Subject: Helium lifting ability
From: Jeff Wisnia
Date: 21/07/03 03:05 GMT Daylight Time
Message-id:

So what *is* the pressure in a typical rubber party balloon. IIRC, it gets
*lower* as the balloon expands. I think it's the same with soap bubbles.

It's higher as the balloon expands or there would be negative work being done
to inflate the balloon against the elasticity of the rubber.


I suppose the pressure in one of those mylar party balloons must be pretty
close to one atmosphere.

It can be anything up to what the material will withstand. It can't be lower
than atmospheric though because the internal and external pressure will
equalise back to atmospheric via a change in volume of the balloon. Once the
balloon (given the relatively inelastic material) has reached its maximum
volume the internal pressure will rise as more gas is pumped in. The lifting
capability will then decrease though as the mass rises for no corresponding
gain in volume and boyancy.

For a balloon made of inelastic material the maximum lift occurs when the
internal pressure is the same as external and the ratio of volume to mass is
highest. i.e. fully inflated until the pressure just starts to rise above
atmospheric. For a balloon made of elastic material the maximum lift also
occurs when the ratio of volume to mass is highest, but the pressure for that
depends on the strength and elasticity of the material.


Dave Baker - Puma Race Engines (www.pumaracing.co.uk)
"How's life Norm?"
"Not for the squeamish, Coach" (Cheers, 1982)