The term Airship can be used to describe any form of vehicle that can travel through the air. Usually through lighter-than-air technology, but not exclusively. Airships are more often than not out-fitted with one or more large dirigible-balloons and fins, sails, and propellers for maneuvering. Some airships have engines for producing hot air, steam, or other gasses for fuel.
Lighter-than-air refers to materials (usually gases) that are buoyant in air because they have densities lower than that of air (about 1.2 kg/m3, 1.2 g/L). These gases can be used as lifting gases in lighter-than-air airships, which include free balloons, moored balloons, and airships, to make the whole craft, on average, lighter than air.
Aerostats (The general term for all balloon-based Airships) use buoyancy to float in the air in much the same way that ships float on the water. They are characterized by one or more large gasbags or canopies, filled with a relatively low-density gas such as helium, hydrogen, or hot air, which is less dense than the surrounding air. When the weight of this is added to the weight of the airship structure, it adds up to the same weight as the air that the craft displaces.
A powered, steerable aerostat is called a "dirigible". Sometimes this term is applied only to non-rigid balloons, and sometimes "dirigible balloon" is regarded as the definition of an airship (which may then be rigid or non-rigid). Non-rigid dirigibles are characterized by a moderately aerodynamic gasbag with stabilizing fins at the back.
Hot air Hot air is frequently used in recreational ballooning. Hot air is lighter than air at ambient temperature.
Neon Neon is lighter than air and will lift a balloon. However, it is relatively rare on Earth, expensive, and is among the heavier of the lifting gases.
Steam The gaseous state of water is lighter than air, and has successfully been used as a lifting gas. It is generally impractical due to high boiling point and condensation.
Ammonia Ammonia has sometimes been used to fill weather balloons. Due to its relatively high boiling point (compared to helium and hydrogen), ammonia could potentially be refrigerated and liquified aboard an airship to reduce lift and add ballast (and returned to a gas to add lift and reduce ballast).
Methane Methane (the chief component of natural gas) is sometimes used as a lift gas when hydrogen and helium are not available. It has the advantage of not leaking through balloon walls as rapidly as the small-moleculed hydrogen and helium. (Many lighter-than-air balloons are made of aluminized plastic that limits such leakage; hydrogen and helium leak rapidly through latex balloons.)
Hydrogen and Helium Hydrogen and helium are the most commonly used lift gases. Although helium is twice as heavy as (diatomic) hydrogen, they are both so much lighter than air that this difference is inconsequential. Hydrogen has about 8% more buoyancy than helium.
In a practical dirigible design the difference is significant making a 50% difference in the fuel carrying capacity of the dirigible and hence increasing its range significantly.
Heavier-than-air airship must find some way to push air or gas downwards, so that a reaction occurs (by Newton's laws of motion) to push the airship upwards. This dynamic movement through the air is the origin of the term aerodyne. There are two ways to produce dynamic upthrust: aerodynamic lift, and powered lift in the form of engine thrust.
Gliders are heavier-than-air airship that do not employ propulsion once airborne. Take-off may be by launching forward and downward from a high location, or by pulling into the air on a tow-line, either by a ground-based winch or vehicle, or by a powered "tug" airship. For a glider to maintain its forward air speed and lift, it must descend in relation to the air (but not necessarily in relation to the ground). Many gliders can 'soar' - gain height from updrafts such as thermal currents.
A fixed wing airship is an airship capable of flight using wings that generate lift due to the ship's forward airspeed and the shape of the wings.
A rotorship or rotary-wing-airship is a heavier-than-air flying ship that uses lift generated by wings, called rotor blades, that revolve around a mast. Several rotor blades mounted to a single mast are referred to as a rotor.
Some Devil Fruit abilities can be used to make ships fly. One such example is the Fuwa Fuwa no Mi.
The first known airships we're developed by the Skypieans living on St. Peters Archipelago, many years ago, after they discovered the plant Hydrocane, and uses for hydrogen gas. They use it primarily to travel around the archipelago, and the white-white sea in general.
Later a hot-air powered airship was developed by Franky, during his younger days with Tom and Iceberg. It was called Battle Franky 20,000, because he was hoping to take it 20,000 feet into the air. Unfortunately, the dirigible balloon ruptured at about 18.000 feet, they were able to land without getting hurt, but afterwards he turned his attention back towards battle-ships.
The Straw Hats used an octopus balloon to reach the Blue Sea safely after leaving Skypiea. Even though it was more like slow-falling, as they were unable to change their course, at on point they used a combination of Breath and Flame Dials to increase their altitude.Shiki the Golden Lion used his devil fruit ability of levitation to make his ship, and those of his armada able to float freely in the air. But since his defeat, those ships effected by this power have become earth-bound once again.
A small number of other pirates also use flying ships. The best known examples of this are, Jayde D. La Coeur and her Pirates De L'air, who use a lighter-than-air airship named the Lily of Sha'ron, while the Red Fang Pirates and their captain, Emperor Kiba, use a hybrid rigid-dirigible/rotorship called the High Dragon Palace.