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Definition 2024

ballonet ‎(plural ballonets)

Any of several small balloons, inside a dirigible, that can be inflated or deflated to control buoyancy during flight.
- 1929, Frederic Manning, The Middle Parts of Fortune, Vintage 2014, p. 27:
  An observation balloon, sausage-shaped and thickened at one end by small subsidiary ballonets, lifted itself, almost as though it were being hoisted by a series of pulls, out of one of the hollows beneath them […].