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Old Jan 28, 2013, 11:09 PM
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Methane, like hydrogen, must be mixed with oxygen to burn. However, hydrogen has a much wider range of mixtures it will burn at than methane does. Methane will only burn when it makes up 4.4% to 17% of an air mixture, while hydrogen will burn in the range of 5% to 75%. So you could have quite a bit of air contamination in a methane balloon without worrying about it being flammable. The reverse situation is where you have the explosion risk-- which is a room full of air contaminated with a much smaller amount of methane. That's where you get the problematic 5-17% range that causes the scary explosion stories you hear in the news. In a balloon filled only with methane you would not be anywhere near that mixture, even if you were very sloppy in filling it or started with an envelope that already had some air in it. Methane has about half the lifting force of helium though, so you would need quite a bit more of it. This does not mean your blimp would have to double in length though, since volume increases in a cubed relation to it's individual dimensions. For example, if you had a 15 ft long blimp with a volume of 132 cubic feet, you would only need to proportionately scale it to about 18.5 feet to double its volume to 272 cubic feet. That still keeps the envelope small enough to make flying with methane viable, you just have to keep in mind that you would be buying twice as much of it as the helium.

Ammonia is another gas that can be used for lift, but it has even less lift than methane and has very noxious fumes.

Flammability Limit Reference:
http://en.wikipedia.org/wiki/Flammability_limit
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Old Jan 29, 2013, 08:44 AM
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one good thing with methane is that the molecules are really big copare to helium or hydrogen
so the ris of leak by porosity is almost null!

loss per day will be minimum !
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Old Jan 29, 2013, 10:15 AM
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Methane does seem like a good alternative that could keep RC blimps going even if the helium shortage makes helium unobtainable, at least with larger size blimps. It's much cheaper, much more available, much easier to keep from leaking over time and much less flammable than hydrogen when contaminated with air. The only down side is having half as much lift as helium, but that is at least a manageable problem.
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Old Jan 31, 2013, 09:15 PM
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I still get helium for less than 10 cents a cube... truckloads or it
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Old Feb 01, 2013, 10:14 AM
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Quote:
Originally Posted by patrickegan View Post
I still get helium for less than 10 cents a cube... truckloads or it
Assuming you are talking cubic feet, that is still 25 times more expensive than the methane price mentioned here, which was .4 cents per cubic foot.
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Old Feb 01, 2013, 12:34 PM
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Yeah, but with half the lift.
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Old Feb 01, 2013, 01:37 PM
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OK, adjusted for lift it is 12.5 times more expensive. Plus you lose more due to leakage.
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Old Feb 02, 2013, 01:46 PM
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Depends on your fabric and some other conditions.
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Old Feb 02, 2013, 10:44 PM
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For any given envelope, fill that same envelope with methane and it will lose gas at a slower rate than helium. The smaller the molecule, the easier it slips between the "cracks." Helium has the smaller molecule, which is of course why it also has more lift.
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Old Feb 03, 2013, 11:48 AM
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True.
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Old Feb 12, 2013, 03:19 AM
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and you can use methan to power gas motors so no more buyoancy problems!
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Old Feb 12, 2013, 08:58 PM
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This is actually what the Graf Zeppelin did. The gas cells were divided into two sections,with the lower one being filled with a propane-like gas called "blau gas" that had the same density as air so that there was no change in buoyancy as it was consumed by the engines. I guess they decided it took up too much volume though since this practice was discontinued on the airships built after the Graf.
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Old Feb 14, 2013, 06:20 PM
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We have a gig this weekend and had no problem getting helium albeit at a higher cost than before.
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Old Mar 06, 2013, 11:48 PM
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Update:

Just thought I should check back in. I read how far this thread has come along and it is pretty interesting. When I had trouble obtaining helium earlier this year I went and looked at a variety of different lighter than air gases as well as gas mixtures and had come across both methane(I decided for my application it was too heavy, but I could see this being used effectively in a different lighter than air application) and ammonia(I decided this was far too dangerous to work with after reading about all the nasty side effects it has such as corrosion, that type of thing).

As for obtaining helium... I was fortunate enough to be able to secure enough helium through my school to fly my prototype aircraft and it was a success. I've also found a source though for helium if I need more later through one of my relatives that has been a long time welder. It is definitely not cheap. I believe it was $300 or $400 or so for a large tank which would just fill my craft as it had been originally designed.

I am proceeding with the project and am currently working on the final design for my UAV version of the design. I'm planning on designing it to be capable of flying with different gases and then the owner can determine what is the most economical and safest gas to fill it with for their application. One thought I had for safely using hydrogen was a vacuum pump integrated into the filling mechanism. Image a two prong filling handle where one side is a vacuum pump and the other is a nozzle for the hydrogen. You could insert the mechanism, pump any air out of the inside of the craft, stop the vacuum/close that valve and then fill it with research grade hydrogen, then that valve is shut and the entire gas bladder is isolated from the electronics. You could also empty it remotely from a distance so that encase of an ignition you would only lose the craft. Just one of my thoughts for how this could be done semi-safely.

Also. I've been asking around in the UAV forum for some suggestions as to what kind of a payload I need to make this capable of carrying for it to be useful to others involved with UAVs and so far I really haven't gotten much feed back. If any of you guys have an opinion or a suggestion I'd appreciate it if you would post it.

Here's that thread: http://www.rcgroups.com/forums/showthread.php?t=1845789

If you would like to take a look at the aircraft's design it can be found under the heading "Aerospace", sub heading "UAV", as well as the heading "News" at( www.awkengineering.com ). I'm very excited about this design and the platform in general and I have high hopes for it in the near future.

If there is an interest in it I wouldn't mind starting a build thread/discussion for it, but I'm a bit at a loss for where I would put it. This aircraft/near space craft falls under so many categories its hard to categorize in a single area if that makes sense. It incorporates electric power systems, scratch building, airframe design, it's a UAV, it's an airship, it's a VTOL craft, it's a multi-rotor, and it's an electric jet just to mention a few of the categories it could fall under.

Any thoughts as to where this belongs and if there is any interest in it?

Thanks.
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