Aero Efficiency of Motorhome/RV vs Car & Caravan

[BamZipPow]

Originally Posted by s_t

Thanks, hadn't seen that before. Size is similar to what I'm looking at (mine slightly smaller perhaps). It's Cd is less than 0.3 according to Welcome to Vixen 21 Motorcoach Website. Makes me wonder how much the teardrop shape on the roof profile, that I was planning for aerodynamics, would improve it - if at all??? Also found a link to the NASA study Inter Action which indicated good results by just by adding a boattail.

That roof profile that I sketched ended up being pretty close to the profile discussed http://ecomodder.com/forum/showthrea...-c-9287-1.html, but drops slightly more steeply near the end. This shape would be 'less usable' during motorhome use - simple square provides more interior space and would be easier to make too. Has anyone seen a motorhome following a more optimised profile along these lines?


Will check that thread out. Sounds like similar efficiency is possible either way - one piece motorhome or car pulling trailer, but perhaps the trailer option requires a little more optimisation. Look forward to seeing the wind tunnel results.

http://ecomodder.com/forum/showthrea...ler-26997.html

[BamZipPow]

I fergot to ask...how many people does this need to support? A single wheel setup may not be able to fit yer needs even if you load a teardrop trailer on there with some compromises.

[s_t]

Originally Posted by kach22i

A very large old thread on this topic.

http://ecomodder.com/forum/showthrea...lly-13149.html

Thanks, that's a good read! Still to get through it all.

Originally Posted by Hersbird

........What is the gas vs diesel cost at the pump in AU? Around here it's pretty close but some places they are different.

I just saw the V80 you posted, that will definitely beat a forester towing. What about a custom 5th wheel, popup camper pulled by that V80. That would tow like a dream, have lots of room, and be able to be dropped at a location while the cab is used by itself. A 5th wheel can have much closer gaps as well if designed right. I wish we had something like that here. The Fiat with the 3.0 VM Motori v6 diesel is about the closest (and I think it may not even be available anymore). A stripped down cab and chassis is about $35,000 US while a new Forester is just over $20,000.

Diesel is generally slightly more expensive I think, but not much in it (don't have a diesel at the moment, so haven't been paying attention to pump prices). Would only go the V80 to build a motorhome on the back, or possible tow a large 5th wheeler. Would want a caravan to be pulled by a SUV type vehicle.

Originally Posted by freebeard

The retractable tongue may be a bad idea. It might be better to have a retractable nose that moves back and forth on the tongue.

A retractable tongue would certainly need careful and detailed engineering to ensure it is safe. A movable nose is a good suggestion that would likely be easier to implement safely.

Originally Posted by Jez77

From a practical point of view I'd stick to the trailer idea.

RVs can only be used while your on holidays where the Suv cab be used every day.
Many interesting places in OZ are difficult to get to with our skinny roads and parking a nightmare in an RV. Van can just be unhitched and left a campsite.
Ducking into town for food or beer means packing up entire campsite in an RV, van can just stay put.

Personally I've chosen the motel path for most my trips now, I've worked out for me with the purchase cost, maintainence, rego, insurance and extra fuel of a van just doesn't make sense and apps like Wotif means you can find a motel on the fly nowadays.

Motels
Sure, financially you may be correct. But I'd much prefer to go to bed in this setting:



or wake up to something like this:



Yes, caravans do have benefits, but so do motorhomes. When noise/disturbances across the road at 11pm has you wishing you'd chosen another spot to spend the night, just walk through to the drivers seat and drive away to another spot. Did this a few times during our around Australia trip. In general, motorhomes are quicker and easier to set up for the night, and get going for the morning. So better for shorter stays. Caravans, as you suggest, have benefits for longer stays.

Originally Posted by BamZipPow

http://ecomodder.com/forum/showthrea...ler-26997.html

Ah, thanks, but too small! There are currently 4 of us, but the two boys are now teenagers so I'm designing this thing just around the wife and I. But we will do long term extended travelling in it, so has to have some creature comforts (composting toilet, queen sized bed, fridge etc) It's a long term project - will be at least 5 years till this thing is ready to go...

What I haven't detailed yet are my hair brained plans re electric and solar. I figure I can fit 2kW of solar panels onto the roof of a motorhome/caravan of the size I'm looking at, as well as another 2 - 4 kW tucked away or underneath them (eg. slide-out arrangement). The latest flexible panels using polycarbonate rather than glass are thin and lightweight (though less scratch resistant, and lower UV resistance).

If I can get an efficient enough vehicle arrangement, along with say 30kWh of lithium, this is enough to get me travelling at least 100-150km per day on solar power alone (most days!).

So when I'm reading through these aerodynamic discussions, I'm considering not just the aero side, but also the impact on solar panel mounting and output.

This is planning for a long term relaxed travelling lifestyle, that is completely free of typical constraints re fuel stations and the like. A slow meander through the Aussie outback, wherever we feel like going.

Sure, it's probably going to cost me somewhere around $60-90k to do this, as well as a fair bit of my labour, but people routinely spend more than that for equivalent gas guzzling machines (or just an ordinary caravan). And I love the engineering challenges associated with building something myself - something that will most definitely be unique

[Hersbird]

Originally Posted by s_t

Yes, caravans do have benefits, but so do motorhomes. When noise/disturbances across the road at 11pm has you wishing you'd chosen another spot to spend the night, just walk through to the drivers seat and drive away to another spot. Did this a few times during our around Australia trip. In general, motorhomes are quicker and easier to set up for the night, and get going for the morning. So better for shorter stays. Caravans, as you suggest, have benefits for longer stays.

We used to have a slide in camper that goes in the bed of a long bed full size pickup. A motorhome would be even easier to get on the move, but we did love that. My big plus was my wife and daughter are slow to get moving in the morning. A few times in Yellowstone park I would just up and start driving while they were still in pajamas sipping coffee in back. Then stop and get breakfast 30 mins later at a new spot, then again with lunch, etc. Sure there are $10 crappy burgers here and there in Yellowstone but only where the crowds are. Last trip we had the camper, nicer to sleep in, but we were on the road later every morning and usually stopping back by at some point during the day at least once. Not as good as having everything on your back, but it was easier to go more places without the heavy camper in the bed.

[freebeard]

Originally Posted by s_t

(composting toilet, queen sized bed, fridge etc)

What I haven't detailed yet are my hair brained plans re electric and solar...

So when I'm reading through these aerodynamic discussions, I'm considering not just the aero side, but also the impact on solar panel mounting and output.

Sure, it's probably going to cost me somewhere around $60-90k to do this, as well as a fair bit of my labour, but people routinely spend more than that for equivalent gas guzzling machines (or just an ordinary caravan). And I love the engineering challenges associated with building something myself - something that will most definitely be unique

(definitive design criteria)

Here is the motorhome design I showed earlier as a class-C.



While my donor chassis of choice was the Beetle, this could easily be a Sprinter van. The defining characteristic would be a second door frame cut into a wedge and added to the existing frame (aligned on the hinge line) so the door seals to it. This eliminates the dogleg most Class-Cs have.

You'll notice the geodesic layout can be read as diamonds, hexagons or triangles. The diamond layout define bands that run around the body. If the altitude of the diamond equals the width of your flexible solar panel they would integrate well.

Instead of folding solar panels, perhaps lightweight reflectors that would concentrate on the [minimal] panels?

[s_t]

Originally Posted by Hersbird

We used to have a slide in camper that goes in the bed of a long bed full size pickup. A motorhome would be even easier to get on the move, but we did love that. My big plus was my wife and daughter are slow to get moving in the morning. A few times in Yellowstone park I would just up and start driving while they were still in pajamas sipping coffee in back. Then stop and get breakfast 30 mins later at a new spot, then again with lunch, etc. Sure there are $10 crappy burgers here and there in Yellowstone but only where the crowds are. Last trip we had the camper, nicer to sleep in, but we were on the road later every morning and usually stopping back by at some point during the day at least once. Not as good as having everything on your back, but it was easier to go more places without the heavy camper in the bed.

Sounds like some nice holidaying

Originally Posted by freebeard

(definitive design criteria)

Here is the motorhome design I showed earlier as a class-C.



While my donor chassis of choice was the Beetle, this could easily be a Sprinter van. The defining characteristic would be a second door frame cut into a wedge and added to the existing frame (aligned on the hinge line) so the door seals to it. This eliminates the dogleg most Class-Cs have.

You'll notice the geodesic layout can be read as diamonds, hexagons or triangles. The diamond layout define bands that run around the body. If the altitude of the diamond equals the width of your flexible solar panel they would integrate well.

Instead of folding solar panels, perhaps lightweight reflectors that would concentrate on the [minimal] panels?

Hadn't thought of creating a second door frame, that angles the door out gently! If it weren't for the window, it would be easier to just put an outer skin over existing door, tapering in thickness. But if I can get enough living area (in plan view) from longer length, then might not need to widen the body beyond the existing cab/vehicle width (lower frontal area).

Understand re the bands. The best is never easy is it - why can't the optimal shape be a rectangular prism though even that isn't too bad with some relatively simple mods according to the Cd figures in that NASA Dryden truck study!

[s_t]

from: http://ecomodder.com/forum/showthrea...c-9287-67.html

Originally Posted by freebeard

Why not go with the curved header and then a Tropfenwagen style rear with a flat top and tapered sides? It would have more interior space.

This design would give more floor space in which we could stand up. Perhaps greater volume too. But less floor area, which is a disadvantage in fitting everything in.

However, I want to spend some time looking at the possibilities a bit more closely. Maintaining max height has some advantages I might be able to utilise (eg. Murphy bed, or bed that stores up flat against the ceiling). You've also got me thinking about possibilities relating to designs where the rear vertical walls rotate outwards to 'open up' the rear of the motorhome when camped (to create close to a square rear). I've got some questions about the aero effect of keeping the roof and floor in a rectangular shape (to the back), extending out past the sides which are tapering in at the back, but I'll have to sketch it to explain.

[s_t]

I found the following presentation useful in reinforcing some of the concepts I've read on this forum (though got a bit lost in the last quarter or so).

http://www.bakker.org/dartmouth06/engs150/11-bl.pdf

In other threads I've seen comments about achieve better milage figures after a clean/detail. How sensitive is turbulence and separation to surface finish, in objects of car/motorhome size? Roughly what sized Reynolds number is involved?

Whatever I build is going to be 'homebuilt', and I haven't been planning on having to get it mirror smooth! Motorhomes/caravans have windows on the sides, which involves surface irregularities of a few mm. I'm planning polycarbonate sealed solar panels on the top, which are of about 3mm thickness (1/8"). I was just planning on gluing them on as is, leaving the 3mm vertical edge around each panel.

Will these kind of surface imperfections have no/little effect, or be beneficially tripping the boundary layer and causing minor turbulence that assists laminar flow beyond that (like a golf ball, or slide 20 in that presentation link above), or just destroy laminar flow and mess up aero properties?

[freebeard]

Even a Bonneville Salt Flats where there are smooth shiny finishes they still use duct tape as needed.

With a 1/8" step, maybe a bead of caulk to fillet the edge?

[aerohead]

Originally Posted by s_t

I found the following presentation useful in reinforcing some of the concepts I've read on this forum (though got a bit lost in the last quarter or so).

http://www.bakker.org/dartmouth06/engs150/11-bl.pdf

In other threads I've seen comments about achieve better milage figures after a clean/detail. How sensitive is turbulence and separation to surface finish, in objects of car/motorhome size? Roughly what sized Reynolds number is involved?

Whatever I build is going to be 'homebuilt', and I haven't been planning on having to get it mirror smooth! Motorhomes/caravans have windows on the sides, which involves surface irregularities of a few mm. I'm planning polycarbonate sealed solar panels on the top, which are of about 3mm thickness (1/8"). I was just planning on gluing them on as is, leaving the 3mm vertical edge around each panel.

Will these kind of surface imperfections have no/little effect, or be beneficially tripping the boundary layer and causing minor turbulence that assists laminar flow beyond that (like a golf ball, or slide 20 in that presentation link above), or just destroy laminar flow and mess up aero properties?

The mechanism which rules the transition from a laminar,to,turbulent boundary layer,occurs at a scale smaller than the surface roughness of glossy paint.
It's called 'critical surface roughness' and there's nothing to be done about it.
Even on a Messerschmitt fighter aircraft,the difference between a polished surface and flat paint makes no difference to performance.
On a laminar airfoil it would,but only at flight conditions,at very high distance from the ground,in calm air.
You won't have any laminar boundary layer on your vehicle,and that's actually a good thing.
Minor surface defects may be embedded within the thickness of the boundary layer,not affecting the laminar inviscid flow outside the boundary layer.
For the hard edge on the PVs,you could soften that with caulk.