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

[skyking]

Don't worry about the front, the dryden test and all the wind tunnel data scattered all over this site says otherwise. The 'money' is in the back

[slowmover]

Split A/C unit

Here's one post from a long thread on AIR regarding a split HVAC unit on an Argosy travel trailer (also diesel powered, no propane). Argosy was the bargain version of an Airstream, painted instead of shiny. (A custom build by a highly knowledgeable owner stretching himself. His tow vehicle carried a Mini Cooper and pulled the trailer. Great read).

[aerohead]

Originally Posted by orbywan

I didn't mean it's never been done before, I've just never seen one on an RV, except in PhotoShop. An 18" boat tail seems really short, but it was a short vehicle, goes to show how much they make a difference I guess.

Doing a belly pan on this RV is going to be a pain but I kind of have to finish it. If I put a full pan on it, should I also make a front air damn?

Both my CRX and T-100 both showed an improvement with an airdam even though they are both bellypanned.
Ford's Probe-IV had an active deployable airdam which lowered at speed to block air from under its fully bellypanned underside.
Many of the lowest drag concept cars will at least lower the car at speed to reduce airflow underneath.
So that's kinda a 'yes' for the airdam.

[aerohead]

Originally Posted by orbywan

I don't drive it much now but it's good knowing it's efficient for it's size. I still have to finish the A/C unit, skin the undercarriage (that's going to be serious fun) and do some mods to the front so I'm hoping there's better numbers ahead. Time will tell.

I was surprised the Dryden tests didn't show better numbers for rounding off the front, so I went after the tail first. I found another chart on a truck products company's web site, which I can't find at the moment, that shows rounding a brick shape object in the front reduces Cd by 50%, which I think is the way the average Joe thinks, including me until I tried the boat tail.

Anyway, I'm going to try several things up front, there's serious room for improvement there also.

Moller improved the Volkswagen 'brick' from Cd 0.76,to Cd 0.42 with front radii.
Walter Lay,in 1933,built a full-size brick on a GM truck chassis.Leading edge radii cut drag from Cd 0.88,to Cd 0.42.
In 1963,GM investigated 18-wheeler drag.A fully radiused front was just a smidgeon ( sp?) better than an 18-inch radius.
In 1936,Fachsenfeld modified an Omnibus bus,with an inflatable boat tail which brought the drag down to Cd 0.158.His tail is the best I've seen so far.It's in Hucho's book.
The NUNA-3 team boat -tailed 'everything' on the car and achieved Cd 0.077.
The back really is where the pot-of-gold it hiding.

[orbywan]

Originally Posted by aerohead

Moller improved the Volkswagen 'brick' from Cd 0.76,to Cd 0.42 with front radii.
Walter Lay,in 1933,built a full-size brick on a GM truck chassis.Leading edge radii cut drag from Cd 0.88,to Cd 0.42.
In 1963,GM investigated 18-wheeler drag.A fully radiused front was just a smidgeon ( sp?) better than an 18-inch radius.
In 1936,Fachsenfeld modified an Omnibus bus,with an inflatable boat tail which brought the drag own to Cd 0.158.His tail is the best I've seen so far.It's in Hucho's book.
The NUNA-3 team boat -tailed 'everything' on the car and achieved Cd 0.077.
The back really is where the pot-of-gold it hiding.

OK, I’ll play with the air dam when the belly pan is finished, thanks for the information Phil, and everyone else for weighing in on this, the comments and advice are very appreciated. I know enough to be dangerous at this point. I just spent the last few days before and after work building the front part of the A/C fairing, photos are below.

It was probably more trouble than it was worth but I wanted to add a rear looking roof cam to my system and this way I was able to incorporate that into the fairing. It’s kind of long but I still have room for solar panels. Probably would have been easier to just take it off and relocate it but it’s done for now.

[skyking]

Good looking work there man. Pic #6 took me back to 1977, R2D2 in the x-wing fighter
Shark-gill louvers on the back should allow the AC to work with that fairing on.

[orbywan]

Originally Posted by skyking

Good looking work there man. Pic #6 took me back to 1977, R2D2 in the x-wing fighter
Shark-gill louvers on the back should allow the AC to work with that fairing on.

I love those domes, they're perfect for small stuff. Now that I've got an extra camera feed up there I can vent the rear cover and remove the tape from the A/C unit, mount a camera off to the side temporarily and watch it as it goes down the road to see if it's over pressurizing the cover or trying to come apart.

Once I'm sure the A/C unit can work properly I'll rebuild the rear cover with metal. That or remove it all together and replace it with a split unit like the one in slowmover's link, or a heat pump mounted in the top of the boat tail. This will have to do for now.

Next up is finishing the belly pan. This is where I wish I had a hoist.

[aerohead]

Originally Posted by orbywan

OK, I’ll play with the air dam when the belly pan is finished, thanks for the information Phil, and everyone else for weighing in on this, the comments and advice are very appreciated. I know enough to be dangerous at this point. I just spent the last few days before and after work building the front part of the A/C fairing, photos are below.

It was probably more trouble than it was worth but I wanted to add a rear looking roof cam to my system and this way I was able to incorporate that into the fairing. It’s kind of long but I still have room for solar panels. Probably would have been easier to just take it off and relocate it but it’s done for now.

According to Sighard Hoerner's book,your AC unit,with fairings should now be down around Cd 0.07.There is nothing lower out there.Sweet!

[orbywan]

Originally Posted by aerohead

According to Sighard Hoerner's book,your AC unit,with fairings should now be down around Cd 0.07.There is nothing lower out there.Sweet!

Cool, thanks Phil.

[slowmover]

Originally Posted by orbywan

Cool, thanks Phil.

Yes, thank you!