Aeromodifications for large motorhome

[aerohead]

Originally Posted by crmears

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

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.

[winkosmosis]

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

[crmears]

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

[orbywan]

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.

[skyking]

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

[crmears]

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.

[Frank Lee]

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

[euromodder]

Originally Posted by cleanspeed1

This is from a side view, correct?

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.

[basjoos]

Originally Posted by orbywan

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.

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.

[KamperBob]

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.