And that's with propulsion!
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I think an ultralight dirigible might be possible by inventing frame spars that weigh less than aluminum or fiberglass. [that's all]
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Of course, the envelope cannot be the 150-pound heavy hot air balloon fabric, it should be plastic film with many mylar lift bladders inside. These are WAY cheaper than fabrics. The ultralight weight rule is forcing the designer to be so minimalistic and so simplistic that the ship must come in under ten grand. And that's the whole point. With hot air balloons starting at $25 grand, we're never getting off the ground unless we invent something ourselves that'll do it. Right?
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254 pounds. They sure don't make it easy. But there's one BIG reward ~ you don't need a pilot license nor a certified airship! That's enough encouragement to work the problem hard!!
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UP! Now!

Shrink wrap will hold the skyboat's shape because it only goes 30 knots, not a lot of deforming wind pressure on the nose.

There is a book that talks about building small blimps. He also talks about ultralight blimps.

A practical guide to building small gas blimps

 I think he said the ones he has seen were really only suitable for indoor flight. But I don't remember for sure.

Probably good to look at in any case.

Would frame rods made from hollow carbon fiber rods (like the ones used to make arrows) work?

Probably, might be expensive though. I think Nav has talked about using rods like that, but I don't remember if the material was fiberglass or carbon fiber or what. 

I talked about fiberglass kite spars.  They are cheep but only go up to 3/4 inch, which might be ok.  http://www.kitebuilder.com/frame.htm

I'm now experimenting with epoxy & glass cloth over styrofoam core spars.

Fiberglass is very heavy. I would think carbon fiber is the way to go. Common PVC pipe might be worth
looking at. Comes in all sizes with ready made couplings and long lengths. Might be strong enough
if unsupported runs are not too long.

I've personally rejected PVC pipe as too bendy. Sure is cheep but by the time it is stiffened with fiberglass covering, too heavy.

Carbon fiber is the darling of composite fabrication. It's not as horribly expensive as it was a year ago but....   would someone like to compare its price to the same thickness of glass cloth? I expect it's 5 to 10 times the price of glass?

My problem now is to buy a selection of thin, thin glass fabric and figger out a mold for casting a tube spar about 2 inches in diameter.  I might do someth8ing with stryofoam or I might go with a removable mandrel type of rig which sounds like a real pain in the bass.

OR,dig this, BUY 21 foot-long, tree-trimmers' poles for a hundred bucks each! Might be cheeper and certainly eezier than laying up the darn things. Check em out at   http://www.nettechdi.com/products/FG%252d6%7B47%7D3-Jameson-Set-of-4-Hollow-Poles.html

I might buy one to trim my own trees at home. I have a big coconut palm that needs its nuts cut off.

A windsurfer mast is about two inches in diameter, light and well made. You might be able to find one used. Any chance of getting some specs from Marvin? The weight of his materials would show us a lot.

I wrote Marvin again at his motel in Ardmore OK. He hasn't answered from December 19th. Guess everyone is after him.

I'd love to use new windsurfer masts if they were under 100 bucks?  Used ones might have interior bruises?  MIght be worth a look.

I have tried to use shrink-wrap on my airframe, but the airframe was too loose and the shrink-wrap kept sliding off. Here is the link;  http://www.youtube.com/user/michaelcweir#p/a/u/1/0xYK6gq0Tnc
The white curled up stuff in the midsection is the shrink-wrap. Plus the the melting temperature is 225 F, so you would have to run at a lower temperature than you could a
could with fabric. I was thinking of using aluminum next.

The next geodesic airframe I am going to make is going to be made with triangles, with either fabric or aluminum
wrapped around the triangles. then the triangles will be joined to make the airframe. It will be much sturdier than the
shapes I have been testing.

Part of the problem with geodesics is that they are unstable while being assembled. Their strength comes after the
are assembled. Second, geodesics tend to vibrate and loosen their bolts after a while. That's why I am working on
nodes that will remain fixed.

It may be possible to make a plane in this fashion, but not a dirigible under 254 pounds. The airframe that the video showed
was slightly under 10 pounds. It would be unable to get aloft with hot air.

Michael

Looks cool. I am impressed by the size and I look forward to seeing the next, larger implementation. You are the only member (I know of) who is actually building models. That, I admire.

<<<I have tried to use shrink-wrap on my airframe, but the airframe was too loose and the shrink-wrap kept sliding off.>>>

   Yes, it's difficult to attach shrink wrap to a frame. It will have to be an innovative, unusual frame that provides a lot of lightweight support points for the twelve-to-forty-foot-wide ribbon of shrink film.

   I expect a lot of taping/glueing would be used to hold the seams shut against the wind buffeting effect when the skyboat is underway. A shrink wrap video guide is at
and at
and this shrink wrap sculpture shows that the frame could be reinforced with shrink wrap and then a shrink wrap envelope heat  "welded" to that frame. 

   Shrink film is tough and much cheaper than fabric. It is worth a lot of experimention to develop a new-style supportive frame which will look different from the frames we are used to seeing.

    But that's why the Ultralight category carries the reward of being free of regulations, it's a hard field in which to excel but quite worthwhile. As far as advancing the entire field of aviation construction techniques, Ultralight is the cutting edge.

Notice the first video shows that SW can be clear material. This means the envelope of the SW ship can transmit sunlight so the builder may install solar cells on the floor, INSIDE the envelope instead of topside where they are so heavy they will roll the ship over. [40 pounds for each 200 watt panel]

   {Yes there are thin solar cells but they are expensive and back-logged and basically unavailable to the cost-conscious retail market.}

If you think the shrink-wrap can withstand the temperatures generated through the heating of the air for lift off, the triangular pieces of a geodesic can be individually wrapped, the airship assembled out of the shrunk-wrapped triangles, and the final product shrunk-wrapped again. The airframe probably will weigh in at 50 ponds for fiberglass frame at 30 foot diameter. If the shrink-wrap comes in at 50 pounds, that only leaves 150 pounds for carriage and motor. is that enough?

    That's a good idea, make the individual modular triangles with shrink wrap and then SW them all together to make a shell that would unitize the geodesic structure!  That would be a type of plastic monocoque construction. Nice.
~
   I expect the frames of the triangles could be made from quite lightweight fiberglass kite spars with just-the-right bevels cut on the ends so they could be epoxied neatly into triangles. The small triangles could be cemented into larger and portable triangles  and then assembled at the flight site. Sounds like a neat and potentially workable system. Anybody care to do a model of this unique system?

[Incidentally, shows an RC clear blimp operating on INTERNAL solar cells ONLY. I LIKE it. I think we have to do a real one that way.]

It is a little more complicated than that.. Yes, the fiberglas rods should do fine, but your method dealing with the nodes will fail. Imagine a ten foot fiberglas rod flexing a 1/2 inch at one end  relative to the to the epoxy joint. The forces brought to bear will crack the joint. Geodesics are tension systems and need to reduce the effects to zero, from tension, if they are to survive for any length of time. That is the main reason that most geodesics that exist as structures are concrete or foam. Even the wood domes are over-engineered because of this fact.

That is why I have been working on the hubs that you see in my videos. I now have a design that that will stronger than the rods by a factor 100, I believe.
Here is the link for a look at the nodes.

The nodes are probably only 10 times as strong as the wood rods, even when they were deformed.
They take some of their strength from the fact that they also are geodesics.

I was reading some the comments you and others were making about getting more attention.
If you really would like to get more attention, make a blimp hangar. Make it the biggest in the world, at least 
80 feet tall. For nothing else, you can store your blimps ready to be inflated, in the hangar.

The boat shrink wrap would be perfect when used with the triangles geodesic.
There is no issue with the shrink-wrap continuing to shrink when the blimp is heated. 
The hangar would probably cost in the neighborhood of 20 grand for the rods and boat wrap.

The hangar would also be a good first test of the system, without actually flying it.

Michael

Takmens: You are one smart man. I now believe I can go prospecting for five grand.

  <<<Yes, the fiberglas rods should do fine, but your method dealing with the nodes will fail. Imagine a ten foot fiberglas rod flexing a 1/2 inch at one end  relative to the to the epoxy joint. The forces brought to bear will crack the joint. Geodesics are tension systems and need to reduce the effects to zero, from tension, if they are to survive for any length of time.>>>

   It's hard to explain things well using only text.  But I'll try to make it clear. I am picturing the fiberglass rods' beveled end joints being pulled together by the shrink wrap. The force-direction of the shrinkage will tighten the joints, not crack their epoxy glue. But still, it wouldn't hurt to perhaps include a tiny bit of glass cloth reinforcement to strengthen the apex joints.
   But I was thinking of 12" triangles and those are too small. It would save time to use longer rods for bigger triangles. The kite spars are available to four feet. These longer tubes would bend an inch or two inwards from the pull of the shrink wrap. [We'd probly want to shrink the wrap fairly tightly to add stiffness to the triangle.] So, there would be an opening between each edge of the triangles.
   This tube-bending might be ok if we only connected the triangles at their apexes and left their edges alone. Then we would cover the finished geodesic lenticular shape with an exterior layer of shrink wrap which would cover the openings between individual triangles.
    As Dude explained making a geodesic lenticular shape, "imagine a geodesic sphere and then take a slice out of the middle and bring the two pieces together and it makes a lenticular shape."  Pretty.
   Maybe I beams or T beams would resist bending better than tubes but they seem too costly, if they're available at all. Don't seem to be. And too hard to make at home?  ?And hard-to-make joints?  Doesn't seem worth the extra effort.

    Viva la lifting body!! 

Perhaps a little experience would help you. Get some wooden rods (3) and do your best to assemble a triangle -epoxy, fiberglass, whatever. Once the adhesive has set, just move the triangle  until something breaks. If the joint breaks,as it most likely will, this is an indication of micro-movement first, before the joint breaks. If the wood breaks first, then you have a joint that is stronger than the rod, and will more likely to survive micro-movement.

Why are the nodes so important? A geodesic is a structure designed to resist tension forces. When the structure is built on the ground, the primary force that is resisted is gravity. In a geodesic, the center of the geodesic is mostly composed of level rods. The force of gravity translates into nearly horizontal forces that have to be transferred to the ground, through the nodes.

 But if you have variable wind loads, as you always do, then the movement of the any part of the nodes relative to other parts of the node, especially if they are made of metal, causes them to flex and have metal fatigue. Eventually the metal cracks, and the much larger tension forces from gravity cause the failure of the node.

If you look and see how nature handles this problem, go look at nodes on a tree. The nodes on a tree are where the branches are joined together.  Several things stand out. The nodes are always, always thicker than the two branches that are joined together. If you cut the node apart, the reason for the extra thickness becomes apparent. The fibers of  of one branch is intertwined  into the fibers of the other. So the node is really a type of netting where the movement of the branch is interfered with by the fibers of the other branch. Micro-movement is restricted.

In the billions of years that nature has had in designing trees, this is the best answer that nature has for joining two similar materials to resist the changing tension forces for a long period.

Michael