Solar "Hump" for aerodynamics for Class A RV

eXodus

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I'm currently fitting large 435W on my RV. (82x 40 inches)

My idea is to fit the long edge (82 inches) across the roof and create a kind of spoiler hump in the front of the RV. (Primary use - is to clear above the vent)

Rvision Solar Hump.jpg

This is recently done in Bus design and I saw it on some newer Class C RVs -
slide_29.jpg


There is lots of science regarding this hump. Just wondering about the forces involved, how much uplift is this going to create? What issues can you see with this idea?

Every time I look on any RV roof - I see an aerodynamic nightmare, and fuel economy proves it :p

I'm already overdesigning the mounts for the panels - Each panel is going to be attached with 12x m6 screws to the 6x mounts and the each of mounts are going to have 4x screws into the roof surface (which is a 1/2 OSB + aluminum struts) (Total of 24 screws)
Further all the mounts are going to be glued down to the roof surface with a Silka Flex kind of compound + I dip all the screws in that stuff before I screw them in :p
 

12VoltInstalls

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Aerodynamics is a funny thing. In 1969?ish some engineering students discovered and reported that a standard VW bus (a brick) at 50mph had less coefficient of drag than many high-dollar sports cars. Peter Brock made similar statements.

Anyways if you don’t leave an “open” area behind or to the sides uplift shouldn’t be a concern. But I’m no engineer.
Basically the hump moves leading edge forces that create that turbulence (drag) further back and the hump makes the area of force spread over a large area so in my opinion negative pressure in psf should be much less.
 

AndyRonLI

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So Cd is not drag, Cd times area times dynamic pressure is drag. So one published value of the Cd of the Mazda Miata is 0.38, and one for the VW Caravelle ( the current van) is 0.37. So the Miata has a very slightly higher Cd, but look at them from the front and its clear the Caravelle has over twice the frontal area. So the Caravelle has twice the drag. The biggest contributor to the drag on a big bluff bus is that big flat back end. As I am an engineer and fluid dynamist by trade, don't worry to much about uplift on any truly boxy shape. You would need a lot of lift to have any effect on a RV. But you could do some stuff to reduce drag. Round vs sharp corners are always good, but the 1000 lb gorilla will always be that big frontal area, and that big rear end. I could design you a very low drag RV, say get the CD down from the high 0.3 low 0.4s range to a low 0.3 type value, maybe even lower, but... if I keep the same length, height and width, I will reduce your useable volume. If I keep the same useable volume it will get 10 to 20% longer. Remember that dynamic pressure is proportional to the SQUARE of the velocity. Drop you speed from 75 mph to 55 mph and the drag force will be reduced by almost half !! Speed is the big knob
 

12VoltInstalls

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So Cd is not drag, Cd times area times dynamic pressure is drag
Thank you. I’m not an engineer. My wording as a lay person is vernacular at best.
I get the length thing- bigger boats by length all else being equal often don’t require larger motors or significantly more sail. To a point anyway. If being bigger displaces more water they need more power.

So do you think the ‘hump’ makes a significant difference? Or is it mostly appearance or marketing lip service?
 

eXodus

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So Cd is not drag, Cd times area times dynamic pressure is drag. So one published value of the Cd of the Mazda Miata is 0.38, and one for the VW Caravelle ( the current van) is 0.37. So the Miata has a very slightly higher Cd, but look at them from the front and its clear the Caravelle has over twice the frontal area. So the Caravelle has twice the drag. The biggest contributor to the drag on a big bluff bus is that big flat back end. As I am an engineer and fluid dynamist by trade, don't worry to much about uplift on any truly boxy shape. You would need a lot of lift to have any effect on a RV. But you could do some stuff to reduce drag. Round vs sharp corners are always good, but the 1000 lb gorilla will always be that big frontal area, and that big rear end. I could design you a very low drag RV, say get the CD down from the high 0.3 low 0.4s range to a low 0.3 type value, maybe even lower, but... if I keep the same length, height and width, I will reduce your useable volume. If I keep the same useable volume it will get 10 to 20% longer. Remember that dynamic pressure is proportional to the SQUARE of the velocity. Drop you speed from 75 mph to 55 mph and the drag force will be reduced by almost half !! Speed is the big knob
Thanks for the insight.

I'm not concerned of the lift on the RV - mainly the lift on the panels trying to get them securely attached.

When I'm not mistaken - the front part of the "spoiler" should actually provide downforce - but the tail-end up? It's still a large box.

I'm getting about 10mpg when I set the Cruise control at 63mph and 8mpg when I got to 70mph.
Actually when I got between 55 and 60 - the gearbox starts shifting into the next lower gear with a little headwind - and my fuel economy gets worse. So I hang out just above that point where I can keep in it in the highest gear.

So do you think the ‘hump’ makes a significant difference?

Many European Long Range Bus corporations have adopted the shape. Fuel is 3x the price in Europe from US - so it must have some merit.
flix-bus.jpg


My idea is - at this point it's not costing me any additional money - I got the panels and the variable height mounts - instead of installing them flat I proposing to build a inverter V - the hump and see.

Just don't want to create something which gets ripped of the roof :p
 

justgary

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You would need a lot of lift to have any effect on a RV.
Lifting the RV is not the worry, but too much pressure on the panels is. I think that is the problem to solve here. Down force on the leading panel and up force on the trailing panel could be fairly large at highway speeds. If you could give him a feel for the size of those forces with specific panel sizes and angles in the wind, it might help him make an informed decision.
 

Paul_R

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You mentioned anchoring to aluminum struts. Be sure you use the right type of screw, regular sheet metal screws and even tapped holes without a nut will come loose in no time.

Thread-Forming Screws for Soft Metal

Not much else to add other than I wouldn't want my solar panels deflecting bugs, rocks and other road debris..
 

eXodus

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You mentioned anchoring to aluminum struts. Be sure you use the right type of screw, regular sheet metal screws and even tapped holes without a nut will come loose in no time.

Thread-Forming Screws for Soft Metal

Not much else to add other than I wouldn't want my solar panels deflecting bugs, rocks and other road debris..
thanks for the input.

The material of the roof:
EPDM membrane
3/8 or so OSB Wood
1 inch Aluminum square tubing
2-3 inches of Foam
1/8 Luan as ceiling

So due to the panels being 40 inches wide - and the roof wood sheets are 48 inches - I can only screw one side into the aluminum tubing. The leading or the trailing edge.

At 10 feet above the ground I'm not worried about rocks and road debris - more like - low hanging branches :p
Down force on the leading panel and up force on the trailing panel could be fairly large at highway speeds.
Exactly that's what I'm worried about. Which is the more critical side? trailing or leading?
 

Cajunwolf

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The first thing that comes to my mind is lift; someone mentioned it. It's not going to lift the bus; it will lift the panel right off if not anchored very well. I've seen this happen to people more than once. I have an aircraft-quality one-piece aluminum roof that I don't want to drill any holes in that I don't have to. With that in mind, I thought of having an aluminum frame made to mount the panels and then mount the frame to the RV's roof only where need be.
 

STW

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I filled my tires with helium…. Dang thing almost floated away!
 

eXodus

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The first thing that comes to my mind is lift; someone mentioned it. It's not going to lift the bus; it will lift the panel right off if not anchored very well. I've seen this happen to people more than once. I have an aircraft-quality one-piece aluminum roof that I don't want to drill any holes in that I don't have to. With that in mind, I thought of having an aluminum frame made to mount the panels and then mount the frame to the RV's roof only where need be.
there are already a ton of holes in my roof. A few more screws don't make a difference.

Just dip the screws in Sealant/ Construction Adhesive before I screw them in - never had problem on getting water in.
I find it fascinating that some youtubers mount large panels with 4 screws each. - 4 Screws is for Roof mount stationary on a house.

The Fastener have a Pull-Out Value:
Metal Roofing Screws in OSB is about 160-200lbs per Screw. (Dry mounted, when I glue them in - should be a bit more)

My current plan I'm having 24 Screws going into roof Substrate. - Which should add up to about 3,840 lbs of uplift resistance. Per Panel.

Many people apparently get by with 4 screws - which is somewhere between 640 and 800 lbs of Pull-Out Value (Metal roofs are better then wood - but most RVs are wood)


Add a fairing to the area forward of the panels.
I thought about that. Just a small metal lip to direct the air up so it can't get under the first panel.
 

justgary

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Exactly that's what I'm worried about. Which is the more critical side? trailing or leading?
Add a fairing to the area forward of the panels.
The more critical side is the one with the most force on it. The best way to figure that out is with an aerodynamic study that includes the shape of the vehicle. I am not an aerodynamics engineer, so I won't volunteer to do it for you.

Adding a fairing will probably cause uplift behind it, but you don't know how far aft the air will again contact the panel with positive pressure, so it could be worse. You could mock this up with thin plywood panels and test it out with smoke tubes and video cameras, but you might want to make sure that you do it in a place that allows any catastrophic failure to not cause harm to anybody behind you. An alternative is to pay the coach maker to run their aerodynamic model with your proposed configuration.
 

eXodus

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I'm sorry but this reminds me so much of this entertaining video:

lol :p

you know that aero on a Brick of the Size of my RV makes a huge difference? When you go from 8 mpg to 10 or 11 mpg , that really has daily application impacts. First - my fuel Tank is 40 Gallon, that is a fixed value. When I add one MPG - that's 40 miles of more driving on the same tank.

First: The range on RVs is terrible and fuel stops really hold you up. Further, the gas station network is already bad in many parts of country and will get worse as electrification gets more popular, many back country roads Gas stations have already closed - and more to follow. So I'm slowly getting the the point of range anxiety with a gasoline power vehicle.

8 mpg29.4 l/100km
9 mpg26.1 l/100km
10 mpg23.5 l/100km
11 mpg21.4 l/100km

Second:
at this point it didn't cost me any more. Needed to elevate the Panels anyhow to clear the vent - so I just going to try it. Just didn't want to create a hazard.

I've got about half a year of fuel data on the RV without the panels mounted - now I can compare and see if it's better, same or worse.

My Thesis is that at least I'm not creating any significant more drag, with a small glimmer of hope to lift the air above the aerodynamic dirty roof with all the random items up there and reduce it.
 

wattmatters

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you know that aero on a Brick of the Size of my RV makes a huge difference?
I have enough understanding to know the value of aerodynamics. I ran an aerodynamics testing business. Not for larger vehicles but human powered vehicles. Worked on multiple successful world records. Eeking out every last aero drop matters a lot. I get that.

I love your enthusiasm but frankly, your panels are going to make 2/5th of SFA difference.

If anything the sharp edges they present are probably going to increase your effective coefficient of drag rather than lower it (sharp edges like that are bad aero news) but either way the difference will likely be so small as to not be measurable within the precision of fuel consumption figures. At least not in any controlled manner such that you can make verifiable claims, especially considering the multitude of uncontrollable factors which influence such outcomes. IOW it will be lost in the noise.

You might after a while find your fuel economy improves because of a natural bias with your testing, IOW the very fact you have put them up there consciously or subconsciously results in you making changes to your driving habits. Or the changing engine state of tune, tyre wear, type and pressures, suspension, even seasonal and year to year variations in weather will confound any such assessment. And this assumes the driving locations and conditions net out to be the same.

If you want to save fuel while driving a vehicle of that size, weight and shape, find ways to (safely) lose some vehicle weight (this mostly helps on the inclines) and reduce your driving speed (helps everywhere for fuel economy). Also ensure tyre pressures are appropriate and consider the type of tyres you use and make sure they are aligned correctly and your suspension is in good order. Having a well tuned engine of course, clean air, oil and fuel filters, cooling system working as it should, not operating unnecessary devices to place additional drain on the alternator.

I think the biggest benefit of the panels on your fuel economy will be their utility in charging batteries for a tyre pressure inflation device to check your tyres at least weekly to make sure your rolling resistance remains optimal and your vehicle is as safe as it can be:
 
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