Quote:
Originally Posted by turbothrush
I am wondering if you keep the flow attached and you reduce the wake area by say 20% would the drag be reduced by the same amount and would the mpg increase by about half of that?
I am building a fiberglass/foam truck camper ( profile is done but I can still taper the sides) hope the "areo experts " chime in.
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turbothrush,I am without my workbook here and cannot give a definitive answer.
If I had to rely on memory( quite dangerous now! ),I'd say that if you were working with an airship,fuselage,or submarine hull,that the relationship would be basically linear.
Pressure drag,the target of streamlining,is a function of the pressure differential between the vehicles forward stagnation pressure,and the base pressure of the wake.
If you move the separation point back,such that the wake is reduced 20%,you'll be picking up pressure recovery,as the point of separation is now in a regime of greater pressure as the air decelerates down the tapering body cross-section.
The reason you won't get a linear benefit is that along with the pressure drag,your still looking at surface friction drag ( which is actually increasing as you lengthen the vehicle),cooling system drag,induced drag,interference drag,and the rolling resistance drag of the powertrain and tires,which all conspire to overshadow the benefit from the reduction in pressure drag.
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If the vehicle is completely streamlined excepting for the boat tail,then as the boat tail is lengthened,you do see an arithmetic relationship of diminishing return as you approach about 70% of the 'Template' aft-body,for example.Between 70% and 100% there's virtually no change in Cd.