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Old 12-07-2016, 05:23 PM   #1 (permalink)
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40ft bus into RV aero

Alrighty everyone, I'm saving up to start converting a rear engine school bus into a full-time RV. We plan to do quite a bit of highway travelling, and aerodynamics come into play quite seriously for the brick shaped things both in terms of fuel economy and in speed given the usually sub 200hp engines pushing 25,000+ lbs of steel. I'm already eyeballing a rear extension similar to semi-truck trailer "tails", a smooth underbody (with storage!!) and wheel covers. What else you got? The bus will be 40ft long, the limit for an RV is 45ft, so we have a few feet to play with. Maybe something to smooth up the big flat face?

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Old 12-07-2016, 05:35 PM   #2 (permalink)
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Have you considered sloping the roof down before it gets to the back (sides too, if possible)? The longer you go, the more difficult it'll be to maneuver and to find parking. A lot of RV park and even forest service type sites have a length limit. Going with a collapsible semi trailer tail would definitely help with that issue.
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Old 12-07-2016, 06:16 PM   #3 (permalink)
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There's an old, slightly dirty joke:

Q: Why are turds pointy on the end?
A: So your butt doesn't slam shut.

The atmosphere doesn't like slamming shut. Any taper you can add to the tail is good. Since you're planning adding tail length, plan ahead to taper it. If you can extend that tapering further forward then you can make even more of a difference - the goal is to reduce as much of the absolute tail's cross section as possible, while still adhering to the ideal aero template (find it in the Aerodynamics sticky). Don't forget you can taper in the sides as well as the roof. You give up storage capacity but pick up a smoother, quieter ride and lots of miles per tank.

As to the nose, increasing the radii of the corners is good. It doesn't have to be structural and you may be able to make that take the form of new rear view mirrors, or simple add-ons at the front corners below the level of the windshield.

With the rear engine you can do all kinds of smooth underbody, which you already have on your do list. That will yield big dividends.

Big, big project. When you start your build thread, I'll be subbing that.
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Old 12-08-2016, 02:57 AM   #4 (permalink)
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Welcome to Ecomodder.
Quote:
I'm saving up to start converting a rear engine school bus into a full-time RV. We plan to do quite a bit of highway travelling...
What else you got? Bluebird pusher, or what? National campgrounds or art fairs? Number of people X number of days? First thing I would do is put it on a scale and confirm that '25,000+ lbs [12 1/2 tons] of steel'. For issues of wheel alignment or sizing water tanks seek out a member named slowmover.

On a vehicle that size, I'd want a chipper/shredder/alchol still for fuel and a 'cherry picker' for extravehicluar activities or to hold up a tent. Skylights and solar panels?

Rather than the external taper that is popular here, look to the annular ring and wheel well treatment on Dr. Morelli's urban car. You have a very favorable fineness ratio — 5:1 in plan and 4:1 in elevation! There is enough length for reattachment so you have clean air into whatever you choose to do in the rear. Also search for Mair and Englar.

Also, commercial box vans will be getting plasma actuators. A ring at the back at least but any edge or wheelwell is a candidate.

Your vehicle could benefit from many of these techniques from trucking:



Note the passive porosity in lower left.
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Old 12-08-2016, 12:11 PM   #5 (permalink)
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The bus for this project hasnt been purchased yet. I'm currently in an '84 International Amtram 35 footer, but with the new lady looking to move in next year it's gonna get tighter. Looking to get into national parks and various fairs and festivals, fulltiming in it.

Sloping the roof: I was planning to likely have to raise the roof of the new bus at least a foot, as most buses are too short for my 6'1" head to fit under, and I will be adding insulation. But to add a slope, I've never seen that done on the skoolie forums. At first glance, that looks exceedingly difficult to pull off. And I want to cover the entire roof in solar panels with a "raise and tilt" mechanism, which would be harder with a sloped roof. I do plan to have fairings around the panels when in the stowed position.

Plasma actuators?!?! Looks a bit far into the future, and having actually built HV "plasma" brown-effect lifter/fliers before, I can tell you the power consumption to move a small amount of air is pretty high. It took 25,000 volts at several watts to lift less than 3 grams.

Weight: My current 35ft bus completely stripped down was estimated at 16-18,000lbs. Its gvwr is 32k. I havent put it to a scale yet, but these are typical numbers for this size bus. I have plenty of reason to expect Bus 2.0 to be up to 25k given it's longer length and all amenities fully installed, including a massive 48v battery bank and hundreds of gallons of water for long term boondocking.

Occupancy time: 2-3 people, "as long as possible" I'm seriously considering an incinerating toilet running on the solar to eliminate the blackwater tank to make more room for gray and fresh water for example.

Got a link to the Morelli car? Google isnt being forthcoming for me today.

Chipper/shredder still? Wouldnt that be alcohol fuel? I dont think my diesel engine will like that much.
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Old 12-08-2016, 03:27 PM   #6 (permalink)
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My 19ft Cortez weighed 7000lb. I compared to twice that, but the Cortez was the 'sports car' of motorhomes and didn't have a battery bank.

I wouldn't slope the roof, except for maybe the last 5-10ft. Instead of messing with the roof, I'd add one foot through the side window area and over the top of the windshield.

Forum member PranayBajjuri only posted twice but he said:
Quote:
Power needed is very low. Our preliminary testing shows an average power consumption of 1 watt for 1 meter long actuator .
http://ecomodder.com/forum/showthrea...tml#post511046

It's not trying to lift the vehicle, it's an ionic wind — fast moving ionized molecules drag adjacent air along with them.

I defer to slowmover on extended occupancy (we both live in our 35ft trailers, but mine is up on blocks and skirted), but instead of an incinerator, consider a 55-gallon drum batch composting system that swaps out through an external hatch.

The chipper/shredder/still was a fantasy I had about parking a conventional cab bus on BLM land and cooking down wood slash. It would have had the chipper at the front bumper running off the engine with the chip blown through a 4" pipe into a bin on the rear roof. The (18" dia., 7ft tall) still would be in the left rear corner with the composting toilet in the right rear and a new rear wall 5ft ahead of the original rear door.

What do you think of using a cherry picker as a tent pole. If it would reach 12ft to the side you could have a 24ft diameter 'big top' tent with a ring of wall poles.

Morelli:

Quote:
In 1999 Morelli & Di Giusto modified a Fiat Punto, adding a Kammback and blower-type rear wheels, calling it the Fluid Tail:

Images of MetroMPG, Basjoos and others come to mind. The duct tape is familiar to all of us. Note that their Kammback has a clear plastic partial end cap. Don’t ask me how they opened the hatch, though.
The rear wheel rims were designed as radial blower fans, pumping air into the area behind the wheel, acting as a sort of air-pillow wheel fairing, as I understand it. Note also the Audi A2-like wheel well bulge (what for?).
Here’s their patent text: An improvement to devices for reducing drag in vehicles - Politecnico Di, Torino Dipartimento Di Energetica .

More tail modifications:

Compared to the standard blunt end (a), adding what they call attikas (fascias or parapets; flat, parallel panels) (b) delays separation and moves the tail vortices back (reduces Cd by up to 10%); these can be set inward (c), further reducing the size of the „dead zone“ (reduces Cd by 0.06, which is about half as good as the ideal tapered tail); the open-ended Kammback made of flat panels (d); and „Fluid Tail" (e).
Before you go out and get a bus, watch the castle truck video:


He started with a flatbed truck with 8ft ram jacks at all four corners, and added pop-outs, slide outs and swing-outs. The cherry on top is the fruit drying rack in the 2nd floor ceiling.
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Old 12-08-2016, 04:02 PM   #7 (permalink)
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Not sure what you mean about adding a foot through the side window area...

Power consumption of 1 watt to move that much air still sounds suspect to me. My point about the flyer was it took much power just to move enough air to affect a little bit of balsa wood and tin foil. I can blow harder through a straw that that 25,000 volt device could blow, so I find it difficult to believe it can significant alter the course of a gail force wind on a vehicle. But I guess we'll see if it's ever actually installed on a truck

Part of my motivation for the incinerator vs composting was, if I dont have a permanant base location or somewhere to dump the partially composted waste, what do I do with it? That said, at the end of the day, the price might get me.

What kind of cherry picker?? Seems there are many things called that these days...
Personally, I was considering having a hinged cover, or another layer of solar panels, over the rooftop solar that folded out to form an awning over one side for extended shelter.

So the Morelli "annular ring" you're talking about is a modified Kamm Back eh?
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Old 12-09-2016, 03:18 AM   #8 (permalink)
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One foot was your idea. Take out all the windows, cut the posts alternately at the top and bottom and raise the roof the height of the original window opening. Move it one window forward or back, or split it and move half each way and put a hatch in the middle. Connect the posts and stack pairs of windows. You'd have to panel in half the window openings with something else.

The windows in a school bus are too low when your standing anyway.
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All the plasma device has to do is speed up the air locally, right at the surface, inhibiting separation. Then the air further away doesn't have to move as much; sort of like judo. If I understand correctly. One watt per meter sounds low to me too; but what do I know. At 25Kv that's milli-amperes.
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I take your point on the composting toilet. Two 30-gallon drums? Ship them home via Fed-Ex?
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By cherry picker, I meant a utility bucket. Except I'd want to replace the bucket with a lounge chair with a little umbrella. You could run it up inside an Apple tree and sit there to read books.

Or place the 'forearm' parallel to the ground and throw a tarp over it to make a tent.

Or use it as an engine hoist.
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The solar panels are expensive but you could have an aluminized shutter that you could open to reflect more sunlight onto the panels, or as an awning. Split down the middle or hinged/latched from either side.
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It's more like the Kamm-back is an incomplete Morreli 'fluid tail'. Kamm, Morelli, Hucho and Jaray are all authoritative. Look to aerohead's posts. Hucho talked about retractable tails. Here's my own attempt:

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Old 12-09-2016, 08:42 AM   #9 (permalink)
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Alternate-cutting the window posts is a brilliant idea. Cut the roof straight across right behind the forward cap and just ahead of the aft cap. Shift the whole roof upward and aft one window width. Cut out one window width worth of the roof at the rear and move that to the front, weld it back in at the front and reattach the front and rear caps in their appropriate places. That makes the cutting and welding all in straight lines around wide radii, about as uncomplicated a major body job as you could hope for. Of course you'd still have to come up with ways to close up the gaps that project would open up.

Maybe it would just be easier to cut the whole roof, jack it up and then weld in a band.

As I recall those windows are significantly more than a foot high, so you'd pick up quite a lot of height that way, more like about 18".

That's just excellent. That's an awesome idea.
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Old 12-09-2016, 10:10 AM   #10 (permalink)
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re: the roof raise, the way 90% of them are done on skoolies is to weld jacks to each corner, remove windows, make cuts on all the "hat channel" frame members that run beside the windows, then jack it up to desired height, level, and weld new iron in the gaps. The bus windows generally are leaky noisey things so most attach sheet metal over the gap then install a few RV style windows.

Bucket lift, LOL, sounds pricey on it's own, how would I mount it to the bus so it didnt severely disturb the aero? :P

Aluminum reflectors for solar: While that would technically work and be cheaper, the higher operating temperatures that would result in the panels would both reduce their efficiency and their calendar lifespan. Would need liquid cooled panels

Something else that bears mentioning aero-wise: I'll likely be towing a little Honda Insight behind the bus as our run-about vehicle.


Last edited by dalez; 12-09-2016 at 10:37 AM..
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