Aeromodifications for large motorhome

[aerohead]

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

[crmears]

Thanks for doing the math and offering some hope for the 11 mpg goal. The roof has 2 large AC units, 2 satelitte dishes, 2 crank out air vents, a stove vent, a dryer vent , a ladder and a skylight. The sides have 4 awnings w/folding support poles. There are 2 large rear view mirrors. There are lots of joints and seams that abruptly protude 3/8 of an inch. I will take it out this weekend and gets some data on coast down times. Then I will start taking things off. If I can get 11 or 12 mpg I am willing to take everything off that does not affect safety. Also I am happy to drive 55 mph to get to the 12 mpg target. Thanks again for the info and ideas.
Randy

[skyking]

Take a bunch of pictures for us, please. I'll do the same on my 5th wheel.

[crmears]

Here is a link to some photos of the motor home. I have not inserted a link before so I did this two ways. Might have to paste it in address bar. Sorry if this looks messy. Randy

Motorhome Aero Mods pictures by crmears55 - Photobucket

Motorhome Aero Mods pictures by crmears55 - Photobucket

[Weather Spotter]

It looks like you have the rear drag of a semi, I would recommend a boat tail.

[botsapper]

The usual boat tail (folding) wings @ recommended taper angles. While we're at it, shallower nosecones & boat tails on the AC units. (Optional: awnings removed). Front & rear wheel well covers. Just the start...

[cleanspeed1]

Originally Posted by botsapper

The usual boat tail (folding) wings @ recommended taper angles. While we're at it, shallower nosecones & boat tails on the AC units. (Optional: awnings removed). Front & rear wheel well covers. Just the start...

Looks like a real cool spaceship!

[crmears]

Originally Posted by botsapper

The usual boat tail (folding) wings @ recommended taper angles. While we're at it, shallower nosecones & boat tails on the AC units. (Optional: awnings removed). Front & rear wheel well covers. Just the start...

Your modifications to my photo amazed my wife...You thoughfully allowed for the tow vehicle by leaving room under the boat tail. Also you allowed room for the radiator to vent. Your mods to the wheel wells are exactly what I had planned. Thanks for the design ideas. I was thinking of extending the boat tail an addition 3 or 4 feet and bringing it to and egg shape. I actually plan to go more radical on the roof. Everything will be coming off. All the awnings will be coming off. I plan fairing in front of the rear dual wheels that will direct wind to the outside. I have 2 questions. Would a front air dam to about 4 inches be worth the effort?? My front axle is at about 6 inches above ground level. Is it important to do something to the panel seams, lips and edges around the windows??

Randy

[skyking]

That front end looks pretty good to start with. I really like the mods that botsapper came up with. That looks good! maybe you don't have to delete those Ac units after all.

[orbywan]

Nice photo mods botsapper! I'm torn between making my boat tail fold up, or making it a permanent addition back there that we can also use for storage. I'm going to build a full blown mock up first and do some testing next. If the results are not a no brainer then it's not happening. Getting the boat or the quad back there on a trailer and having it all work will be a challenge, but maybe just keeping the tail up high like your photo is the easiest solution.

The dually tire issue is a good question. Is it better to split the air flow and direct air around both sides of the wheel sets, or try to shoot it all to the outside? Anybody know? Is there wind tunnel software out there where you can run simulations that doesn't require a super computer to run?

I think the air dam in the front is a good addition, as long as it is conveyor belt or some kind of rugged material that can take an occasional scrubbing and bounce back.