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The airships operate by the Principle of Archimedes: "Bodies submerged into a fluid receive from it a lifting force which is equal to the mass of the displaced fluid. The airship is filled with a lifting gas (Helium). The atmospheric air has a higher specific weight than the lifting gas. The airship envelope filled with the light gas generates a lift that is equal to the weight of the displaced air and the airship floats in the heavier air.

Because the pressure inside the envelope is very low, (about 5.0 millibar or 1/15 psi), a hole in the envelope results only in a very slow leak, taking hours or even days to affect the airship's performance.

As the airship rises, the helium expands and helium contracts when the airship descends. In order to maintain a constant pressure within the a ballonet is installed (or in some airships multiple ballonets. These are simply bags containing air, which are inflated or deflated to maintain a constant pressure inside the envelope. This allows the helium to expand and contract. When the ballonet is completely empty the airship is said to be at its "pressure height." The initial design of the ballonet size will determine an individual airship's maximum change of altitude capability.

In addition to the lift provided by helium, modern airships derive aerodynamic lift from the shape of the envelope as it moves through the air, as an airplane does. Maximum payload capacity may be achieved by making a running takeoff in an airship, much like an airplane. The speed gained on the ground is converted to lift when the pilot raises the airship nose. Once airborne, airships can perform much like helicopters, remaining nearly geo-stationary for extended periods of time.

   Types of Airships
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Rigid Airships

Rigid airships have a rigid internal framework, which maintains their shape. The infamous Zeppelin airship (which caught fire just before landing in 1937) was an example of this type. In general, rigid airships have a good weight to volume ratio only when their length exceeds around 120 m. The solid internal framework is considered too heavy for a small rigid airship. The use of composite material can perhaps obviate this.


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Semi-rigid Airships

Semi-rigid airships were more popular earlier this century. They usually comprise a rigid lower keel construction and a pressurized envelope above that. The rigid keel can be attached directly to the envelope or hung underneath it. The airships of Brazilian aeronaut Alberto Santos-Dumont were of this type. One of the most famous airships of this type was Italia, used by General Umberto Nobile in his attempt to reach the North Pole.


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Non-rigid Airships

Non-rigid airships, also known as Blimps, are the most common type nowadays. They are large gas balloons whose shape is maintained only by their internal overpressure. The only solid parts are the passenger car and the tail fins. All the airships currently flying for advertisement purposes are of this type; the Goodyear Blimps, the Budweiser and the Metlife Blimps in the USA, and the Fuji Blimp in Europe.


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Hot Air Airships

Hot air airships, also known as thermal airships, are counted as a fourth kind although they are technically part of the non-rigid category. Hot air airships are derived from traditional hot air balloons. Early models were almost like balloons with an engine and tail fins added. Later, the envelopes were lengthened and the tail fins and rudder were pressurized by air from the wash of the propeller. Newer hot air airships maintain their shape with internal overpressure in the whole envelope, a feature which older models did not have.



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