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Originally Posted by Otto
Consider Sighard Hoerner's Fluid Dynamic Drag book, specifically his section on external stores (i.e., bombs and drop tanks) and the interference drag created when two otherwise streamlined shapes operate in close proximity. I think this may be instructive re. your problem regarding shear forces. Basically, the flow past the wing gets tangled up with the flow around the drop tank, increasing drag well beyond the arithmetic sum of the respective bodies.
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That's a good point and I can see how this could still be explained with shear forces. As the air velocity between those two bodies would be accelerated (similar to a venturi nozzle), it would generate much higher shear forces than if one simply calculated with the aircraft velocity.
And in this case it is worse than with a car, since both bodies are moving and not just one surface and if one compares the Hagen-Poseuille equation for two moving parallel surfaces against just one moving surface the 2 moving surfaces have a 12 times higher resistance (if everything else is kept equal and again assuming laminar flow, which is not the case here).
A rounded nose aerodynamic vehicle would also increase shear forces, as the rounded nose would accelerate the air moving underneath the car and therefore disproportionately increase shear forces (which would to some extent also explain why a pitched down round nose is more efficient than a pitched up nose).
Having higher air velocity would also get air to reach a turbulent regime sooner which would increase shear forces even more.
However, I think that this wouldn't play a significant role with a passenger car, as the air would be turbulent almost from the start and the nose isn't usually very round either.
So I still think that any practical lowering (>3" above surface), wouldn't be detrimental as long as the back end is not lowered more than the front end.
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Originally Posted by aerohead
If you can get a look at any of Hucho's books,you'll find a section on 'Effective-Bluffness','Effective-Fineness Ratio',which examines the role ground clearance plays with Cd.The Schl'o'rwagen is investigated completely for G.C. effects.
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Thank you for the tip and Happy Thanksgiving. I ordered the 2013 edition of the German-Hucho-Version (I live in a German speaking region) and look forward to receiving it.