Class a motorhomes aerodynamics

[woodydel]

I know it's hard to make a box aerodynamic but just wondering if anyone knows of any links where people have made the attempt.

[Old Tele man]

...also lookup what the trucking industry does with their long and short trailers.

[PaleMelanesian]

What he said. Mainly, deflect air away from the wheels and the rough underbody, and taper the back end.

[woodydel]

I liked the Prototype tapered rear on the home page. A class A motorhome with a taper in scale with that car might be 15 feet long. The undercarriage of a class A is huge so smoothing it seems impratical. I was thinking of the underside of the front being streamlined and streamlining in front of all wheels. Coming up with a proper shape to streamline the wheels I'd rather copy what has been tried. I will be making wheel skirts. I looked at air tabs for the rear. Seems tapered rear would be better. I don't tow a car like most Class A owners do.

So anyway, if anyone has seen anything done to a big fat motorhome....

I will take a look at the trailer stuff again. Some of those truck sites focus upon the gap between the tractor and the trailer.

There are some "aerodynamic" mud flaps out there. Supposed to smoothe out the spray with slots in the mudflap surface.

[aerohead]

I've not yet seen any streamlined motorhomes except for earlier efforts by GMC and Airstream,plus one-off units built from aircraft fuselage.
The motorhome would benefit from anything which improves a typical van.Hucho's thing is to add length while carefully reducing the cross-section to maintain attached flow,which is the "boat tail."
If I had a motorhome,I'd construct a trailer which forms the boat tail,with complete gap-fillers.( the accordian-bellows material used with articulated buses would be a great way to go ).
I've had good success with a van,coupe,and pickup truck,using boat-tails.NASA has some images of their boat-tailed van research at Edward's Air Force Base.This is one of the 'bird-in-hand' low-drag technologies.
Last December I attempted a road test of a boat-tail trailer without the gap-fillers.I lost a wheel early on and had to abort the trip and limp home with my tail between my legs.
Considering that the trailer shouldn't work without the gap closed off,I was relatively pleased with the 30-mpg under the winter driving conditions and undercarriage damage.Until I can repair and re-test,she remains an unknown quantity,so I can't give you any 'real' numbers until then.On paper,the trailer will increase mpg.

[euromodder]

Have you got any pics ?

Originally Posted by woodydel

I liked the Prototype tapered rear on the home page. A class A motorhome with a taper in scale with that car might be 15 feet long.

It also helps if you only add part of a boat-tail.
On trailers, flat plates angled inboard and around 1m / 3' long already help.

The undercarriage of a class A is huge so smoothing it seems impratical.

If it's huge with a lot of stuff sticking out underneath, it's also an area where serious improvements can be made

Fairing any big external fittings on the roof (vents, windows, airco ?) would also help.

[slowmover]

Truck fuel economy is pretty well cast-in-stone. Still, the the difference between drivers of otherwise identical rigs (big trucks) is 30%. That's a huge spread. Some drivers just get in and go. Others (the winners) know the details of the day in advance.

Trip Plan is the truck industry name for best utilization of equipment in re time & distance. It involves the use of different analytical tools. Most parameters are absolutes (staying on pavement, obeying traffic laws), so the margins are exploited to gain small increases that accrue over time.

Were I using a motorhome (and I have, going back over 30-years) my first thought is that the shortest engine-on time is a goal to post. Second is the distance to be covered (where the origination and destination are both known). Third is weather. Fourth is daylight hours available.

Within these are the items which can affect total mpg: best roads (even if a bit longer), estimated fuel burn (where do I re-fuel; consequently, where/when do I eat & rest); what major metro areas must I traverse (they tend to be at least 100 miles wide on any road), what temps, winds, conditions will I encounter? Altitude changes? Etc.

One needs to know the point-to-point distances to correctly estimate driving time on a daily basis. 50 mph for all hours is a standard, to include moving or stopped. But, as you are NOT on a schedule in the same manner, a different numbers may work, but you MUST keep a log of how things go in order to isolate and improve.

In other words, before the key turns one must know not only routing, but EVERY stop beforehand; right down to how to enter and exit a particular location (GOOGLE Street View). Some choices are far better than others, as all truckstops or rest areas may not be open, or be unpaved, or exist after three turns and three controlled intersections away from the highway, etc.

Motorhomes are notorious for terrible steering (sloppy, huge dead-center) so that is where I would expend any funds prior to aero aids. Second is brakes. Then FE alignment. Depending on year model, ignition, exhaust and fuel delivery if gasoline. A close eye on hub & tire temps/pressures, etc.

The fewest stops/starts is key. The smoothness of each is secondary, yet additive.

Within this, aero starts to work. Unlike a car one cannot just follow the front wheels around and keep gains consistent. The percentage difference of a 1/4-mpg on a moho are huge (and practically meaningless for a car), and easily, easily lost.

The trip plan means all contingencies have been addressed (takes experience, too). If high winds are expected in the afternoon, one might be advised to avoid same by a late, long, lunch and an early stop to the day (campground), for example.

The "aero" motorhomes are low to the ground in their class: VIXEN, FMC, GMC. Note mirrors (big deal) and other items hanging off body. Chin spoilers seem to have been attached to some Class C types since the 1970's.

Modern motorhomes (SPRINTER-based) rely on excellent drivetrains/gearing to achieve high mpg. A boat-tail on one of those would likely be the "capacity" (motorhome abilities) versus fuel burn champ.