Note from Darin (admin): this installment is part of a series posted by Phil (aerohead) about the effectiveness of various aero mods - with quotations and citations to source data.
See the aero mods data index here.
End note.
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I figured this was going to come up at some point so I wanted to nip it in the bud before spammers started marketing "miracle" finishes for cars, promising higher mpg and we got a flurry of inquests.
This one's short and it's from Kelley and Holcombe at General Motors Research Laboratories:
"The nature of the boundary layer is such that a critical speed exists below which improvement of a particular body surface smoothness has no effect on reducing surface drag. For the finished portion of an automobile body the surface smoothness is of the order of 0.5 to 1.0 microns or 20 to 40 Mu in. Any improvement will not reduce surface drag,since the grain size of the surface ( 0.02-0.05 mils ) is below the critical diameter for any land vehicle operation."
Of interest to Bonneville racers whom will enter into
transonic flow, Hot Rod Magazine's June,1962 issue published the following:
"By just waxing the surface of the C118 ( DC-6 ) airplane the Air Force was able to save an hour's time,or 370 gallons of avgas between Honolulu and Japan."
Bottom line, if you've washed and waxed your car,that's as good as you're going to do. Surface friction is still going to claim 9% drag from you.