Air Flow design

[McFish1951]

I am designing a Foamie. The trailer is a 6 x 8 utility bed with side rails. Does anyone have hard data about the gas savings provided by slanted roof line in front vs a straight box? The box is much easier to build and has more storage. With a box I can place a bulkhead at the front to stiffen the box and create storage for headboard type items like cpap, keys, glasses and clothing.

[Tom&Shelly]

I am designing a Foamie. The trailer is a 6 x 8 utility bed with side rails. Does anyone have hard data about the gas savings provided by slanted roof line in front vs a straight box? The box is much easier to build and has more storage. With a box I can place a bulkhead at the front to stiffen the box and create storage for headboard type items like cpap, keys, glasses and clothing.

Short answer is no, unfortunately.

This comes up from time to time on tnttt in various forms. Of course, "gas savings" converts to ease of movement of the camper through the air, behind a tow vehicle. Axle friction (which presumably changes relative to how much the air is pressing down on the camper) and the creation of turbulence take energy out of car/camper systems and so are to be avoided as much as possible. There are three ways to estimate these relative to the shapes of the units (this works with aircraft too, which is why I have the ready answer): 1. Wind tunnel, 2. Extremely numerically intense computer simulations, and 3. Experiment. As teardrop builders, we can't afford 1 or 2. You might see some attractive looking software on the internet that would make you think you could do 2, but read the fine print: it's 2D code used for educational purposes that looks qualitatively similar to the real thing. But to make any real world estimates, especially quantitative ones, you need 3D code and a super-computer.

That leaves experiment. You can build two copies of a camper, and then test them with your tow vehicle. Of course, that costs about twice as much, takes twice as long to build, etc. etc. Not likely anyone cares enough about the answer to actually do it.

Now soft data, that's a different story. Search tnttt and you will find several interesting threads. And most of us will opine that the more streamlined the shape the better it's likelier to be.

Tom

[Pmullen503]

I am designing a Foamie. The trailer is a 6 x 8 utility bed with side rails. Does anyone have hard data about the gas savings provided by slanted roof line in front vs a straight box? The box is much easier to build and has more storage. With a box I can place a bulkhead at the front to stiffen the box and create storage for headboard type items like cpap, keys, glasses and clothing.

That will depend alot on how the air is coming off the tow vehicle. If the trailer is taller or wider than the tow vehicle the front of the trailer is a bigger deal than if it fits in the TW's slip stream.

[tony.latham]

provided by slanted roof line in front vs a straight box?

The rear of your camper is just or more important than the front since the front is shielded by your tow vehicle to one degree or another.



Tony

[McFish1951]

Thanks everyone, especially Tony. OK, so a lot of measured data doesn't exist, or is difficult to locate. I guess I must default to anecdotal evidence. For example, based on what we have seen in previous posts, we got about 60% mpg (vs no trailer) with a box, and 75% mpg with a square drop. What I really need to do is make a guestimate as to the box style penalty in gas savings so I can decide if the extra pain to add a drop is worth it.

[tony.latham]

What I really need to do is make a guestimate as to the box style penalty in gas savings so I can decide if the extra pain to add a drop is worth it.

There are online wind tunnel websites you may find useful. Make sure you add in your vehicle. I have not seen any that are three-dimensional so I don't know how useful they are. Camp-Inn says their Raindrop model gets better mileage than their traditional teardrop. I felt a noticeable difference when I went from a 4' wide to a 5' wide.

Traditionally shaped teardrops are an airfoil, and they do create some lift because the versions with Fantastic fans raise the lid and induce a low pressure inside the cabin. (Fantastic fans only have one side of the lid supported.) Because of this, Camp-Inn only uses Maxxair fans that have double-lid supports. I have a lid lock on my Fantastic that I use when we're towing and I no longer get dust coming in through the window vents.

Shape matters.

Tony

[Pmullen503]

Besides better aerodynamics, a curved roof adds rigidity and isn't that hard to do on a foamy.

[working on it]

provided by slanted roof line in front vs a straight box?

The rear of your camper is just or more important than the front since the front is shielded by your tow vehicle to one degree or another....

Mine's not a foamy, but it is a squareback "box", so I was trying to mitigate air "drag" when building my 4x8. It was supposed to end up under 1000 lbs, but wasn't even close, made of 3/4" plywood with lotsa steel inner bracing/hines & latch hardware/bolts etc; but I tried to fit it within the wind shadow of my wife's '08 Chevy Cobalt (it was going to be her trailer, but I took it over) or of my '09 Chevy HHR Panel, both with a towing capacity of 1000 lbs.

I wanted the front of the trailer to have a sloped roofline, to match the side profile(s) of those vehicles, but they were different, at approximately 35 degrees for the HHR an 55 for the Cobalt. I compromised at 45 degrees, which made cutting the plywood side profiles & doors much easier, and determined that the height of the leading edge of the front slope should match the height of the Cobalt's rear wing, 40" (or 42", I forget) off the ground. That fit nicely with the floor height of 24", which left 16-18" for footroom below the angled front slope (in this trailer, you sleep feet forward the nose). Just for reference: the drag coefficient for the Cobalt is .32, and the HHR is .354, which we'll come back to, later.

Trailer designed to follow either HHR or Cobalt, 45 degree angle slope a compromise.jpg (190.66 KiB) Viewed 3663 times

At the rear, I knew there'd be drag induced by the vertical hatch, so when I covered the gap between hatch and main body, I used excess semi-rigid conveyor-belt material, which would rise up into the airstream, and cause eddies to reduce the suction.

rear hatch (gap) cover acts as an airfoil.JPG (84.18 KiB) Viewed 3663 times

I did have some road dust infiltrating past the rear hatch into the galley/storage area, but I added weatherstripping and right-angle clamps/latches, which eliminated the problem.

Since the trailer was way too heavy for either of the pre-build designated tow vehicles, I tow it with my '04 Chevy 2500HD pickup, whose open (or semi-open, now covered) 8ft bed made the "in the wind shadow" effect moot. I never tried to evaluate mpg figures with and without the trailer attached, because the heavy truck (with 1000+ lbs of tools & gear in the bed, and equipped with 4.10 gears and a thirsty 6.0 liter gas engine) gets about the same mpg at 75 mph, towing or not (as I found out while towing my old Chevelle drag car on a tandem axle open trailer). In fact, it hardly notices the 4x8 squareback trailer back there.

When I resume camping (after a year's interlude), I might just use my '01 BMW X5 E53 as the tow vehicle. I got it as my final project car, intending to camp and soft-road travel in it (before my health problems got a bit worse), but it can easily pull the 2225 lb trailer (whether or not I install a brake controller in it...it will cost a bunch to adapt off-the-shelf, non BMW stuff to it), at least to local state parks around N.Texas or S.Oklahoma. The X5 has a good drag coefficient of .36, so it might provide the "wind shadow" effect I initially was hoping for, with the smaller vehicles I was designing the trailer for.