mods-data-%change Cd or drag(installment#3 surface drag)
 

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.

Good post!

Fuel economy is the most poorly understood (by the general public) parameters of auto preformance.

Thanks to high-priced gas we are seeing old charletans coming out of the woodwork hawking "solutions" that never worked before and counting on the public's short attention span.

As we know, there is little new in the world of engineering. Stuff works or doesn't work for fundamental engineering reasons.

Phil,

Have a look at this blog entry at AutoSpeed.

I haven't read the book yet, but from the article it looks like it's possible to deal with friction drag at our speeds.

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friction drag
 

I looked at Barnes & Noble,and Hasting's for the book.No luck.I'll keep on trying.I did run across something of potential interest in Hermann Schlichting's Boundary Layer Theory involving laminar airfoils which may have significance to the book.I'm gonna chew on it for awhile and post when I think I've got a handle on it.P.S.,all of the Navier Stokes stuff is in Schlichting's book.

Would a car have more drag if it's dirty? I don't know that much about aerodynamics, but the original post suggests to me that it might. My very informal observations suggest that washing may improve mileage.

dirty vs clean
 

The surface drag is only 9-percent of the total drag,however,as mentioned in the installment,the relationship to a clean and freshly-waxed finish is firmly established.It's picking nits but its one of the things we do have control over.When I get all my "pick and shovel " work done,I'd like to spray a nice finish on my monstrosities and keep 'em maintained.