Airship Design Issues
A rigid airship is provided by a vast aluminum hull structure completely filling the outer envelope. The lifting gas was then contained within a number of individual gas cells contained sequentially front-to-back within the hull structure. The gas cells themselves had virtually no pressurization. They simply floated against the top and sides of the hull structure to keep the airship aloft. Rigid airships are much more expensive to produce than the nonrigid variety primarily because of the complexity of the aluminum hull structure.
The biggest drawback of a nonrigid is they are limited in size by the strength of the fabric used in the envelope. Even though they are only slightly pressurized, the larger a nonrigid airship gets the greater the stress in the fabric even if the internal pressure remains constant. In the 1920s and 1930s the state of the art of fabric technology only allowed the construction of small blimps, hence all large airships were rigid out of necessity. Almost all airships proposed for construction in the 21st Century are nonrigid.
When an airship climbs the lifting gas within it expands as atmospheric pressure decreases. As this occurs the lifting gas must be allowed to expand for two reasons. First, to try to contain it under increasing pressure puts unnecessary stress on the envelope. Though an airship may appear to be highly pressurized, the pressure inside the envelope is maintained only slightly above ambient (less than 1 pound per square inch) to maintain its structural integrity. Second, because the pressure and density of the atmosphere decreases with altitude as the airship climbs, the lifting gas must continue to provide the same amount of buoyant lift and must be allowed to expand to displace additional ambient air.
In a nonrigid airship this is accomplished by incorporating separate, smaller envelopes called ballonets within the main envelope. The ballonets are filled with ambient air and expand and contract opposite the lifting gas. Before takeoff the ballonets are filled with air and the rest of the envelope with helium. As the airship rises and the helium expands within the main envelope, air in the ballonets is released into the atmosphere and the ballonets contract. The pressure height of the airship, which is generally the maximum operational ceiling, is the altitude at which the ballonets are completely emptied of air and helium fills the main envelope. When the airship descends and the helium contracts the ballonets are refilled with atmospheric air to compensate for the shrinking helium and maintain the same relative pressure and total volume of gas within the main envelope.
The design pressure height of an airship is important because it determines the proportion of total envelope volume allocated to air in the ballonets - more air means greater pressure height, but it also means less of the main envelope is allocated to helium at takeoff, which means less lift. An airship that is going to take off at sea level and climb to 10,000 feet en route must have approximately 30 percent of its total envelope volume taken by air in the ballonets at take off to allow room for the expansion of helium during the climb. This means the amount of helium available for lift is only 70 percent of the total envelope volume. If that same airship only had to climb to 3,000 feet, however, the ballonets need only be filled to 10 percent of the total volume so 90 percent could be filled with helium. All other things being equal, this means an airship that had to climb to 3,000 feet on a mission could take off with 28 percent more payload by weight than an identical airship that had to climb to 10,000 feet, the difference the 90 percent helium fill versus 70 percent fill.
This tradeoff must be considered during route planning for an airship, as it could be more efficient to deviate several hundred miles on a transcontinental mission to avoid an 8,000-foot mountain range instead of climbing over it. The additional payload available due to a lower pressure height would probably more than make up for the fuel required by the slightly longer route.
If ballonets are placed fore and aft in the vehicle, they may also be used for trimming the aircraft in lieu of aerodynamic trim. Pumping more air into a front ballonet and less out of a rear one while keeping the total volume constant is essentially a transfer of ballast (the air), which shifts the center of gravity of the airship forward. This is more efficient than using aerodynamic trim which increases induced drag that, in turn, increases fuel consumption.
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