1.2 Aircraft
1.2.4 Lighter-Than-Air Aircraft: Balloon and Airship
1.2.4.3 The Airship
k k used as a means of lifting a test object, such as a parachute or vehicle, to an altitude where it can
be released to study aerodynamics, flight dynamics, or other characteristics.
Unlike the gas balloons that expand as they ascend, the superpressure gas balloon is designed to maintain a constant volume at all altitudes. The gas envelope of a superpressure balloon is con-structed of a high-strength polyester film that can bear the high loads as the gas pressure changes.
Superpressure balloons can stay aloft for months, making ideal long endurance, high altitude sci-entific platforms.
The hybrid balloon combines features of the hot air and gas balloons. The hybrid balloon gener-ates its buoyancy from a combination of heated gas from a burner and the carriage of an unheated, lighter-than-air gas such as helium or hydrogen. De Rozier attempted to cross the English Channel in a hybrid hot air–hydrogen gas balloon. Since de Rozier’s time, hybrid balloons have been used for several long distance flights, including a solo, around-the-world flight by Steve Fossett in 2002.
Fossett’s circumnavigation in a hot air–helium hybrid balloon took over 14 days.
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Fabric covering Structural airframe
(a)
(b)
Gas bags or cells
Engine and propeller
Buoyant gas bags Gondola
Horizontal stabilizer
Horizontal stabilizer Vertical stabilizer
Vertical stabilizer Envelope
Air ballonet
Air ballonet
Gondola
Engine and propeller
Figure 1.40 Components of airships, (a) rigid airship and (b) non-rigid airship.
airships typically may have multiple power cars or engine cars – separate nacelles where the inter-nal combustion engine and propeller are installed. These engine cars may be mounted from the gondola structure or from other locations on the envelope. The propeller mounting may be of the pusher (facing aft) or tractor (facing forward) configuration. The multiple engines allow for the application of asymmetric thrust, which is used to help steer the airship. Movable, horizontal and vertical, control surfaces, located at the tail of the airship, are also used for steering and to control the attitude of the airship. To descend, the airship can vent gas and to climb, it can drop ballast.
Longitudinal trimming of the airship can be accomplished through weight shifting, by pumping water or gas fore and aft inside the vehicle.
Takeoff, landing, and general ground handling of an airship requires unique facilities and a large ground crew. An airship can take off or ascend much like a balloon, but it can also use its engines to assist in the lift-off. The landing is made by slowly descending towards a ground crew, dropping ropes for them to grab, and being anchored to the ground. A mooring mast may also be used, where the nose or bow of the airship is attached or moored to the mast. As might be imagined, a large number of people are required in the ground crew for ground handling of the airship. The process is more difficult in gusty or high wind conditions.
Perhaps the most famous rigid airships were those built in the early 20th century by the German Zeppelin Company. One of the most famous Zeppelin passenger airships was the LZ-129 Hindenburg (LZ stood for Luftschiff Zeppelin, German for “Airship Zeppelin” and “129” is the airship designation number). The Hindenburg was 803.8 ft (245.0 m) in length and 135.1 ft (41.2 m) in diameter. The giant airship contained over 7 million ft3(200,000 m3) of hydrogen gas.
Powered by four Daimler-Benz 16-cylinder diesel engines, the Hindenburg had a cruise speed
k k of 76 mph (125 km/h) and a typical cruise altitude of only 650 ft (198 m). The Hindenburg had
a flight crew of 39 men, an additional dozen chefs and stewards, and a doctor on board. Luxury accommodation for 72 passengers included private cabins, promenade observation areas, a dining room, a lounge with a grand baby piano, and a smoking room, which was kept at a higher than ambient pressure to prevent any leaking hydrogen gas from entering. For the passengers, flying in a Zeppelin passenger airship was much like taking a voyage on a luxury cruise ship. These large passenger airships were the first commercial airliners, able to cross long distances, including routinely flying across the Atlantic Ocean.
Even though these Zeppelins were filled with highly flammable hydrogen gas, they had a very good safety record and flew all over the world carrying passengers and cargo for almost a decade.
However, on 6 May 1937, the Hindenburg caught fire while attempting to dock at the Naval Air Station in Lakehurst, New Jersey, reducing the huge airship to a skeleton and ashes in less than a minute. Although never definitively proven, the leading theory for the cause of the fire was the ignition of leaking hydrogen gas by a static electric spark. This high profile disaster, along with a series of other airship accidents, contributed to the decline and ultimate demise of the airship as a viable means of commercial air travel. Replacing the flammable hydrogen gas with helium made the airship safer for flight, but the advancements in fixed-wing airplanes soon made the airship obsolete.
Airships are still used today, but mostly for applications where flying “low and slow” is desired, such as aerial advertising, tourism, remote sensing, and aerial observation. There has been renewed interest in using airships as long duration, very high altitude, scientific and commercial platforms, similar to high altitude balloons. An advantage of the airship is its ability to maintain a constant location over a point on the earth, similar to a stationary satellite. Scientific applications of airships include astronomical or weather observations. Commercial uses include acting as telecommunica-tions platforms.
An example of a modern airship is the Zeppelin NT, shown in Figure 1.41. The Zeppelin NT is a semi-rigid, helium-filled airship with a gas volume of 290,450 ft3(8255 m3). With a length of 246 ft (75 m) and a diameter of 46 ft (14.2 m), the Zeppelin NT has a gross weight of about 23,500 lb (10,700 kg). It carries a crew of 2 and 12 passengers at speeds up to 77 mph (125 km/h) and altitudes up to about 8500 ft (2600 m). The airship is powered by four 200 hp (149 kW) Lycoming IO-360, air-cooled, piston engines.
Figure 1.41 A modern airship, the Zeppelin NT, 2010. (Source: User: Stefan-Xp, “Zeppelin NT” https://
commons.wikimedia.org/wiki/File:Zeppelin_NT.jpg, CC-BY-SA-3.0. License at https://creativecommons .org/licenses/by-sa/3.0/legalcode.)
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Another related aircraft, the hybrid airship, combines elements of the lighter-than-air airship and the heavier-than-air, fixed-wing airplane. The hybrid airship obtains its lift from a combination of aerostatic (buoyant) lift and aerodynamic lift. By virtue of its more aerodynamic shape and higher cruise airspeeds, as compared with a conventional airship, the aerodynamic lift of a hybrid airship can approach 50% of the total lift.