Displacement of Air - RC Groups
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Aug 27, 2017, 05:11 AM
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Displacement of Air

Hello, I am currently working on a project dealing with helium balloon and buoyancy and I find myself needing to understand what displacement really means and how it works. How does one actually displace air? Is it the object being a certain size that displaces the air? Are certain shapes better than other shapes or does it matter? I understand that the helium in the balloon being less dense than the surrounding air gets pushed against by the air forcing it upwards. Does it have to be a sphere shape to work? Can it be a Square? Box? Is wider balloon work better than a narrower balloon? More surface area better than a small surface area?


Michael Cote'
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Aug 28, 2017, 05:31 PM
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Most simply displacing air is like blowing up a balloon, it starts out basically empty with small volume, you force air or another gas into it, and it starts to fill and displace more volume (as it either stretches or fills out from the flattened condition). If you use an elastic vessel like a latex balloon it takes extra pressure to stretch the material as it gets bigger. If you have a deflated gas bag, not so elastic, it only takes atmospheric pressure to fill plus just a hair extra.

The (bouyancy) simple version is simply if you fill the vessel with a gas lighter than air, at some point the sum difference in the weight of gas vs air exceeds the weight of the vessel and the whole thing becomes bouyant and rises. Air is relatively compressible so its density changes significantly with altitude, so your vessel will rise until the air density is low enough for it all to reach equilibrium. (as opposed to say a vessel underwater where it basically will or will not float, sinks to the bottom or floats on the surface, water density changes very little under pressure in comparison to air, the same concept does apply, but very precise densities in a small range govern the altitude*). Several other factors constantly change for your helium vessel though. Best review the ideal gas law. Temperature in particular can vary quite a bit.

A sphere is essentially the most efficient and common vessel for helium, like a balloon. It has the least surface area for the volume. It's also relatively stable fabric structure (holds its shape well), but there is nothing preventing the use of a cube or other shape, it's just going to tend to bulge into semi sphere shape if it's made from a fabric or sheet material that is free to flex. But that doesn't affect the lift or bouyancy. If you make a fabric cube and fill it with gas and then increase the pressure it will start to look like a sphere with eight tits. Balloons are under pressure because you need the pressure to stretch the latex. (pressure inside the balloon greater than the pressure outside). Most any helium or other balloon will be under some overpressure to make it keep its shape, otherwise it gets limp and folds over etc. unless it has a rigid frame confining it in some way.

Hope that helps, always good to review the basics. What's up? Are you having a nerd birthday party or something? Cap

* even engineered to some extent these galileo theremometers will rarely have a bulb floating midheight, the slight changes in temperature will drive them up or down, rarely stable at midheight as shown in the optimistic photo below, typically they are grouped top and bottom, without the stray midheight bulb. (ie they are digital thermometers not analog...)
Last edited by capricorn; Aug 28, 2017 at 05:47 PM.
Aug 28, 2017, 11:28 PM
Registered User

Displacement of Air

Hey thanks for that explanation. Best I have heard in a long while. So....The shape doesn't matter to an extent except for efficiency.
I am thinking about making a sort of rigid body frame, rounded rectangle lifting body with x lbs of payload and x lbs of lift and go in this direction in my project.

thanks again for the info.

Michael Cote'
Sep 18, 2017, 01:43 AM
Registered User
thanks again for your response to my question. I have another question referring to my first post, maybe you can help me here also? I am looking to design a modular LTA craft. when I say modular I mean set a module that has X number of payload capacity and net lift. Then if you need more lift then add more modules. I used the balloon performance calculator at high altitude science website http://tools.highaltitudescience.com/# not sure how accurate it is for getting the correct values. But I used a balloon size of 3000 grams, payload of 13,607.8 grams (30lbs), net lift of 4,535.9 grams (10Lbs). It came up with a requirement of 760.14 cu. Ft. of Helium; burst altitude of 27,580 m. (90,485.6 ft.). So, I chose to make it 800 cu. Ft. I rounded to the nearest whole foot. So I made it 16 ft. x 16 ft. x 4 ft. which gave it 1024 cu. Ft. Now this is a rectangle shaped envelope with rounded ends, no sharp edges.

Now my question is how do I accurately calculate for the proper cubic feet to make a modular LTA for a given material? what type of material would you suggest? Would carbon fiber or some type of fiber glass? I have heard of non-rip nylon fabric. I read somewhere where a guy made his own blimp with a bladder inside of a nylon envelop. Would this suffice? I could add an external ribcage to form the shape of the rectangle?
Michael Cote'
Oct 10, 2017, 06:04 PM
Registered User
Michael, So I gather the reason you want them rectagular is so you can stack them together (modules). I think you need some sketches, I assume the modules need to be fastened together and have a way of hanging the payload? Probably will need frames to keep their shape and to connect to. The weight of the frames will decrease the lift obviously.

Probably the design will be governed by how severe of weather you want/need to have it survive. Cap
Oct 11, 2017, 02:31 AM
Melbourne, Australia
You might be better off using spherical units, and using a tetrahedral-style close packing arrangement :-). Trying to force round balloons into square holes will cost you a lot of weight, while spheres will keep things simpler..! It can still be modular if you attach/glue linkage points on your spheres...
Oct 12, 2017, 07:20 PM
Registered User
Good idea pegacat.

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