Class a motorhomes aerodynamics

[aerohead]

please see:
# 198 ( permalink )
last sentence, second blocked in section.

[sregord]

Originally Posted by aerohead

.... as long as care was taken with the wheel-wells, to prevent splash and spray broadcasting material onto the 'top' of the fabric.
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yes, splash or spray or any other debris...wouldn't that be the case with any bellypan ...coroplastic, ABS, marine plywood or aluminum.

[aerohead]

Absolutely !
And never, ever, drive through deep water, or shallower water where 'waves' from other vehicles could 'swamp' the belly, 'submerged' it.
Water is 833-X more dense than air, and moving in it at just a few miles per hour, will be enough to rip the whole thing loose from the underbody.

[j-c-c]

Originally Posted by aerohead

please see:
# 198 ( permalink )
last sentence, second blocked in section.

"care was taken" but a vehicle driven maybe 600 miles continuously in foul weather must be designed to prevent ANY collection of increasing water build up with a system subject to "ponding". "care was taken" does not seem to me to rise to the level required to prevent that occurrence, ie it sounds a bit casual.

[j-c-c]

Originally Posted by sregord

yes, splash or spray or any other debris...wouldn't that be the case with any bellypan ...coroplastic, ABS, marine plywood or aluminum.

The catch here is the "ponding" effect, something stiff or rigid materials are not as prone to like flexible fabrics are.

[aerohead]

Originally Posted by j-c-c

"care was taken" but a vehicle driven maybe 600 miles continuously in foul weather must be designed to prevent ANY collection of increasing water build up with a system subject to "ponding". "care was taken" does not seem to me to rise to the level required to prevent that occurrence, ie it sounds a bit casual.

I believe that sregord is aware of the ponding potential, and how important it will be to design for eliminating any chance that anything can ever get 'above' the pan.
If it can be maintained 'dry', then the ponding potential goes to zero.
I've not done an RV, but I've 'bellied' all my cars since 1980, and have never had a failure.
With mandatory annual vehicle inspections, I've always removed the belly pans, as a courtesy to the guys inspecting the exhaust system and anything else they needed to look at.
It's a good way to get a 'status report' on how a belly pan is performing.

[j-c-c]

I've lost track here, is your shared experience with fabrics/vinyl's or rigid materials regarding belly pans?
Not sure cars can ever collect enough water to have ponding a serious concern due to size, whereas an RV/trailer/semi its very possible, and with their much greater load capacity the driver might never sense any water build up is occurring prior to failure. It is also a catch 22 issue, in that the better one seals the space to keep out water, the better it is to trap it.

[aerohead]

Originally Posted by j-c-c

I've lost track here, is your shared experience with fabrics/vinyl's or rigid materials regarding belly pans?
Not sure cars can ever collect enough water to have ponding a serious concern due to size, whereas an RV/trailer/semi its very possible, and with their much greater load capacity the driver might never sense any water build up is occurring prior to failure. It is also a catch 22 issue, in that the better one seals the space to keep out water, the better it is to trap it.

I don't believe your question is germane to the topic.
I do have some experience with steel sheet, aluminum sheet, FRP sheet, fabrics, coated fabrics, elastic fabrics, Fiberglass, Kevlar, Pre-Preg Epoxy Carbon-fiber, Honeycomb, concrete fabrics, tapes, woven roving, mat, etc. along with formal instruction in automotive, marine, and aircraft upholstery, aeronautical composite training by Alexander Sport Air, @ Oshkosh, Wisconsin as an EAA member, advanced composites training at ABARIS, Reno, Nevada, 30-months around experimental aircraft at Edwards AFB, California and I've been aero-modding since 1974; on top of a B.S. in Mechanical Engineering Technology in 1980.

[j-c-c]

Originally Posted by aerohead

I don't believe your question is germane to the topic.
I do have some experience with steel sheet, aluminum sheet, FRP sheet, fabrics, coated fabrics, elastic fabrics, Fiberglass, Kevlar, Pre-Preg Epoxy Carbon-fiber, Honeycomb, concrete fabrics, tapes, woven roving, mat, etc. along with formal instruction in automotive, marine, and aircraft upholstery, aeronautical composite training by Alexander Sport Air, @ Oshkosh, Wisconsin as an EAA member, advanced composites training at ABARIS, Reno, Nevada, 30-months around experimental aircraft at Edwards AFB, California and I've been aero-modding since 1974; on top of a B.S. in Mechanical Engineering Technology in 1980.

So the answer is "yes" maybe?

[aerohead]

A client has a 55-foot Prevost motorhome.
* 'Chord' = 55' ( 16.764-meters )
* 'Wingspan' = 8.5' ( 2.59-meters )
* 'Aspect ratio' = 0.154
* 'Wing area' = 467.5 square-feet gross ( 43.432-sq-meters ) [ we can ignore the wheel house areas for simplicity ]
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A cubic-foot is 12-inches on a side, with 1728- cubic-inches volume.
A cubic-foot of 60-degree F water weighs 64.2-pounds.
There are 7.707- gallons in a cubic-foot of water.
Each gallon of that same water weighs essentially 8.33-pounds.
A one-inch 'slice' of a cubic foot of water weighs 5.35-pounds.
One square foot contains 144-square-inches.
One square-inch of water, one-inch 'deep' weighs 0.0578-pounds.
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The World War-II, Waco Aircraft Co. CG - 4G 'Hadrian' assault glider, ( featured in 'Saving Private Ryan )' is cloth-covered, reaching up to 150-mph ( 241.4 km/h ) while towed behind the Douglas C-47, 'Skytrain', then glides to it's target landing zone at around 75-mph ( 120.7 km/h ).
The cloth-covered wings have a design wing loading of 8.33-pounds per square-foot.
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A 'cloth'-skinned belly pan, built to the specification of the Hadrian's wing would support 8.33-pounds per square-foot, at normal loading.
8.33-pounds per square-foot requires a live-load of 1.557-inches of standing water, netting 3,894-pounds of water on top of the pan.
If the Hadrian is designed to a 4g safety factor, positive and negative, then specifications would allow a survivable depth of 6.22-inches of standing water on 'top' of the belly's membrane, still short of structural failure, be it the membrane, ribs, stringers, or 'wing spar'; yielding a live-load of 15,577-pounds of standing water without failure.
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I'm not advocating for allowing 'any' water to make its way onto the top of any belly pan, regardless of materials chosen.
The wheel-houses and inner fender wells will be the focus, as this is where splash and spray originate.
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Most homes and apartments I've experienced have 'ceilings'.
And they're typically constructed with paper-sheathed gypsum board ( Sheetrock' if you will ).
And even though their complete and total destruction is just one, leaking roof away, we continue to construct them, while inspecting, and maintaining roof integrity.
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Seems like a stretched-fabric membrane, securely fastened to it's supporting structure, could function reasonably well as a belly pan, as long as its weaknesses are respected in the 'build.'