"Stephen McNaught" wrote in message
...
Might be bad in case of electrical failure. What about having two
"opposite"
bleeder like valves, that when there is a pressure difference, one opens
up
to equalize the outside, and inside of the floats?

Well, first...what you're talking about would be simply an open passage. So
forget the "two 'opposite' bleeder like valves" idea. If you have two
valves, one of which allows air movement with a negative pressure difference
and one of which allows air movement with a positive pressure difference,
then you just have the equivalent to an open passage for air to move.

Second, for it to work, you'd need a float that worked correctly when
inflated only to ambient pressure. Most "inflatable" objects (the floats in
question included) generally maintain a higher pressure inside than ambient,
and this is what gives them structure. It wouldn't be appropriate to
operate the floats when the internal pressure is the same as ambient
pressure. You really need the internal pressure to remain at least somewhat
higher than ambient.

Third, an inflatable object is pretty good at resisting an internal pressure
higher than ambient, and a calibrated valve certainly would allow one to
maintain any specific pressure or relative pressure difference desired. But
it would be hard to get the internal pressure *below* ambient, which would
be required in order to allow external air to flow back into the object.
Without any structure built into the object to prevent it from collapsing
(such as that found in self-inflating camping bedrolls, for example), an
increase in ambient pressure would simply make the object smaller. It
wouldn't take on more air.

There are a variety of solutions to dealing with the pressure changes due to
altitude. But in general, they all involve enough complexity and weight
that you're better off just getting rigid floats if you anticipate
significant altitude changes. Inflatable floats do exist, and they are
appropriate solutions for some aircraft, but just not those that climb any
significant amount.

Pete