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Van #2 2021 EXT
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So, I thought about it a bit. I'm pretty dense, but I finally understand that minimizing frontal area, with streamlined nose and tail is much better than a front slope/dam to keep air from getting underneath the roof clutter. To that end I am
  1. proposing to take my trusty cordless DeWalt circular saw to my cross bars and reduce the rear from 3" to 1.4" to match the solar panels and the front to 1". I cannot shift the x-bars as they directly transfer the moment load from the canopy arms,
  2. Relocate the PV panels on top of the uni-strut and snug them up to the rear x-bar
  3. Obtain some sheet aluminum and fabricate an aerodynamic shell for the front bar and a nose for the rear PV panels leading edge. I'll use construction adhesive to glue the sheets to the bottom of the xbar, then hand form the remaining shape, perhaps use pop-rivets for the tails, or some vhb tape. There will be some experimenting before the final assembly.
  4. I dunno what I'll do with the flood lights. If I move them, I'll need to patch the roof. Should be easy enough with some RTV and a metal disk on either side of the hole.
  5. Maybe skin the bottoms of the PV. Awaiting a response to #58's question!
The question I have ( @GaryBIS and @RV8R?) is what weight and alloy aluminum? I don't have sheet metal tools so I'll probably get on-line metals to cut the strips, then do the initial fold on my universal metal brake (the table saw edge) and hand form the rest. So, .020? .040? I think the latter might be too stiff for my old hands, the former too delicate for all the tree branches that seem to regularly swoop down and attack my roof.
View attachment 86730
I suppose I could just rip some 1" and 1-1/2" PVC pipe to make a semi-aerodynamic nose/tail and glue them down.
Hi @larry barello

If it were me, I like to pick the low hanging fruit. I do not have a wind tunnel & am relying upon my experience in aviation & general science.

@GaryBIS has done testing & might be able to provide better than my gut feelings but here they are;

1) Biggest bank for the buck is reduction of frontal area
2) 2nd biggest is disturbing the laminar flow as best as possible
3) The Leading Edge is a big item
4) The Trailing Edge may not be worth it
5) Bottom skin the solar panel ,, yup some benefit, but how much I do not know.
6) You want air flow over the van’s roof & do not want to create lift (positive or negative) with the solar panels. This is nearly impossible in my mind.

I have worked with aircraft aluminum of several types of alloy


I would not necessarily buy aircraft aluminum or even aluminum. You might get a good price at a roofing shop & the roofing companies I deal with can brake sheet metal with a computerized brake that is phenomenal these days.

I think 0.020 is thick enough if fastened close enough.

Even a 1/2 pipe would be quite effective.

Here is a link to one of the Experimental Planes I have built. Might give you some ideas. “Van’s Aircraft designed “pressure recovery wheel pants” & they plus the gear leg fairings really work (increased cruise speed about 20 miles per hour from naked undercarriage);

 

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Thanks @RV8R for your thoughts. I'll start with minimizing frontal area and adding simple noses.
I believe that will be your easiest biggest bang for your buck / labour.

Some of the rest might not be worth the hassle/costs.

The actual “testing” & mileage recording can be quite tricky with many hard to uncontrollable / account for variables (dialing out the variables). Especially if only measuring small increases.

On A Footnote;

I find Van Building much like Aircraft Design in this manner ,,, It just seems to me to be a series of compromises 😁. I would have solar in a heartbeat on my roof if it were not for the mileage penalty & potential heat gain in the summer when traveling south. We have purchased 2 159” EXTs now both “White” where we are both attracted to the dark “Granite Colour”. Both examples of compromises of an extremely long compromise list. 🙃
 

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2014, 138WB, High Roof, Gas, SW MT
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Discussion Starter · #64 · (Edited)
Hi,
A couple more thoughts and a little more data on roof racks.

Again, the two things you can control to reduce drag of a rack or rack elements are frontal area and drag coefficient - things that reduce these will reduce drag.

This is a video made by Yakima that gives a little data on drag for various shaped rack cross bars

This is a still from the video...
Font Rectangle Slope Parallel Screenshot


It shows the drag force on various cross bars at 62 MPH.
There is a 7X difference between the best and worst of the bars, AND the worst bar is far from the worse possible case in that it is still and "Aero" bar - a rectangular bar would likely be quite a bit worse.

These bars have roughly the same frontal area, so what we are seeing in the 7X reduction is the effect of lowering drag coefficient.

This lines up with what you get if you look up drag coefficients for various shapes - rectangles are over 1 and well streamlined shapes are down around 0.05 Cd - showing a potential for as much as a 20X improvement.

So, it looks like there can be a very good payoff for using low drag shapes for crossbars.
Also keeping the height of the crossbars as low as possible reduces frontal area.

There are outfits that sell elliptical metal tubing - someone posted a link a while back.
Another option might be to use the crossbars from a commercial rack - something like this Yakima low drag cross bar that is available up to 80 inches long.

Of course, eliminating the cross bars altogether and attaching the PV panels to the roof with small angle brackets that have almost no frontal area would be better yet.

Running the cross bars inline with the PV panels rather than below the PV panels would reduce frontal area and be beneficial.

Mounting things as far back as possible and avoiding the area high speed airflow where the air is transitioning from the nose/windhield of the van onto the roof would also help.

And, of course, flexible panels mounted directly to the roof sheet metal are an aerodynamic thing of beauty.

Gary
 

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And, of course, flexible panels mounted directly to the roof sheet metal are an aerodynamic thing of beauty.

Gary
Not to get too sidetracked from your very thorough analysis of the MPG penalty, but if you were starting over (or advising a new builder), would you go for flexible solar panels?

There seems to be a cost premium along with some concerns about longevity and heat transfer.
I presume the extra cost of flexible panels is negated by not having to build a roof rack.
 

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Discussion Starter · #66 · (Edited)
Not to get too sidetracked from your very thorough analysis of the MPG penalty, but if you were starting over (or advising a new builder), would you go for flexible solar panels?

There seems to be a cost premium along with some concerns about longevity and heat transfer.
I presume the extra cost of flexible panels is negated by not having to build a roof rack.
Hi,
I've read the same mixed reviews on the longevity of the flexible panels, but if I were going to put solar on a next van, I'd probably go that way after trying to get a more in depth understanding of their longevity issues (if any). I think @Baxsie has them - I wonder what his experience has been?

Compared to a full rack approach that might hit you with as much as a couple MPG, the savings in gasoline would pay the extra. Just 1 MPG over 150K miles would be about 400 gallons - around $2K at today's gas prices.

I have to say, that even though I am the biggest fan of solar and I run my house with solar and have a big DIY solar website, I'm not sure I would include solar at all in a new van. We drive almost every day and have enough batteries to be able to stay somewhere at least two days with no charging. We use propane for heating and cooking, so our electrical demands are only about 60 amp-hrs a day.
Even after concluding that we were really not making a lot of use of the solar on the van, I figured it was quite handy to keep the lead acid batteries topped up between trips, which is critical, but for the new LiFePo4 batteries, its better just to leave them around 50% SOC between trips, so no solar for that. So, for our style, maybe we don't need solar (can't believe I'm saying that:).

Gary
 

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Hi,
I've read the same mixed reviews on the longevity of the flexible panels, but if I were going to put solar on a next van, I'd probably go that way after trying to get a more in depth understanding of their longevity issues (if any). I think @Baxsie has them - I wonder what his experience has been?

Compared to a full rack approach that might hit you with as much as a couple MPG, the savings in gasoline would pay the extra. Just 1 MPG over 150K miles would be about 400 gallons - around $2K at today's gas prices.

I have to say, that even though I am the biggest fan of solar and I run my house with solar and have a big DIY solar website, I'm not sure I would include solar at all in a new van. We drive almost every day and have enough batteries to be able to stay somewhere at least two days with no charging. We use propane for heating and cooking, so our electrical demands are only about 60 amp-hrs a day.
Even after concluding that we were really not making a lot of use of the solar on the van, I figured it was quite handy to keep the lead acid batteries topped up between trips, which is critical, but for the new LiFePo4 batteries, its better just to leave them around 50% SOC between trips, so no solar for that. So, for our style, maybe we don't need solar (can't believe I'm saying that:).

Gary
This is a very interesting point. The cost of a 270 amp alternator and 100+ amps of DC-DC charging is far less than a solar setup. For full-timers they probably want the solar, but weekend warriors wouldn’t.


Sent from my iPhone using Tapatalk
 

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If you don't have any solar, how are you going to keep the van aux battery pack charged up and able to run the various appliances so that it is ready to go when needed / desired?

I can see how it is different for various regions though - as in the upper US / Canada, sunlight is more limited - but my approach is to use solar in this area is to use solar as the primary energy source.

I just added a panel to my explorer and using the "fingers as a sensor" the sheet metal is quite a bit cooler under the panel shade than elsewhere. Not really cold or anything but very noticeable. After some simplistic testing, I suspect that the reason for the roof temperature being higher in some cases vs lower under the panels is simply the panel to roof surface distance.

@GaryBIS has his located very close to the roof for lowest drag. On my Explorer, the only viable way to mount them was 4 inches of gap - a bit much but it was the only viable method.
 

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We have been entirely happy with our flexible solar panel setup. So much simpler than all the rack noise. If I had to do it again, I would not hesitate to use high quality flexible panels again. I'd probably just use some kind of good sealant instead of messing with the double sided tape - - that would make it even simpler still.



 

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Solar significantly extends our boondocking capability.
Solar is great if you use your van in activities like Boondocking.

When I joined The Forum there was references around solar on the van being “Free” once a DIY has purchased & installed it.

Free, IMO is tragically wrong. It ain’t “Free”, but I do agree with your note about extending your Boondocking. And it can be extremely useful.
 

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If you don't have any solar, how are you going to keep the van aux battery pack charged up and able to run the various appliances so that it is ready to go when needed / desi

At other extreme, what would happen if we view camper electrical loads similar to an electric car? You charge, use it as needed, charge again to extend use. An extreme case for discussion, but what would it take? More battery capacity? No doubt. For many campers who get by with as little as 1 kWh per day and do not boondock for a week at a time, just how impractical (and costly) would it be? Much like EV, we’d be placing all eggs in one basket (i.e. — invest mostly in battery capacity).

For me it would never be an issue because I’m not boondocking remotely in one place for a week. Will never happen. And a large battery bank for two or three days shouldn’t be a problem; provided I don’t need air conditioning. And if I did require A/C, +/- 1 kWh of solar isn’t going to meet my needs anyway (assuming total of 2 kWh daily production for comparison).

Full timers who live off the grid have different needs, but they must be a small minority compared to average campers. For me simplicity has value.
 
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