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 accommodati on was moved inside the airship hull, which made a much larger area available for passengers. The historic zeppelins and airships like the "Schütte-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. Today, however, this no longer make any financial sense either.
Any gas or gas mixture that is "lighter than air", and therefore has a lower density (weight per cubic meter) is suitable as a lifting gas. Under standard conditions (0°C, 1bar) air has a density of 1.28 kg/m³.
Air that has been heated to about 70°C has a lower density than atmospheric air. Its density is approximately 1.02 kg/m³ and therefore significantly higher than that of hydrogen or helium. This results in less lift (only approx. 260g/m³). A great disadvantage is that heat has to be added to the air constantly because it cools down and consequently loses lift (density increases with cooling). Hot air is used with small hot air-airships and of course with hot air balloons. A large-scale technical application of hot air for airships is impossible, because high heating costs and the low amount of lift make it uneconomical.
Hydrogen is a low-density gas and easy to produce. It exists as the H2 molecule and is therefore larger than helium. This is relevant for the gas tightness of the airship envelope. Under standard conditions (0°, 1bar), one cubic meter of hydrogen has a density of 0.09 kg/m³. However, together with oxygen, hydrogen is flammable and in a certain mixing ratio with air can form oxyhydrogen which is highly explosive. Therefore, hydrogen is no longer used for commercial airship operations.
Helium is an inert gas and therefore chemically inert, not flammable and appears as a single atom He. It is found in the earth's crust and is a by-product of natural gas extraction. At 0.18 kg/m³, its density is clearly greater than that of hydrogen, but still much smaller than that of air. Because of the small size of the helium atom, it was a problem for a long time to manufacture gas-tight envelope materials. With modern materials it is now possible to better confine the diffusion within the envelope material. CargoLifter envelope material loses less than 1% of helium per year.
Buoyant lift is the influence on the weight of a body created when the body density differs from that of the medium surrounding it. A person in the environment, for example, experiences lift that corresponds to the amount of air volume he takes in. This buoyancy lift is utilized in airship aviation. The balloon envelope is filled with a gas (see lifting gas), that has a lower density than air. It is "lighter than air". If, for example, one were to take a balloon that has a volume of 1 cubic meter and fill it with helium, one would be able - under simplified assumptions and deducting the weight of the balloon - to lift approximately 1 kg of service load. (In this example the air density is assumed to be at 1.28 kg/m³, the helium density at 0.18 kg/m³, the weight of the balloon at approximately 100 g.)
The lift created through flow around bodies is called positive lift. One example is the lifting surface of an airplane. Because of the special profile with a more prominent curvature on the upper side an air particle has to cover a greater distance, and therefore flow around the profile at a higher speed. This creates force which in turn works on the profile i.e. the body. In pre-war airship aviation, positive lift, especially with rigid airships, did not play a big role. Today positive lift is applied selectively. Most airships today start "heavier than air" and also use positive lift for take-off.
Gas bags were cylindrical balloons - used in the historic Zeppelin airships for example - which were built into the rigid rib structure of the airship and contained the lifting gas. The LZ 127 "Graf Zeppelin" airship contained 17 of these gas bags. It was possible to trim the airship by deliberate release through gas valves and therefore to correct its position along the ship's axis. Gas bags are no longer used in modern airships, because of new types of envelope material and since, due to weight considerations, the costs outweigh the benefits.
The airbags inside the envelope are called ballonets. These usually take up between 10-30% of the total volume. One has to imagine a ballonet as a kind of "balloon within a balloon". For all non-rigid airships and semi-rigid designs one or more of these balloons are necessary. Since air density decreases with altitude, the lifting gas in a completely filled envelope would expand and thereby influence the pressure difference between "inside" and "outside". But if the ballonets inside the envelope are filled with air on the ground the gas expands when a higher altitude is reached. Therefore ballonets ensure a consistent form in which the pressure is constant, while the volume of the lifting gas fluctuates. The air is simply forced out of the ballonets into the environment during ascent. Ballonets can also be filled with air either via special blowers or air ducts that channel air from the airstream of the drives. This process takes place during descent. In this way the ballonets function as "equalizing tanks". If an airship has several ballonets, these are often connected to each other and help the airship to trim. In this way the ballonet located in the nose, for example, can be filled with more air than the ballonet in the back. This is can also be done by pumping air through a connecting tube. As a result of the extra air, the airship nose becomes heavier and tips downward.
Air resistance is the most important factor in determining the propulsive output of an airship. Although the aerodynamic design of a modern airship has a low resistance coefficient (known as the cw-value in the auto industry), the area relevant for calculating air resistance is huge. Frictional resistance, created by currents around the airship, is also a factor. In the boundary layer, the air around the airship body must also be accelerated. Attachments like the tail unit, for example, can also have an effect on air resistance. Because air resistance increases by a power of two in relation to speed, it is also a factor in determining the overall maximum speed airships can reach.
Gas pressure is particularly important for non-rigid and semi-rigid airships. The excess pressure of the lifting gas gives the envelopes of these airships a tight, firm shape just like a balloon. However, the excess pressure is relatively low: an airship envelope is kept tight with an excess pressure of approx. 450 to 650 Pascal, (0.0045 to 0.0065 bar); whereas a car tire requires an excess pressure of approximately 2.0 bar! But even this low excess pressure acts upon the envelope with a force of 450 to 650 N/m² or, expressed in weight, approximately 45 to 65 kg/m².
Pressure altitude is the height at which there is no more air in the ballonets of non-rigid and semi-rigid airships, when the lifting gas takes up the entire envelope volume. Depending on the airship, it lies at around 2000 meters. With rigid airships at this altitude the gas bags are expanded to capacity.
Superheating refers to the effect created when interior lifting gas heats up more than the surrounding air. The volume of the lifting gas increases, helium density decreases and in turn the liftlift increases. Pressure altitude can be lower, though, because a lower ballonet volume with constant lifting gas level remains.
There are different ways in which the tail units with control surfaces can be mounted on the airship. Today there are only three types of rudder/tail unit configurations: the + - configuration, the X-configuration and the reversed Y-configuration. Only the + - configuration has clearly defined elevators or side rudders, with the other variants angled control surfaces influence both altitude- and rudder control. Therefore, these are called "ruddervators" in English. The use of these mixed rudders demands complex electronic steering devices for accurate altitude- and left/right steering. An advantage of the X-, and reversed Y-configurations is that because of the missing lower side tail unit it is possible to realize steeper take-off angles. The reversed-Y configuration also saves the weight of an entire tail unit plus rudder, which makes this model particularly attractive for small airships.
During flight, as a rule an airship is steered through control surfaces on the tail units. An airship can also be trimmed or started through ballonets (if available) leading to either aerodynamic lift or downward pressure, which means that it can be steered at an altitude. There are also airships which use a vector thrust, i.e. a swiveling driving gear able to create horizontal thrust (in the longitudinal direction of the airship) for the forward- and vertical thrust of the elevator control. This is a particular advantage for take-off and landing, because airships with vertical thrust can take off and land horizontally like a helicopter. Today's latest airships have an additional driving gear, that produces traverse thrust for improved rudder control.
The gondola is an attachment to the airship housing the cockpit and the passenger area. The historic Zeppelin airships had two types of gondolas: the control gondola and the engine pod. The control gondola housed all relevant steering apparatus and spaces such as the navigation room, radio room etc. and - until the LZ 127 "Graf Zeppelin" - passenger cabins. The engine pods housed the enormous engines. They powered the propellers and had to be maintained and adjusted by mechanics around the clock.
