Feet forward Benroy - 180 degree change

[TimC]

Wondering if anyone has done a Generic Benroy cabin with a feet forward design. I don't know how awkward that might be to enter and exit with the door in the traditional location. I'm not a big person and fortunately still have plenty of flexibility in my joints. My goal is reducing drag with the frontal area as much as possible. I'm open to aerodynamic lessons if this seems foolish.

Some details...
-The generic Benroy plan has a 19" radius on the top front corner. I've drawn a front wall profile with the high point of that 19" radius at 42" (as opposed to 48") to improve aerodynamics behind my tow vehicle which is 58.5" high. The 16.5" difference is the height of the trailer chassis above the road. Utilizing the full 48" of the sheet goods for the front wall profile would result in 8" sticking up above the tow vehicle roof. Just to be clear... though I love Slim's ingenuity I don't want to strap a snow board on the back of my car

-Continuing toward the back, the roof would slope up another 3" to just forward of the bulkhead location. Maximum exterior height would be 45". At the center of the door I would have headroom of 35" above the 6" mattress and be about 36" near the back of the mattress. I've found I need a minimum of 32" to be comfortable sitting. If I could find a firm 4" mattress I could lower the profile a little more

-To allow for sitting and reading I'm thinking of running the bulkhead straight up from the floor to the hatch hinge and eliminating the inside storage space (cabinets) over what is normally the foot of the bed. A less substantial storage area might be up front over my feet. This will be solo camper on a four foot chassis so storage space needs will be minimal. With a twin mattress of 39" wide I will have about sixish inches on the road side to build some small cubbies for storage (no road side door).

Thoughts? Other than maybe a weird orientation would this work for you?

Edit... added a rough sketch


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[tony.latham]

Thoughts?

How about a sketch?

Keep in mind, one reason the teardrop has stood the test of eighty years or so is the basic design works. I see over and over folks trying to improve...

Tony

[Pmullen503]

I think it will be fine. The standard config has galley or storage space over your feet. If you eliminate that for headroom it should be no problem.

[GTS225]

My goal is reducing drag with the frontal area as much as possible. I'm open to aerodynamic lessons if this seems foolish.

You're thinking backwards. As an example, look at the profile of an airplane wing. The thick portion is leading in the direction of travel, not trailing. This is because it requires not a lot of energy to seperate the air with the fixed object, (the wing), but it does cause a lot of drag on the trailing portion as the airflow seperates from the surface of the solid object, and causes a low pressure area (vacuum), as it does so. Leading in the direction of travel with a "blunt" object, then slowly tapering the shape, so that the airflow stays on the surface until there's no surface left, uses much less energy to move the object.
(Visuals would be great right here.) So I'll just suggest you google things like "teardrop", "laminar airflow", and "aerodynamics", so that it will give you at least a modicum of understanding on the subject.

Something else (in the real world). Look at the back of semi trucks today. See those big flaps that can be folded down to the rear, or propped out at an angle? That's to break up the vacuum around that huge square back end that causes ddrag, and a related reduction in fuel economy. Those flaps never used to be on the rear of cargo trailers. That's a relatively new thing.

Now.....all that being said. As for aerodynamics, you're not going to be zipping along at 300MPH, so making it more "slippery" through the air won't net you as much as it does for an airplane. If, however, you're shooting for maximum MPG, then it's something to consider.

Roger

[TimC]

My goal is reducing drag with the frontal area as much as possible. I'm open to aerodynamic lessons if this seems foolish.

... If, however, you're shooting for maximum MPG, then it's something to consider.

Roger

Thanks for the info. I edited the original post with a rough sketch.

And, I'm looking at maximizing MPGe (maintaining range or miles per kilowatt), but that detail doesn't make a difference in my goal. Weight is my biggest concern, then aerodynamics. I can confidently design the weight reductions. I'm hoping for under 600# GVWR. The aerodynamics are another story but with some advice on tweaking the profile I hope to avoid big mistakes...

Tim

[Tom&Shelly]

Well, if I got in, I'm so used to the head forward I'd bump it and find myself in backwards before I knew what hit me!

What Roger says is right, it's the blunt tail that causes most of the drag. The air becomes turbulent and that energy to create the turbulence comes from your gasoline (or EV). Actually though, with a Benroy it probably won't make too much difference. Sorry to be so negative, but I wouldn't try it for aerodynamic reasons.

Guess I agree with Tony, they got the design right a long time ago. But, as they said at Woodstock about the purple acid, "hey, it's your trip!"

Tom

[TimC]

...

Guess I agree with Tony, they got the design right a long time ago. But, as they said at Woodstock about the purple acid, "hey, it's your trip!"

Tom

Thanks for your input Tom. As I mentioned my focus is to reduce the flat frontal area. My tow vehicle already has a pretty flat rear end so the Benroy profile doesn't change that there is a drag "problem" behind me somewhere. Unless I enclose the space between tow vehicle and trailer I can't fix the turbulence that that area creates.

I'm confident that you aren't suggesting I apologize for making changes to the Benroy design. I guess I don't really understand which changes I am suggesting are going over the line. It still looks like a Benroy profile, yet shorter. My first Benroy was six inches taller than the generic plan.

Traditionally, the Benroy is the ultimate design IMO. A future owner has the option to flip the pillow and sleep according to tradition and add some cabinets to the footboard. And as you say, how I sleep in it is my trip.

[Pmullen503]

The teardrop shape is more aerodynamic than a Benroy when you look at it from a side view. But slice it horizontally, and you are looking at rectangles which is not the most aerodynamic shape.

You also have the problem of the sharp corners all the way around. Think what a cross section at a 45' angle would look like and what the 90' corners do to that cross section.

Then you have the interior volume which needs to be practical for you to use. You also have construction techniques limited by what the average builder can accomplish (ie. square corners not rounded). The end result is that there are real limits to what you can achieve aerodynamically, especially with flat panel construction.

My next build is also meant to be towed by a Prius and I'm thinking primarily of keeping the weight low and keeping the frontal area as small and as much in the "shadow" of the TV as possible. I'm also thinking of doing a compound curved roof with rounded corners.

I've noticed on my current trailer that I actually get a more mileage reduction at 55 MPH than I do at 70 MPH. Counterintuitive I know but watching what happens while driving during a rainstorm at 55 vs 70, I seem to get a lot more turbulence between the trailer and TV at 55 MPH than 70 MPH so I'm thinking about what I can do to smooth out the transition in that area.

[tony.latham]

My goal is reducing drag...

Then using the Benroy profile is counterproductive. That boxy rear end will increase drag. Go with a traditional teardrop with a much better aerodynamic profile.





You might also consider the Camp-Inn Raindrop front profile. Camp-Inn says it gets better MPG than its traditional profile.





Tony

[Tom&Shelly]

I'm confident that you aren't suggesting I apologize for making changes to the Benroy design. I guess I don't really understand which changes I am suggesting are going over the line.

Certainly not! I don't think you are going over any lines!

I suspect, however, the changes won't improve aerodynamics or, therefore, mileage. It's the tapered back end that makes all the difference there, but that comes at the cost of an annoyingly less useful shape for the galley. (Not that I've used the galley of a teardrop with a more pointy back; just what we decided as we considered our design.)

BTW, we did a Benroy to maximize the interior volume with the 4 x 8 plywood we used for the core of the walls. Then, the flat roof gave us trouble as the PL glue we used expanded. We should have used Titebond III there. If I had it to do over though, I'd use Tony's profile, as the curve in the roof adds strength, makes the expanding glue a non-issue, and doesn't actually cut out very much volume.

Just my thoughts. I don't want to discourage your (or anyone else's) innovation!

Tom

[TimC]

Thanks again Tony, Tom and Pat. I haven't given up totally on the Benroy as it is my favorite profile, but I do see the validity of the argument against what I was attempting to do. I'm going to go back to the drawing board and sketch some other profiles to see what I come up with. I only have an 8' chassis (49.5" wide by 8' floor is finished). Maybe I can push the front up onto the tongue frame a bit to give more room for a shorter tail section.


I'm thinking a swing out countertop in the galley will help. I don't need a lot of storage but hope to have at least 14" deep on the galley floor to accommodate my fridge.
Tim

[TimC]

A quick sketch to address some of the suggestions. Keeping an eight foot length there's just enough room in the galley for needed items. Kind of like backpacking on wheels. 33" head height at doorway. Some space for cabinets above foot. Drawn with a 6" mattress. Finding a firm 4" mattress would make headroom even better.

Hatch clearance is a problem being only 5' high at this sketch position. Could swing it higher or maybe double hinge hatch. Changing the bulkhead to a "Z" configuration might help by having hinge forward compared to this sketch. Something to think about...

[Tom&Shelly]

Yes, that'll be more aerodynamic. And it looks really nice as well.

Tom

[tony.latham]

33" head height at doorway. Some space for cabinets above foot. Drawn with a 6" mattress. Finding a firm 4" mattress would make headroom even better.

33" means you can't sit up, which reduces the practicality of a camper. If the weather is nasty, we'll eat dinner inside.



A 4" mattress isn't thick enough. I had one in my first teardrop, and my hip could feel the floor when lying on my side.

Generally, in an 8' long cabin, the mattress space goes nearly all the way to the rear of the cabin--underneath the countertop.



There's a reason the standard teardrop has--in a sense--ruled for eighty years. If you follow the generic Benroy plans here, you'll have a fine looking and fine camping teardrop.

But... build what you want.

Tony

[Pmullen503]

A quick sketch to address some of the suggestions. Keeping an eight foot length there's just enough room in the galley for needed items. Kind of like backpacking on wheels. 33" head height at doorway. Some space for cabinets above foot. Drawn with a 6" mattress. Finding a firm 4" mattress would make headroom even better.

Hatch clearance is a problem being only 5' high at this sketch position. Could swing it higher or maybe double hinge hatch. Changing the bulkhead to a "Z" configuration might help by having hinge forward compared to this sketch. Something to think about...

The Z bulkhead idea not only gives you more height but moves the raised lip of the hatch forward.

Just another aerodynamics thing, I've noticed running down dirt roads with the trailer at speed raises an impressive dust cloud. So I wondered about turbulence under the trailer and what kind of drag that might be causing.

I got several large coroplast signs leftover from political campaigns and thought I could use those to cover the underside of the trailer to smooth it. I figured I could attach the coroplast panels with zip ties or plastic rivets.

Light, cheap, why not?