Thread: Biggest gains are at the back???
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Old 01-08-2012, 02:39 AM   #23 (permalink)
Ken Fry
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Hi Aerohead,
Good observations. I imagine you are thinking about more than you are actually writing. For example, you mention the wheel-less Aptera/Morelli shape which (several sources agree) is a .05 -.06 shape, as is a typical bomb, torpedo, etc. But what you did not mention is that the actual CD with wheels, suspension, ground proximity and drive shafts is at least .15, and the X Prize test results suggest a little higher than that -- although I don't have an accurate frontal area.

Quote:
*Sailplanes are governed by skin friction drag
Sailplanes are all about lift/drag, and fuselage drag is a small part of the overall drag. Induced drag is the big part. If you were writing only about the fuselage, then there is a lot more than just skin drag. It would be simple to make a sailplane with a cylinder for a fuselage, but such a shape produces, (for the fuselage alone) a Cd close to 1. So I think your statement is a little shorter than it needs to be to be valid.

Quote:
*the 1957 MG EX 181 registered Cd 0.12 on 'real' tires


The MG, Bonneville streamliners, the .143 Sunraycer, etc all taken together show that there is something about them that is dramatically different than a Malibu -- it takes more than tapering the back to make a really low Cd.

Quote:
*Sub-0.12 coefficients of aerodynamic drag can result from wheel streamlining.
I think you meant to say, perhaps, that sub .12 Cd's can result from wheel streamlining if everything else has been optimized. The MG's wheels are already quite well streamlined, and the body is extremely well streamlined, front and rear, but still it is not sub .12. The Aptera had well streamlined wheels but is a long way from sub .12. The VLC has well-streamlined wheels but is also a long way from .12

As of 2010, the .143 number for the Sunraycer still stood as the lowest recorded at GM.

Quote:
*the 'Template' is a streamlining tool to correct flow separation with production automobiles operating on public thoroughfares.Wind tunnel investigations have demonstrated Cd 0.12 with vehicles with the 'Template' aft-body.
This makes it sound as if one merely puts a boat tail on a car according to the template, and a Cd of .12 results. I imagine that's not what you intended. My old Citroen SM was a reasonably good fit for the template, but gained very little from extending the rear into a full boat tail, vs the existing Kamm back. Its Cd was around .30 either way.   Reply With Quote
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