Okay, here's a challenge (mod my Winnebago!)

[Christopher Jordan]

I have no idea how to use them on an R.V., but those wind "wings" on the roof of an 18-wheelers are very common. If you have air conditioning or something on top; a smaller wind deflecter might help.

Or you could use a normal size "wing" like those that swings down in front when moving

countersTrike

[MetroMPG]

Originally Posted by NovaEona

Boat-tailing the rear may or may not be possible, but I'll look into it.

The rear of the vehicle is where your largest gains are to be made. The other details mentioned are worth investigating, but the back is where you should focus your attention.

bestmapman: great image! I can't tell if you're just having fun, but I can't resist pointing out that aerodynamically it's not quite right.

The best front shape for a large vehicle is something more hemispherical, like the NASA modified van images trebuchet posted.

Also, the rear angles as drawn (copy/pasted? ) are far too steep to be effective. Anything sharper than your Prius image is too much. Or, again look at the rear of the Nasa van tail and extend the lines out further. Of course with a giant cross-section like this, drawing the boat tail out to its pointy conclusion might make it 20+ feet long!

[trebuchet03]

Originally Posted by MetroMPG

The rear of the vehicle is where your largest gains are to be made.

I second that

[LibbyMetro]

als o being as your motorhome is adiesel see where it does its final shift and if its close to 55mph and you have tha patience for that speed, you should be able to hold that speed rather well , with the torque from the diesel, we had a winne just like that except with a 440 gas, and it got about 6 miles to the gallon, but at 55 feathering the throttle it got about 9 -10 you should do better but its a huge improvement

[tasdrouille]

Originally Posted by MetroMPG

The rear of the vehicle is where your largest gains are to be made.

If it was possible to further round off the edges at the front there would be a large reduction in Cd.

With sharp edges at the front, the flow does not stay attached past the corners to the sides, which leads to a LOT of drag.

With something as boxy as this winnebago the drag reduction can be in excess of 40% just with rounding off the front.

There is an example in the introduction of Hucho's 2nd ed with a 1969 VW van.

[aerohead]

Your clutch fan has to be in good working order,or it will drag down the engine.The thermostat must not prevent the engine from reaching design temp or engine oil viscosity will be a constant drag ( SHELL ROTELLA?).I can't tell how much of the grille is "active",however,if there are openings that don't actually direct cooling air onto the radiator,oil-cooler,transmission-cooler,AC condenser,etc.,any of those openings could be safely blocked for a little drag reduction.Also hard to tell,is front bumper.If you look under the front bumper,and its lower edge is not as low as say the front axle bottom,then a shallow airdam could be fabricated to block the air around to the sides of the RV,where its got a straight shot to the rear,a small drag reduction.The rear of the rooftop Coleman air conditioner unit is aerodynamically dirty,but I don't have a quick fix for that(I think such things should be integrated into the body of RVs as they are in ships,airplanes,and cars).Watch those tire air pressures.Don't try and buck headwinds or crosswinds.Take advantage of tailwinds and downhills (not MOUNTAINS!).Accelerate gently through town if traffic will allow it.Try and time the traffic lights as much as possible.Your big frontal area and high drag coefficient are enemies of mpg on the open road.A 5-mph difference in speed will show up at the pump.Uphill grades taken at lower speeds will conserve fuel.Of course,be mindful of the other motorists behind you.I'd almost say that pulling a truly aerodynamically designed trailer,one specifically designed to fit your rig,would offer you the greatest savings,but they don't exist yet.There's just no silver bullet for your project.Your driving style will probably have the biggest impact on your fuel bill.

[Whoops]

I can't say much, although if you check out the link on trailer aerodynamics, that was posted recently, it might give you some practical ideas.

I can tell you something I once did that enhanced things substantially, in a very conter-intuitive fashion. We once had a 82 Plymouth Reliant station wagon. It is a fairly boxy shape, but we typically got 22 mpg or so, on trips. I built a car top where the front of the car top followed the windshield angle. I modified it so it was about 18 inches tall because at the 24 inches I initially had built, it got about 15 mpg and I couldn't go over 60. The top and rear were just square.

I later added a shape similar to a the top half of an airplane, oriented with what would be the trailing edge, to the front. What would have been the leading edge was aligned with the back edge of the car top.

When I made this change, I went from 16-17 mpg back up to about 21 mpg. Additionally, it made a substantial improvement in our ability to accelerate and I could drive 65, before I could feel the aerodynamics starting to really drag the car down, where previously this had occurred at 35 or 40.

I initially discovered this when I had loaded a stack of fire wood on top of the cargo top, at the back 1/3 of the top. I put about 4 inches or so of wood, across the top, for about 18 inches or so, at the back. The difference it made was noticable upon acceleration, and when I had to purchase gas. However, the car only had a 2.5 liter engine and it was a 3 speed automatic, with no torque lockup.

I don't know if it would help, but I would get about 10 2*4 boards, stack them on top, at the back and give it a try.

Sounds weird, can't really explain it, just know it worked, on that car.

[NovaEona]

Wow, these are a lot of great suggestions. I'll try as many as I can as soon as I'm able, and I'll keep you all up to date on my successes and failures. 'Till then, keep 'em coming! Thanks again.
-Rob

[Who]

Outside of maybe a small light fairing on the roof to shroud the AC and vent equipment along with maybe some more aero mirrors, I'd focus all efforts on keeping the weight minimized. Many aero mods could just add weight. Your frontal area is such that you should resign yourself to slowest speeds and visit places instead of driving in strong headwind situations. You may be able to benefit more FE-wise by keeping the weight low for climbing hills. Besides, you aren't aero enough to take advantage of it on the other side.

[RH77]

I have to throw-in the component of driving style into the mix.

At high speeds, you have to be moving a massive amount air out of the way, just to cruise. In addtion to physical modification, I wouldn't discount the possibility of driving efficiently. The difference between 60 and 70 MPH may mean all the available aero mods you're willing to affix.

I'll admit that it's a tough challenge, and easy to defeat this "mod", but if you hold in there on the long trips, you won't be disappointed.

RH77