Featherdrop - An Over-Engineered Teardrop

[timm-TNT]

Hey guys, so I've been through several iterations of trailers and this is what I've settled on. I'm calling it the "Featherdrop" trailer as it's going to be very light (estimating around 600 lbs. all up). I'm a mechanical engineering student in university right now, so I've been applying my limited engineering knowledge as much as possible.

It's been designed with a couple design parameters in mind:

Aerodynamic

• Rounded corners everywhere
• Tapered Rear
• Air Splitter between TV and trailer
• Wheel Fairings
• Extendable top to minimize size when towing and maximize size when parked

Light

• Getting a custom trailer frame fabricated to cut undesirable weight and unnecessary strength
• Using foam to construct non-load bearing components (air splitter, rounded corners)
• Using construction techniques similar to old-fashioned airplanes
  • Using a CNC to lighten plywood sheets by cutting pieces out. (~100 lbs. saved)
  • Covering wood panels with canvas and exterior paint then assembling panels.

Easily Buildable

• Delegate as much cutting as possible to the CNC
• Use as many right angles as possible
• Light panels that are easy to handle
• Very simple window design

With that out of the way, here are some pictures of the design.

Side Profile

Front. The trailer doesn't stick out past the car very much. The wheels might be slightly closer to the trailer depending on the axle I get.

Rear. The tail lights may change yet, these are placeholders.

This is how the rounded corners will be created, the white pieces are plywood pieces and the red is foam.

The foam can simply be cut into a rectangle and glued into place. I'll be 3D printing a curved sanding drum and sanding the corners into the correct shape.

The design is essentially 2 plywood boxes, one inside the other. The inner box is mounted to the plywood floor. The outer box is connected to the inner box by 10 drawer slides oriented vertically, it is raised with 4 custom pneumatic cylinders.

The door and door frame are removable and are replaced by a travel cover which covers the hole left by the door when the trailer is collapsed.

Here's an animation of the setup process.
You can also see the lightweight skeleton style frame for the roof.
[youtube]sRWdZ4DMGN4[/youtube]

The front, right, and rear windows can open 20%. They're designed such that rain won't enter the cabin even if there's some wind. They'll be hinged at the top and the hinge will be covered by a 3D printed flashing. The window in the door doesn't open. There are 2 holes on either side of the front window for the computer fans I'll be using for ventilation.

Cutting specific sections while leaving others allows me to keep most of the strength of the sheet while decreasing the weight significantly. Here's one of the panels (inner box right side). Cutting these pieces out reduces the weight of this panel by about 45%. The 3 wider vertical sections are where the drawer slides will be attached.

I haven't delved into the electrical much, but the plan right now is to run very basic 12V wiring to the computer fans, several interior lights, and one or two exterior lights. There will be a car battery either inside the air splitter in the front or possibly inside the rear hatch depending on how my tongue weight works out.

The pneumatic cylinders are already partially built, I'll be running some pneumatic tubing to each of the corners and pressurizing the system will a small car tire compressor. The roof should lift around 30-40 psi and once it's up I'll place supports in each corner and halfway along the longer walls and then depressurize the system.

The kitchen area in the back has a countertop 40" wide and 35" deep which should give me plenty of room for a propane camp stove and anything else I might need to use. The entire area behind the cabin is storage and there should be plenty of room for a cooler and several containers for food and other supplies.

There should be enough room inside the cabin for clothes and a couple other things if needed.
This is a size comparison to the Generic Benroy plans I found under design resources. The blue lines are the interior of my design and the black lines are the Benroy. My design has a ceiling about 12" higher and the cabin is quite a bit longer as well.

Thanks for looking this over! I realize it's a pretty long post, but I've already done a lot of work on this so I don't want to leave stuff out.

If you have any thoughts on this thing I'd love to hear it!

 

[OP827]

I like your ideas and CAD design, subscribed. Did you estimate trailer weight via CAD by applying density to the materials? I would consider foam sandwich vs. skeleton plywood to be lighter. Back edge is better be left sharp, not rounded, for better air flow separation.

Sent from my Moto G Play using Tapatalk

 

[Andrew Herrick]

Hi Timm! Me again

I love where you're going with this, btw. Here are a couple of thoughts in random order:

- You've probably checked for this fault, but be 100% certain that can turn the vehicle 90 degrees to the trailer without jackknifing into the frame or air splitter.
- You'll want to adjust the frame height based on your desired approach and departure angles. That rear hatch looks like it might be dragging the ground driving around hill country. Expedition Portal-type camper trailers typically have a mitered rear end (not the technical term for it, can't recall it off the top of my head) to increase departure angle.
- If you have the money, splurge on a painted epoxy exterior rather than PMF. And why PMF/glass before assembly?? Lot of seams to fix ....

- As another poster pointed out, you have two basic lightweight construction options:
- Skeletonized (your current design): A load-bearing sectionalized plywood core with cavity insulation and non-loadbearing skins
- Sandwich: A version of stressed-skin (composite) design where the core is responsible for shear stress and the skins responsible for bending and torsional stress

Both are good. Both work. Skeletonized is easier. Sandwich structures are superior (full disclosure: that's what I use), but you must have the right equipment (e.g. vacuum press).
Here's a practical difference: A skeletonized wall panel might have 1/8-inch wall skins, since all fasteners screw into the loadbearing core. A sandwich panel might have 1/4-inch wall skins, since the skins have to carry all the weight, and therefore you need that 100% increase in thickness. Point loads are a challenge for sandwich structures and require some creative solutions. Anyhoo, like I said, skeletonized is easier and more forgiving. I'd stick with that. But as an aficionado of lightweight campers myself, I could easily shave a 100 lbs of your structure using my stressed-skin design. Something to think about for the future

Random note: I like how you sectionalized your plywood cores based on a truss. Don't know if that was intentional or not, but thumbs up.

- Electrical: You'll need a home run-style system. There's almost no spaces in your assembly for junction boxes, and it's poor design to stuff electrical junctions into a sealed wall panel. Find some place for your main fuse/circuit breaker panel, leave lots of room around it, and then run dedicated wires to each fixture - no junctions in between. No daisy chaining!

- Custom trailer: I'd build you one if I was nearby! Grrr. It's hard to find anyone local who knows how to work with aluminum and not charge a bazillion dollars. If you must choose steel, remember, it's not a utility trailer. You don't need channel cross members and the like. The two most important junctions are where the tongue ties into the frame and where the suspension mounts to the frame; those needs to be strong AND stiff! Beyond those two points, cut weight like crazy.

 

[timm-TNT]

I like your ideas and CAD design, subscribed. Did you estimate trailer weight via CAD by applying density to the materials? I would consider foam sandwich vs. skeleton plywood to be lighter. Back edge is better be left sharp, not rounded, for better air flow separation.

Sent from my Moto G Play using Tapatalk

Very close, I got the volume of each part and then applied density in an excel document to make comparing plywoods easier. Foam sandwich construction would be nice but pre-fabbed panels are pretty expensive and I don't have the tools to make them properly myself.

You're completely correct about the rounded tail, I've changed it.

The car can not jack knife completely with the trailer. Right now it can do about 70° max before there's contact, which is just enough to pull a u-turn as hard as the car can do. I don't think any more than that is really necessary, maybe nice, but not necessary.

The departure angle is 12°, which isn't a lot, but I'm not planning on off-roading with this thing. If it's a huge issue I can always flip the axle and raise the frame another 2-3".

Unfortunately, I don't have the money to do epoxy and glass. I figured doing PMF before assembling the panels would make applying it easier, especially since I need the skin to be nice and tight over the cutout holes. Maybe it would be better to apply PMF on the inside of the panels separately and then cover the outside all at once? I'm pretty new to PMF so I'm not entirely sure what the best approach here is...

Yep, they're based on truss designs. One of my Uni courses was about stress analysis in trusses actually.

I could put the main breaker box in the splitter at the front and run all my wires from there. The fans are right at the front, same for the exterior lights.

As far as the custom trailer goes, I have a fab shop willing to do it for $540 (materials and welding). Just the steel frame pieces weigh about 140 lbs, not including axle, coupler, etc. I could cut more weight by using thinner steel (using 1/8" right now) but then the issue of rust becomes a lot larger. I'm using 2" x 1" steel for the main beams, 1.5" angle for cross members, and 2.5" x 1.5" for the tongue pieces.

 

[Andrew Herrick]

Thanks for all the thorough answers!

There's a part of me that wants to argue for better jackknifing capability. You're right; it won't be too difficult to improve the departure angle via under- vs. over-slung axle. But the jackknifing angle? That's more or less set in stone once the trailer is fabricated. While 70 degrees may be the maximum when driving forward, it's definitely not the maximum going backwards! Only takes one mistake to ruin your air splitter. And your trailer is long enough that you'll need all the maneuverability you can get. How much would adding, say, 3-4 inches to the length of the A-frame tongue improve the jackknifing capability?

The one thing to add to the departure angle is that you'll have to engineer some solution if you want to mount any rear stabilizer jacks. They'll have to go inside the frame, against the floor. I'd have your welder add in oversized triangular gussets in the rear steel frame if you want rear jacks.

PMF on the inside? Nothing wrong with that. Would be easier to get foam-safe spray adhesive (3M 7 and a "headliner" material - canvas, vinyl, auto headliner. Probably a little more expensive, but much less mess. Considering the exterior PMF, remember, canvas shrinks after it's initially wetted and dried. As long as you have the ends secure, the canvas will dry tight against the shell.

The trailer design looks decent. I'd put a reinforcing cross member in the tongue, though, about a foot short of the coupler. Double-check the spring hanger welds, though. You don't have much metal there (I'd consider reinforcing the spring hangers with a bracket extension). If you go any thinner than 1/8-in steel, costs might skyrocket as well, since you might have to pay for custom-bent/welded 12- or 14-gauge sheet metal. If you plan to use reamer screws for the floor, there's no need to change the frame design. If you prefer bolts, then you'll want the welder to add some L-brackets on the frame. Whatever you do, make sure you tie into the main rails and NOT just the cross members.

 

[timm-TNT]

The trailer frame itself can completely jackknife, but the air splitter limits it to about 70°. I'm planning on using a hot-wire cutter to make the splitter from a large block of foam, so it will be easily adjustable. Also, in the event of contact between the splitter and car, the splitter should just crumple and prevent damage to the car.

I could go with triangle gussets for stabilizer jacks, I was also considering keeping the jacks separate.

Putting a nicer headliner material on the inside is a good idea, it would make the inside a bit more inviting.

Why the cross member by the coupler? I have considered doubling up the main beams right by spring mounts. I was planning on attaching the trailer to the frame with L-brackets.

 

[Andrew Herrick]

Why the cross member by the coupler?

Main function is to reduce torsional deflection of the tongue on uneven roads. Great place to mount a spare tire or other accessory. If the trailer is ever jackknifed, it'll help prevent the tongue rails from collapsing. It's not necessary, but it is helpful.

 

[Shadow Catcher]

Why not go aluminum?

 

[Andrew Herrick]

Why not go aluminum?

Not to speak for Timm, but if he can't afford epoxy and glass, aluminum might be beyond his budget, too! Timm, if you do look into aluminum: Remember that 6061-T6 aluminum loses about 80% of its strength when welded. It's got a nasty HAZ. You must add a lot more metal to the joint to get equivalent strength to steel (unless your welder has a giant oven for heat treatment) and even then, 6061-T6 has no useable toughness (defined as ability to plastically deform without fracture). Don't get me wrong - I love (and build) aluminum trailers, but it requires a different set of fabrication skills than steel.

 

[timm-TNT]

Why the cross member by the coupler?

Main function is to reduce torsional deflection of the tongue on uneven roads. Great place to mount a spare tire or other accessory. If the trailer is ever jackknifed, it'll help prevent the tongue rails from collapsing. It's not necessary, but it is helpful.

That makes sense, my tongue is already pretty beefy though.

I haven't actually looked at aluminum much, I've just kinda assumed it was out of my price range. My cost estimate for the trailer is sitting around $2,500 CAD right now. I also have to add a hitch and wiring to my car, as well as a transmission cooler. Then I actually have to outfit the trailer with gear and whatnot, so I'll probably spend closer to $3,500 - $4,000 on the whole project.

 

[OP827]

I like your ideas and CAD design, subscribed. Did you estimate trailer weight via CAD by applying density to the materials? I would consider foam sandwich vs. skeleton plywood to be lighter. Back edge is better be left sharp, not rounded, for better air flow separation.

Sent from my Moto G Play using Tapatalk

...
Unfortunately, I don't have the money to do epoxy and glass.

If you do an objective comparison of materials cost between Epoxy Fibre Glass and PMF you may be surprised. Canvas would generally require more TB2 by volume than FG cloth and epoxy is 100% solids while TB2 is 50% solids. In the area where I live the cost of epoxy($100/gal) is twice of the price for TBII($50/gal). You can do the cost comparison I am sure and then test samples to make your decision. I did most glass lamination in separate panels on a flat surface, it is less work and easy. I placed dry glass cloth over the dry surface and then poured epoxy and used squeegee to distribute. It is not difficult to do - https://youtu.be/EvBJOQBZqYc.

Some more points:
1. Did you think of putting wheels inside wells to match track of your tow vehicle and also streamline the trailer.
2. Did you consider ultralight frame design from Design Library here?
3. If this is your first trailer build then you may likely underestimate the amount of time, money, tools, and effort it will require to build. I am not trying to discourage you, just trying to warn you based on my own experience. A good size scale working model helps, but of course might not reveal all issues, especially if there is no previous experience with building similar designs. Good planning helps a lot.
4. Lifting mechanism needs to provide same lifting speed at all four corners. You are mechanical engineering student and probably already know that air cylinders will not lift at the same speed if their loads and friction forces are not absolutely equal on all four corners. I know you are incorporating sliders, and they might work to compensate for that, but are they really needed of there could be another more economical design to do the lifting? How did you arrive to your decision on the air lifting design and what other options did you consider if you don't mind sharing?

 

[timm-TNT]

Alright, so I looked around just a bit and it looks like glass and PMF could be pretty similar in price.

However, another reason I'm thinking PMF will be better is that the cloth needs to bridge the cutout holes in the plywood, and I'm thinking fiberglass won't do as well compared to PMF. The PMF will have a bit of give while the fiberglass will be more stiff, but also more fragile.

I haven't done any work with fiberglass, so I could be wrong on this.

I did consider putting the wheels inside the trailer, but I would lose room inside the trailer. I would either have to widen the trailer to get that room back which would be a lot worse aerodynamically than the fairings.

I know I'm probably underestimating the time and effort but I'm pricing everything out as I go so I think I'll be pretty close on that area. I'm hoping to finish the trailer in about 3 months of weekends and evenings.

As far as the lifting mechanism goes, I considered a couple different methods:
- Linear actuators
- Pneumatic cylinders
- Manually lifting from inside

Linear actuators are really expensive and quite difficult to make so I would have to buy them and that would be ~$300 without any controllers. However, they would all extend at the same rate.

Pneumatic cylinders are quite expensive as well, but they are much simpler to make at home, the seals are the most difficult part to make and since I'll be using them to extend and retract and not hold weight for long periods of time the seals are ok if they leak a bit. They aren't guaranteed to extend at the same rate, but I think if I add weights to balance the upper box and roof so each corner weighs about the same amount it should be ok.

Just climbing inside and lifting it manually is definitely the cheapest and simplest option, but since I'm estimating the roof will weight about 130 lbs. that would be difficult. Also, since the roof is a skeleton frame with pretty small members I don't think that's a great idea.

I considered creating a mechanism that ensures all the corners lift at the same rate but I figured it would get overly complicated when I could just walk around the trailer as it lifts and make sure it's all lifting the same.

I've already built a cylinder and so far it looks quite promising although I haven't done any testing on it yet to see how much leakage there is around the seals. The nice thing about it is that most of the parts are 3D printed so I can make changes to the models and then re-print them for almost no cost.

This is the design I'm building, it's 1 1/4" PVC pipe with a pushrod of 10 mm steel and a lot of 3D printed pieces (blue).

It has an 18" stroke and since PVC has such a high pressure rating (around 1200 psi for 1 1/4" schedule 40) I can basically lift as much as weight as the seals can take without leaking like crazy.

 

[OP827]

Are you familiar with this method: http://www.tnttt.com/viewtopic.php?f=2&t=52750#p1145889? It seems to be very light and not hard to do. It could even work with canvas instead of dacron, as TB2 glue causes the fabric to shrink while drying. This would need some testing of course. That might work, I would put some synthetic insulation in the voids, as it will also help with blocking the light where unwanted. It might look pretty nice showing the structure of your design. Rounded corners can still be made with foam.

As for lifting mechanism on my build, I did cables and pulleys system actuated by a single screw similar to a popup trailer, but simpler. Cable routing can vary, of course. I can report that it works on my build.

 

[Tony Latham]

...the fiberglass will be more stiff, but also more fragile.

The Rutan aircraft guys and fiberglass boat boys might be rolling their eyes if they come across that.



Tony

 

[OP827]

I agree with Tony. Best way to decide is to see other campers made with PMF and FG and/or do your own test panels and then decide what is best for your build .

 

[Pmullen503]

I would just make the top with foam. It would be easier to round the corners.

But if you want to go the skeletonized plywood route I would fill the voids with foam. Not too heavy and you don't have to bridge to voids. Glass cloth/epoxy doesn't shrink and it will be tough to get it tight over the voids. Canvass and glue will shrink tight over the voids but I would still fill them with foam.

Years ago I build a 4x8' lifting bench and used Acme rods in the four corners with a long chain and sprockets to turn all four in unison. The posts were just metal pipes with flanges and Acme nuts welded to the tops. I had a gear motor and sprocket to move the chain but you could use a crank for a "no power" solution.

 

[timm-TNT]

Are you familiar with this method: http://www.tnttt.com/viewtopic.php?f=2&t=52750#p1145889? It seems to be very light and not hard to do. It could even work with canvas instead of dacron, as TB2 glue causes the fabric to shrink while drying. This would need some testing of course. That might work, I would put some synthetic insulation in the voids, as it will also help with blocking the light where unwanted. It might look pretty nice showing the structure of your design. Rounded corners can still be made with foam.

As for lifting mechanism on my build, I did cables and pulleys system actuated by a single screw similar to a popup trailer, but simpler. Cable routing can vary, of course. I can report that it works on my build.

I haven't seen that post before, but it's the same idea as PMF with slightly different materials. The advantage with canvas is that it shrinks so getting it tight over the voids will be easier than with a fabric that stretches when wet. I wasn't planning on adding insulation as I figured cutting the foam pieces to shape would take while to get right, but soft insulation could be pretty quick.

Using pulleys could work, but then I have to route the cables somewhere. Using pneumatic cylinders is nice cause I can just run the small air hose almost anywhere.

The Rutan aircraft guys and fiberglass boat boys might be rolling their eyes if they come across that.

Perhaps a poor choice of words on my part, I don't think fiberglass does well when stretched over voids like I'm planning on doing. I haven't been able to find anyone who's done it at least.

I agree with Tony. Best way to decide is to see other campers made with PMF and FG and/or do your own test panels and then decide what is best for your build .

I'll definitely do some test panels with PMF to make sure it works, and if it doesn't work I'll look into other skin ideas. I'm still thinking PMF is the way to go here.

I would just make the top with foam. It would be easier to round the corners.

But if you want to go the skeletonized plywood route I would fill the voids with foam. Not too heavy and you don't have to bridge to voids. Glass cloth/epoxy doesn't shrink and it will be tough to get it tight over the voids. Canvass and glue will shrink tight over the voids but I would still fill them with foam.

Years ago I build a 4x8' lifting bench and used Acme rods in the four corners with a long chain and sprockets to turn all four in unison. The posts were just metal pipes with flanges and Acme nuts welded to the tops. I had a gear motor and sprocket to move the chain but you could use a crank for a "no power" solution.

Threaded rods and chains could work, but then I'm looking at several either several very long chains or 8' shafts to drive all 4 corners at once. Also, routing chains or shafts is going to be more difficult than pneumatic tubing.

I'm really enjoying hearing all these new ideas to do this!

 

[Andrew Herrick]

Timm,

So, if I'm understanding this thread correctly, you plan to leave the wall cavities empty and overlay them with PMF?

To be blunt, this doesn't make much sense to me. You're missing out on a host of benefits: thermal insulation, acoustic insulation, superior waterproofing, theft security, wind resistance, etc. Remember: When towing this thing down the highway at 70 mph into a 30 mph headwind, in the rain, you're essentially sending this thing into a Class II hurricane. And you're crossing your fingers that some cheap single-fill canvas rolled with house paint is going to keep out the water?? What if you camp in a freak hailstorm? What if you cut a suburban corner too close and a tree branch punctures the unsupported canvas? What if a tiny bit of water leaches into the canvas, slowly rots it out, and six months later, the only thing that's bridging the wall cavity is a 0.090- inch thick web of house paint?

It's possible I'm missing something, of course. After all, I've never tried this. But based on my current knowledge: Do yourself a massive favor and fill those cavities with a substrate, either rigid foam, spray foam, Coroplast, whatever. You mentioned that you weren't planning on adding rigid insulation due to time constraints when cutting, but if you use the plywood cores as a template, and either cut the foam with a jigsaw or sharpened putty knife, you'll be done in an afternoon.

 

[timm-TNT]

Yeah, that's pretty much the plan. My thinking is that aircraft used to use a similar technique (canvas and dope) and it worked very well. Admittedly dope is a better sealant than exterior paint, but aircraft also move significantly faster than my car. A freak hailstorm would definitely do damage, but a hailstorm would do damage to a teardrop regardless of what the roof was made of, at least my roof can be repaired very quickly and inexpensively.

I can add foam for the reasons you mentioned, but I think the canvas and paint would be ok alone.

 

[Andrew Herrick]

Yeah, that's pretty much the plan. My thinking is that aircraft used to use a similar technique (canvas and dope) and it worked very well. Admittedly dope is a better sealant than exterior paint, but aircraft also move significantly faster than my car. A freak hailstorm would definitely do damage, but a hailstorm would do damage to a teardrop regardless of what the roof was made of, at least my roof can be repaired very quickly and inexpensively.

I can add foam for the reasons you mentioned, but I think the canvas and paint would be ok alone.

I will freely admit that I am nothing close to an aircraft guru, and my knowledge of the aircraft canvas-and-dope technique has been gleaned from the last 10 minutes of scurrying around the Internet. In the end, I agree that canvas and paint would be "ok." From what I can tell, a genuine Grade A Cotton-and-dope covering uses significantly better materials than PMF and is subject to much more stringent QC.

And I have to say: A freak hailstorm shouldn't (within reason) do damage to a properly built camper. A well-built camper is like a little Panzer and can deal with, among other things, bouncing rocks that would crack a glass windshield. And while patching PMF can be a quick and easy fix, if you get rot or mold in the canvas, it'll be anything but. Mold requires moisture, spores, heat and oxygen in order to reproduce. Reducing airflow is the #1 easiest way to prevent mold, and adding a substrate to your PMF will do just that. Yes, patching your PMF skin will be easy at home, on a Saturday afternoon. But if you're out camping on a rainy weekend and a tree branch pokes through the skin, and then you have to deal with a big ol' hole for a weekend in the woods ... bring some duct tape!

It's obvious you're putting a great deal of thought and care into your project. As I said, I'm no aircraft guru, and the unsupported PMF might do just fine. If worse comes to worst, you could always renovate your camper later. Or you could do it for $50 and an afternoon's work the first time