Trailer roof edge question.

[ThomCat316]

I'm in the "thinking about it" stage of designing and building an aerodynamically slippery ultralight fifth-wheel travel trailer. One thing I'm not finding in the wild is information about turbulent drag over side-to-roof corners.

The simplest build for the center section of the trailer would be a rectangular box, or I could (almost as easily) put a radiused (anywhere from "sharp" to 6") edge on it. Any references, information or educated opinions about which way I should go?

[freebeard]

The worst starting point for the center section of the trailer would be a rectangular box

Not saying it can't be done; you could add a radius equal to half the trailer width on the front top and sides. It would look like this:

Any references, information or educated opinions about which way I should go?

It's hard to know where to start. Maybe with the tow vehicle? The Silver Streak Clipper pictured doesn't even have side-to-roof corners. But it might experience lift from side-winds. A trolley top would fix that. "An aerodynamically slippery ultralight fifth-wheel travel trailer" isn't going to be a box, it will want to look like an Art Deco zeppelin, so let it. Here's a design for a Class-A motorhome conversion, but it could be adapted to a 5th-wheel.

[ThomCat316]

So, to use a thousand words where one lousy drawing would suffice:

I will be using a Dodge Dakota as the tow vehicle, and will be modifying it for a bit more aerodynamic efficiency. The nose of the trailer will be fitted as tight as can be reasonably achieved to the cab, with gap fillers installed. The nose of the trailer will match both cross-section and plan view of the cab, and increase gradually to the max "box" dimensions of "less than 7 ft." height above ground by 82" wide, then tapering back down (and up, starting at the wheels) to a 6' wide by 4' tall tail.

The intuitively obvious observation is that "sharp roof edges bad", but all the modelling I've found is for semitrailers and it doesn't show induced vortices being generated over those long sharp corners. I know that radiusing semitrailers is a non-starter due to the cargo constraints, so there's not going to be a lot of study there anyway.

My real question is one of ease of construction and maximization of interior storage space. I'll have eight feet in the middle that will have a flat roof and could have from near-zero to about 6" radius from the roof to the sides, with benefits to the cabinet space on the interior accruing with smaller radii. I'm trying to gather any information to indicate what order of magnitude this induced drag might have, if any, so I can balance needs vs. economy.

[ChazInMT]

Hey Thom, I can't give you a figure on magnitude or anything but I can say that a radius on roof to wall will make the rig way more "forgiving" of any aerodynamic upsets such as cross winds and design issues where pressure variations are in close proximity to each-other.

Took the liberty of drawing a rough shape of what would be the best thing to do for a 5th wheel. It is a 25 footer with 7.75 foot ceiling max and 6.2 foot in the very back.





Full Size Version of Pic Here

I can whip up a drawing like this pretty quick, lemme know if you want to see it changed somehow and I'll do it.

[Sven7]

Square sections are doable...



...but you're going to want radiused edges if you're serious about it.



If you're willing to adopt airplane building principles, ignore these and build freebeard's zeppelin. But if you're more of a weekend carpenter I'd go with a box frame and generous radii.

I've been poking around regarding a pickup bed camper and have been banking on some pop-up elements to give more standing and sleeping room without sacrificing aerodynamics too much (5 degree taper on top). Maybe you could cut the frontal area significantly by popping up the top of your trailer!

Tent type

Hardtop type



Now, maybe the hardtop ones would be more applicable to your uses, but they could allow a much larger interior volume and better aero without giving up much but weight. If you were really into it, maybe you could fiberglass the top that hinges up, thereby avoiding the issue of building compound curvature with sheet metal.

[Hersbird]

I love the custom 5th wheel idea. My plan would take it a step further and make a chopped minivan the tow vehicle and a pop up roof on the trailer be no higher then the stock minivan roof. Imagine how cool a new Honda would look, like a bullet train.

Anyway, I plan on using standard galvanized ductwork opened up to 1/2 rounds on the front edge of my travel trailer to simulate a radius edge where a 90 is now, especially as mine is wider then the tow vehicle. That ductwork is less expensive then other materials I looked at.

[freebeard]

I'll have eight feet in the middle that will have a flat roof and could have from near-zero to about 6" radius from the roof to the sides, with benefits to the cabinet space on the interior accruing with smaller radii.

Overhead storage changes the picture. How many feet overall? How many axles? How long is the Dakota's bed?

With a vehicle body as tall or taller than it is wide, plan taper is more important because there's twice as much of it. Even if the overhead cabinet doors are flat, the inner [outside] wall can be curved.



The Tropfenwagen shape would be simple to construct. But the roof should be treated like an airplane wingtip. In fact the T-wagen looks like it would be better upside down.

A compound curve shape doesn't have to be hard, use bulkheads and stringers.



Cut your bulkheads like this:



That's a squircle. Here's the shape positioned halfway between a circular and square cross-section (but it can more square-ish or more circular, all laid out with a calculator and a ruler):



Then the stringers can be skipped and covered with fiberglass or butted like a redwood strip canoe (and covered with fiberglass). I have a friend who lived deep in the woods of coastal Oregon who went with plywood ribs, 1x2 stringers, and tensioning wires, then covered the outside with laborously hand carved wooden shingles and had the whole interior sprayed out with poly-eurathane foam. I'd have carved it smooth and painted it with that paint that forms a crust when exposed to fire, but he lived in it for years that way. It was basically a camper sliced out of an egg shape; and appeared in one issue of Shelter but I don't have the page number.

Here's an aerobody added over an egg shape:



If you increase the edge radius, you can bring the cabinet doors together to gain storage space.

[aerohead]

[ThomCat316]

Originally Posted by Hersbird

I love the custom 5th wheel idea. My plan would take it a step further and make a chopped minivan the tow vehicle and a pop up roof on the trailer be no higher then the stock minivan roof. Imagine how cool a new Honda would look, like a bullet train.

If only minivans didn't suck so much... I can't get an affordable diesel minivan with a stickshift in the US, and I'm pretty sure I can't modify any of the available ones to make it work... :-( That said, imagine an early Sprinter built as a close-coupled semi!

Anyway, I plan on using standard galvanized ductwork opened up to 1/2 rounds on the front edge of my travel trailer to simulate a radius edge where a 90 is now, especially as mine is wider then the tow vehicle. That ductwork is less expensive then other materials I looked at.

I am thinking of using 8"-12" diameter PVC pipe as a male mold to lay up my radius sections in 17oz. biaxial cloth, which will then get structural foam strips laid up inside with another layer of 17oz. glass on the interior. One benefit of this method is that if you want your edge to follow a curve all you need to do is force a curve in the PVC pipe that matches the design. If you need mirrored pieces you lay up a 180 deg. section and split it to two 90 deg. bits.

[ThomCat316]

As is typical, the community is amazing!

My simplest question is thoroughly answered - at the very least I put a 4"-6" radius on the longitudinal/parallel roof-side edge to improve the handling - good enough for me.

Chaz, thank you for the offer! I can send over a PM when I have a minute with a dimensioned sketch of the truck and the trailer - I haven't had time to put it all together just yet. The idea is similar to your drawing, but with the bed sides cut down for tighter vertical clearance and the front of the trailer blended more smoothly behind the cab. Additionally, the area behind the (single) axle will taper up for road clearance.

Aerohead, I really appreciate the bus drawings - they point out what I already had intuitively suspected, but in a very clear and definitive way! I am planning on keeping the cross-sectional area to a minimum, the tapers gradual, and the rear area relatively small. One interesting point will be at the back of the wheel wells, where the trailer is already getting narrower but the wheels haven't.

Freebeard, I love the full-aero shapes!! Pity is that I have constraints that keep me from taking full advantage of them. I have to have flat roof space for solar panels, standing headroom (while maintaining small cross-section) and storage at the edges of the "overhead" space in the largest sectional area. I don't intend to have an overall height of more than about 7 ft, which leaves no overhead room to speak of. One aerodynamic concession I've planned to make all along is an "ideal curve" taper (top, sides and bottom) from the largest section to the tail, over a length of about 8 ft., with the tail being a squared oval of as small an area as I can manage without detaching the flow.