Quote:
Originally Posted by sregord
All good stuff.
I'm wandering a bit in the : Aeromodifications for large motorhome ...stream.
Very nice photoshop rendering by Botsapper...
...and your Reverse-Engineering Randy's motorhome..mod to CD ...
I'm still self "imaging" the boattail concept. Giving up the rear-mounted ladder will simplify whatever collapsible/fold-able option I go with.
I've already removed one of 2 roof ACs...there is so much unused space (for us) below the floor.
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Just for drill, I did a thought experiment for a Motor Coach Industries ( MCI ) J400 Motor Coach, Cd 0.363, Af approx. 97.678 sq-ft, wake area 94.131-sq-ft, 8-mpg @ 65-mph, Diesel, BSFC 0.314806 lb/hp-hr, thermal efficiency= 39.256%, 2% accessory losses, 88.666% powertrain efficiency, CfR-R= 0.008725 @ 65-mph, 7.1089-lb/gallon, 136,775-Btu/gallon, 2546-Btu/hp; modified to get 9-mpg under the same test conditions.
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* The fuel economy allowed calculating the gross-hp.
* Thermal efficiency gave brake-horsepower.
* Accessory & powertrain losses gave Road Load Horsepower *.
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* I calculated the aero-hp and subtracted that from the Road Load, yielding the rolling resistance horsepower, to be used in further calculation.
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* To get to 9-mpg would require reducing the brake-horsepower by 19.04-bhp, 16.45 less aero hp, 24.778% less wake, 23.3% less base area, near the '1st-22-degree angle location on Mair's contour.
The only avenue open is to reduce the aerodynamic HP by 24.778%, which requires Cd 0.27305.
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* If we accept that we can reduce the drag a certain %, simply by reducing the wake size by the same %, then we'd need a wake = 70.7318- sq-ft.
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* I chose to use the 'width' of the motorhome for streamlining.
* And I chose to use W.A.Mair's boat-tail profile of 1969.
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After a bit of noodling, it turned out that I would 'hit' 70.7176-sq-ft with a tail 63-inches in length. It 'burns' 7.687-inches off the top, two sides, and bottom OEM dimensions, yielding a new:
Height = 117.375-inches, and Width= 86.635-inches.
I used a photo-enlargement of Mair's boat tail, and just 'scaled' all the dimensions off of it.
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Ignoring the wheels, and working only with the motor home's main body cross-section, according to the 'experts', this is essentially all the research we need to accomplish.
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Identical 'ribs' 63-inches by 7.687-inches,with the Mair contour along the curved 'outer', spaced all around the original aft perimeter would define where the 'skin' would be attached. Lights and license plate would be moved onto the 'new' rear. And it could be side-hinged, allowing one to 'swing-away' the tail for whatever reasons.
If constructed like a commercial awning, with aluminum skeleton, and stretched-fabric 'skin', it's weight would be 'meaningless, as far as adding any rolling resistance.'