We want to build a helium balloon (larger than a standard party balloon) for use in an school project. We've found templates for traditional hot air balloons. If we fill a balloon with helium instead of hot air, will it still float the same? If we build it so that we can close the hole on the bottom and the helium won't escape... will it work? Will the traditional hot air balloon shape be appropriate for a helium balloon?

I mean a regular toy balloon, filled with Helium. How high up does it go? What happens at maximum altitude? Please cite your source, if you don't mind. My 3 year old let his "Trader Joe's" balloon fly out of his hand in the front yard yesterday, and is demanding a full explanation as to why Daddy was unable to retrieve it. No kidding.

A helium balloon, with a combined mass of the balloon and helium of 20 kg, is used to life a 70kg payload. At an altitude of 30 km the balloon stops and remains stationary. Explain, using physical principles, how the balloon with its payload can remain stationary at an altitude of 30km.

Is there a formula showing how big a helium balloon or blimp would have to be to lift a certain weight to a certain height? I'd be interested in building an aircraft runway on the roof of the blimp. Let's say the runway is wide enough for the biggest plane and long enough for a safe landing or takeoff, how deep would the helium part of the blimp have to be?