A ballonet is an inflatable bag inside the outer

envelope An envelope is a common packaging item, usually made of thin, flat material. It is designed to contain a flat object, such as a letter (message), letter or Greeting card, card. Traditional envelopes are made from sheets of paper cut to one o ...

of an

airship An airship, dirigible balloon or dirigible is a type of aerostat (lighter-than-air) aircraft that can navigate through the air flying powered aircraft, under its own power. Aerostats use buoyancy from a lifting gas that is less dense than the ...

which, when inflated, reduces the volume available for the lifting gas, making it more dense. Because air is also denser than the lifting gas, inflating the ballonet reduces the overall lift, while deflating it increases lift. In this way, the ballonet can be used to adjust the lift as required. Ballonets may typically be used in non-rigid or semi-rigid airships, commonly with multiple ballonets located both fore and aft to maintain balance and to control the pitch of the airship. The image illustrates the principle of a balloon within a balloon. The outer balloon represents the airship's outer envelope or gasbag, while the red inner balloon represents the ballonet. In an airship the ballonet would be much smaller relative to the size of the gasbag; for example, in the French airship Lebaudy Patrie the volume of the ballonet was approximately one-fifth that of the envelope.

History

The ballonet was first described in 1783 by Jean Baptiste Meusnier, then a lieutenant in the French Army. However his own airship project was unsuccessful. In 1784 professor

Jacques Charles Jacques Alexandre César Charles (12 November 1746 – 7 April 1823) was a French people, French inventor, scientist, mathematician, and balloonist. Charles wrote almost nothing about mathematics, and most of what has been credited to him was due ...

and the

Robert brothers Les Frères Robert were two French brothers. Anne-Jean Robert (1758–1820) and Nicolas-Louis Robert (1760–1828) were the engineers who built the world's first hydrogen balloon for professor Jacques Charles, which flew from central Paris on ...

built an elongated, steerable craft that followed Jean Baptiste Meusnier's proposals. Their design incorporated Meusnier's internal ''ballonnet'' (air cell), a rudder, and oars for propulsion, which proved useless. On July 15 the brothers flew it for 45 minutes from Saint-Cloud to

Meudon Meudon () is a French Communes of France, commune located in the Hauts-de-Seine Departments of France, department in the Île-de-France Regions of France, region, on the left bank of the Seine. It is located from the Kilometre Zero, center of P ...

accompanied by M. Collin-Hullin and Louis Philippe II, the Duke of Chartres. The absence of a lifting gas release valve meant that the Duke had to slash the

to prevent it rupturing when they reached an altitude of about 4,500 metres (15,000 ft).Federal Aviation Administration - F.A.Aviation News, October 2001, Balloon Competitions and Events Around the Globe, Page 15
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It was not until after Meusnier's death that the first successful application took flight. On 8 August 1884 the first practical airship, ''La France'', flew for the first time.Walker, P.; ''Early Aviation at Farnborough, Vol. I: Balloons, Kites and Airships'', Macdonald (1971), Page 176. Ballonet air valves were originally of a

butterfly Butterflies are winged insects from the lepidopteran superfamily Papilionoidea, characterized by large, often brightly coloured wings that often fold together when at rest, and a conspicuous, fluttering flight. The oldest butterfly fossi ...

type, actuated by springs. If pressure rose in the ballonet, a spring would pressure the spindle of the butterfly valve to turn, relieving the pressure. In more recent implementations, the valves are actuated electrically either by a

linear actuator A linear actuator is an actuator that creates linear motion (i.e., in a straight line), in contrast to the circular motion of a conventional electric motor. Linear actuators are used in machine tools and industrial machinery, in computer perip ...

(driven open/closed) or a linear

solenoid upright=1.20, An illustration of a solenoid upright=1.20, Magnetic field created by a seven-loop solenoid (cross-sectional view) described using field lines A solenoid () is a type of electromagnet formed by a helix, helical coil of wire whos ...

(spring return), the latter being the favored fail-to-safe arrangement.

References

{{reflist Airship technology