Aerostatics

Aerostatics is the study and practice of flight sustained by buoyancy in a surrounding fluid. It underlies balloons and other lighter-than-air craft.

Buoyancy: The upward force a fluid exerts on an immersed or surrounded object, equal to the weight of displaced fluid. Aerostatic lift is buoyancy: the craft displaces air heavier than its total weight.
Balloon: A flexible enclosure inflated with gas, commonly used for recreation, observation, or transport. Balloons are the basic aerostatic craft, from simple free balloons to complex airships.
Free balloon: An untethered balloon that moves with the wind and is not steered by propulsion. Free balloons demonstrate buoyant flight without active steering.
Heavier-than-air aircraft: An aircraft whose weight is supported by aerodynamic lift generated through motion in air. It contrasts with aerostats, which can remain aloft through buoyancy without forward motion.
Archimedes' principle: A principle stating that a body immersed in a fluid experiences an upward force equal to the weight of fluid it displaces. It quantifies the force that supports a balloon in air.
Airship: A powered lighter-than-air aircraft that steers through the air while supported by buoyant gas. It adds propulsion and control to buoyant flight.
Tethered balloon: A balloon secured to the ground or a vessel by a cable. Tethering lets aerostats hold position for observation or communications.
Glider: An unpowered heavier-than-air aircraft that stays aloft by exploiting aerodynamic lift and rising air. Gliders rely on wing lift rather than displacement-based buoyancy.
Lifting gas: A gas used to make an aerostat buoyant because it is less dense than the surrounding air. Gas density sets how much air a balloon displaces for a given payload.
Envelope (aeronautics): The outer fabric or gas-tight structure of a balloon or airship that contains its lifting gas. The envelope holds the volume of gas needed to displace air.

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