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The problem as I understand comes to play when the size of the object is scaled down to fit in a wind tunnel. I still can't wrap my head around it but apparently you have to increase the wind speed on a scaled model to get an accurate Cd measurement. To me it would seem if you had say a 1/10th scale model, you would run it at 1/10h the speed or 6 mph vs 60 mph but it's not the case. If speeds had so little effect on Cd then it wouldn't matter and models would be just as good as the real thing without messing with speeds.
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