New and seeking aero advice for my "brick"

[Farraider]

Hi Everyone,
I have a camper I built from a 1994 square-box-style ambulance. It has a 7.3 IDI diesel and get about 13mpg at 55mph. It has a huge flat front area that just grabs the wind and I keep thinking about putting something up there to make it more aerodynamic. I am trying to find a way of figuring out the benefit I might see with a more sloping front...and what other mods I can do to benefit mpg in a cost effective way. I'll try to post a pic but imagine any box ambulance you have seen before.
Thanks for any help/advice (accept "sell it")
FR

[kach22i]

The classic response to your inquiry is that drag is created at the aft section of the vehicle, after the high point in the roof (at the rear).

However after looking at your "brick" I see there is ample opportunity at the front where attachment of the air built up from air pressure occurs.

The air pressure build-up after the cabin as it hits the box is off the scale, those rounded corners help, but I cannot but help to feel your pain.

Let me try and sketch something up, a picture is worth a thousand words.

EDIT:
I marked up eight things off the top of my head, looks like a lot of work. If you are going to do that much work, look at the Ram-Aero camper thread of a few days ago.

Link:
https://ecomodder.com/forum/showthre...udy-36886.html

[aerohead]

There's great paper available online from NASA and their 'Shoebox' project from the 1970s at Edwards Air Force Base,California.
Maybe GOOGLE NASA truck aerodynamic research.

[kach22i]

Originally Posted by aerohead

Maybe GOOGLE NASA truck aerodynamic research.

I was thinking of the Dryden projects, but found this under scholarly articles, it's a book more than a paper I think, and I have not seen it before. And it seems to go on forever, keeps loading more pages and more images every time I scroll.

https://books.google.com/books?hl=en...search&f=false


Here is the NASA Dryden project page: (a fragment of what's out there)
https://www.nasa.gov/centers/armstro...-100-DFRC.html

EDIT: Speaking of books, this is new to me as well.

https://www.amazon.com/Fairing-Well-Aerodynamic-Monographs-Aerospace/dp/1780398999


Click "LOOK INSIDE" in the upper left hand corner of the page.

Spoiler: Lifting-Body content in preview.

[kach22i]

Here ya go, some numbers to go with the theory and forms.

Aerodynamics Research Revolutionizes Truck Design
https://spinoff.nasa.gov/Spinoff2008/t_3.html

Rounding the vertical corners on the front and rear of the cab reduced drag by 40 percent while decreasing internal volume by only 1.3 percent. Likewise, rounding the vertical and horizontal corners cut drag by 54 percent, with a 3-percent loss of internal volume. Closing the gap between the cab and the trailer realized a significant reduction in drag and 20 to 25 percent less fuel consumption. A second group of tests added a faired underbody and a boat tail, the latter feature resulting in drag reduction of about 15 percent. Assuming annual mileage of 100,000 driven by an independent trucker, these drag reductions would translate to fuel savings of as much as 6,829 gallons per year.........................

Perhaps most importantly, drivers of vehicles fitted with Airtabs have reported improved stability and handling and dramatically reduced fishtailing of trailers—an effect where the trailer sways or slides from side to side independent of the tractor, potentially causing catastrophic loss of control—effects that are especially important with the double trailers found in North America and the famous quad-trailer “road trains” in Australia. Increased stability also means that the trailer does not scrub on the sides of the road as much, increasing the life of tires. Drivers also report better handling when being passed in the same direction by other large vehicles.

There is half-hearted acceptance of Airtabs in the forum in my opinion, speaking for myself, I was curious but cautious about jumping on any bandwagon I didn't completely understand. Maybe I will take a second look now.

Farraider, as far as adding a #9 to the sketch for Airtabs I will leave that up to you.

[freebeard]

Thanks for any help/advice (accept "sell it")

Accept ≠ except. Sorry, but you asked.

One of our forum members has had success implementing this design as a way to radius a square corner without cutting into it.



At the doors, it might need a concave inner half. I've had less success with this:



But modern bus design follows the principle of this stap-on:


https://ecomodder.com/forum/showthre...tml#post494966

So maybe you could add a cab-capper with a little added frontal area and show a net gain in Cd. Would you be interested in a retractable boat tail back porch? What's the use case anyway?

[Frank Lee]

Why add the frontal area?

If those protruding radii at the vertical leading edges of the camper work, why not turn 'em horizontal and slap one on the forward leading edge of the windshield/roof junction with no added Af?

That whole ****ing Greenshield thing makes me want to puke.

[kach22i]

Originally Posted by freebeard

But modern bus design follows the principle of this stap-on:


https://ecomodder.com/forum/showthre...tml#post494966

So maybe you could add a cab-capper with a little added frontal area and show a net gain in Cd. Would you be interested in a retractable boat tail back porch? What's the use case anyway?

I posted that graphic and the comments that disputed it's working. I also said something similar to what Frank Lee just said.

I like the idea, but Aerohead has answered this same question many times in the forum and said the additional frontal area just doesn't pay off. At least that's what I recall, will stand corrected if my memory is bad.

Just clarifying that is a bad graphic, hence it's inclusion into the "Random UnAerodynamic Cars and Trucks Thread".

If newer buses do have this feature built-in I question if it's just a marketing gimmick.

[freebeard]

Bad example. I found it first when I was looking for this:


https://ecomodder.com/forum/showthre...ics-17717.html

Originally Posted by Frank Lee

Why add the frontal area?

Adding interior volume. So long as the curve respects The Template, flow can reattach to the (admittedly short) flat top. There's normally a low pressure area behind the top edge.

Alternatives would be a row of vortex generators or a blown slot.

[Farraider]

Thanks for the sketch Kach, great looking ideas. Freebird, I rushed the post and caught the error right after I hit submit but had to head out the door. Or may be I meant to have to accept selling it, hmmm, that's it!

Anyway thanks all for the ideas & suggestions but I'm trying to get an idea of my cost and effort for reward ratio. Is there a site to help me compare different modifications and what the energy savings would be? I'm sure I could spend $10G making it look like a rocket but I'm not going to do that. Im trying to find that sweet spot in the ratio (effort<=reward) and what to expect.

Question: If I have a square meter box, what is the aerodynamic reduction of force required to push that through air at 60mph if I instead it had a 45 degree leading edge? Is this an applicable question for the front box of the truck?

If that greenshield thing works, maybe I could mimic the idea and use it to hide my spare up there. Reasonable?

Thanks again everybody.
FR