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Old 04-10-2008, 06:57 PM   #33 (permalink)
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fineness ratio

Originally Posted by SteveP View Post
Maybe you mean something different by "fineness ratio" than I understand it to mean, so perhaps you should provide a short definition. Mine is "width/length" (or, for a cylinder, diameter/length). I therefore don't understand how lowering a car can alter its fineness ratio.

Also, would you be so kind as to expand on your comment that "Since road vehicles all suffer from the "mirroring" effect of the ground,anything which can be done to increase fineness-ratio is a shoe-in for lower drag". What does fineness ratio have to do with the ground effect? I could see it if you were *narrowing* the car, but lowering it doesn't do that.

Steve,good question and I'll try and do it justice.From experimental investigation,it was discovered,back in the 1920s,that a body of revolution (say a Zeppelin body/fuselage) which had a drag coefficient of X in free air,would have a drag coefficient of 2X near the ground.The drag behaved as if you had placed the object on a mirror,and the air was striking both images,hence "mirroring".In early windtunnel photographs you will actually see car models joined at the wheels,one right side up,the other upside down,hanging in the air stream.When viewed from the side,if you drew a line which enveloped the double-car combination,hitting all the high points and spanning the voids,the line might describe an egg,as in the VW Beetle.The length of the "egg" divided by the height of the egg would define it's "fineness ratio".Its kinda like the aspect ratio of a wing,where the double-car is the airfoil.Kinda weird! Anyway,since all vehicles are in ground-effect,they all suffer from this virtual "mirroring" effect.So if you divide the length of your car by its height and say its 3-to-1,which would otherwise be very efficient,as far as the air is concerned,its only 1.5-to-1,an extremely blunt body,and not a good candidate for low drag.Hucho's first English translation edition from 1987 shows this whole affair on page 200.By lowering a car,you effectively stretch out the "egg",improving it's fineness as you approach 6-to-1,the ideal for anything in ground-effect.If you go longer,profile drag improves at a lower rate than skin friction which begins to climb,canceling out the benefit of any additional length.A wing has extremely low profile drag but extremely high skin friction and is unfit as a model for car bodies.A symmetrical airfoil of 6-to-1 aspect ratio,cut in half lengthwise,would describe the lowest drag car body.Not really practical because of the necessary length,however it is the "pumpkin-seed" form chosen for world-class energy sippers.Our challenge is to get as close as we can to the efficiency of the pumpkin-seed while addressing all the other parameters of automotive operation.
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