Featherdrop - An Over-Engineered Teardrop

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Re: Featherdrop - An Over-Engineered Teardrop

Postby Pmullen503 » Sun Dec 02, 2018 8:34 am

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.
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Re: Featherdrop - An Over-Engineered Teardrop

Postby timm » Sun Dec 02, 2018 10:25 am

OP827 wrote: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.

tony.latham wrote: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.

OP827 wrote: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.

Pmullen503 wrote: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!
Last edited by timm on Mon Dec 03, 2018 5:36 pm, edited 1 time in total.
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Re: Featherdrop - An Over-Engineered Teardrop

Postby Andrew Herrick » Mon Dec 03, 2018 2:54 pm

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.
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Re: Featherdrop - An Over-Engineered Teardrop

Postby timm » Mon Dec 03, 2018 6:02 pm

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.
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Re: Featherdrop - An Over-Engineered Teardrop

Postby Andrew Herrick » Mon Dec 03, 2018 7:55 pm

timm wrote: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 :p
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Re: Featherdrop - An Over-Engineered Teardrop

Postby tony.latham » Mon Dec 03, 2018 8:41 pm

I built a "rag and tube" aircraft decades ago. It was covered with aircraft grade dacron. It was finished with nitrate dope, and then silvered butyrate (because of the UV problem) and finally butyrate paint. It took a roll of duct tape to get home after a bad hail storm in Pierre, South Dakota ––but N716TL is still flying even though I don't own it.

They quit using cotton for aircraft in the late fifties. You can buy aircraft (shrinkable) dacron from Aircraft Spruce along with the dope and powdered aluminum. There should be plenty of Youtubes on how to apply it.

Andrew is spot on with the rock chips.

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Re: Featherdrop - An Over-Engineered Teardrop

Postby friz » Mon Dec 03, 2018 10:06 pm

tony.latham wrote:I built a "rag and tube" aircraft decades ago. It was covered with aircraft grade dacron. It was finished with nitrate dope, and then silvered butyrate (because of the UV problem) and finally butyrate paint. It took a roll of duct tape to get home after a bad hail storm in Pierre, South Dakota ––but N716TL is still flying even though I don't own it.

They quit using cotton for aircraft in the late fifties. You can buy aircraft (shrinkable) dacron from Aircraft Spruce along with the dope and powdered aluminum. There should be plenty of Youtubes on how to apply it.

Andrew is spot on with the rock chips.

Tony
If you want a poor man's version of aircraft covering and finishing, try to find information on Fisher Aircraft. They are a low cost kit aircraft company. I dont know for sure if they are still around.

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Re: Featherdrop - An Over-Engineered Teardrop

Postby timm » Tue Dec 04, 2018 8:40 pm

Andrew Herrick wrote: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 :p


You're right, I should just do it properly the first time. I'll put 1/2" foam in the spaces.
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