Cargo Airships Prospective - Faculty of Aerospace Engineering

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significant structure weight reduction, but on the same time, does not allow high loads; literally only slow cruise speed and small payload is allowed. Thus blimps are usually the smallest in the variety of airships, and used mainly for purposes of local surveillance, airborne advertising and as a small tour vehicle. Fully rigid airships, as were the giant zeppelins of the past, are at the opposite side of scale. They have full internal framework that is not only intended for the loads but for keeping the external form as well. This makes ballonet system unnecessary to some point, but on the other hand, increases vastly the structure weight, on the expense of the potential useful load. Semi-rigid airships are basically a trade-off option between the two. It is similar to non-rigid ones, with the difference of owing some sort of load-bearing internal structure; by this achieving higher load tenability, with reasonable penalty of the additional weight. Progress in the field of the building materials, as will be presented in the next paragraph, has allowed building stronger and tougher semi-rigid airships, thus making them a preferred option. While not utterly disbanded, rigid airship designs are rare nowadays and most of the current designs are based on semi-rigid hulls for cargo airships and non-rigid for lighter applications. Figure 6. Three examples of main airship structure types presented. From left to right: Goodyear nonrigid blimp, Zeppelin NT semi-rigid airship and USS Shenandoah rigid zeppelin. IV. Technologies History of flight is fully bound with the technological progress. New technology development, has allowed incremental improvement of the flight vehicles, beyond the previously known frontiers. Same principle is relevant for all types of aircraft and for lighterthan-air vehicles as well. The evolution of advanced designs and its competitive advantages (compared to the past), took place mostly because of technological progress in materials, avionics, systems and propulsion. With no doubt, one of the greatest progress advances in airship technology was made in the areas of structure and materials. Massive metal internal structure was substituted by lightweight and strong composite framework. Implementing aircraft based experience on composites, lead to building much lighter gondola, engine nacelles and systems casing. Polymer based, multi-layered, impervious and durable envelope material took place of former heavier and more permeable fabrics of the airship's envelope. Zeppelin's skin was usually made from heavy cotton fabric, which was soaked with rubberized coating and externally laminated with aluminum coating for better weathering durability. The lifting gas was stored inside it, in separate gas cells, made from intestines of cattle and goats. Contemporary airship's skin is usually made of at least three layers – load bearing layer made of tough material (like Kevlar or Vectran), non-permeable layer made of Mylar and additional polymer coating (like Tedlar), for increased imperviability and weathering resistance. The results of improving envelope materials are; lesser weight, better lifting gas sealing and higher structural strength. Constantly improving synthetic and fiber materials development and
manufacturing processes, allows assuming further empty to total weight ratio reduction. This is the main reason why most of the present cargo airship projects, presents concepts of semirigid airship, as a default. Fly-by-wire (FBW) controls for engines and control surfaces, coupled with advanced avionics and systems enhance stability and control, as well as navigation. Large size of the airship's hull, derived from its nature, require the use of fly-by-wire system much more than in fixed wing aircraft. FBW can contribute for a weight saving of airship's systems, which is an important feature in a platform, where each kilogram counts. FBW, coupled with specialized computerized avionics system, simplifies control and navigation of an airship. As the airship is more susceptible to gusts, compared to fixed wing aircraft, due to its large cross-section, an active control system is required. Finally, airships today use smaller, more powerful but more fuel efficient internal combustion engines than before. In the early days of aviation, Zeppelins used the available aero-engines, which were heavy, bulky and weak. Over the time, engines of internal combustion developed further and turbojets were invented. Turbojet invention led to creation of the turboprop engines, which are most suitable for cargo airships, due to its power requirements. Turboprop engines are usually smaller than comparably powered internal combustion engines, but they are slightly less efficient. In recent years, due to environmental and regulation policies tendency, there is a trend to plan future projects, towards hybrid or even fully electrical propulsion, with integrated solar panels on the airship's hull. Figure 7. Several of the key technologies that make airship fly V. Challenges and Opportunities Along with their great potential, airships do have several inherent disadvantages. Compared to aircraft they are more limited in bad weather operations; due to their large size they are more susceptible to winds and precipitations. Their buoyancy is affected by direct sunlight and surrounding air density and temperature. Cargo airships are generally restricted to operations under 10,000ft, due to increasing typical wind speed profile and decreasing air
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