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I'm aware of that. Just thinking of using ANY space, that can be filled - like all hallow room in the plane that cannot be made use of.

I'm not an engineer, probably it is more reasonable to fill the remaining space with battery ;)

It would reduce the plane's weight by a trivial amount, like eating ice cubes to lose weight (nutritional "calories" = 1000x heat calories).

Also, helium requires airtight containment - which itself would add weight.

Not belittling you; trying to explain the problem better for you.

No offense taken, thanks for the explanation. Makes sense.