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Old Mar 19, 2014, 04:18 PM
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Melbourne, Australia
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A further development on ballonets. A common problem with any scale model blimp is that they are tail heavy. The distribution of gondolas and engines that works for the prototype often doesnít for a model; fins with servos are always heavier and push the cg aft, so you need ballast at the nose, a waste of weight. Itís hard to put anything useful there, except perhaps batteries, but then you have problems with the long cables.

An alternative solution is a nose ballonet. A fixed volume ballonet in the nose pushes the helium aft and restores balance. You need less ballast (if any) at the nose, and use less helium too. See sketch below. The main ballonet is variable volume, controlled by blower and valve.

I have done this on my model ZPG-3W (see postings above). I actually used a latex balloon, with a filler poking out the nose. I blow it up with 45 litres of air from an air mattress hand pump inflator, then seal it. Then fill the main bag with helium. Saves 50 grams of ballast at the nose, and works fine. The only thing to watch is that an air filled latex balloon inside a helium envelope will expand as helium diffuses in. I tested this, it swelled by 5% over 4 hours.
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Old Mar 21, 2014, 01:25 AM
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Joined Feb 2009
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Wow, that thing looks so realistic I could swear I was watching video of the real blimp! Just slow down the frame rate so the rapid motion wasn't a give-away and you could really convince people they were looking at the real thing. Nice job!
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Old Mar 21, 2014, 03:04 PM
Go small or go home
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Canada, SK, Regina
Joined Mar 2008
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Quote:
Originally Posted by Alan Sherwood View Post
The only thing to watch is that an air filled latex balloon inside a helium envelope will expand as helium diffuses in. I tested this, it swelled by 5% over 4 hours.
I've found a product at the craft store for sealing helium in latex balloons. 'Hi float' or something. It's a Liquid rubber of some sort. Squirt a bit in the balloon and run it around the intrepid before filling. It really does seal the helium in, at the cost of a little more weight.

In our flying gym there was a common latex party balloon stuck in the ceiling for over 4 weeks. I eventually recovered it with another balloon on a string. Even after all that time it was still well inflated with good lift. An untreated latex balloon will be drooping in a day. I'm thinking of rubbing it in my envelopes, or at least along the seams.

I'm not happy with the pressure changes my micro blimps go through. A ballonet is probably the answer, but I can't see his it could be made light enough. I've seen self inflating Whoopi cushions that have a foam sponge inside. Maybe a flat foil bag with a light weight 'sponge' material to act as a mechanical pressure spring. Just enough to absorb temperature pressure changes for a flying session
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Old Aug 07, 2014, 05:33 AM
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Melbourne, Australia
Joined May 2005
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Another video of my ZPG-3W. Works a lot better when somebody else holds the camera!
IMG 0581000428 029 001348 567 (9 min 22 sec)
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Old Aug 07, 2014, 10:42 AM
Go small or go home
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Canada, SK, Regina
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Wow! She's a beauty Alan. Please post more details!
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Old Aug 07, 2014, 12:23 PM
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What kind of film did you make the envelope with? It looks like the metalized side is on the outside (assuming it was heat sealed).
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Old Aug 12, 2014, 09:36 PM
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Melbourne, Australia
Joined May 2005
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Some info on my ZPG-3W blimp:
1.45 cubic meters volume, 4 m long
Typhoon Micro brushless motors, rated at 15 A each (although I haven’t been game to run at full throttle in flight, it’s a handful at half speed)
Controls are rudder, elevator (mixed) and throttle (fwd only, no reverse)
Envelope is made from 15 micron plain aluminised nylon film. I built it about 20 years ago before foil balloon material became available; I had to scrounge around packaging companies for some samples. The gores are glued with contact adhesive. It had stood up reasonably well over the years, needing occasional peeling seam resealing. It does leak now, although at a tolerable level.
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