Really really cheap lift bags

Instead of huge lift bladders, I propose using 150 smaller lift bags. The food industry has some gas-resistant product bags that are awesome at keeping oxygen and moisture in or out of a package.

  Yesterday was Thanksgiving Day and I noticed the bag for my StoveTop Stuffing had kept the contents exceedingly dry and flavorful. It was a three-layer bag; paper, aluminum foil and plastic film.

   Makers of food packaging film can probably supply this film retail. Maybe in bag form with three sides already closed, in which case we can insert a filling valve and seal the other end.  Faster, easier and cheaper than sewing big lift bladders.

Also safer against leaks and blowouts, since you've got a hundred and fifty bags supporting you rather than just two or four. The ship could never go too far above pressure ceiling because one by one, the bags would start popping and lowering the ship to below pressure altitude so it could safely fly away.

Pack your bags. Your lift bags, that is.

What shape would these bags be? Cubic? The bag material would need to be very thin and lightweight. We might also consider the bag shapes used in the great zeppelins. I believe they placed each bag between each set of structural rings.

They'd be pillow-shaped rectangles like monster potato chip bags. Probly the size of a door, 3x6' depending on how wide the rolls are supplied. 4x10' might work but if they get very long, they won't neatly pack against the ceiling.  I don't want to waste any of the food package film cutting circular balloons, it's affordable but not cheap in the amount of footage that I need.
  I'm looking for cheap netting to divide the fuselage interior into segments to prevent the lift bags from avalanching "uphill" when the craft pitches and rolls.  {8-o>
How bout orange safety netting for $25 for 400 sq feet?

What about a cutting each bag to the cross section of the ship and sealing it with a long circumscribing strip?

Well, you could weld them together to make the big cross sectional pieces.  Even if they are only 3' wide. 

Would packing pillows work? They are cheep and hold up well. Uline co. hase nice ones called rapid fill.
www.uline.com/BL_7700/Rapid-Fill

You gotta remember that every gas bag (Bladder) has to have a vent to relieve excess pressure as you
gain altitude. Also gotta have a fill tube to inflate. If you have a hundred small bags with a hundred
tubes and valves your going to add a lot of weight and complexity. A few large bags (Maybe 3-5 ft. in diam. x 20 ft long) I think is the way to go. Link a few bags together with a short coupling and lead one back to a manifold or plenum for filling and venting. Let the excess pressure vent into a holding bag to not lose the gas.

Re "packing pillows;" these are cheep and therefore interesting. We are not sure how well they hold helium, which is harder to hold than air. Preliminary thoughts are that mylar and aluminized mylar are somehow better containers of small atoms. Care to do some research on how different plastic films hold gas?
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Re; gas bag vents, that's right, attaching tubes and valves to each bag would defeat the purpose of cheep and eezy gas bags.  My thoughts were to just let the gas bags start poppin when their pressure height was exceeded. They won't all pop at once but serially, according to individual strength.
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What's the worse thing that can happen?

Up forever.

I think bursting is not an option. They would all burst within a very small vertical column.
The blimp ideal is not to lose gas or ballast as in a balloon but to maintain equilibrium.
The worst that could happen is death when all the bags have popped.

On the other hand, if you plan to operate close to the ground (as I do) sealed bags could be an option.
Just under fill by a bit and you might have 2 or 3 hundred feet for flying.
But if for any reason you get into an uncontrolled ascent you could be in serious trouble.

The equipment to make good bags is probably not cheap, nothing else is. I propose to check with whoever makes the party balloons to see if they do custom work. We don't want to overfill our gas bags anyway, so don't worry about perfect shape, they will conform. Mylar is super light, the extra just won't matter.
Like ebgeek says, a manifold for filling and draining would be a good idea. An overflow bag is a great idea.

I'm thinking common poly sheet. As far as I know it can be heat sealed with a simple household iron.
I found a hand held roller type sealer for $150.00 new.
I plan to make some tests with this as soon as the weather starts to warm.

Plastics will stick to irons, make a mess. I have a cheap 'rotary heat sealer' which is teflon coated. It is a soldering iron type of thing, with the roller on the end. Not like a pizza cutter, the roller is a pain to keep on course. If you roll too slow, it will cut through the plastic, not necessarily welding the plastic. If you roll too fast, no seal at all. What a pain. I'm sure the pros have a  motorized unit and they work out the speed and heat on scrap before doing the job. The only ones I have seen were very expensive. Post a picture if you can, I'm interested to see what is available. If you see anything like a spot welder, let me know.

I think its just a matter of getting the heat and speed right.
A little practice should do the trick.
Here is the roller I was looking at
http://www.usplastic.com/catalog/item.aspx?itemid=23802&catid=792

Re, just letting the lift bags pop when they go above pressure height, a careful reading will show that I said they would pop serially, meaning one after another, and not all at once.

Say there were 100 lift bags. Each one lifts 1% of the ship's weight. After one or two pop, the ship physically has to descend and no more pop.

Actually, normal flight buoyancy is usually less than the ship's weight, ie, the Goodyear blimp flies 100 pounds heavy and ascends on engine power, not on positive buoyancy so, strictly speaking, an airship is always "under-buoyant."

The ship we are talking about in this forum has the hybrid quality of aerodynamic/static lift so whenever the ship descends it cannot simply fall like a flat empty balloon, it maintains a semi-airfoil shape and therefore has no choice but to glide down.

 There would be some urgency to rapidly discovering a good landing spot but death could not occur  unless the pilot doesn't see the high voltage power lines   ;-] 

You can cover an iron with a "sock" so that it doesn't stick. I have a hobby iron and a "sock" that fits it. You can get this at any hobbytown type place. I don't think we will have any danger of an uncontrolled assent in a hybrid airship unless you are in an extreme situation, like flying over a forest fire or loosing a heavy airship component. If you have power, you can control your altitude. If you loose power, you will lose altitude. 

If we are going to use multiple flat sided bags, we need use bags of different sizes. This will increase packing efficiency and reduce weight. We could valve bigger bags and not worry with smaller bags.

Some of my comments are not strictly related to the hybrid type. My own project is for a floater.
Low and slow. Still I can't see any advantage to losing the bags and the gas.

If the poly sheet idea works out, you can build bags of just about any size or shape.
And for cheap. I have seen rolls 20 ft. x 100 ft. for about 40 or 50 bucks I think.

Your roller looks very good, has heat control and the roller is going in the right direction. The sock idea is fine. With practice, this sealer just might do well.
They have a spot welder! It looks just fine. I will get the price, looks like it will do just what I want. After the holidays I'll try to contact my plastic supplier for a sheet. A standard shape dirigible will do fine for a test model to test construction technique.

Are we talking about welding polyethylene or polycarbonate?

Our bags need to have low gas permeability. And the level of performance we need is dependent on the gas we plan to contain. Here is a paper that examines several different materials and their permeability. We need to make sure that the material we plan to use can contain our lifting gas without unacceptable leakage. 

http://www.ag.iastate.edu/wastemgmt/Mitigation_Conference_proceedings/CD_proceedings/Manure_Storage_Amendments_and_Covers/Kolbasuk-Gas%20Impermeable%20Film%20and%20Sheet.pdf

If you take a rectangular peece of hullmaterial, close it to a tube, then close the front end to be a horizontal line (inside armoured with a 2" tube), close the rear end same way but vertical, then you get an easy cheap done hull, on which you can install 2 front ailerons for  /down or roll control and a rear vertical end to install a finn for side orders. The air drag coefficient is extremely low, well suitable for muscular powering ( a two-seated version can be easily made of a cockpit of a TWIKE with attached aerodynamic end.
Have fun
George