Aero RV (custom boat tail for '95 Ford E-350 Class C motorhome)

[orbywan]

About a month ago I decided to build a boat tail for my 95 E-350 Ford 25 foot turbo diesel Class C motor home. I read the aerodynamics study NASA did at Dryden Air Force base back in the late 60’s on the squared off E-150 van about 20 times, as well as several other studies, drew up some plans and three weeks later and some serious sweat the prototype is done and tested.
Photos of the process are posted below. I used ½” square tubing for the ribs. I determined the shape I wanted, hammered out a ‘control’ rib, taped it to the bench and then built 28 more just like it, so the four sides have the same shape coming inward. Next I built the rear frame, then made it bigger, then added ribs one at a time until I had the shape.
The basic frame weighs about 75 pounds, which is about 25 pounds heavier than the rear bumper that was removed, so there is a net gain of about 25 pounds plus the weight of 5 sheets of coroplast, and a bunch of aluminum pop rivets.
I tried my best to replicate the rounded corners of the body on this vehicle because it makes sense to me that going from a rounded corner right to a 90 degree square corner wouldn’t be conductive to smooth air flow.
This creates lots of compound curves, which are a pain to cover, but as far as the prototype skin is concerned it wasn’t too bad. The four corners were the worst so I used monocoat to cover them, which is heat shrink plastic made for covering model airplane wings and fuselages, available at most hobby stores. The larger areas are covered in coroplast. You can see in the photos where the panels had to be cut and trimmed to allow for the compound curves. Not much modification considering the shape.
In an attempt to keep the air flow over the roof area good, I also faired the roof A/C housing. According to the tell tales, that worked out really well, although now that I know the overall fairing design is good, now I have to get the A/C to work with the fairing in place. I think maybe if I louver the hell out of the final cover it will be good.
I’ve only done one road test of the new design so far but it was very encouraging. Getting up at 3AM to beat the heat and wind, we (my wife insisted on going, she thinks the boat tail is great, how cool is that?) drove from Tucson to Phoenix, which is about 1,000 feet lower elevation, to Apache Junction, a total distance of 135+ miles, one way, with lots of subtle hills and valleys in between. We didn’t use the A/C and we ran right at 57-58mph, since that is the ‘sweet spot’ for this drive line (7.3L turbo diesel Ford, A4OD auto trans, 4.10 tail gear).
The drive up yielded 17.26 mpg, compared to an average of 13.1 for the same trip sans the boat tail. I have to say by the time the tail was completed I was having my doubts this would make much of a difference so I was really happy with that. The return trip we used the A/C the whole way, and had an intermittent crosswind of about 10 to 20 mph and got 14.68mpg. That works out to a two way average of 15.97mpg. Not bad for a first try under those conditions.
Anyway, it will take a lot more testing to know the accurate numbers of course but I’m very happy with the initial results. I also have a ton more work to do before this is anywhere near ideal or finished. I still have to skin the rest of the under carriage. For this test, about the last 3 feet of the belly pan was installed and I’m guessing it was catching some air in there. So far the cost of this extravaganza is about $225 for materials, which was way worth it just for entertainment value, for me as well as my friends. If I had 5 bucks for every time someone has said, ‘What the HELL is that?’, the materials would be paid for. : )

[skyl4rk]

Very nice job, professional.

[skyking]

nice job man. This is another inspiration!
How about some other pics of the rig, it looks pretty slick.
I've been looking at mini split heat pumps as a replacement for failed AC units. It would get them off the roof, you could hang them on the back of the rig. They are also a great deal more efficient.

[jime57]

Great job! Wish I had you here in Richmond to work on my diesel pusher.

I think you might be a bit unfair to yourself with your math. If your baseline was without A/C, then that is the only data that really counts. But I certainly know how difficult it is to get good data. Initially, I think it looks very encouraging.

[orbywan]

Here are some more photos of the process. I wasn't aware they made heat pumps for RV's, Skyking, do you have any info on that? I'd love to get that big pile off the roof. Thanks.

[skyking]

Mini Split Air Conditioner AmericAire 120000 Heat Pump

They are not specifically designed for RV's but a quick search and you will find plenty of folks "down under" who have retrofitted them on Rvs with great success. They have extreme heat and also extreme rough road conditions in Australia, so I consider that a good testament to the durability.
What gets me going is the lower power draw of the 12,000 BTU units. It is 2/3rds of the rated draw of a typical Coleman 13,500 BTU unit. I suspect that a Honda EU2000i generator just *might* be able to start it.
The other huge bonus is when you are plugged into a full hookup situation. Now you have a 12,000 BTU heat pump, also much more efficient. Keep that propane in your tank.

In your case you would hang the outdoor unit on the back, find a suitable wall and route the line set and cables, and finish off with an Rv skylight to fix the hole where the old AC was.
RV Skylight

[Frank Lee]

Looks good; very inspirational!

One question: is the flow any good along the bottom? Seems too much rise down there for attached flow... Looks like the tufts are swirling around in pic 7.

[larrybuck]

Congrats on a great start! Perhaps full side skirts soon?

[orbywan]

Originally Posted by Frank Lee

Looks good; very inspirational!

One question: is the flow any good along the bottom? Seems too much rise down there for attached flow... Looks like the tufts are swirling around in pic 7.

I'm hoping that completing the belly pan will help help the flow down there because it's not very good. I may have to straighten the curve at the bottom some. So far it's only skinned about 3 feet forward from the rear. That could be effecting the flow in itself.

[orbywan]

Originally Posted by larrybuck

Congrats on a great start! Perhaps full side skirts soon?

OK, I'm new at this, what is a full side skirt? If you're talking about something kind of like a side air dam, I hadn't considered it because we take this vehicle off road a fair amount. The plan is to skin the rest of the undercarriage and fair both duallies and see how that does.

Maybe full skirts would work if they were pretty flexible. If I skin the undercarriage should I also put a front air dam on it, or is that redundant?