Airships
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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