Checkout MPG penalty for roof racks and roof mounted PV

 

That's a really high rack but it looks like it's also very strong. My guess is that it will not be effected by wind or buffeting from truck turbulence. Rather than trying to make one big blunt fairing like you show (looks like a lot of wind resistance to me) you might consider making smaller tapered edges on all the front surfaces of the panels and any other flat edges so they will cut the wind better. Then just let the wind flow underneath. This might also reduce wind noise.

Over the years I have had to taper crossbars and other rack members while hauling canoes and kayaks. It helped to stop wind noise and let the air flow straight thru under the boats. It worked better than trying one front fairing.

Ultimately you may just have to try a couple of methods and see what works for you.

 

Hi,
I'm not a fan of those fairings mounted just behind the windshield on the initial slope of the roof.

They add quite a bit of frontal area (which means more drag).

This transition from the windshield slope onto the roof gets a lot of attention from the van aero designers - you want a smooth transition from the windshield onto the roof and you very much want the flow to stay attached to the roof and not separate (which causes a lot of drag).
When you add one of these fairings just above the windshield I think you are not only adding frontal area but almost guaranteeing that the flow will separate from the roof at the sharp break at the back of the fairing.
From an aero drag point of view, I think its a bad idea that will end up adding quite a bit of drag, and the link in Larry's post above tends to confirm this.

If you are concerned about drag from your PV panels, I think the following things will reduce PV panel drag.
1) Move them as far as practical aft on the roof, and mount low to the roof.
2) Attach them to the roof with right angle brackets that add almost no frontal area - ie don't mount them on a rack which just adds a lot of frontal area.
3) Add an elliptical fairing to the front edge of the PV panel frame - this reduces the drag coefficient of the PV panel (picture below)
4) It may be beneficial to use some thin sheet metal to skin over the bottom of the PV panels so that its a smooth aero surface.
5) Avoid placing anything in the critical flow area at the forward end of the roof where the flow is transitioning from the windshield area onto the roof.

Gary

 

Hi,
I'm not a fan of those fairings mounted just behind the windshield on the initial slope of the roof.

They add quite a bit of frontal area (which means more drag).

This transition from the windshield slope onto the roof gets a lot of attention from the van aero designers - you want a smooth transition from the windshield onto the roof and you very much want the flow to stay attached to the roof and not separate (which causes a lot of drag).
When you add one of these fairings just above the windshield I think you are not only adding frontal area but almost guaranteeing that the flow will separate from the roof at the sharp break at the back of the fairing.
From an aero drag point of view, I think its a bad idea that will end up adding quite a bit of drag, and the link in Larry's post above tends to confirm this.

If you are concerned about drag from your PV panels, I think the following things will reduce PV panel drag.
1) Move them as far as practical aft on the roof, and mount low to the roof.
2) Attach them to the roof with right angle brackets that add almost no frontal area - ie don't mount them on a rack which just adds a lot of frontal area.
3) Add an elliptical fairing to the front edge of the PV panel frame - this reduces the drag coefficient of the PV panel (picture below)
4) It may be beneficial to use some thin sheet metal to skin over the bottom of the PV panels so that its a smooth aero surface.
5) Avoid placing anything in the critical flow area at the forward end of the roof where the flow is transitioning from the windshield area onto the roof.


View attachment 86727


Gary

Gary, any idea where an "elliptical fairing" can be purchased? I've attached my panel similar to your pic.

 

Thanks yall. I need the ladder rack to haul gear so the panels have to ride up front and high like that. I guess I will just leave it as is and see how it goes.

 

Maybe look at home depot on PVC moulding trim? you can always sand it down too.

 

I agree with Ken. If it's solid and works, you can keep track of your mileage and see if little adjustments like your deflector can make a difference.

We get in the habit of trying to make things as close to perfect as possible, which might be more understandable in this case considering how expensive fuel is. The MPG penalty of going from no rack to a normal one is about 1-2MPG. If you're in that range, I'd leave it alone.

 

The MPG penalty of going from no rack to a normal one is about 1-2MPG. If you're in that range, I'd leave it alone.

At that fuel penalty I would need solar badly to justify the cost. For my type of use, it would be far more economical to charge from alternator. I suppose owners who camp much of the time and drive much less than I do could justify the lower MPG. I could never justify a 1~2 MPG reduction.

 

vantech sells a "wind foil" for their racks with square crossbars. Looks like a triangle wedge you attach to the flat front facing edge. a bit pricy but looks like a good starting point for fabrication project.

 

I think most people here do a fair amount of boondocking or at least try to make their builds as off-grid as possible with the idea that you only run your engine when you need to go somewhere. Plus, there's a tendency to end up using more power than your original design plan as you make improvements to your rig over time, so it's nice to have the additional self-regenerative capacity even if you can work around it with your alternator.

 

We put a fairing on our Transit, when we still had it, and it introduced a major issue immediately upon getting on the highway. It created so much turbulence that our maxx fan, mounted directly behind it, shook a tremendous amount. I think the more blunt fairings introduce a lot of air turbulence. If you look at the fairings from the roof rack co's you'll notice they tend to have a small gap or taper to them to allow for air flow beneath them.

This is purely anecdotal given research from FT users but the 3rd party ones seem to do more harm than good when it comes to noise and drag.

 

We put a fairing on our Transit, when we still had it, and it introduced a major issue immediately upon getting on the highway. It created so much turbulence that our maxx fan, mounted directly behind it, shook a tremendous amount. I think the more blunt fairings introduce a lot of air turbulence. If you look at the fairings from the roof rack co's you'll notice they tend to have a small gap or taper to them to allow for air flow beneath them.

This is purely anecdotal given research from FT users but the 3rd party ones seem to do more harm than good when it comes to noise and drag.

Good Comment ,,, size, shape & especially “location” matters.

@GaryBIS positioned his solar panel closer to the rear of the van with as small of a frontal area as could be & also close to the roof. I image in that location the “relative wind” is uniform (not as turbulent as the front roof area) & “straight on”

If placing a leading edge aerodynamic fairing near the front slope of the van’s roof this is introducing it into possibly already turbulent air from rising up & just onto the roof. This is probably one of the worse spots to place air dirty items on the roof.

There is a high spot on the PM roof that I utilize to place my Maxxair Fan. A spot that I suspected might be into cleaner air, but if aerodynamics is important I would not place anything forward of the aft end of that roof fan.

The idea is to disturb the laminar flow as least as possible.

I believe the PM roof corrugations (peaks & valleys) serve 2 purposes;

1) Manly added strength to the thin sheet metal roof skin
2) Also as a streamlined feature to somewhat control the airflow over the roof

a 3rd design consideration from Ram could be water shedding (front to back rather than sides)

 

Thought y'all might be interested in jimandboni's results

Solar Panel wind fairing...custom or bought?

Hey Guys, Just finished mounting my solar panels to my roof rack. I'm hyper anxious about these things becoming airborne during driving. I couldn't use the Renology mounts that were provided with the panels so I bought some Rafter connectors to mount the solar panels to my roof. Once installed...

www.promasterforum.com