Foamie aerodynamics.

[KCStudly]

Makes me think of those collapsible backpacking cups.

My first thought is that whatever style sweeper seal you use between the sections (garage door seal or RV slide out) will be interrupted by the exterior shelf std.'s. I'm assuming that the std.'s you referred to are the channel type with slots at regular intervals. Water will be able to go through the slots and travel through the channels bypassing the seals.

Otherwise I think it is a great idea... if you must have the ultimately short trailer over the road. But is it worth the effort to build it?

Engineering and design are always a compromise between design goals, cost and ability to manufacture (i.e. method... which is the same as cost).

Method and cost don't seem to be a problem on this scale, but the design goal of keeping water out may not be worth the design goal of maximizing towing efficiency. Internal space vs. aerodynamics. Is that worth the challenge of the complication of keeping it water tight?

Just $.02 worth of wax.

[GPW]

Here’s an idea , keep like it is , but make just the center section pop up , the front would be storage, the rear the galley .... Much easier to lift ... still streamline ...collapsed ...

[Bogo]

I think the challenge is worth it if I can keep the overall weight low enough that it could be towed with a very high efficiency vehicle. My quick guesstimate is 1000 to 1200lbs. It has close to twice the wall, floor and ceiling area of a MicroRV pickup back design I did weight estimates for. Then I added a more for the tires, and frame. The build here would be a bit lighter as I don't have a full perimeter exterior frame, and I'm planning on FRP skins over foam. My guess could be wild. After the shape is firmer, I'll put all the parts into a spread sheet to calculate their weights and distribution. That will give me a number that should be within 10%.

Water seals, etc. for the joints between the sections. At the bottom inside of the top and middle sections there will be a stiff brush seal with a rubber flap at the bottom. That will keep all but the hardest driven dust, dirt, mud, and water from going up between the walls when they are down. When lowered, a lip at the top of the middle section will seal against the top of the base section's wall. There will be a rubber gasket there. The top section will seal against the top side of that lip on the middle section. The seals will be stacked right above each other and provide a clean load path. So far I've managed to keep it all able to be made out of standard gaskets, parts and metal shapes with only simple machining used. Using custom extrusions would make it much simpler, but there would be the cost for the die, and around 300 lbs of extrusions for each shape. That would be much more length than I would need for a trailer.

When raised the middle section is lifted by a lip at the bottom inside of the top section. That lip is a little higher than the brush and flap. The lip engages a similar lip at the top outside of middle section. They have a gasket to keep the elements out as well as stop air flow. BTW, this lip is designed to not slip off. It is interlocking. A similar lip set is used for the other joint, but it is for limiting how high the top can be raised. There is also a pair of flaps as that joint may not be pressed tight.

On water and the shelf standards for the inside ones. They don't extend into the area where water sealing is done so no issue there. The outside standards could get water into the slots that the shelf brackets lock into. The bottom of the standard would need to be setup to drain. Also the back of the standard would need to be water sealed so water doesn't get into the structure of the wall.

The lift spring mechanism I'm thinking of using would be simple to gear for any load. The rear storage area may end up with the air conditioner embedded in it. The front one now looks like it will mostly be cover for the generator, etc. while in motion. It will lift with the top, but the generator, etc. will stay at the hitch platform level.

[mezmo]

Hi Bogo,

Just throwing out a couple things to consider as you refine your
design.

The front can be the same shape as the rear in-so-far as the
'aero-ness' goes. Since a trailer is in the 'punch-hole' through
the air which the tow vehicle has already made, all that it's front
edge needs is to be within that tow vehicle cross-sectional area.
'Aero' shapes on the front of a trailer there are really just 'styling'
as andrew has maintained many times. If you like the look of the
semi-circle front radius design, then go for it. But if you are thinking
to just have it be a cover for a tongue box of sorts, [as you stated],
make the front the same shape as the back. That'd be easier to build -
no kerfing and fitting for a radius - - you could build two identicle farings
for the front and rear and then attach them to the center box sections. The
more verticle front faring would also give you more usable space in the
tongue box.

Also, remember to radius all the edges and corners, even if it is only
a small amount of a 1/2 in to an inch or two. Again per andrew, and
what I've read, this is an essential detail.

Here are a couple links to the Australian offroad Kimberley Karavan.
[A fantastic product in my view.] They show their front and rear
faring shapes that are very similar to your rear shape, and the edges and
corners have that small edge and corner radius detail. For towing, the
whole Karavan fits pretty much within the tow vehicle's cross-sectional area.

http://www.kimberleykaravans.com/index.php?item=home
http://www.kimberleykaravans.com/index. ... n_overview
http://www.kimberleykaravans.com/index. ... y_overview

Just a couple observations...

Cheers,
Norm/mezmo

[Junkboy999]

3 sections? I working on designing a pop-up but just one section and it is driving me crazy.
I guess I’ll just have to keep an eye on this ideal.


As far as sealing it. This is how I plan to seal mine. I believe Sharon posted this pic of a pop-up seal.
The one on the right is how My top will seal to the inner wall with a Kinda-of slide past contact seal.

[Bogo]

This design is 80" wide on the outside, an artifact of having 2" thick walls and two lifting sections. That adds 10 inches to the overall width over a simple wall with fixed roof. Most cars I'd get, my small pickup, and 4Runner are on the order of 68" wide. Any frontal area that is wider than the TV needs to be properly aerodynamic, and due to cross winds, the aerodynamic area likely needs to extend a good foot or more in from the width of the TV. A highly aerodynamic vehicle like the Prius does a very good job of getting the air flow to come together quickly behind it. At 5 feet back the zone of disturbance is likely around half the size of the back, guesstimate from seeing a wind tunnel smoke image. For my pickup and 4Runnder that make no effort at air flow control the zone will be a much higher percentage of the size of the back of the vehicle. So, yes frontal aerodynamics is important. It's really the cross winds, which are the norm, that cause the need for front aerodynamics, and they are a real consideration at the sides of the front. the lower middle front could be vertical, but higher up it needs to be sloped where the smooth air flow from the TV hits it.

For the front and rear shapes, I'm much more likely to be cutting and carving blocks of foam. I'll likely have ends of the front curved into the middle for better aerodynamics. The hard edges on the top will be radiused. Likely from 1" to 2". At this point, my Sketchup drawing is a concept. It is a very rough approximation so I can get an idea of looks and visualize how it all fits together. That is allowing me to see where I need to tweak the design without having to maintain a complex model in my head. As an example the storage that is in the front and rear aerodynamic cones will be high up near the ceiling. Much more so that the previous design. The bottom of the top section is 80" off the ground when raised up. That would be inaccessible from the ground. It is making me lean towards using some of that space for heater and air conditioner. and other similar dedicated uses. They are still reasonably accessible from the inside, but on average higher up. I wouldn't be wanting to put heavy modules into one of them. Clothing would work.

Why is it that I can buy a 3k BTU RV air conditioner in Europe, but not in the USA?

Got side tracked doing some weight estimates and forgot to get the above post finished and posted.

This design is two lifting sections, and one fixed base section. Overall lowered height is 50.5" with 31" off road tires, and lower than 48" with Prius sized tires.

[Bogo]

This is all preliminary stuff as some design stuff isn't finished yet, like floor and frame design, but curiosity was burning a hole... I also stayed with aluminum skins and full perimeter frames instead of using fiberglass skins and fiber glassed exterior corners as I haven't looked up the data yet. An additional thing I realized is I don't have the weights for the lifting mechanism included. It would be at most 50 lbs.

I took the spreadsheet I did for estimating the Micro RV and plugged in the numbers for this trailer. I'm not done yet as I have to figure out the weights for the front and rear aerodynamic ends. So far it is looking good. Using Foamular 250 as the foam I'm getting 1213 lbs for the trailer with body, add on whatever the aerodynamic ends are for total weight. It looks like there will be roughly 900lbs of bed, appliances, water tanks propane tanks, etc. including a Honda EU3000iS generator. All water and fuel tanks filled adds an additional 430 lbs. I'm figuring with luggage, etc it'll be under 3,000 total towing weight. A bit higher than I expected, but within reason and the towing ability of my current vehicles. The body has come in close to the high end of what I expected. It is the utilities and appliances, etc. that added up much more than I thought they would. There is no way I could get this down to 1,000 lbs for towing with a Prius. This design for my requirements and uses will never be towed by a small light vehicle so using two lifting sections for keeping it low looks like it maybe isn't worth it for me. I can build taller because my tow vehicles are taller.

Because it is a spreadsheet and I have weights calculated in it I can quickly change the length and width and see how it effects weight. For each additional inch of length added it is only 5 lbs. An inch of width adds 7 lbs. Adding 20 inches makes the length of the box portion of the top 12' or the same as the width of 3 sheets of foam. I get 9 sq feet additional space for only 101 lbs. Adding 12" to the width adds 90 lbs more plus allot more wind drag so I'm not sure it is worth it. It would allow the bed to go crosswise which would make the bathroom/kitchen area much larger. It also messes up having the same wheel track width as the pickup and 4Runner. I'll have to think more on that change.

For small car towing... Shortening by 32" and narrowing by 6". Shrinking the inside length and width to that of a queen bed. That drops 196 lbs to bring the core trailer to 1017lbs. For towing by a Prius, I think the tires could be narrowed a couple inches from what I have and have the same track width as the Prius. That brings the body and frame down 208 lbs for 1005ish lbs plus aerodynamic end weights. Note, that still includes the weight of the 31" off road tires, trailer or Prius tires would be less. I have 160lbs for tires, and 120 for axle. The 1005 lbs doesn't include any bed, utilities and appliances. Just the wheels, axle, frame, and box. Some shrinking of the height could also be done which would allow some more weight savings. It's only a few lbs per inch. Now, my design uses 2” thick foam to clear the frame of Peninsula Glass's 1800 series double paned windows, going to 1” thick foam allows an overall shrinkage of 38” in length and 8” in width which drops the weight of the queen bed sized one to 944 lbs without aerodynamic ends. That weight is even more reasonable, but still high for a small tow vehicle. BTW: a Prius is 68.7” wide and the 1” insulation standie is 72” wide. Note: That overall shrinkage is based on the top lifted section. For somebody who has a small car it may work for them in stripped form.

An issue I'm running into is the height of the storage space under the bed. The module concept for cabinets, counters, table, etc. is nice for those things, but for refrigeration it is problematic. Some regular ice chests are under 14” high, but all of the reasonably sized compressor based ones are higher. Ultimately I'll be getting one of them. The coolers end up on the very limited entry/bathroom/shower floor space, or have to be stowed in the tow vehicle. Neither is what I want. Stretchingg the length 20 inches helps alleviate some of the cooler issue. Note: even my Coleman 5 day Extreme is 18” high so it won't go under the bed. The ARB Compressor coolers, 37q to 82q, range from 16.85” to 20” high. Raising the bed height to get the coolers under it raises the seat height of the sofa. I'm already on the tall side for seat height.

Note: Walls are all Foamular 250 foam with 0.040” aluminum sheet bonded inside and out. Ceiling is the same thickness with 0.080” top surface, and 0.040” inside surface. Floor is 0.125” aluminum diamond plate floor surface with Foamular 250 foam core, and 0.032 aluminum bottom layer. I still need to get the numbers for Baltic birch plywood, and 1/4” laminate flooring. I need to rework the floor allot as I need to handle the shower pan area, and move the insulation to the bottom so the basement area can be heated. I can go to much thinner aluminum on the top surface for basement use. I'm expecting to end up with something like a 3/4” Baltic birch floor with 1/4” laminate flooring glued on top of it for most of it. Where the shower pan is, the floor will be the shower pan and the framing will be lowered some to allow the shower pan lip to be flush with the main floor. Then an insert will raise the effective pan floor to the height of the regular floor. I'm expecting it to weigh more, but it will also look nicer.

I hope I got all the numbers right. Part way through I figured out I wasn't including the axle and tire weights into the base trailer.

[Bogo]

Looking again at that NASA link a few pages back.

Another thing I've been curious about since finding about the technique decades ago is air turbulators like AirTabs. http://www.airtab.com/main.html I noticed they were mentioned at the NASA link. Looks like for just a tractor/trailer gap between cab and trailer they can provide 3% to 4%: http://www.airtab.com/MIRAReport.pdf Of course this is a test report on their web site, but it is inline with what I've read elsewhere in the past. At the back off a squared off trailer they should help allot. They may also help for the gap between the TV and trailer.

Interesting report from an expediter on his Sprinter: http://expeditenow.com/truckgear/173-an ... rtabs.html A front bug shield cut 3.5% to 4% off his fuel economy. Before and after wind tunnel tests.

They are $2.75 per tab and are to be placed every 4 inches. That's 34 for the back of my three tiered popup trailer. Figuring 15 MPG towing it behind my 4Runner, $4 gas, 3% fuel savings, the breakeven payoff is around 12,000 miles of towing.

For a little fun, Mercedes playing with aerodynamic trucks.

Note the rear edges of the trailer. Not turbulators, but a set of roughly 6" inch deep extensions of the side walls and roof beyond the rear doors.

[kudzu]

Interesting this older thread popped up again & that you mention the Airtabs. I've been considering them for my Ford Transit Connect van. Have also thought those could help on the back end of my future toy hauler build. I don't like the aerodynamics of the larger, squarish rear of my current design (which is still all in my head) but that is the most practical for loading/unloading toys & gear. Rounding the angles just a bit will help also, even if it means the doorway height is reduced somewhat but I can't see getting as far as a teardrop shape. It would mean a hatch which isn't as easy to build or as useful. I prefer barn doors. So keeping the larger rear but adding Airtabs would seem like a good plan.

Am confident Airtabs on the van would pay for themselves but not sure about the trailer. Can't calculate possible fuel savings without specs on my plan but since my trailer would be used primarily for weekend trips it may not get enough miles for the fuel savings alone to warrant the cost of Airtabs. Yet, when I consider the likely reduction of wear & tear on the TVs engine/drivetrain from reduced trailer drag I can start to justify adding them. And then...

Hey, it's a foamie. I can just sculpt Airtab equivalents & glue those on. That should work, no?

[kudzu]

Interesting report from an expediter on his Sprinter: http://expeditenow.com/truckgear/173-an ... rtabs.html A front bug shield cut 3.5% to 4% off his fuel economy. Before and after wind tunnel tests.

In there it mentions the increased drag of dirty, bug splattered vehicles. Haven't read this entire foamie aero thread yet but am wondering if the trailer finish has been addressed. Would it be worth the extra work to get the finish smoother? BTW, when I mentioned that part about add'l drag from dirty vehicles my BF just rolled his eyes. His cars are always clean whereas mine tend to be ... um... well, left to nature.

[Bogo]

I'm just dropping aerodynamics stuff into this old thread as it has a bunch of other stuff on aerodynamics.

I never wash my vehicles, but then I drive dirt and gravel roads allot. I let the rain clean them off. I'll clean bugs off the window, but elsewhere I let the hummingbirds pick them off.

The new VW XL1 (europe only) has a drag coefficient of only 0.189 which makes it the lowest for a production car ever. BTW: 261 mpg hybrid electric/800cc turbo diesel drive system. I want one!!! http://www.wired.com/autopia/2013/05/vo ... ewall=true

Take a look at how much they curved the front end.


Look at how they pulled that rear end in, and the sides again have that bit of extension past the rear wall.


Oh, they mention using a diffuser on this car, so it has a full belly pan to smooth off the underbody area.

[Wolffarmer]

Some strange ideas I have had to lessen drag with a trailer.
#1 telescoping tongue. As speed increases the tongue shortens up. Higher speed vehicle is going the less turning it will do so can get buy with a shorter tongue. When speed goes down tongue lengthens. Could really scare the pea wadings out of somebody when you hit the brakes and tongue goes full length all of a sudden. Could be tied into the vehicle electronic system for speed sensing.

#2 gap filling panels between TV and Trailer. Hinged on the vehicle and going very close to the Trailer. Actuating rods attached to trailer tongue and to the flap so when turning the inside flap pushes out to allow turning.

Some of my ideas but it does seem to me that the gap between the TV and Trailer is probably the biggest drag maker.

Oops, in idea #2 that is with a rather square backed SUV or pickup with camper shell type vehicle.

Randy

[Bogo]

#2: The panels could fold up when going slow, and open up when going fast.

[mezmo]

I've often thought you could do such panels in polycarbonate
or such clear plastic sheets so as to not obstruct rear vision as
opaque panels would.

Also, I wonder if a heavy spandex like stretchy fabric panels
would work in a similar fashion between the tow vehicle and trailer.

[kudzu]

i think a spandex sock would measurably reduce drag though not as well as a solid surface. Could it hold up on th road? It works as a body sock for recumbent bikes but you"re.not going over 50 mph on bikes.