Non-rigid airships, or "Blimps" as they are affectionately called, are members of the Lighter than Air family of aircraft, which includes balloons as well as rigid airships such as the German Zeppelins. Blimps have no structure of any kind within the envelope, only helium and air. All but one airship model seen flying today are non-rigid, and use a gas called helium, instead of wings, for lift. Helium has a lifting capacity of 1.02 kg/m3 (0.0640 lb/ft3). This may seem quite small, but the ABC airships' envelopes range from 1900 m3 (67,000 ft3) to 4800 m3 (169,000 ft3), resulting in very useful payloads.
Using the advanced technology available today in materials and computer aided design, the modern airship seems a distant relative to those of only 50 years ago. The envelope, which when inflated is aerodynamically shaped and contains helium for lift, is a proprietary design made of modern synthetic materials. These are stronger than steel by weight, and impervious to ultraviolet light degradation, a problem with earlier envelope life.
Helium gas is a natural fire extinguisher and, while once rare, is available world wide as a byproduct of natural gas production.
It is important to understand that the pressure inside the envelope is very low, about 1/15 psi: equal to that of a column of water 4-5 cm (11/2 in) high. Because the pressure is so low, a hole in the envelope results only in a very slow leak, taking hours or even days to effect the airship's performance.
The figure to the right illustrates the way an airship's ballonet system accommodates the changing helium volume. As the airship rises, the helium expands. It contracts when the airship descends.
In order to maintain a constant pressure within the envelope, 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 airship nose is raised by the pilot.
Once airborne, airships can perform much like helicopters, remaining nearly geostationary for extended periods of time. Helicopters must use nearly 100% power to hover, but airships reduce power, fuel consumption, noise and vibration. In fact, there is no more comfortable airborne platform for extended missions than an airship.
Since airships usually do not rely on speed to provide aerodynamic lift, they do not require a great deal of power or large powerplants. The small powerplants contribute to quiet cabins, low vibration levels, lower costs, and ample room for payloads and crews, as show here. (Click on the photograph for a 45 kB enlargement of the A-150 10 place interior for air tours.)
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