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Old 07-02-2008, 09:11 AM   #11 (permalink)
Harebrained Idea Skeptic
 
Join Date: Jun 2008
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Quote:
Originally Posted by Bror Jace View Post
"drag increases exponentially with velocity"

That's the key. Spend lots of time at or above 100mph? Then washing and waxing becomes crucial. I'm always amazed when I look at older aircraft with exposed rivets, etc ... what a waste of efficiency!
Apparently, a number of you are disregarding what I said about pressure and friction drag on cars, maybe because I have no longstanding reputation here and haven't built up credibility. I really dislike pulling out this "hammer" because folks reflexively call it arrogant, but here it is: I'm an aerospace engineer and know what I'm talking about. There, I said it.

As I said before, pressure drag predominates on cars, not friction drag. Waxing does not become "crucial" for cars at any speed. In fact, having a rough surface enhances attachment on objects with low length/diameters, such as cars. Thus, waxing your car could make drag worse. In any case, the change is negligible.

As to the exposed rivets, it depends on the aircraft. It cost a great deal extra to manufacture it with a smooth surface, especially 40-60+ years ago. The P-38 was very smooth overall because the design led to thin boundary layers (i.e. no long fuselage), but the back end of a B-52 has many exposed fasteners/rivets, which isn't as important because the boundary layer was thick back there. Also, many combat aircraft were expected to have short operational lives due to attrition or obsolescence, so the extra $$$ was not always deemed worth it, especially when many had to be pumped out quickly.

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Old 07-02-2008, 07:41 PM   #12 (permalink)
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Question

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.
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Old 07-03-2008, 07:49 AM   #13 (permalink)
Harebrained Idea Skeptic
 
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Quote:
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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