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Old 09-09-2020, 04:20 PM   #10 (permalink)
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Originally Posted by aerohead View Post
' [T]he drag of the basic body ( Cd 0.09 ) is achievable. To what extent this can be approached in the development of a production vehicle is therefore more a question of the balance of the requirements of the specification than of technical feasibility.' Hucho, 2nd-Edition, page 209, circa 1986
The 'law of the Paris dressmaker' is what stands between us and low drag.
I really think you've put yourself at a disadvantage working from such an old copy of the book. I have the 4th edition (1998), which isn't even the newest but is the last edition edited by Hucho.

On p. 53, under the heading 1.4 Aerodynamics and Design, Hucho writes:
This discord between physics and art was resolved only when the automobile manufacturers started to do their own aerodynamics. As members of a vehicle engineering activity aerodynamicists became converted to vehicle engineers. The daily contact with design gave them an understanding of this kind of art. The aerodynamicists learned to respect the designers' creativity, and they realized how much the designer's freedom is already restrained by requirements, be they technical or legal. The designers, on the other hand, realized that aerodynamics is not a black art, but a rational discipline. The strategy of detail optimization, the step-by-step procedure, provided a sound basis for cooperation. Both the need to improve fuel economy and the desire to make the economics of a car visible by its style made designers open to aerodynamics. This openness increased as they realized that this technology is far more reliable than the temper of fashion. The plot of drag coefficients versus model year in Fig. 1.58 shows how fast the change of attitude progressed.

However, the strong position now held by vehicle aerodynamics is not welcomed by everyone. Two arguments are repeatedly made against aerodynamics: one technical and the other emotional. Aerodynamicists are well advised to carefully consider both of them if they do not want to lose the status they have now achieved.
You can go ahead and think that car design is all some sort of conspiracy, and that aerodynamicists have no say or are blinded somehow by corporate culture into producing shapes that have much higher drag than the lowest-drag experimental models or concept cars. But the reality is they work under constraints: people, who are motivated by cultural norms, buy cars; people buy cars for practical reasons and emotional ones; exterior design is dictated by both stylistic considerations (4 wheels, a standard-ish dimensional footprint, proportional wheels, aggressive faces, etc.) and technical ones (crash test performance, NVH, fuel economy, handling and performance, etc.).

The reason we don't have cars that look like tadpoles is simple: hardly anyone would buy one. Why? They look "abnormal." With a long tail, they won't fit in a standard garage, and if you make them small enough to do that they end up being tiny inside. They compromise packaging, interior volume, sightlines, and practicality.

Probably the best example of this so far is the original Honda Insight. That car is as close to a no-compromise car as have ever been brought to market. It had low drag and low weight. Consequently, it was a sales flop. People didn't want a two-seat car that didn't have an insane amount of power. People didn't want to pay extra for an all-aluminum body. People didn't want a tiny car with wheel skirts that looked different than every other car at their local Honda dealership.

It's stupid, yes, and buyers consistently buy impractical cars with feeble justifications--but this is not because of some conspiracy preventing their rational behavior. It's because we're irrational already.

We're seeing this start to change, as low-drag design becomes a premium feature, with cars like the Model S and 3, Taycan, and now S-class returning lower drag coefficients without significant changes to exterior design features. It remains to be seen how much lower they can go.

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