Aero RV Belly Pan(s)

[forums123]

Galane,

I really like your suggestion of the Angled Trail Skirts for big trucks and trailers with Dualies in the back. I have seen semi-truck trailers with those skirts on them and wondered what they were for. Orbywan, maybe you should try the trailer skirts to smooth flow the air around your back dualie tires?

Dave

[orbywan]

Originally Posted by forums123

Galane,

I really like your suggestion of the Angled Trail Skirts for big trucks and trailers with Dualies in the back. I have seen semi-truck trailers with those skirts on them and wondered what they were for. Orbywan, maybe you should try the trailer skirts to smooth flow the air around your back dualie tires?

Dave

I just want to reproduce something similar to the photo of the smoke stream in front of the Calibra in Huchos book on page 183. That would take a huge amount of guesswork on so many of these shapes.

Thanks for the suggestion Dave. This brings up the guess and by golly thing again. If you look at the photo of the smoke stream going under the front end of the Calibra on page 183 of Hucho's book, he says that the air coming over the front and rear wheels can be diagonal anywhere from 15 to 60 degrees from the center. In other words, the air going under the air dam and then over the front tires of, in this case an Opel Calibra, is hitting the front tires and to a lesser degree, the rear tires at a 15 to 60 degree angle coming from the center. WTH?

That statement (and the photo backs it up) and the range of variation in the angle, kind of complicates the hell out of designing the shape of wheel skirts, or fairings. I'm going to order a smoke generator so I can run it on my rig in various places and record the results with the camera system, and hopefully find out exactly what the air is doing under there. Other wise it's like my grandma used to say, it's all by guess and by golly.

[forums123]

ok, that sounds really cool. Please post pics of the smoke stream around the wheels when you get them

Dave

[orbywan]

Originally Posted by Galane

I'm thinking what might work well for belly pans is to go from the front to just ahead of the bottom of the firewall, then on the next section curve it up to fit to the bottom of the firewall/floorpan area, cutting as needed to fit closely around stuff.

Then you'd have an outlet for engine compartment air through a slot that shapes and smooths the airflow, and as little as possible of that hot air getting between the floor and the rest of the belly pan.

There is so much going on in that area I'm not sure that's possible but I'll try to incorporate that into the original plan which is to just open up the pans in that area so the frontal air can get out.

One article I found when I looked this stuff up was a paper on the effect of various rear mud flap designs. A full width flap increased drag and reduced MPG but one with a gap in the middle increased MPG over no flap at all. Have to find the article to see what the numbers etc. are.

I'd love to see that Galane, let me know if you find the source again, OK?

What's the average MPG for semi trucks these days? I recall not long before the flat front tractors died out, some company built one with every aero trick they could come up with and squeezed 10 MPG out of it, which was bleeping excellent for any semi.

Yes, if you want to see designs better than today, frequently you just have to look and see what they did back in the old(er) days. We (me included) seem to be stuck in having to re-learn the past sometimes. Especially true with aerodynamics and politics. Thanks for the suggestions.

[orbywan]

Originally Posted by forums123

ok, that sounds really cool. Please post pics of the smoke stream around the wheels when you get them

Dave

What I had in mind was some short video clips. Is that possible to do on ecomodder? If not maybe some UTube videos. Can you post a link to a UTube video on ecomodder?

[orbywan]

Originally Posted by forums123

After reading this entire thread: I realize I am relatively a noob when it comes to aeromodding but here are My thoughts and suggestions to get a little more out of your project:

1) Change all lubricating fluids to 100% full synthetic such as Royal Purple or Amsoil (engine, tranny, differential, wheel bearing grease)

Synthetics are superior to organic, but I've been that route before and saw no difference mpg-wise. I'd do it to the differential anyway but everything I read says it will eat the seals up on a 95.

2) Extend your boat tail further instead of making it square in the back

According to the NASA test at Dryden AF base in the late 60's, and several other studies, the last several feet of a boat tail only reduce drag by a very small amount. That's good news because if I take the tail to a termination point then I'm another 4 to 6 feet long, with a pointy tail and no place to hang my license plate and tail lights.

3) Flat panels over the wheel openings

Done. In the back anyway.

4) Air Bag suspension, lower cruise height on flat smoooth highway runs

That would be awesome, and I bet it would help a ton, but it's a huge amount of work and very expensive. It'll have to wait for now.

5) Boat Tail Nose of vehicle

Now you're just pissing me off. Ha. Just kidding. How much that would help depends on what studies you read, but the bottom line is to change the vehicles design to that degree is beyond my interest at this point. It would be a massive re-design of the front of the vehicle. So, I'm doing the next best thing, which is 'smoothing out' what is already there. Most studies say the biggest, easiest gains are in the rear by far.

6) Replace Both Door Mirrors with Video Cameras (you're already setup for this)

Good suggestion, that's in the works.

7) Get Aftermarket Computer Chip - Retuned for better fuel economy (some kits have handle held controller with both economy and performance switch)

I'm looking into that, and propane injection, later. If anyone knows of a specific econo chip for a 95 7.3 I'd really like to know about it.

8) Add Overdrive Unit or change rear gear ratio.

High on the list. 4.10 tail gears are not making this any easier. Overdrive is pretty pricey but I could swap the diff out with a 3.83 or lower for not much since I could sell my current unit. Have to research that further to see exactly what that would do for the rpms.

9) Drive a little slower

Call me Sammy because I can't drive 55. OK for 5 miles. Actually I'm only going 2 or 3 mph over that, I just feel like the spirit of impending doom is gaining on me at 55. Stupid, I know.

I hope to see you break 25mpg soon... If anyone can do it, I believe you can do it after seeing your craftsmanship and work ethic.

Keep it up

Dave

Thank you very much Dave, for the suggestions and the support. I'm giving her hell.

[Galane]

This page has some semi truck aero research data. Aerodynamic Resources : Aerodynamic Resources for Tractor/Trailers

On boat tails. There's plenty of research from the 60's when they were all the rage on race cars. Early wind tunnel tests found that a sharply cut off "Kamm back" tail worked almost identical to a full tail, as long as the angles were correct to keep the airflow attached.

Look up some pictures of those old racers, especially the Shelby Cobra Daytona 427 Super Coupe, and you'll see that a concave back panel was often used. The Daytona Cobra Coupe also used a sharply cut off tail with a concave panel, but the effect was spoiled by the addition of a spoiler on top to counteract lift.

[slowmover]

I'm looking into that, and propane injection, later. If anyone knows of a specific econo chip for a 95 7.3 I'd really like to know about it.


PM Big Dave. He's conversant with 7.3L mods. Powerstroke Nation and FTE are some enthusiast boards to wade through on the subject.

Do you have corrected tach/speedo readings? FF/RR certified scale axle weights? Measured tire diameter? All that sort of thing will be necessary to weigh alternatives for any proposed mods.

I'd certainly do a search on ambulances contemporary to your MH. Common problems in reliability and longevity exacerbated by hot, hard runs. Pickup trucks -- no matter the use -- are like greyhounds compared to your bulldog.

.

[Midniteoyl]

Hey orby.. great job so far! Joined just to follow your work

Anyways, as I was reading about your tail, the first thing that came to mind for your bottom side detached flow problem was strakes or fences. These help keep the air from 'circling' and force it more into a straight line. If it does spill over, it'll 'trip' over into a vortex. This idea is already in use on autos to help remove the air from under the car (rear diffuser). Cheaper and easier to try than redesigning the tail.

grrr - cannot post links or pics... Just google 'rear diffuser'


You are correct on needing to have more outlet for air in the belly pan. I believe what you did was trade under carriage drag for increased frontal and cooling drag. Try to think of how to get any air smoothly out..

Good Luck!

[orbywan]

Originally Posted by Midniteoyl

Hey orby.. great job so far! Joined just to follow your work

Anyways, as I was reading about your tail, the first thing that came to mind for your bottom side detached flow problem was strakes or fences. These help keep the air from 'circling' and force it more into a straight line. If it does spill over, it'll 'trip' over into a vortex. This idea is already in use on autos to help remove the air from under the car (rear diffuser). Cheaper and easier to try than redesigning the tail.

grrr - cannot post links or pics... Just google 'rear diffuser'


You are correct on needing to have more outlet for air in the belly pan. I believe what you did was trade under carriage drag for increased frontal and cooling drag. Try to think of how to get any air smoothly out..

Good Luck!

Thanks for the suggestions midnite, the strakes are a great idea. Since I'm going to extend the hitch back far enough to tow things, I decided I'd just do a quick rebuild of the lower area and drop that down about a foot or so. That will improve the air flow hopefully, and enclose some of the bracing I'm going to do for the hitch. After doing that, if the air still swirls to the right as my friends described, the strakes will go on. Good idea.

I totally agree about the pans. The engine cooling was doing unusual things and I'm sure it had to do with the air flow backing up in the frontal area, probably increasing my bow wave considerably, which is why I saw what seemed like to me almost no gain from the pans. It was also only one test run but when you work full time and it's 900 degrees outside and diesel is $4 a gallon, it's hard to do test runs sometimes.

The summer temps are coming down nicely lately, I've just got to find the time to do the changes in the back, vent the pans, build some wheel fairings, finish the diff, and do some more changes to the front. If I can bust 20 mpg I think I can get this beast in an RV magazine or two maybe. I do business with several RV manufacturers and dealers and I'd love to use this vehicle to get them looking seriously at aero mods for RV's and big vehicles. What was it someone said, these babies are 'target rich' for improvements. Uh, yeah.