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Old 08-28-2020, 11:46 AM   #15 (permalink)
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fineness ratio

Originally Posted by JulianEdgar View Post
In the decades over which I have been writing about cars, I've met many people with undergraduate engineering qualifications who know very little, so it doesn't much surprise me to find another.

Hucho (1987) page 200 for anyone who wants to look. There's just one index reference to fineness ratio in the whole book! And the current - fifth edition - drops 'fineness ratio' entirely from the index. So as I said:

There seems to be a lot of oversimplification happening here - eg "Since drag is directly related to fineness-ratio". No, that's not the case on any real-world car.

If we were striving for shapes with the lowest drag in free air, then I'd imagine fineness ratio would be important. (Say, in the design of airships.)

But we're talking here about cars, so more misleading material from Aerohead.

What a great analogy! And so, using your logic, the parallel to fineness ratio would be the house's north-south versus east-west length ratios! That's all we need to do to assess the energy efficiency of these houses - just measure their shape....
1) '... the shape of a body in front of the largest cross-section has only minor influence on the total drag. The main contributions to the drag force originate from the rear part of the body.... it is very important to design a rear body surface which brings the divided streamlines smoothly together. Optimum shapes are 'streamlined' bodies having a very slender rear part.' Hucho
2) ' ... the optimum shape in terms of drag is a half-body, which forms a complete body of revolution together with its mirror image- produced through reflection from the roadway.' Hucho
3) ' OPTIMUM FINENESS RATIO' is presented in Hoerner, Page 70.
4) Maximum aft-body contraction geometry necessary to prevent flow separation is given by Mair and Buchheim et al. in Hucho. ( 22-23 degrees)
5) The lowest drag streamline body which satisfies the aft-body contour limit is a 2.5:1 fineness ratio, provided by Hoerner, and illustrated in Hucho's drag table (derived from Hoerner's data ), page 61, TABLE 2.1, 3rd from bottom.
You're looking at the 'aerodynamic streamlining template.'
By definition, this shape provides the lowest drag, three-dimensional flow, half-body, free of flow separation, with minimum surface friction and pressure drag. It's a known quantity. A sure thing. Defined by Hucho, with supporting evidence by same.
I've given you all this information before.
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Last edited by aerohead; 08-28-2020 at 11:51 AM.. Reason: typo
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