Cds and comparative wind tunnel values

[aerohead]

In Hucho's 2nd Edition,in his chapter on wind tunnels,he gets into the difference in Cd values obtained at different wind tunnels around the world.
Of most interest with regards to recent posts is that with respect to the ideal 0.05 blockage ratio,as of 1986,only two wind tunnels,worldwide fulfilled the necessary 40 m-square nozzle exit requirement: GM's,and the Deutsch Niederlandischer Windkanal ( DNW ).
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Society of Automotive Engineer's papers which Hucho cited in his chapter are:
*SAE Paper # 800 140 ,1980
*SAE Paper # 800 139 , 1980
*SAE Paper # 810 187 , 1981
*SAE Paper # 820 374 , 1982
*SAE Paper # 830 301 , 1983
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Hucho mentions that between all tunnels tested,that results were all within a statistical mean standard deviation of +/- 2.5%
The greatest single discrepancy was around 9% in one case.
Lift coefficients did not fair as well as Cds.
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Hucho also mentions that Cd will vary with any particular car as a function of load ( inclination of body on suspension ),window position.
Not mentioned would also be the cooling system,should a viscous-coupled fan be engaged or not.
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And not mentioned by name,but I think it is Lockheed's Marietta tunnel,individual users use company-specific wind tunnel correction factors for the test data obtained.
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Anyway,those wanting a deeper understanding of our elusive Cds are directed towards these papers.