Featherdrop - An Over-Engineered Teardrop

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Re: Featherdrop - An Over-Engineered Teardrop

Postby timm » Wed Jan 02, 2019 4:46 pm

I wasn't planning on using PVC as the supply lines, I was thinking pneumatic tubing would handle the vibrations and stresses of being on a trailer. The PVC would only be used as the cylinders, which should lift the roof with only about 30 PSI. I can also enclose the cylinders with plywood pretty easily if I need to run more pressure in the cylinders.

I'm thinking the air supply would be a small 12v compressor running off the car battery in the trailer. I'm not too concerned with the extension time as the pressure is pretty low and the volume being pressurized is relatively small. I can build a fifth cylinder and bring it along in case one fails.

If I can't get the roof to extend properly I might have to add a threaded rod system to help everything extend together or completely switch over to threaded rods. I think that potential risk is worth the simplicity of the pneumatic system.

I won't be using actual aircraft dope, I'll be using regular exterior paint so the foam should be fine.
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Re: Featherdrop - An Over-Engineered Teardrop

Postby fpena1962 » Sun Sep 20, 2020 12:23 pm

Very cool design and some nice cad work too.
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Re: Featherdrop - An Over-Engineered Teardrop

Postby wysedav » Mon Sep 21, 2020 5:51 pm

google SOF kayaks. They are made with Nylon fabric on a rib and spar type of frame. the Nylon is adhered/waterproofed with a flexible epoxy. It's super strong and lightweight.

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Re: Featherdrop - An Over-Engineered Teardrop

Postby KCStudly » Fri May 20, 2022 8:49 am

I'll add my insight to your thoughtful design. The reason people are advising against multiple cylinder air actuation is because it has been tried by others... and failed.

There were several times during my build... which is still not finished... where I chose not to take the advice of more experienced builders, and later came to learn that if had taken their advice I would be further ahead with a better result.

If you are set on air, I would suggest a single cylinder operating pull "cables" made of light weight 550 paracord over a nylon pulley system, configured like a traditional pop-up.

IIRC, the issue with PVC is that the pressure ratings are misleading. It doesn't like to be pressure cycled. So in a shop air environment where you are shutting down the compressor (or at least isolating the distribution system) every day, it can lead to problems. Same for a dynamic cylinder with pressure cycles. Maybe not such a problem for low pressures, but the concern is well documented. IMO, PVC should be reserved for low pressure drains and, perhaps, electrical conduit (CPVC).

Another concern is that the OEM's control the OD very closely for compatibility with the fittings, but there is a wider tolerance on the ID. Have you checked any samples for circularity on the ID? Rather than 'o'-rings, cup seals might be a better choice.

I'm not sure if I interpreted your plan correctly, but it almost sounded like you were planning to skin the "skeleton" areas and only paint the foam. If so, the foam is quite fragile and would quickly deteriorate. Plan on pmf'g the foam, too, or better yet use epoxy/glass weave. Two plys of 6 oz cloth saturated with epoxy provides a substantial amount of armor, does stay relatively flexible (contrary to common mis-belief) and, surprisingly, weighs virtually the same as 10 oz canvas pmf.

One last observation, I am concerned with the proximity of the peak of you splitter fin to the trunk lid on your TV. This gap will close when transitioning onto an up slope or off of a down slope (approach and departure angles). Yes, foam is fragile and somewhat compressible, but can sustain significant pressures over larger areas (IIRC the common pink stuff is 25 psi). So while the pointy bit might not do much harm, once it starts to flatten the contact area will increase and it will no longer yield as much. I could see it easily denting a sheet metal panel. Once reinforced with pmf or epoxy/glass it won't yield much at all.
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Re: Featherdrop - An Over-Engineered Teardrop

Postby tony.latham » Fri May 20, 2022 9:20 am

it has been tried by others...


His last post on this was four years ago. For whatever reason, his dream must have walked away or been put on indefinite hold.

:thumbdown:

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Re: Featherdrop - An Over-Engineered Teardrop

Postby KCStudly » Fri May 20, 2022 11:30 am

One more thing... and I really don't intend this all to be critical, just constructive... is that the way you have your rear shelf and storage arranged doesn't seem very efficient to me. If the hatch could double as the work surface, eliminating the weight and hassle of the portable panel you could save weight and set up effort. Even if you have to make the hatch flat instead of arched, you aren't losing aero.

But regardless, in that configuration the folding strut consumes too much storage space. In the least you would have to move stowed items to deploy. Once set up, as soon as anything is placed on the table surface, access to the stowage would become a hassle.

Perhaps consider a portable folding table to be placed on the ground nearby (packed suspended at top of trunk so it can be deployed w/o having to move other stowed items). That way you can keep the arched trunk, compensate for extra weight of portable table by eliminating separate work surface panel and prop, and avoid having to add side access doors to trunk.
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Re: Featherdrop - An Over-Engineered Teardrop

Postby KCStudly » Fri May 20, 2022 2:24 pm

Actually I'm still poking away at it... slowly, but that's another topic.

<edit> Oh, yeah, I see what you mean now, Tony. OP vaporized. I've been catching up on my reading and lost track that this was an old post. :?
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