An airship or dirigible is a type of aerostat or lighter-than-air aircraft that can be steered and propelled through the air using rudders and propellers or other thrust mechanisms. Unlike aircraft that use aerodynamics, such as fixed-wing aircraft and helicopters that produce lift by moving an airfoil through the air, aerostatic aircraft stay aloft by having a large 'envelope' filled with a lifting gas that is less dense than the surrounding air. In early dirigibles, the lifting gas used was hydrogen, despite widespread concern due to its flammability.
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