Class a motorhomes aerodynamics

[sregord]

Hi,
Been busy, at work, preparing for retirement.

Updates:
I have an UltraGauge all configured, so I hope to get a "baseline" on the camping trip later this month.

I did the overlay of "the template" on Moby...Great Tool-BTW... ummm... it looks like I would best be building a trailer.
Starting to look like the dirigible/airship you are referring to.
... a good place for an ATV &/or kayaks

[aerohead]

Shooting from the hip, my thoughts go to a either a superlight truncated boat-tail, or super-light, full-tail, one-wheel trailer, attached as Burt Munro's bumper-mounted 1920 Indian Streamliner, with zero-rake at the front fork pivot, and connected at the axle shaft 'pivot.'
My second trailer was done this way, except if had a bit of rake, which caused it to 'lean' during turns.
Since Moby is probably 'taller' than it is 'wide', you'd use its width as your reference for the streamlining, rather than the roofline.
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An aluminum skeleton for either, could be fabricated at a commercial awning company.
The stretched-skin covering, likewise. They use hypalon-impregnated aerospace fabric like GOODYEAR used on their blimps. It's UV stabilized and will last for decades.
It would be like an early doped- fabric-covered tail-dragger aircraft. Extremely light for it's 'visual' mass.
I have the rear wheel, swing-arm, hydraulic disc brake, and mono-shock suspension from a HONDA Hawk sport bike I'll be using for my next trailer. The trailer would be so light that trailer-brakes wouldn't really be necessary.
The tires are radial, DOT rated, and already have the load rating necessary for a superlite. Mine's a Dunlop, already speed balanced.
With drop-down 'pogos' ( Lockheed U-2 Dragon Lady ) either the boat-tail or trailer can be handled by one person at a campsite or RV pull-in.
The boat-tail could be double-purposed as an auxiliary sun-shade awning, or 'tent.' Same for the trailer body.
The trailer COULD carry light, fluffy gear, like sleeping bags, pillows, and bedding, but it's prime purpose would be fooling the air into thing that Moby is a SMART car.
Just some thoughts.

[sregord]

I like your "from the hip" thoughts. A single wheel "tail"(ala Burt Munro) definitely could add a dimension to the Moby theme. Your careful nature(scaredy-cat) offers me confidence in bouncing ideas off ALL this group.
Last thing I want is for anything to fall off Moby at 55mph.
I will control my view of the iRV2 Forum on aerodynamics. drafting semis? yikes.

A boat-tail: Would a streamlined/narrowing extended "kammback" along the top w/long triangular "gussets" to the bottom of each side, be the reasonable starting point...to provide hinge points for the boatail sides to fold under? or support a flexible gap filler for a trailer?

A trailer option: I know other MH owners tow another car or enclosed trailer behind them. Some are sooo long, I think they're just showing off. A not too heavy single axle "GolfCart" trailer (10-12') was what came to my mind, with a kind of clam shell hinged enclosure for the just right (Bezier curve) shape. The challenge is the gap between the trailer & Moby. correct?

Moby is 11' tall & 8+' wide, "use its width as your reference for the streamlining, rather than the roofline". So the truncated end would be short- vertically, wide- horizontally? or tall- vertically, narrow horizontally? or proportionally rectangular?...lower vertically than center.

[aerohead]

https://ecomodder.com/forum/member-a...ing-effect.jpghttps://ecomodder.com/forum/member-a...g-rig-aero.jpghttps://ecomodder.com/forum/member-a...ilkisons-t.jpgAn inflated structure can serve as both a tail, or a gap-filler between Moby and a trailer. According to NASA, this alone is good for a 16% drag reduction.
Here's a UK product which never made it to the USA.
https://ecomodder.com/forum/member-a...l-trailers.jpg
Here's my first inflatable, which helped push SPIRIT to 32.5-mpg @ 65mph.
How you do your attachment is going to decide the engineering.
AeroStealth has a Prowler 5th-Wheel we're wanting to boat-tail. We think that rather than drill into the trailer, it will rest on, and be clamped to the rear bumper, with tension members running forwards over the roof, using the roof curb of the AC unit as a cinch point to hold the tail snug. ( no holes).
This arrangement also allows us to 'picture-frame' the 'mount' if we like, hinging the tail on one side, which, when swung away, would allow access to the roof ladder.
On the 8' dimension, W.A. Mair's boat tail contour, sides and top would be a known quantity. NASA's tail is similar. As mine.

[freebeard]

The sides 'want' the same taper but there're two of them. Think Tropfewagen.



All taper in elevation and none in plan gets you a teardrop trailer.

I propose a single axle trailer with a castering wheelie bar. It could fold flat for parking.

[sregord]

Interesting, I would have thought this would be more effected by crosswinds.
More shark like than whale
This certainly makes for more stack-able room in a tall trailer. or easier to construct foldable boattail.

[sregord]

Aerohead..the Mair drawing/graph shows a proper (truncated) boattail of a "torpedo/bullet", but we're only trying to duplicate the top half ... correct?

The attached had me going in a different direction.
Now I'm thinking rotate 90*?

[freebeard]

I gave it my best shot at Permalink #12

You even thanked me on (05-23-2022).

[aerohead]

Originally Posted by sregord

Aerohead..the Mair drawing/graph shows a proper (truncated) boattail of a "torpedo/bullet", but we're only trying to duplicate the top half ... correct?

The attached had me going in a different direction.
Now I'm thinking rotate 90*?

Exactly! Mair's model was a long, body of revolution. It CAN be morphed into a square or rectangular section ,as ASRAE engineers do with ductwork ( square-to-round, and visa versa ) as long as the sectional density is maintained per unit length.
The underbody flow has very little energy and cannot withstand a sudden change in cross-section like above it. For the best aero performance we'd want a diffuser. That would require full belly panning upstream.
Tractor trailers may be constrained by weird, DOT regulations about DOT bumpers. Don't know. I believe that we're not held to the same metrics. Personally, if I were going to rear end an eighteen-wheeler, I'd prefer to drive into an 'air bag' rather than a steel jungle-gym.
In the photos of Orbywan's MH, you can see how he modified his diffuser over time, to 'relax' its angle.
The 10-degree SAE 'departure' angle, up from the back of the rear tires ,does need to be respected or there'll be a ground-strike.
With Moby's long overhang, you may already have to ascend ramps at some oblique angle.
Your attached thumbnail photograph depicts a tail which would in all probability, suffer separation like NASA's. In plan-view and side elevation. Bottom for sure.
The thing would be to transition inwards and downwards to the 'safe' 22-degree downslope, and then, just hold a constant 22-degrees from there, for as long as your willing to go, knowing that there's a high confidence for fully-attached flow.
Simultaneously, your 'square' roof/sides corner intersection would begin to morph to a gradually-increasing corner radius.
General Motors 'Optimum' tail had this feature. ( 'Tailwind Project', Texas Tech University, circa 1994, Professor Carver and Scott Funderburk ).
This kills any propensity for vortex formation ( one of the highest sources of drag ).

[sregord]

Freebeard, yes you did… ...you’ve discovered my 2>3D side not kicking in.
I remember looking at your boattail and thinking taillights, rearview camera & ladder. Still am, but if the top of the boattail can be a less steep angle...er tangent... down. I think I need to build a scale model as you did.
Attaching anything to Moby without drilling holes ...at the bumper?
Actually your “scalloped” edge has me thinking. Attaching using the screw shear in the side of Moby, not through the back ie, pulling straight out on the screw. Its all fiberglass skinned, with a steel frame in there somewhere.

Aerohead, I know I was the one with the inflatable youtube...but the already bad experience with the awning being ripped off in a crosswind & plus Moby’s ladder location has had me scratching my head with the inflatable boattail... concerned about adding a parachute to the back. But you’re correct, its much more forgiving than “jungle gym”(trailer-Tail, but not Orbywan's?). What ever it is, I need to keep it light & strong, flexible & forgiving.

BTW: Moby’s long overhang is 9 feet ...from aft of rear wheel to bumper end there is a 5” rise. -Diffuser angle 2.65*
..there is 21" ground clearance at trailer hitch - Departure angle 11*