Paul, as with most things , it depends on several things. The material

for the bladder can't be bike inner tube since it needs to resist the heat
of cure. Around 250°F. There are several silicone tubes that will work.
A simple tube will work if you use a flexible rubber.
Bill Jepson

Paul, I have two updates on this process. First; we, that is the composite guys 
I work with have found a strechable bagging
material that would be perfect for the prop bladder. Second; for the tip 
areas a solid could be the real ticket. A small
designed in cavity allows weights for balancing.
Bill Jepson

 
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