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Old 06-29-2008, 05:47 AM   #1
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Trying for 12 mpg on a MoHo

With an overall average of 9.5 mpg, I managed to make 11 on a trip from Texas to Colorado last year. Did that by not going over 60 and a good coat of paste wax on the front. This year I want to beat that record and wonder if any other forum members have had success. Im driving a 30 ft. aerodynamic Airstream Land Yacht with a 7.5 liter Chevy gasser with Allison automatic. Should I try 55 mph, pulse and glide, dropping into neutral on downhills, all of the above, or what suggestions can you give me.
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Old 06-29-2008, 06:04 AM   #2
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Lower speed

Coast when practical

I doubt if pulse and glide will help due to poor aerodynamics, maybe at lower speeds like 35 MPH.

regards
gary
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Old 06-29-2008, 06:28 AM   #3
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I'd "redecorate" the back end like this...

Get a roll or two of closed cell foam insulating strip, "Draft insulation" with self adhesive backing, at least 1/4 of an inch tall.

Cut this into lots and lots of 1 foot strips.
Look at the sides at the back and mentally plan a line 18 inches in from the back end, all the way round, or lightly mark it with a pencil.
Starting at the bottom, stick a strip on at an angle such that it's leading end touches your 18" line and it's trailing end, at the rear, is 3.5 inches higher.
Place the next strip above it with a spacing equal to the height of the strip. If the strips are approximately square in section, leave the backing on one strip and use it as an inter-strip spacer.
Continue this up each side and around the top until they meet in a Vee on the roof.

You have now added a turbulation device to the rear of your RV that will allow air to "tuck in" behind it more effectively, reducing base drag. See the thread "turbulence friend or foe" for more background.

A steady cruise speed within 200 rpm below and 100 over the motors torque peak should now give you good mpg. Actually, make that within 90% of peak torque, if it's one of those real flat torque curves that has a small bump high up that's technically peak, but it starts pulling at 1500 or so.
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Old 06-29-2008, 07:30 AM   #4
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I'd "redecorate" the back end like this...

Get a roll or two of closed cell foam insulating strip, "Draft insulation" with self adhesive backing, at least 1/4 of an inch tall.

Cut this into lots and lots of 1 foot strips.
Look at the sides at the back and mentally plan a line 18 inches in from the back end, all the way round, or lightly mark it with a pencil.
Starting at the bottom, stick a strip on at an angle such that it's leading end touches your 18" line and it's trailing end, at the rear, is 3.5 inches higher.
Thanks for this. A few questions...
1. What width should the 1 foot, 1/4 inch strips be; or minimum/maximum width? Wider means fewer, narrower means more strips and less holding power...Your thoughts?
2. The back of the RV is radiused. Should the 18 inches be measured from the actual back or from the point where the flat side ends, and the radius begins?
3. Would you think the leading edges of the strips should be streamlined or left square?
4. I might run into the windows on each side and that could interfere with opening them. Can those strips be shorter than a foot, or better to place them further back?
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Old 06-29-2008, 08:44 AM   #5
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Ideally they should be squareish section or close to it, 1/4x1/4 or 3/8x3/8 but 1/4 x 3/8 would be fine. If you found triangular section stuff or rubber "wiper" type strip that stuck on standing straight up, that would work too, but spacing should be based on their height.
18 inches from the radius is good, we need a bit of surface for the energised boundary layer to attach to before it tries to turn corners.
Wedging the leading edge of the strips may be slightly more efficient, doubt it's measurable but I guess it would look cleaner.
Windows... you could either go ahead of them or behind them with shorter strips, but don't go closer than about 3 inches to where the back end radius begins.

They would "work" if you put a band of them around the midsection of the RV but they do cause an increase in skin friction/surface drag, so as near the back as possible while still leaving a surface for the flow to reattach to is best.

If it would make things tidier you could go to 8 inch strips 3 inches from the start of the radius, with a rise of 2 1/4 from the horizontal at the back. Basically between about 15 and 25 degree angle. On that size vehicle I'd guesstimate that they would begin to have marginal effect as you got towards 6 inches long.
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Old 06-29-2008, 01:09 PM   #6
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That's a lot of strips!

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Originally Posted by RoadWarrior View Post
Ideally they should be squareish section or close to it, 1/4x1/4 or 3/8x3/8 but 1/4 x 3/8 would be fine... spacing should be based on their height.
They would "work" if you put a band of them around the midsection of the RV but they do cause an increase in skin friction/surface drag, so as near the back as possible while still leaving a surface for the flow to reattach to is best...
What do you mean by "a band of them?" If I put the strips on the entire sides and roof the added skin friction might negate any benefit they provide; is that what you are saying? At a quarter inch spacing I would need 24 strips per running foot which estimates to 576 feet to do the whole surface. Realistically, how many strips in groupings of how many strips would truly help the turbulance issues without increasing skin friction drag? Has anyone experimented with this setup where I might find photos of actual installations?
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Old 06-29-2008, 01:56 PM   #7
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I was meaning, that they'd provide about the same base drag reduction however far forward you put them, but that because the turbulent flow that helps the base drag reduction will increase surface drag, you want them as far back as possible while still leaving a little bit of surface for the turbulent/energised flow to "stick" to to make it round the corner. The small amount of surface drag they'll make on the rear 3-6 inches would be negligible to the amount of base drag they'll reduce, but could become significant if you provoke that turbulent flow over the whole of the back half. This would be the case with any turbulation/vortex type devices.

Yes, wow 576 feet is a lot, these type of turbulator are a combination delta and fence type. In fence type formation*, which would be simple vertical strips, they give optimum effect in groups of 3. So you could cut down to spaced groups of 3 strips, leaving a 3 3/4 inch gap, let's say for the 1ft or 2 1/2 inch gap for the 8 inch size. So that would reduce it to 173ft or 154ft...

Now pricing it in the 5m/16ft rolls of black foam strip I can find in the "everything for a dollar" stores here comes out at a reasonable $10 for the 10 rolls to do the 8 inch strips, which should pay in about 22 gallons of fuel use, or start paying after 264 miles of driving, but I guess if you can only find the stuff in $10 rolls at the hardware stores, and can't find "Contractor packs" of the stuff or similar savings then it's looking like it's uneconomical in terms of this trip since I figure it's about 1600 miles to get there and back.

Edit: * fence type formation you could just do three strips of it banded vertically around with 72ft of the stuff, spaced their own height apart and 3-6 inches from rear curvature, but angling them in delta formation provides a benefit in induced (shape) drag reduction by correcting "downwash" in the rear airflow. Also organises the flow a little better as the fence type flow can become more chaotic sooner (i.e. passing semis and crosswinds throw you around a tad more) Hummm what's better way of saying that, it'll be chaotic whatever, I mean that fence type may form large scale chaotic vortices (bad vortices) sooner that are big enough to tug you around. They actually reduce that effect somewhat over "normal" but the angled install should reduce it more.
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Old 06-29-2008, 03:06 PM   #8
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... Yes, wow 576 feet is a lot, these type of turbulator are a combination delta and fence type. In fence type formation*, which would be simple vertical strips, they give optimum effect in groups of 3. So you could cut down to spaced groups of 3 strips, leaving a 3 3/4 inch gap, let's say for the 1ft...
That sounds better. Is the 3 3/4 inch gap arbitrary or calculated? Would 4 strips be better than 3? Will the 12 inch strips work more efficiently than 8 inch? In other words, how much can I alter these values and which would you personally recommend if the cost of the material is not in the formula? I am certain I can add more strips later if they are needed. And thanks again for your assistance.
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Old 06-29-2008, 03:14 PM   #9
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Old 06-29-2008, 05:10 PM   #10
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That sounds better. Is the 3 3/4 inch gap arbitrary or calculated?
That's the 3 1/2 inch at the back rise plus the assumed 1/4 inch spacing. That should allow "full coverage" still where the air tumbling off the leading edge of the first strip in the group just misses the trailing edge of the last strip in the previous group.

Quote:
Would 4 strips be better than 3? Will the 12 inch strips work more efficiently than 8 inch? In other words, how much can I alter these values and which would you personally recommend if the cost of the material is not in the formula? I am certain I can add more strips later if they are needed. And thanks again for your assistance.
3 strips were considered optimum when used on sailplanes in the '70s, to promote a strong turbulation effect, most of the air gets "pumped" three times into nice tight and small energetic vortices. I guess the additional momentum transfered by the fourth and fifth strips had negligible further effects. I guess it's kind of redundant anyway to go with the extras. Sizing is a question of scale of the vehicle, on a longer, taller semi-trailer for instance you'd probably need 1/2 inch x 18 inch devices to perturb the boundary layer enough. On minivans and SUVs 1/4 x 6 inch devices would be adequate. Personally I'd go with the 12in length if you can only find 1/4 high strip and 8 inch if you can find the 3/8 high. It is kind of educated guesswork on my part, I've run the actual numbers before, and got a feel for the ranges, but don't have the books available at the moment to run them for the precise dimensions of your vehicle. But stuff like this you can be 25% off either way and get 90% of the results.
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