"Louis L. Perley III" wrote in
:

Saw an article today about an aircraft that would change it's
buoyancy, so it would float up like a balloon and then change buoyancy
and glide down. Interesting stuff, although I doubt we'll see it
anytime soon. Is something like this even practical? The article says
they take the aircraft up 10 miles to get 400 miles range,

In theory, yes. In practice, probably not. G It's called a blimp...

One of the things that limits your aircrafts speed (or range - somewhat
interchangable for this discussion) is induced drag. That comes from
using part of the aircraft's power to create lift instead of forward
thrust. If we reduce the weight of an aircraft (keeping everything else
identical) then we can now lower the angle of attack and maintain level
flight at a much lower power setting (or higher speed at the higher powr
setting).

Nothing new here... You can see it in any C-172 that's either light, or
at gross.

So now we seal off part of the plane and replace the air in there with,
well, nothing would be best. We reduce the weight of the plane by the
weight of the air that we pumped out. We can go faster and/or farther
on less.

Now the problem... First off, we lost part of the volumetric capacity of
the aircraft. Second, we played great games with the CG. Third, by
having to seal the area, we increased the "empty" weight some (even if
we maintain 1 atmosphere by pumping back in helium). Fourth, we got
only minimal improvements... maybe a few pounds savings.

That last is the big problem. If we want REAL weight savings we have to
increase the volume. That's called a blimp, and to get useful weight
savings it must be BIG BIG BIG. Now we have a Bonanza that weighs 100
pounds ... but it's 400 feet long and 100 feet across. And NOW we have
to add back in the parasitic drag that all this increased wetted area
generates, and now we are down to 35 knots TAS at full power.


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James M. Knox
TriSoft ph 512-385-0316
1109-A Shady Lane fax 512-366-4331
Austin, Tx 78721
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