I spent a week and a half playing around with Dr.Frank Buckley's drag data from the late 1970s 'Sedan/Airstream' wind tunnel study at The University of Maryland.
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*I used a 1975 Chevrolet Impala 4-door Sedan as the tow vehicle:4,222-lbs,Cd 0.508,Af = 24.6 sq-ft (est.),CdA= 12.49 sq-ft.
*I used data for a 1975,31-foot,5,035-lb Airstream 'Sovereign',estimated at 86.66% gross frontal area,or, 58.66 sq-ft.
*As a tow vehicle,the Impala undergoes a 43.4% drag reduction,or, Cd 0.287,giving it's wake to the Airstream (D.M.Waters),and its power requirement drops to 12.52-hp,down from 16-hp.
*Using 94% of the rolling force coefficient estimated for the Impala (no differential,no propeller shaft,no transmission tail shaft) I estimated the power required to overcome the friction and tire losses of the Airstream,at 8.979-hp.
*The 34.6-hp of the 'rig',minus the power for the Impala leaves 20.07 hp to move the Airstream.
*Subtracting the 8.979 hp for it's rolling resistance leaves 13.090 hp to overcome aerodynamic drag.
*At Af=58.66 sq-ft,and solving for Cd,we get 0.196 for the Airstream in the shadow of the Impala.(equal to a trailing NASCAR in a 2-car draft,not quite as low as a Cd 0.167 bus,drafting another identical bus).
*Multiplying by it's frontal area to get it's CdA,adding the CdA of the Impala,and dividing by the Af of the Airstream,we get a coefficient of aerodynamic drag for the Sedan/Airstream combination of Cd 0.314.
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*Cd 0.314 is 40% lower than some Cd 0.53 combinations presented by Hucho.
*A 40% drag reduction would equal a 20% fuel economy advantage to the Airstream at 55-mph as tested.
*Airstream advertizes a 20% fuel economy advantage for it's trailers.
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*Based upon width and height,the Airstream has about 6.3% lower frontal area than the 'box' trailer it is compared to,which is part of it's lower drag.
*The bulbous nose,upper edge radii,and trailing surface radii,and belly pan would make up the difference in drag .
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*Testimonials from owners suggest that,on a frontal area basis,a longer,or shorter Airstream will require about the same amount of fuel to pull regardless of weight gain/loss,unless experiencing aggressive road grades.
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*Without the rest of Dr. Buckley's research,these values must be viewed as only an estimate.And of course,different tow vehicles will alter things.
