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

[aerohead]

Originally Posted by orbywan

OK, I'll try that. Can't wait for it to cool off a little so I can get back on this. Do you think lengthwise strakes running front to back on the belly pans are benefitial enough to try them?

To put it a different way, if you only had time to do one or the other, would you build fairings for the duallies and front wheels, or put lengthwise strakes on the belly pans?

I'm afraid that my answer won't be as good as your question but here's what comes to mind.And I hope some of the other members chime in.
*with the belly pan,the air furthest from the ground should be moving close to the velocity of the air along side the vehicle.If so,then there would be no spanwise flow.Strakes,as developed for aerospace,are to work as a flow fence to sequester air and prevent it from flowing sideways to a region of lower pressure,where it would spin up into an attached vortex.This pressure differential would not be great on a road vehicle.One of the recent land speed record motorcycle streamliners developed in the GALCIT tunnel at Cal Tech exhibited no under body flow disturbance except very close to the wheels,which self-corrected a small distance downstream.They used nothing under the bike except nice smooth bodywork.
*Air under and beside the vehicle is basically flowing back in a straight path,with the lowest kinetic energy unlike over the roof of a car.
*Low-drag vehicles will have wheel fairings to mitigate pressure spikes in the region of the wheels and provide for sectional density of the overall vehicle,allowing only very gentle changes in body cross-section.
*I'd be inclined to go after the wheels before I went into micro-managing airflow.
*get all the pick and shovel work done first,then go after the minute details.
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I do have 'strakes' under my diffuser,but they are structural reinforcements for a long sheet metal span and were never intended as a streamlining device.
They could actually hurt in a crosswind.Don't know.

[orbywan]

Originally Posted by aerohead

I'm afraid that my answer won't be as good as your question but here's what comes to mind.And I hope some of the other members chime in.
*with the belly pan,the air furthest from the ground should be moving close to the velocity of the air along side the vehicle.If so,then there would be no spanwise flow.Strakes,as developed for aerospace,are to work as a flow fence to sequester air and prevent it from flowing sideways to a region of lower pressure,where it would spin up into an attached vortex.This pressure differential would not be great on a road vehicle.One of the recent land speed record motorcycle streamliners developed in the GALCIT tunnel at Cal Tech exhibited no under body flow disturbance except very close to the wheels,which self-corrected a small distance downstream.They used nothing under the bike except nice smooth bodywork.
*Air under and beside the vehicle is basically flowing back in a straight path,with the lowest kinetic energy unlike over the roof of a car.
*Low-drag vehicles will have wheel fairings to mitigate pressure spikes in the region of the wheels and provide for sectional density of the overall vehicle,allowing only very gentle changes in body cross-section.
*I'd be inclined to go after the wheels before I went into micro-managing airflow.
*get all the pick and shovel work done first,then go after the minute details.
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I do have 'strakes' under my diffuser,but they are structural reinforcements for a long sheet metal span and were never intended as a streamlining device.
They could actually hurt in a crosswind.Don't know.

I was hoping you would say that, I know just enough to be hazardous, but I agree. When I figure out the shape for the fairings I'll run that by everyone for some feedback before I build them.

Driving in a crosswind was one of the reasons I don't like the idea of strakes, seems like that would cause a serious amount of turbulence the entire length of each strake. That plus I didn't want to mount them on top of the belly pans. There's enough stuff down there as it is.

I know this will make everyone laugh, but my goal is 20mpg, straight and level, no load, no A/C. If I can improve the numbers by about 18% I'm there so I'm hoping some redesign work on the the front, plus the front air dam, plus the wheel fairings will put me in the neighborhood. If they do, then I'll do the last mod which will be 3:55 gearing or as close as I can get. Maybe an under drive if the final drive works out to 3:55 or close but I think it will be much lower than that, which I don't think will make for improvements.

Actually the last consideration will be HHO and/or adding propane. The propane is questionable for sure, but I achieved a consistent 20% increase in mpg with my Tundra with an HHO system, but the paraphernalia and increased 'figit' factor required to deal with the truck's ECU was not worth it to me. Not to mention the fear of burning valves from running it in the 17 to 18:1 mixture range. The best results were in the 16:5 to 17:1 mixture range with minimal HHO flowing believe it or not. The 7.3 doesn't have the O2 sensors to deal with so it should be much easier.

Anyway, it's all hot air until you do it, so I'll shut up until it's done.

[aerohead]

I perused the 1995 Sunrayce Technical Report again last night.
They made mention that with one of the George Washington University cars. When full wheel fairings were added, the overall drag was reduced over 34% (a 17% 'fuel' savings right there).
Hucho says that with low-drag vehicles,the wheels can constitute 50% of the drag.
Only more perspiration will tell!

[orbywan]

Originally Posted by aerohead

I perused the 1995 Sunrayce Technical Report again last night.
They made mention that with one of the George Washington University cars. When full wheel fairings were added, the overall drag was reduced over 34% (a 17% 'fuel' savings right there).
Hucho says that with low-drag vehicles,the wheels can constitute 50% of the drag.
Only more perspiration will tell!

Wow, that's great. I needed some encouragement Phil, thanks. It's on the drawing board, just have to find the time to build them. I was just out in the shop showing the mods to someone. It's like an oven out there. The new weight loss program, lose 2 pounds in 15 minutes.

[orbywan]

Originally Posted by aerohead

I perused the 1995 Sunrayce Technical Report again last night.
They made mention that with one of the George Washington University cars. When full wheel fairings were added, the overall drag was reduced over 34% (a 17% 'fuel' savings right there).
Hucho says that with low-drag vehicles,the wheels can constitute 50% of the drag.
Only more perspiration will tell!

I looked at the Sunrayce Technical Report some last night. I'm looking for some photos and any info I can find on wheel fairings for some inspiration on the shape of the wheel fairings, especially for the dually rear wheels. Anyone have any reference materials or links?

Something interesting happened yesterday. I took the Aero RV to get it emissions tested. I didn't think about it until I was driving onto the dyno-like rollers to test the emissions output, but just then I realized the entire undercarriage is covered by belly pans, including all but the last 5 feet of the tail pipe. The guy whips out his mirror, 'on a stick', as Jeff Dunham would say, looks underneath and calls over his supervisor. I'm thinking Oh crap. The super walks over and says, 'Sir, you're entire undercarriage is covered'. I said, yes, I know. He says, 'We can't...' trails off and then says, 'What year is this vehicle?' I said '95'. He said, 'OK, nevermind.'

There's no cat on this vehicle (it's a 7.3 diesel) so they didn't have to confirm the cat was on. Anyway, something to remember when you're building belly pans, if you put a screen or something under the cat to smooth the flow, make it easily removable so the emissions guys can easily identify it.

So anyway, then the first guy sticks the probe in the tail pipe, which is now several feet under the boat tail. He comes back to the cab looking really perplexed and worried and says, 'What the hell is that thing in the back?' I said, 'It's a boat tail.' He just looks at me, deer in the headlights. I said, 'It's to improve the aerodynamics'. All the sudden the furrowed brow turns into a big smile. 'Cool!!', was all he said and we finished the test.

After the testing he prints out the results (it passed), and walks back over to the cab and hands it to me, looks over at the console where the 9 inch camera monitor sits, with four camera views showing, and says, 'What the hell is that?' I said, 'It's a camera system'. He looked at me real serious like, gets a big smile on his face and says, 'Cool!'.

Anyway, that was one trip to the emissions station I enjoyed a lot, for a change.

[aerohead]

If you can get access to Sighard F.Hoerner's book 'Aerodynamic Drag',he has some tables depicting different aircraft wheel fairings and wing tanks which mysteriously appear on solar racers and record cars.
Simple 'canoe' fairings are always a safe bet as long as they have some fillet at the attachment intersection.

[orbywan]

Originally Posted by aerohead

If you can get access to Sighard F.Hoerner's book 'Aerodynamic Drag',he has some tables depicting different aircraft wheel fairings and wing tanks which mysteriously appear on solar racers and record cars.
Simple 'canoe' fairings are always a safe bet as long as they have some fillet at the attachment intersection.

I'll see if I can find a copy. Thanks.

[slowmover]

Originally Posted by orbywan

Slowmover am I reading the specs right, doe your Dodge have 3:05 gears in it? I know those 6 bangers are torquey but wholly canole. I must be reading that wrong. How much does your trailer weigh?

3.73 stock anti-spin AAM rear axle. Single OD 6-speed manual. (305 horsepower with 555 LBS/FT of torque.)

I am considering going to the 3.42 ratio as on later CTD's (which have larger, more powerful engines) to accommodate my "low" GCW of a maximum 17k (truck can do much more weight, but the permanent nature of my rig -- home in all respects -- means I can afford to experiment as there won't be any commercial service or the need to do major contractor jobs as with a home, rental properties, etc as before [divorce]).

The 3.42 ratio would make me slow off the line (and in hillls/mountains), but would keep me much closer to the rpm range "sweet spot" for this engine in town or country. Living at 46-ft above sea level has some benefits at present.

The truck is currently at 7,936-lbs set for solo running (recent set of scale tickets to "see" total solo vehicle weight, and then wheel-by-wheel to dial in tire pressure against load; and then a second set will be done with trailer with & without weight distribution leverage applied [truck and trailer, together and separately]), and the travel trailer -- which has a GVWR or 8,00-lbs -- is currently at just over 7k. OAL is 63' of which the trailer and hitch is 36'.

But, remember, it's very easy for me to make weight changes as it is not with a Class C. I can move around in a skin of fron 14K to 17k fairly easily (although the lower number is not realistic, only possible). Another trailer (asap) may weigh more than this one. That, with a move to a mountainous state (altitude, not just up & down grades) might kill the 3.42 idea. While 1,000-lbs is no cause for notice in performance, a range of 3,000-lbs is.

Aero, for me, will be treating the vehicles separately and not worrying over combined results overmuch. Mods will be more about using the very least amount of force/time (steering, braking, etc) to affect vehicle motions. Then, aero will have it's place (for me).

I believe we covered propane in another thread . . my take was that a "gallon" of propane cost 3/4 as much as a gallon of diesel but with 2/3'rds the energy. Thus, unless adding a big horizontal tank to my truck for purposes of running a generator and thus also increasing the amount available to the travel trailer, the benefit to the truck was singly in extending the range of the diesel carried aboard (and not really a concern unless diesel had become more difficult to obtain). Propane needs to be nearly dirt-cheap to be of "true" mpg benefit.

Am pleased to read about the possible aluminum re-skin plus paint on yours!

.

[euromodder]

Originally Posted by slowmover

Propane needs to be nearly dirt-cheap to be of "true" mpg benefit.

It also make the diesel burn more efficiently and more cleanly.


Over here, we have dirt cheap propane / LPG @ 39% of the price of diesel and 34% of the price of petrol, so it becomes viable to fuel semi's with a combination of diesel and LPG .

[orbywan]

Originally Posted by slowmover

3.73 stock anti-spin AAM rear axle. Single OD 6-speed manual. (305 horsepower with 555 LBS/FT of torque.)

I am considering going to the 3.42 ratio as on later CTD's (which have larger, more powerful engines) to accommodate my "low" GCW of a maximum 17k (truck can do much more weight, but the permanent nature of my rig -- home in all respects -- means I can afford to experiment as there won't be any commercial service or the need to do major contractor jobs as with a home, rental properties, etc as before [divorce]).

The 3.42 ratio would make me slow off the line (and in hillls/mountains), but would keep me much closer to the rpm range "sweet spot" for this engine in town or country. Living at 46-ft above sea level has some benefits at present...

.

OK, that makes sense. 3.42 sounds low to me but I'm sure you know your stuff. I have a question for you if I may. Where do I go here in Tucson to get my Class C weighed? Do they have weigh stations at some truck stops?