Lifting roof XPStream - making improvements

[OP827]

Looking great! Following with interest! Do the images below remain your intent? The gap below the roof filled with the rigid folding panels? Bravo!

Thanks Bluebunny! Yes the intent and trailer profile is practically the same, and yes the gap is filled with folding rigid side panels, but I am having second thougths after I have assembled the walls...
After I've seen the actual size and got a better feel of space in front, I started to think that I may put there a short folding wall 16" high or so. It would be similar to Boxcar build by Osborn http://home.comcast.net/~bdosborn/Boxcar/Boxcar.html, but on the slope and curved roof, short wall in front and tall wall in the back instead of current idea with only a hinge in front. Due to longer trailer, the gain in height with just a hinge in front may not be enough to be nice and comfortable while staying in the lower ceiling part of the kitchen area in front of the trailer. BTW for your design the front hinge may work fine since your floor plan has a bed along the trailer length in front, my floor plan is different from yours. I plan for two double beds in front and back across the trailer.

So.. I am thinking of doing the hatch which will produce the same trailer profile when folded, but when unfolded, there will be a higher ceiling in front. Thinking of its detail design, stepping back from actual building for a few hours, maybe days. I may still do more testing with mockup roof before I go any further.

Doodling a hand sketch of the front side of the hatch and roof. Tried Solidworks 3D model for the trailer to start with, but I found it is a long learning curve before I can do so many moveable parts in Solidworks, so I decided to do manual dimensional sketches and actual build of what I am sure should ( ) not change in the trailer design and then make design decisions as I build and mock up the parts together. Folded streamlined profile, door location, kitchen arrangement and floor plan are not changing, but the lifting roof design has to change from the little mockup photo you mentioned above; after I mocked the walls up in actual size and stepped inside the trailer.

I know it is old school, but I personally like hands on activity more than sitting in front of a computer and learning 3D modeling.
It is a hobby, not a production, so I can afford it.

Cheers.

[Bluebunny]

I know it is old school, but I personally like hands on activity more than sitting in front of a computer and learning 3D modeling.
It is a hobby, not a production, so I can afford it.

Cheers.

Solidworks or sketch up or pencils or whatever are simply tools. The craftsman and his/her ideas bring things into reality.

As to head room, maybe take a look at your roof curvature. Maybe humping it closer to the hinge will relieve the headroom loss nearer the hinge point without too high a penalty on aerodynamic drag. May actually help stabilize the flow at the turbulent leading edge? It will increase the drag AREA, but a larger area with good form drag may beat a smaller area with poorer form drag. Incidentally, several big-rig aero studies have shown that a 12 degree down-slope will provide good form drag. Steeper up to 15 will work pretty well. Also, taking your advice, I modified my roof design to have flat runs fore-and-aft of a single arc nearer the front hinge; still should provide good aero, simplifies construction.

I took the liberty to sketch TWO roof lines on photo your model that would provide varying degrees of more head room, without killing the aero too badly. Offering it simply to whet your thought process. It sure is looking good so far!

[OP827]

Thanks for looking into this with me, appreciate that! I may do some mockup with long strips to play with the curve tonight and see.

[OP827]

Bluebunny,
I did some more measurements of lifting geometry. Non-dependent of the roof shape and profile, just based on hinged lift geometry and proportions, if I want trailer to be same height as my TV (Nissan Pathfinder'12) and with roof hinged in front, then it is about 6" short in above stove location to be free standing almost touching ceiling in that spot. Will have to before considering raising the folded trailer height another 6".

[Bluebunny]

I understand your concerns. Fuel economy is a big deal. Consider also that if you want the box fully within the wake of the tow, it might need to be SHORTER than the tow. The wake flops around behind the tow, dropping down somewhat in the gap. Assuming your sharp hinge point height is fixed, raising the roof well back of that edge may not hurt the aero much or at all if the transitions are gentle. If you are able to get your roof to lift instead of hinge, so much the better. If you cannot solve lifting easily, your original idea with more hump shouldn't kill the aero. Your side curves sure should help highway economy as well. Good luck solving this challenge; we're following with interest.

[OP827]

I understand your concerns. Fuel economy is a big deal. Consider also that if you want the box fully within the wake of the tow, it might need to be SHORTER than the tow. The wake flops around behind the tow, dropping down somewhat in the gap. Assuming your sharp hinge point height is fixed, raising the roof well back of that edge may not hurt the aero much or at all if the transitions are gentle. If you are able to get your roof to lift instead of hinge, so much the better. If you cannot solve lifting easily, your original idea with more hump shouldn't kill the aero. Your side curves sure should help highway economy as well. Good luck solving this challenge; we're following with interest.

Thanks for encouragement and technical support. I do not know if it makes sense to model this difference in Autodesk Flow if I can get a hold of the software.

I also started to think of more complicated roof shape with not just kerfed foam in one plane, but with some spherical element to it. Are you aware of any software that can help design a cutting pattern for a say egg shape or such so I can cut it out of foam and then glue? I am thinking of centre line of trailer ceiling to be higher than sides to help with ceiling height with less price for front section drag, but maybe I just complicate things too far. The roof top planned to be only made of foam with most likely a glass fibre thickened epoxy in kerfs needed for curvature and then glassed on both sides for structural strength. There will be reinforcements for where they needed to tie it down to lower walls.

You know aerodynamics right?

I mostly figured how to make a strong, waterproof front wall and hatch design on the slope, so it could be the way to go, but I just wanted to "step back" and think about all options before I go further. Hatch on the slope is more time to unfold and fold down than with front hinge, but the hatch could be easier to build and be generally stronger or reliable due to less opportunities for twisting/ripping the hinge out with such long roof. Front hinge seemed to be quite an elegant solution: one lift move from the back and walls are up if assisted with gas struts in right places and if I could find some proof that the aero drag not changing with another 6-8" height, I may stick to current concept. Back to concept thinking for now.

[Bluebunny]

Thanks for encouragement and technical support. I do not know if it makes sense to model this difference in Autodesk Flow if I can get a hold of the software.

I also started to think of more complicated roof shape with not just kerfed foam in one plane, but with some spherical element to it. Are you aware of any software that can help design a cutting pattern for a say egg shape or such so I can cut it out of foam and then glue? I am thinking of centre line of trailer ceiling to be higher than sides to help with ceiling height with less price for front section drag, but maybe I just complicate things too far. The roof top planned to be only made of foam with most likely a glass fibre thickened epoxy in kerfs needed for curvature and then glassed on both sides for structural strength. There will be reinforcements for where they needed to tie it down to lower walls.

You know aerodynamics right?

Panel products like foam and plywood can only be formed in a flat arc section of a cylinder or a conic section of a cone. Short truncated sections of cones can come close to spherical. The tricky part are joining these svelte shapes back into your box. Suggest modeling here with stiff flat stock, tape, and scissors?


There are a lot of opinions about predicting efficient flow shapes. My particular opinion is that the trailer is in dirty air, at the mercy of the particular tow shape. Try to sneak the nose within the shrinking shadow of the tow, and then be very gentle with it from there...not very helpful, is it? A lot of research is helpful, if only to show how difficult it is to predict flow. I strongly caution against amateur computer flow modeling; it is tricky to get the conditions correct, and may give you good confidence in a bad shape.

Your intuition of aero seems spot-on in my opinion. Good compromises of form and function. follow your instinct and enjoy your t^3...

[OP827]

Panel products like foam and plywood can only be formed in a flat arc section of a cylinder or a conic section of a cone. Short truncated sections of cones can come close to spherical. The tricky part are joining these svelte shapes back into your box. Suggest modeling here with stiff flat stock, tape, and scissors?

- Yes, true for thin sheet. Kerfing is the example when the flat sheet is not separated to many pieces, but still produces conical or cylindrical shapes AND when the kerfs are crossing each other at some angle then to some extent the two plane curve surface appears. In my case I could come up with a kerfs pattern when cylindrical shape across the trailer transitions into a conical or shallow cylindrical shape towards the back, all that without actuall cutting all the way through the foam, just usual 75% of panel thickness. Schematically, without actual calculation of kerfs places and positions the kerf pattern of the roof could look like this to produce the shape with shallow curves still providing advantage of stiffness increase and aero properties:



EDIT:
I just wanted to go further and have the kerfs extend and crossing in some places when I want that 3d arc for slightly more headroom in front for example, how that pattern then would look like I wonder? Maybe like this:


To make it simplier, I plan on fixing the final roof shape on the trailer in a folded shape and then cut it apart for roof part and lower walls and make side walls to match for unfolded position.

There are a lot of opinions about predicting efficient flow shapes. My particular opinion is that the trailer is in dirty air, at the mercy of the particular tow shape. Try to sneak the nose within the shrinking shadow of the tow, and then be very gentle with it from there...not very helpful, is it? A lot of research is helpful, if only to show how difficult it is to predict flow. I strongly caution against amateur computer flow modeling; it is tricky to get the conditions correct, and may give you good confidence in a bad shape.Your intuition of aero seems spot-on in my opinion. Good compromises of form and function. follow your instinct and enjoy your t^3...

- Thanks, the shape is sloping gradually in height at less than 12 degrees angle, so it should be good. I just need to find that compromise, you are right. . Be back soon.

[Bluebunny]

Bravo! You have a perfect combo of a conical, where the kerfs intersect at a focal point, with a SEPERATE cylindrical, where the kerfs are parallel. This is the limit of bent panels. If you were to attempt a cross- cylindrical or cross-conic on say your front curve, the laws of nature will fight you and win! Alternative is to try to fit say three narrower full length conic/cylinder pieces having wedged edges together, giving you a sort of curve?

[OP827]

Bravo! You have a perfect combo of a conical, where the kerfs intersect at a focal point, with a SEPERATE cylindrical, where the kerfs are parallel. This is the limit of bent panels. If you were to attempt a cross- cylindrical or cross-conic on say your front curve, the laws of nature will fight you and win! Alternative is to try to fit say three narrower full length conic/cylinder pieces having wedged edges together, giving you a sort of curve?

Could you rough sketch that, so I can understand, I am kind of a visual person you know or you're saying that my second sketch with crossing kerfs will work?

[KCStudly]

Even though the kerfs cross in the second sketch, there is no place for the remaining unkerfed side of the sheet to escape to when attempting to form, so it won't want to go into a compound curve. To do that you would need to add a dart, or relief to remove some material strategically. Then the area of the dart provides the relief and the edges of the dart can be glued back together when the desired shape is formed.

A good sheet metal CAD design software (such as AutoDesk Inventor) could help figure this out.

[OP827]

Even though the kerfs cross in the second sketch, there is no place for the remaining unkerfed side of the sheet to escape to when attempting to form, so it won't want to go into a compound curve. To do that you would need to add a dart, or relief to remove some material strategically. Then the are of the dart provides the relief and the edges of the dart can be glued back together when the desired shape is formed.

A good sheet metal CAD design software (such as AutoDesk Inventor) could help figure this out.

KC,
You are right about unkerfed side, it will not be able to escape. You lost me with this "dart" relief term, could you explain a little more?

There should a way to claculate out a cut out shape that I would place on the sides of the roof to add curvature. The cut could be made with hot wire all the way through a foam thickness and the cut out does not need to be wide, but it should be narrow, long at a certain distance to allow for smoother shallow curve and in order to work, what do you think? I could do a quick template out of plywood or sheet metal to be positioned on both size of the foam for a precise cut, but I need to calculate the curve in it to come up with needed finished panel surface.

[OP827]

BTW, I will work with a dimensionally wider and longer piece of foam for the roof part to do kerfing and cutouts and only when it is glued up I will cut the sides to match the sides of the trailer, so everyone understands that there is no concerns about outside dimensions of the panel to be calculated and final before all curves are done.

[Bluebunny]

Cross kerfing is very tricky, darts can allow it, but you are "separating" the pieces at the darts anyway. Things can get complicated in a hurry, especially the edges.
Another caution, you show a rectangle before bending...unless the side walls get CLOSER TOGETHER towards the rear, your roof width will come up short back there. Either taper the wall angles, or taper the roof panel Wider toward the rear. The formed cone will narrow in the width direction when bent.

Separate pieces is also very difficult getting the edges to line up. Watch your panel widths.

If you continue this path please MODEL it at least "a little" with stiff flat stock. I'd get me some thin wood "door skins" to practice with (scissors and glue or tape), but any STIFF flat stock will do. You may be surprised at the edge conundrums.

(Waaaaaiit a minute... why not...thin.....foam!)

I am embarrassing myself with my 3-d renditioning super-techie design package - results shown below... but t'ellwittit...was in a hurry to be responsible to my bride...my apologies to your eyes...

[OP827]

Another caution, you show a rectangle before bending...unless the side walls get CLOSER TOGETHER towards the rear, your roof width will come up short back there. Either taper the wall angles, or taper the roof panel Wider toward the rear. The formed cone will narrow in the width direction when bent.

- as I mentioned above the rectangular plan view roof shape will be made after a roughly larger shape of panel is formed, glued and done will all forms and cut outs. Only when I am happy with the shape I got, I'll to the finishing side cuts.

Separate pieces is also very difficult getting the edges to line up. Watch your panel widths.

If you continue this path please MODEL it at least "a little" with stiff flat stock. I'd get me some thin wood "door skins" to practice with (scissors and glue or tape), but any STIFF flat stock will do. You may be surprised at the edge conundrums.

(Waaaaaiit a minute... why not...thin.....foam!)

I am embarrassing myself with my 3-d renditioning super-techie design package - results shown below... but t'ellwittit...was in a hurry to be responsible to my bride...my apologies to your eyes...

- Hey, I'll try to kerf and cutout some scrap foam piece in order to form a torroidal form I am trying to form here for the central part of the roof and report later. Getting free spring egdes to align would be a challenge. Need to think of a "technique" or some jig.