Quote:
Originally Posted by ryannoe
Hey guys, I see some of you are concerned about digitally finding your frontal area and looking up your Cd values so you can calculate your CdA but why?
CdA is what we call Flat plate Drag (FPD) in the field. FPD is simple to use and is the coefficient needed to calculate drag (or vise versa in our situation). It produces easy to compare values for your baseline and modification data.
Flat Plate Drag uses the same equation we are already familiar with:
Drag = 0.5*density*velocity^2*FPD
(honestly, what it does is set Cd to 1.0 to remove it from the equation, then your comparisons are in fictional areas with a Cd of 1.0)
Do you guys have a test methodology for determining drag?
Is there a standard for testing yet? Is that something I could help put together?
-Ryan
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Road vehicle coefficients of aerodynamic drag,as a convention,are always presented as pertaining to frontal projected area.
If we know the original Cd,if only the shape of the vehicle is modified,and a change in mpg shows up,there is a metric by which we can reverse-engineer the new Cd.
The best method to measure drag will remain the full-scale wind tunnel.At $435-$2,000 an hour,they're kind of pricey (and if you can book time in one).
Coast-down testing is so problematic,that while not 'impossible' to pull off,most auto makers have gone to the wind tunnel for results instead.
Computational Fluid Dynamics,while better than ever,is so expensive that it remains within the domain of universities and auto makers.