all highly suspect.

And what about NACA data, or that great old textbook, Theory of Wing Sections by Abbott and Doenhoff? It's a book I have (I think I have all the major aero textbooks) but it's one that has as little relevance to the shape of current cars as data on low drag wheel covers on a WWI aircraft has. It's all interesting - but tech papers on wheels on low drag cars with smooth undercovers are in a quite different world to a WWI aircraft! (Not to mention the different galaxy of aerofoil profiles measured in free air!)

I read somewhere 'what is the time cut-off for scholarship'? The answer to that question is very easy: when the current evidence suggests that our understanding has changed. Patterns of airflow don't change, but they way that they are measured, modeled and predicted has changed massively over time.

I've recently been reading a 1960s textbook. It's very well regarded - in fact an absolute classic on engine tuning. But the material on tuning air/fuel ratios and ignition timing is so simplistic as to be useless for any current engine. In fact, to be honest, it sounds like they're trying to tune a lawnmower. And yet the author worked on, and helped develop, F1 engines....

If you don't keep up with the current research evidence, you're doomed (I think anyway) to 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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