Aeromodifications for large motorhome
 

My situation is not one typically discussed here but the phyics of fuel economy are the same. My 2008 Class A diesel motorhome gets 8 mpg at 65 mph. I have a goal of getting a solid 11 mpg. It is 26,000 lbs, 8 by 13 feet for a frontal area of 104 sq. ft. It is shaped like a brick (front and rear) with rounded corners. It has 2 large ac units on top and a couple of vents stickins up also. The engine is a Cummins 360 hp, 8.3 liters, variable blade twin turbo, computer controled fuel injection and computer controled transmission.
My assumption is 70% of energy is used to overcome aerodynamic drag so that should be my focus. I own a body shop have the means to create a front end that looks like a Japanese bullet train with a remote controlled liftable air dam 3 inches from the pavement. I could also modify the rear to look like the rear of the bullet train.
Would this be enough to gain the desired 3 mpg? Or could it even give me more gain? Any other ideas.
Thanks,
Randy

Great fab skills & $$! Should be a great thread...

Great elevation pics of your model would solicit plenty of ideas.

You may have to look into improving things under the hood also. Synthetics, software upgrades, exhaust, intake; all things should be taken into account. Pittsburgh Power is probably the best for the Cummins, type them in and see what things they have for the ISC.

Look at my avatar; they make trucks that are shaped like my old W900 ( Peterbilt 389s ) get 8 mpg and better loaded ( 80K gross ) with near 900hp; and that's with engines in the 12.7 to 16L range.

You would get most of your potential mileage gain by building a boattail on the back end, which, if designed properly, could also add a considerable amount of additional storage space (inside the boattail). Next, would be to do what you could (underpanelling, fairings, wheel well skirts, etc.) to clean up the air flows along the underside, sides, and top of the RV. A bullet nose would help, but would have much less effect than adding the boattail.

One way to quickly test what aero mods could do to improve your RV's mileage would be to note what your mileage is when drafting, as that is the mileage you could get by reducing your Cd. I've aeromodded my vehicle to the point where drafting at legal speeds provides very little improvement to my mileage, which wasn't the case when I started.

Pull all the stick-out parts off the roof and go 55.

A boat tail would be far easier for me to do. Are you saying that the rear of this 26,000 brick is more important than the front? I understand about the partial vacume created by the rear but I never thought it could be more important than the front.

I will check into engine type mods but I thought that the ISC 360 should be tuned pretty well already. The shape of a class A motorhome just seems so....terrible. I am also concerned about changing the computer settings and ruining the $2000 computer module some how. I will check with the Pittsburg power website and see what they have.
Randy

I couldn't find it, but sometime in the last month on a thread here, someone posted a drawing from one of the classic aerodynamics research programs showing a brick shape, then rounding either the front, the back, or both and showing the resultant Cd reduction. Rounding the back produced by far the greatest Cd reduction.

There is a lot of room for tuning the engine, and if you drive it right, the mileage will improve. The computer that Pittsburgh Power has is an add on unit that is multi adjustable and does not affect the stock ecm.

While I'm thinking about it, check with Snow Performance about their water/methanol injection. Keeps the egt's down and does boost the mileage.

Pics or model?

Considering you say the front is flat I would think you'd get a lot of drag reduction from making it look like a bullet train... But can you make a new angled/curved windshield or are you stuck with the existing one?

Maybe like the old Peterbilt 372?

http://farm5.static.flickr.com/4067/...42547098_z.jpg

Wow... why don't trucks look like that anymore?


Also...
Aerodynamics Research Revolutionizes Truck Design
http://www.sti.nasa.gov/tto/Spinoff2008/images/t-7.jpeg

Welcome to EM!

Yes the back end makes more of a difference then the front end.

In the case of the 372, it didn't sell well enough, albeit PACCAR didn't push hard on promoting it, especially since diesel was still dirt cheap when it came out. The traditional longnose conventional was what all the gearjammers wanted. And to think it was the first Class 8 to get 10 mpg loaded with a van trailer.

The ones that are left are getting bought up quick by those who know about them.

A boattail will give you a big increase in fuel efficiency, but to do it right, you will need a long boattail, increasing the total length of your vehicle. Do you have an idea of how much longer you are willing to make your vehicle?

There may be some less lengthy options...

Or make a boattail trailer?
Ala Aerohead.

Hey that is a brilliant Idea for this guy!
How about a boat tail trailer with some articulating gap fairings, inside is his favorite car? Maybe road bikes? That would really rock.

The windshield is an issue. A big expensive issue. I plan to build the nose of the front fairing to partially cover the huge windshield. It would cover the lover 12 inches of the windshield. This would change the pressure distribution on the windshield and might make it unsafe so I need to determine how a bullet nose on the bottom 2/3 will affect the pressure on the flat upper 1/3. I will take photos today.
Randy

I think you are missing the boat so to speak Randy. Forget the nose, concentrate on the tail. Cheaper, easier, more effective according to all accounts.

I imagine to make only a 6 or 8 ft. tail. My MH is 40 ft. long, 13 tall and 8 wide. and is a diesel pusher. The really huge radiator is at the extreme rear of the vehicle. Air enters the engine area from the bottom and flows through the radiator and into the vacume at the rear of the vehicle. I assume that the vacume is strong and produces a strong air flow through the radiator. This might be an issue in the design of the tail.
Randy

What would be a less lengthly option?

Does it have a supplemental fan? If so, you could proceed with a tail design anyway. Usually there is too much airflow over engines and through radiators, lots of wasted drag.

EDIT: I will be drawing up my folding boat tail design for my 5th wheel in the next week or two. It is a little shorter in height than your MH, but just as wide and blocky at the back.

I am beginning to come to that conclusion. It is more simple, cheaper, but I did not understand that it is more important. Is that a proven and non debateable fact, that the rear is more important than the front??

Randy in Spokane

It has a 3.5 ft. manual fan with a centrifical clutch. Have you tested your boat tail on the 5th wheel? Is yes then how much did it help?

Hard to believe, right?
Somebody posted info from a test using a brick shape. Fairing in the back was the single biggest improvement, and here is my take on why:
No matter how you smooth the flow going onto your rig, it still displaces X amount in cross section. Still 13 high and 8 wide. No way to fool that.
At the back you can "let" that air off very gently and efficiently, or let it slam shut.
If you let it slam shut the turbulence and vacuum is so great, a car can ride with almost no power input in the wake of your motorhome. IF you think about it, you could pull a smaller motorhome along behind you:)

I just got here myself, and have not designed it yet, let alone built it.

Airplanes (except for fighters) have a relatively blunt nose but a long slender tail. That seems to confirm the relative significance of the rear aerodynamics.

Yes.

If you need to streamline a brick, add a fairing on the rear ...
Counter-intuitive, but that's how it is.


There you go :

Airflow is going from left to right, BTW ;)

http://ecomodder.com/forum/attachmen...1&d=1259337975

This is from a side view, correct?

The tail could certainly be open to the rear for probably IMPROVED cooling flow.

mororhome
 

Randy,if you can't re-locate the rooftop (Coleman?) AC units,could you fabricate a fairing which would better blend them into the roof? If you can get to an image of the 1987 OLDSMOBILE AEROTECH land speed record car you could look at how they ducted their heat-exchangers.NACA submerged inlets are supposed to offer the lowest drag for any inlet.
With respect to the nose,to really alter it would require a windshield which doesn't yet exist.Experts report time and time again,that if you have attached flow at the front ( and I presume you do ),then go after the back as has been already mentioned.
A radical approach would be a dedicated tadpole trailer which takes the form of an ideal boat-tail which moves your RVs separation point all the way back to zero,completely eliminating the diesel-eating turbulent wake.
Since you mentioned the Shinkansen already,you are probably aware that the nose of the train is ALSO the tail.The railway recently experimented with a 53-foot long nose/tail extension for drag reduction.These trains go through tunnels at speed,so they have some peculiar pressure spike phenomena they must address as the flying 'piston' attacks the' cylinder' bore.Something we need not be concerned with.But as a tail,I would be surprised that the train had any wake at all.
I suspect your drag coefficient may already be down around Cd 0.26 if you can get that s--- off the roof or clean it up.
The tail could cut that number in half again.Since it carries no load,the trailer would only have to be,in the words of Clarence Kelly Johnson,'strong enough.'
Any way you attack it,going after the 104 sq ft wake will show dividends at the pump.
Please take a look at the Aerodynamic Streamlining Template for design help.
Also threads 'Full-Boat-tail-trailer with Gap-Fillers for T-100 Pickup',and' 2011 Prius boat-tail trailer' here at the Aero Forum.

Instead of making a bullet nose which will direct air up into the flat windshield, you can make a tub shape to direct air toward the sides, like that NASA truck

Thank you for the detailed response. I am convinced that I should attack the rear and remove the AC units on top. Almost everyone immediately promoted that. I will normally be pulling a tow vehicle so I may use it to form the boat tail. Here is why this is so important to me. At 8 mpg it cost me 54.8 cents per mile. At 12 mpg it would cost me 36.5 cents per mile. So the savings for two years on the road could be could be as high as $4000 depending on fuel costs. I thank you again for the pointers and I will be looking up the resources that you mentioned.

Randy

I have a 96 F-350 class C motorhome, 25 feet, that I'll be applying some aero mods to in the next few months. These responses have convinced me also that the area of most consequence is the rear.

My vehicle is considerably smaller than Randy's so maybe it will be a little easier to fab the rear cone for testing. First up will be a lightweight removable cap so I can do some testing and see what kind of results it yields.

Does anyone know, is a rounded shape like the one shown in the diagram the most efficient, or should the sides be straight? I guess it's time to order that book on fluid dynamics.

Randy I'd be really surprised if you have any cooling issues with your boat tail in place. No matter how well you build your boat tail, it won't take much of a vacuum back there to help the air move through that massive radiator, with the huge fan. Just my opinion. If a push comes to a shove, you can easily fab a sprayer in front of your radiator that taps into your 'house' water supply, with a switch on the dash. If you get excessive heat on the occasional hill in the summer time, push the button, your radiator gets sprayed with a fine mist of water. I've done that on several RV's, it works great.

Good luck with your project, I'll be reading with great interest.

I am sticking with the custom car trailer in the form of the perfect boat tail. It would open up like your own personal bat-cave:)

Let us say that I attach a fabric boattail to the rear of the MH and inflate it just enough to take shape. To about 1 psi above atmospheric pressure or something like that. At 55 mph would the pressure inside the cone increase or decrease? I think that the vacume or pressure difference outside of the boat tail might cause the fabric to become more taught. Or the boattail might deflate from the turbulence. I have no idea what might happen.

I'd think it would flap around a lot unless it had sufficient pressurization inside.

If memory serves me well, these were rods being tested in a wind tunnel - i.e. round, so they would look like this from every angle.

Ideally it should be rounded, tapering on all sides, and ending in a point, but in the real world it is often ended in a small flat plate where a licence plate and tail lights can be mounted, and where ventilation and/or engine cooling exhausts can be discharged to help fill the tiny void behind the small flat plate. Use the Aerodynamic Streamlining Template pictured in the thread with that name as a pattern in designing your boattail.

Two food for thought points:

1. The eddy currents behind a brick flow back along the sides (boundary continuity) but forward up the center (supply stream). If a diesel pusher's radiator in centered and sucks inward (forward) that would seem to cooperate. If outward (backward) then rear flow might be even more messy and a "typical" solution may need some tweaking for optimization.

2. I don't design fish but I've stared at many. Many are taller than wide and their tails seem to taper to a vertical edge. When I think about big rigs that are considerably taller than wide my imagination wants to overlay fish anatomy.