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.
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.
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.
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.
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