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Airships

Balloons, blimps and zepplins have captured the fancy of man since he first aspired to fly. These airships combine desirable characteristics from the standpoint of aerodynamics, travel economy and ecology. Lighter-than-air vehicles can remain aloft nearly indefinitely and are capable of touching down in areas which would be inaccessible for most other conventional transport vehicles. They can be powered by a simple propeller engine and refueling can be accomplished either directly from the ground or by means of a simple umbilical cord from another aircraft.

There are two forms of lighter-than-air [LTA] craft the balloon and the airship; the generic term for both is aerostat. The fundamental difference between the airship and the balloon is that the former is powered and horizontally controllable and the latter is neither. The French connection is how and why the word "dirigible" entered English as a synonym for "airship." It comes from the French word dirigeable the adjectival form of the transitive verb diriger, which means, among other things, to control or steer. Thus, with incontestable Gallic logic, a steerable lighter-than-air craft was called a ballon dirigeable. Eventually ballon was dropped and the adjective became the noun; thus dirigeable remains to this day the French term for airship.

A zeppelin is really just one brand of airship named after its inventor, Ferdinand Graf von Zeppelin. Zeppelins were built from 1900 to 1939 as rigid airships. They were - and still are today - very popular because of their pioneering successes. But there were and are still numerous airship types and brands. Modern airships, with the exception of the "Zeppelin NT" are not really zeppelins.

The term "blimp" is used particularly in the USA as a catch-all expression for all kinds of airships, similar to the way in which the term "Zeppelin" is used in Germany. But strictly speaking this term describes only non-rigid airshipsnon-rigid airships, including advertising airships. According to research done by the Goodyear Company, the term "blimp" originates from A.D. Cunningham in the year 1915. A.D. Cunningham was a commanding officer at the British Capel airship base, who, while inspecting the SS-12 rigid airship flicked his finger against the airship envelope and copied the resulting noise with the sound "blimp". That's why this term is used for this airship type. A strange, but true, story.

There are three types of airship systems.

Non-rigid Airships: Non-rigid airships are airships which, like balloons, keep their exterior shape through the pressure of the lifting gas inside the envelope. There is no interior skeleton or supporting structure. The gondola and tail units are mounted on the airship envelope. The load in the gondola area is transferred to the lower envelope sections via large-surface patches. So-called catenary curtains or cable mechanisms transfer force to the upper envelope sections and thereby ensure the even distribution of weight on top of the envelope section. The engines of non-rigid airships are mounted directly on to or even underneath the gondola. They are also equipped with ballonets.

Semi-rigid Airships/Keeled Airships: These airships differ from pressure airships through their rigid keel structure beneath the airship envelope or a partial structure inside the envelope. The rigid structure helps to ensure even distribution of force to the envelope and as a reinforcement of the entire system. These aircraft - just like non-rigid airships - get their shape from gas pressuregas pressure inside the envelope. Therefore ballonets are needed for semi-rigid airships to keep the envelope pressure constant. According to this model, it is possible to mount engines on more efficient points on the airship body. This was the case during the development of the Zeppelin NT, whose engines were separate from the passenger gondola. This resulted in greater comfort for passengers and more efficient engine operation. Furthermore, all attachments (engines, tail units and gondola) are firmly connected to the interior support structure, which also acts to absor stress. With this type, optimal low weight and greater efficiency for larger airships can be achieved.

Rigid Airships: With this type of airship, the exterior form is determined by its rigid skeletal structure. The attachments (tail unit and gondola) are either part of the structure or connected to it. Within the skeletal structure of large rings fastened to longitudinal girders are gas bags filled with lifting gas. These can be imagined simply as some kind of cylindrical/disc-shaped balloons. The skeletal structure of old Zeppelins was made from aluminum alloys (Duraluminium). It is simply covered with a material that gives the airship a smooth surface and protects the gas bags. These are suspended from the structure in a way that compensates for any volume fluctuations of the lifting gas with increasing or decreasing altitude i.e. for any expansion or contraction of the gas bags. Thus, rigid airships don't need any additional ballonets. With this design, the motors can be placed very efficiently. Another advantage surfaced during construction of the LZ 129 "Hindenburg" zeppelin: all passenger accommodation was moved inside the airship hull, which made a much larger area available for passengers. The historic zeppelins and airships like the "Schtte-Lanz", which have a wooden structure, represent this typeof design. A definite disadvantage is the immense weight of the rigid skeletal structure, which makes it suitable only for large airships.

Small airships up to, say, 300 ft. long are necessarily non-rigid, as there is not sufficient lift to justify a rigid framework. The largest airships have a rigid hull structure because the pressures involved in an envelope of large diameter necessitate very heavy fabric and make a system of compartments essential. Between the two, the semi-rigid seeks to reduce the fabric tensions by the use of a rigid keel girder, but it is doubtful whether this justifies the keel, except as a convenient means of carrying the loads from the envelope.

The frames of most of the past rigid airships consisted of built-up rings and longitudinal girders stabilized with wire bracing. The rings and longitudinals were typically made of aluminum alloy and the bracing was steel. This structure was very light and efficient, even by present standards. However, this construction was highly complex and labor intensive.

The operation of the 1930's airships was as labor intensive as their construction. Control operations of the 1930s are best grasped by appraising the crew sizes. The rigids had very large crews. The Graf Zeppelin had a crew of 44, the Hindenburg 38. Likewise, military rigids had large crews: the USS Akron had 76 crewmen, the 51 USS Shenandoah 43, the USS Los Angeles 45, and the USS Macon 76. The reason for such large crews was the lack of sophisticated flight support systems. In flight large onboard crews were required to constantly monitor and adjust the trim of the ship and maintain nearly neutral buoyancy.

Trim and neutral buoyancy were maintained by one or more of the following procedures: valving lifting gas, dropping ballast, transferring fuel or other materials within the airship, collecting water from the atmosphere and engine exhaust, and moving crew members within the airship. Also, it was not unusual to repair the structure and the engines in flight. Engine controls of the rigid airships consisted of an engine telegraph that transmitted engine control commands from the helmsman to an engine mechanic, who would then manually make the required engine control changes.

Airships all had about the same strucural efficiency, measured by empty weight/gas-volume ratio) despite differences in size, design concept, year of develment, and lifting gas. The insensitivity to size was a reflection of the airship "cube-cube law" (i.e., both the lifting capability and the structural weight increase in proportion to the cube nf the principal dimension for a constant shape). Since fixed-wing heavier-than-air craft follow a "square-cube law," airships will compare more favorably with heavier-than-aircraft as size is increased. Smaller airships have tended to have non-rigid or semi-rigid construction, whereas the larger airships have been rigid, and this would be true of modern vehicles as well.





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