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In the
case of a balloon, the lift force is produced by the lower density of the gas
in the balloon envelope. In the Breitling Orbiter 3 shown in the photographs,
part of the envelope is filled with a helium bag below which is a hot-air
bag that provides a variable component for the lift force. At constant altitude,
the lift force and the weight of the balloon are in balance. For this
system there is no thrust force. The device relies on the wind to provide
the required horizontal motion. The drag interaction between the balloon and
the air provides the necessary coupling to the local wind. Changing altitude
to take advantage of the wind direction at another level in the atmosphere
provides a mode of navigation. The map below shows the course taken in this
first successful circumnavigation of the world by this balloon. |
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