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Old 07-03-2008, 07:49 AM   #13 (permalink)
Shawn D.
Harebrained Idea Skeptic
Join Date: Jun 2008
Location: Alpharetta, GA
Posts: 211

The White Car - '84 Mercedes-Benz 300td
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Originally Posted by Bror Jace View Post
Shawn D, can you rig up a couple quick charts showing the influences of pressure and friction drag at different speeds ... maybe one for "typical" aircraft and another for a "typical" car? Just enough detail to show where they come into play.

Nothing too fancy ... X axis in gradations of 50mph ... Y would be % maybe. You could even draw this with MS Paint, I'd think.
Thanks for fashioning your response that way -- it's refreshing not to have to defend my background!

That would be a great concept if it were not for the fact that pressure and friction drag vary in lockstep with the square of velocity, so the percentage will stay the same throughout the speed ranges we're concerned with.* Note that measures of Cd make no distinction in what type of drag it is because short of doing CFD (computational fluid dynamics), you really can't tell what component is contributing what percent of the total, and again, the percentage for each remains the same. Aircraft also have an "induced drag" component which varies with speed and weight, but that isn't applicable to cars/trucks.

Sorry if this seems to be a non-answer, but it is the answer!

*Things change as one goes through the transonic region (starting at ~0.8 Mach) and into the supersonic region, but that's called "compressible flow," which is another realm altogether! Our land-borne vehicles doing under 600 mph are well into "incompressible flow."

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