I've been living inside Hoerner' and Hucho's world for awhile and have ferreted out some quanta which may shine a light on low drag trailer potential.
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In Horner's section B.1.,'Drag of Motorcars and Locomotives',Fig.9.10,there is a depiction of a self-powered passenger railcar studied by O.G.Tietjens and Ripley,'Air Resistance of Trains and Inter-urban Cars,Transactions,American Society of Mechanical Engineers,1932.
The railcar is credited with a frontal area drag coefficient of Cd 0.08,complete with wheels,on tracks.
This is considered the lowest 'theoretical' limit for drag of such a vehicle.
Within the text of the chapter Hoerner also reports that the "ideal" streamline forebody-drag coefficient is Cd 0.04.
In Hucho's 2nd Ed.,pg. 330,Fig. 8.60'Air drag distribution over vehicle sections for a rounded and sharp-edged front,' if one 'solves' the pictorial algorithm for forebody-drag coefficient,a value of Cd 0.041 is arrived at which is in very close agreement to Hoerner's value.
If we accept the Cd 0.04 nose drag value,we're left with Cd 0.04 for the tail.
Both sections equally share the surface friction component.Wheel drag is reflected.Ground proximity is reflected.
We'd be talking about the Lancair/Legend of railcars.Everything flush,smooth,faired.Completely smooth underside.
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If a wheel fairing package for a trailer were executed as with contemporary solar race cars we might establish Cd 0.04 as a 'talking -point' for the lower limit we could predict for a full-gap-filled,full boat tail trailer of ideal length.
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It's an evocative number.
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My trailer is not ideal in length.I have disparate exit wash velocity issues due to the bed box rear corners.I'm over-built,overweight,and the Viking speedboat hull body is also short of ideal in contour,compared to its integration to the T-100.
The 47.9 mpg between Colorado Springs and Trinidad was a glimpse of what really low drag can do,so I'm in it to the finish.
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A final thought for now for any of you members who are thinking about a project like this.
Kamm's wind tunnel experiments proved the value of plan-taper in the body.If one were to do a full 'Template' style roof contour ( in elevation view) ,without plan taper you're looking at Cd 0.21 capability,versus Cd 0.13 if you include it.That would cost you 38% loss in drag reduction.Yes it's a larger front-loaded investment,but it will pay significant tax-free returns over its entire service life.And when wheel bearings are the only significant moving part that could be a very long service life!
