Friday, November 16, 2007

An airship or dirigible is a buoyant lighter-than-air aircraft that can be steered and propelled through the air. Unlike aerodynamic vehicles such as fixed-wing aircraft and helicopters which stay aloft by moving an airfoil through the air in order to produce lift, aerostatic craft such as airships and balloons stay aloft by filling a large cavity with lighter than air gas.
Airships were the first aircraft to make controlled, powered flight. They were widely used prior to the 1940s. Their use decreased over time as their capabilities were surpassed by those of airplanes. A series of high-profile accidents, including the 1937 burning of the hydrogen-filled Hindenburg at Lakehurst, New Jersey, furthered their decline. Airships are still used today in certain niche applications however, such as advertising.


Rigid airships (for example, Zeppelins) had rigid frames containing multiple, non-pressurized gas cells or balloons to provide lift. Rigid airships did not depend on internal pressure to maintain their shape.
Non-rigid airships (blimps) use a pressure level in excess of the surrounding air pressure in order to retain their shape.
Semi-rigid airships, like blimps, require internal pressure to maintain their shape, but have extended, usually articulated keel frames running along the bottom of the envelope to distribute suspension loads into the envelope and allow lower envelope pressures.
Metal-clad airships had characteristics of both rigid and non-rigid airships, utilizing a very thin, airtight metal envelope, rather than the usual rubber-coated fabric envelope. Only four ships of this type, Schwarz's aluminum ships of 1893 and 189 the ZMC-2 and the Slate "City of Glendale", have been built to date with only the ZMC-2 a success.
Hybrid airship is a general term for an aircraft that combines characteristics of heavier-than-air (airplane or helicopter) and lighter than air technology. Examples include helicopter/airship hybrids intended for heavy lift applications and dynamic lift airships intended for long-range cruising. It should be noted that most airships, when fully loaded with cargo and fuel, are typically heavier than air, and thus must use their propulsion system and shape to generate aerodynamic lift, necessary to stay aloft; technically making them hybrid airships. However, the term "hybrid airship" refers to craft that obtain a significant portion of their lift from aerodynamic lift and often require substantial take-off rolls before becoming airborne. Types
Any gas that is lighter than air can be used to create buoyant lift, however many such gases are either toxic, flammable, corrosive, or a combination of these, limiting their use in airships. Historically, hydrogen and helium have been used in large airships.
In the early days of airships, the primary lifting gas was hydrogen. Ships called thermal airships utilize heated air, in a fashion similar to hot air balloons, as their lifting gas.

Lifting gas
With hydrogen gas being approximately half the density of helium gas, one may erroneously assume that it has twice the buoyant force of helium. In fact, the theory of buoyancy indicates the force is related to the density of the air which the gas displaces more than the density of the gas itself.
The density at sea-level and 0°C for air and each of the gases:
Utilizing the buoyancy equation
As such, hydrogen's additional buoyancy compared to helium is:
However due to the fact that hydrogen is highly flammable heilum is usually used.

Airair) = 1.292 grams per liter (g/l).
HydrogenH2) = 0.08988 g/l
HeliumHe) = 0.1786 g/l
Buoyancy = mass × (1 - ρairgas)
Therefore the buoyancy for one liter of hydrogen in air as:

  • 0.08988 grams * (1 - (1.292 / 0.08988) ) = -1.202 grams
    And the buoyancy for one liter of helium in air as:

    • 0.1786 grams * (1 - (1.292 / 0.1786) ) = -1.113 grams
      1.202 / 1.113 = 1.080, or approximately 8.0% Hydrogen versus helium

      Airships were among the first aircraft to fly, with various designs flying throughout the 19th century. They were largely attempts to make relatively small balloons more steerable, and often contained features found on later airships. These early airships set many of the earliest aviation records.
      In 1784 Jean-Pierre Blanchard fitted a hand-powered propeller to a balloon, the first recorded means of propulsion carried aloft. In 1785, he crossed the English Channel with a balloon equipped with flapping wings for propulsion, and a bird-like tail for steerage. The beginning of the "Golden Age of Airships" was also marked with the launch of the Luftschiff Zeppelin LZ1 in July 1900 which would lead to the most successful airships of all time. These Zeppelins were named after the Count von Zeppelin. Von Zeppelin began experimenting with rigid airship designs in the 1890s leading to the badly flawed LZ1 (1900) and the more successful LZ2 (1906). At the beginning of World War I the Zeppelin airships had a framework composed of triangular lattice girders, covered with fabric. and containing separate gas cells. Multi-plane, later cruciform, tail fins were used for control and stability, and two engine/crew cars hung beneath the hull driving propellers attached to the sides of the frame by means of long drive shafts. Additionally there was a passenger compartment (later a bomb bay) located halfway between the two cars.

      The prospect of using airships as bomb carriers had been recognised in Europe well before the airships themselves were up to the task. H. G. Wells described the obliteration of entire fleets and cities by airship attack in The War in the Air (1908), and scores of less famous British writers declared in print that the airship had altered the face of world affairs forever. On 5 March 1912, Italian forces became the first to use dirigibles for a military purpose during reconnaissance west of Tripoli behind Turkish lines. It was World War I, however, that marked the airship's real debut as a weapon.
      Albert Caquot designed an Observation Balloon for the French army in 1914. The Type R Observation balloon was used by all the allied forces, including the British and United States Armies, at the end of the World War. In 1919, Japan equipped the Imperial Army with several "Caquot dirigeables".
      The Germans, French and Italians all operated airships in the scouting and tactical bombing roles early in the war, and all learned that the airship was too vulnerable for operations over the front. The decision to end operations in direct support of armies was made by all in 1917.
      Airplanes had essentially replaced airships as bombers by the end of the war, and Germany's remaining zeppelins were scuttled by their crews, scrapped or handed over to the Allied powers as spoils of war. The British rigid airship program, meanwhile, had been largely a reaction to the potential threat of the German one and was largely, though not entirely, based on imitations of the German ships.

      First World War
      Airships were operated in a number of nations between the two world wars. The major operators of rigid airships were Britain, the United States and Germany, with Italy and France operating a few. Italy the Soviet Union and United States and Japan operated semi-rigid airships, while blimps were operated in many nations.
      The British R33 and R34, for example, were near identical copies of the German L 33, which crashed virtually intact in Yorkshire on September 24, 1916. The Hindenburg completed a very successful 1936 season carrying passengers between Lakehurst, NJ and Germany. The Hindenburg's 1937 started with the most spectacular and widely remembered airship accident. While approaching the mooring mast just minutes from landing on 6 May 1937 the Hindenburg burst into flames and crashed. Of the 97 people on board, there were 36 deaths: 13 passengers, 22 aircrew, and one American ground-crewman. The disaster happened before a large crowd, was filmed and a radio news reporter was cutting a recording of his coverage of the arrival. This was a disaster which theater goers could see and hear the next day. On that same next day, the Graf Zeppelin landed at the end of it's flight from Brazil, ending intercontinental passenger airship travel.
      There was no possibility of flying the Hindenburg's sister ship, the Graf Zeppelin II without Helium and the United States refused to sell it. The Graf Zeppelin flew some test flights and conducted electronic espionage until 1939 when it was grounded due to the start of the war. The last two Zeppelins were scrapped in 1940.
      Development of airships continued only in the United States, and in a small way, the Soviet Union.

      Inter-war period
      While Germany determined that airships were obsolete for military purposes in the coming war and concentrated on the development of airplanes, the United States pursued a program of military airship construction even though it had not developed a clear military doctrine for airship use. At the Japanese attack on Pearl Harbor on 7 December 1941 that brought the United States into World War II, it had 10 non-rigid airships:
      Only K and TC class airships were actually suitable for combat purposes and they were quickly pressed into service against Japanese and German submarines which at that time were sinking US shipping in visual range of the US coast. US Navy command, remembering the airship anti-submarine success from World War I, immediately requested new modern anti-submarine airships and on 2 January 1942 formed the ZP-12 patrol unit based in Lakehurst from the 4 K airships. The ZP-32 patrol unit was formed from two TC and two L airships a month later, based at US Navy (Moffet Field) in Sunnyvale, California. An airship training base was created there as well. In December 1941 and the first months of 1942, the Goodyear blimp "Resolute" operating from Los Angeles, was operated as an anti-submarine privateer. The only US craft to operate under a Letter of Marque as a privateer since the war of 1812 the Resolute, armed with a rifle and flown by its Goodyear crew, patrolled the seas for submarines.
      The Soviet Union used a single airship during the war. The W-12, built in 1939, entered service in 1942 for paratrooper training and equipment transport. It made 1432 runs with 300 metric tons of cargo until 1945. On 1 February 1945 the Soviets constructed a second airship, a Pobieda-class (Victory-class) unit (used for mine-sweeping and wreckage clearing in the Black Sea) which later crashed on 21 January 1947. Another W-class — W-12bis Patriot was commissioned in 1947 and was mostly used for crew training, parades and propaganda.

      4 K-class: K-2, K-3, K-4 and K-5 designed as a patrol ships built from 1938.
      3 L-class: L-1, L-2 and L-3 as small training ships, produced from 1938.
      1 G-class built in 1936 for training.
      2 TC-class that were older patrol ships designed for land forces, built in 1933. The US Navy acquired them from Army in 1938. Airship Continued use
      There are two primary focuses of current research on airships: 1) high altitude, long duration, sensor and/or communications platforms and 2) long distance transport of very large payloads.
      The US government is funding two major projects in the high altitude arena. The first is sponsored by U.S. Army Space and Missile Defense Command and is called the Composite Hull High Altitude Powered Platform (CHHAPP). This aircraft is also sometimes referred to as the HiSentinel High-Altitude Airship. This prototype ship made a 5 hour test flight in September 2005. The second project is being sponsored by the US Defense Advanced Research Projects Agency (DARPA) and is called the high-altitude airship (HAA). In 2005, DARPA awarded a contract for nearly $150 million to Lockheed-Martin for prototype development. First flight of the HAA is planned for 2008.
      There are also three private companies funding working on high altitude airships. Sanswire is developing high altitude airships they call "Stratellites" and Techsphere is developing a high altitude version of their spherically shaped airships. JP Aerospace has discussed its long-range plans that include not only high altitude communications and sensor applications but also an "orbital airship" capable of lifting cargo into low earth orbit with a marginal transportation cost of $1 per short ton per mile of altitude.
      On January 31, 2006 Lockheed-Martin made the first flight of their secretly built hybrid-airship designated the P-791 at the company's flight test facility on the Palmdale Air Force Plant 42. The P-791 aircraft is very similar in design to the SkyCat design unsuccessfully promoted for many years by the now financially troubled British company Advanced Technology Group. Although Lockheed-Martin is developing a design for the DARPA WALRUS project (see below), the company claimed that the P-791 is unrelated to WALRUS. Nonetheless, the design represents an approach that may well be applicable to WALRUS. Some believe that Lockheed-Martin had used the secret P-791 program as a way to get a "head-start" on the other WALRUS competitor, Aeros.
      A privately funded effort to build a heavy-lift aerostatic/aerodynamic hybrid craft, called the Dynalifter, is being carried out by Ohio Airships. The company has stated that they expect to begin test flight of the Dynalifter in Spring of 2006.
      21st Century Airships Inc. is a research and development company for airship technologies. Projects have included the development of a spherical shaped airship, as well as airships for high altitude, environmental research, surveillance and military applications, heavy lift and sightseeing. The company's airships have set numerous world records.

      Present-day research
      The proposed Aeroscraft is Aeros Corporation's continuation of the now canceled WALRUS project (see below.) This proposed craft is a hybrid airship that, while cruising, obtains two thirds of its lift from helium and the remaining third aerodynamic lift. Jets would be used during take-off and landing.
      There is a case for the airship or zeppelin as a medium to long distance air 'cruise ship' using helium as a lifting agent. Airship passengers could have spacious decks inside the hull to give ample room for sitting, sleeping and recreation. There would be ample room for restaurants and similar facilities. The potential exists for a market in more leisurely journeys, such as cruises over scenic terrain.

      Proposed designs and applications
      The advantage of airships over airplanes is that static lift sufficient for flight is generated by the lifting gas and requires no engine power. This was an immense advantage before the middle of WW I and remained an advantage for long distance, or long duration operations until WW II.
      The disadvantages come from the nature of the airship itself. All things being equal, the power required to propel an aircraft will rise as the square of the speed. Given the large flat plate area and wetted surface of the airship, a practical limit is reached somewhere between 80 and 100 mph. The altitude an airship can fly at is largely a function of how much lifting gas it can lose due to expansion before stasis is reached. The ultimate altitude record for a rigid airship was set in 1917 by the L-55 under the command of Kurt Flemming (who later died in the Hindenburg) when he forced the airship to 24,000 feet (7,300 meters) attempting to cross France after the "Silent Raid" on London. The L-55 lost lift as the descent to lower altitudes over Germany compressed the gas left in the cells, and weight of air displaced. L-55 crashed due to loss of lift.
      So long as the horsepower to weight ratios of aircraft engines remained low and specific fuel consumption remained high, the airship had an edge for long range or duration operations. As those figures changed, the balance shifted rapidly in the airplane's favor. By mid-1917 the airship could no longer survive in a combat situation where the threat was airplanes. By the late 1930s, the airship barely had an advantage over the airplane on intercontinental over-water flights, and that advantage had vanished by the end of WW II. The blimp remained a viable military system only until the conventional submarine was replaced by the nuclear submarine. Today, airships are used primarily for advertising where their size and novelty have an advantage.

      Practical comparison to fixed-wing aircraft
      Hybrid designs such as the Heli-Stat airship/helicopter, the Aereon aerostatic/aerodynamic craft, and the Cyclocrane was a hybrid aerostatic/rotorcraft, have struggled to take flight. The Cyclocrane was also interesting in that the airship's envelope rotated along its longitudinal axis.
      CL160 was a very large semi-rigid airship to be built in Germany by the start-up Cargolifter, but funding ran out in 2002 after a massive hangar was built. The hangar, built just outside Berlin, has since been converted into a resort called "Tropical Islands".
      In 2005, there was a short-lived project focused on long distance and heavy lift was the WALRUS HULA The primary goal of the research program was to determine the feasibility of building an airship capable of carrying 500 short tons (450 metric tons) of payload a distance of 12,000 miles (20,000 km) and land on an unimproved location without the use of external ballast or ground equipment (e.g. masts.) In 2005, two contractors, Lockheed-Martin and US Aeros Airships were each awarded approximately $3 million to do feasibility studies of designs for WALRUS. In late March 2006, DARPA announced the termination of work on WALRUS after completion of the current Phase I contracts.

      See also

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