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Reverse-Engineering Randy's motorhome
I ran some numbers on the motorhome and got some guestimates on numbers:
* 65 mph / 8 mpg = 8.125 gallons/ hour
* @ 6.885 lbs/gallon = 55.95 lbs/ hour
* @ 0.32-lbs/ hp-hour BSFC ( estimated from SAE reports )
* Road Horsepower @ 65 mph = 174.812 Bhp
* @ 95 % differential/rear axle efficiency and 95 % transmission efficiency,Road Load Horsepower @ drive wheels = 157.767 hp
* From GM/Detroit Diesel computer,@ 26,000 lbs,road horsepower to overcome R-R = approx. 57.401 hp ( radial tires )
* 157.767 hp - 57.401 = 100.366 hp aerodynamic load
* @ 100.366 hp,and plugging into the aero power formula,using 104 square-feet frontal area = Cd 0.514,to satisfy conditions.This should be closer to Cd 0.45 for a 'clean' bus and I suspect the rooftop AC units and other 'clutter' are responsible for the higher drag
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* To get to 11 mpg,the drag coefficient would need to drop to Cd 0.357,which was done by Mercedes in 1992 with their EXT-92 concept truck
*Rear wheels skirts,a full belly pan,and roof cleanup might fetch Cd 0.331( data from Renault VIRAGES report)
* To get to 12 mpg would appear to require a bit of boat-tailing.
* Hucho shows a bus with tapered roof and sides of simple 5-degree angle converging which netted a 6%-20% drag reduction.
* Adding a boat-tail,which,combined with the body taper resulted in a 'tail' of 1.44X body height produced Cd 0.20.This would be 13.189 mpg @ 65 mph.
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A 1995 Texas Tech wind tunnel investigation of semi-trailer boat-tails showed a 'GM OPTIMUM' boat-tail.It's about 5.5 feet long
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By 1980,Fiat had demonstrated an inter-city bus with Cd 0.238,although it had an 'Ideal' front end which requires a magic windshield.
-------------------------------------------------------------------------- 1991,Peterbilt/Caterpillar model 372 COE w/ Hobbs 45X 96 trailer,low-profile radials,71,000 lbs,@ ave.51 mph,California to Virginia = 10.49 mpg
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