Airship

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USS Akron (ZRS-4) in flight over the Panama Canal, March 1931

An airship or dirigible is a buoyant aircraft that can be steered and propelled through the air. Unlike aerodynamic craft (e.g. airplanes 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 primarily by means of a cavity (usually quite large) filled with a gas of lesser density than the surrounding atmosphere.

Airships were the first aircraft to make controlled, powered flight. Their widest use took place from roughly 1900 through the 1930s. However, their use decreased over time as their capabilities were surpassed by those of airplanes. In addition they suffered a series of high profile accidents — most notably the burning of the Hindenburg. Today they are used in a variety of niche applications, particularly advertising.

Terminology

In many countries, airships are also known as "dirigibles", from the French dirigeable, meaning "steerable." The first airships were called "dirigible balloons"; over time, the word "balloon" was dropped.

The term zeppelin is a genericised trademark that originally referred to airships manufactured by the Zeppelin Company.

In modern common usage, the terms "zeppelin", "dirigible" and "airship" are used interchangeably for any type of rigid airship, with the terms "blimp" or "airship" alone used to describe non-rigid airships. In modern technical usage, however, "airship" is the term used for all aircraft of this type, with "zeppelin" referring only to aircraft of that manufacture, and "blimp" referring only to non-rigid airships.

"Airship" is sometimes informally used to mean any machine capable of atmospheric flight.

In contrast, balloons are buoyant aircraft that generally rely on wind currents for movement, though vertical movement can be controlled in both.

There is often some confusion around the term "aerostat" with regard to airships, as the term has two different meanings. One meaning refers to all craft that remain aloft using buoyancy. In this sense, airships are a type of aerostat. The other, more narrow and technical meaning refers only to tethered or moored balloons. In this second technical sense, airships are distinct from aerostats. This confusion is often exacerbated by the fact that both airships and aerostats have roughly similar shapes and comparable tail fin configurations, although only airships have motors.

Types

Airships are categorized based upon their underlying structural designs. There are three main groups:

  • Rigid airships (for example, zeppelins) have rigid frames containing multiple, non-pressurized gas cells or balloons to provide lift. Rigid airships did not depend on internal pressure of the lifting gas to maintain their shape. No rigid airships have been built since the 1930s.
  • Non-rigid airships (also known as blimps) use a pressure level in excess of the surrounding air pressure in order to retain their shape. Most airships flying today have a non-rigid design.
  • 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. The Zeppelin-NT airship is an example of a semi-rigid.

In addition, there are two smaller groups:

  • Metal-clad airships have characteristics of both rigid and non-rigid airships, utilizing a very thin, airtight metal envelope, rather than the usual rubber-coated fabric envelope. Only two airships of this type, one built by Schwarz in 1897 and the US Navy ship ZMC-2, have been built to date.
  • 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.

Lifting gas

The two principal gases used in airships are hydrogen and helium. These are sometimes called lifting gas because of their function. In the early days of airships, hydrogen was commonly used because it could be generated locally using a variety of chemical reactions. In contrast, helium is a byproduct of natural gas production and must be transported to its point of use. Hydrogen is less expensive than helium and provides slightly more lift per volume (1.1 kg per cubic meter vs 1.0 kg per cubic meter at sea level). The primary advantage of helium is that it non-flammable whereas hydrogen burns readily in air.

Until the 1950s, all airships, except for those in the United States were filled with hydrogen. The United States (until then the sole producer) was also unwilling to export helium because it was rare and was considered a strategic material. However, modern passenger-carrying airships today are prohibited, by aviation regulations, from being filled with hydrogen. Nonetheless, some small experimental ships still use hydrogen today.

Some small ships, called thermal airships, are filled with hot air in a fashion similar to hot air balloons.

History

The development of airships was necessarily preceded by the development of balloons. (See balloon (aircraft) for details.) It was immediately recognized that for balloons to be generally useful, they would need to be able to move in some direction other than the prevailing winds. It would take nearly 130 years, and the invention of the internal combustion engine, for this dream of controlled flight to come true.

Pioneers

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 developments moved in fits and starts throughout the 1800s.

The first person to make an engine-powered flight was Henri Giffard who, in 1852, flew 27 km (17 miles) in a steam-powered airship.

In 1863, Dr. Solomon Andrews devised the first fully steerable airship, although it had no motor. The aircraft would "swim" forward as it moved upward and downward by means of alternately dropping ballast and venting gas.

In 1883, the first electric-powered flight was made by Gaston Tissandier, who fitted a 1-1/2 horsepower electric motor to an airship.

The first fully controllable free-flight was made in a French Army airship, "La France", by Charles Renard and Arthur Krebs in 1884. The 170 foot long , 66,000 cubic foot airship covered 8 km (5 miles) in 23 minutes with the aid of an 8-1/2 horsepower electric motor.

None of these efforts led to ongoing development. Continuous development that led to today's designs began in 1898 with the work of the remarkable Alberto Santos-Dumont. Santos-Dumont built 11 airships, driven by internal combustion engines. His were the first aircraft to make routine controlled flights.

The zenith of Santos-Dumont's flying career came in 1901 when his airship "Number 6", a small blimp, won the Deutsch de la Meurthe prize of 100,000 francs for flying from the Parc Saint Cloud to the Eiffel Tower and back in under thirty minutes. Many inventors were inspired by Santos-Dumont's small airships and a veritable airship craze began world-wide.

The golden age of airships

Although the blimps of Santos-Dumont's lineage survive to this day, airships are best known for a brief period when large rigid airships, although now extinct, were the most prevalent aviation technology of their time.

The beginning of the "Golden Age of Airships" is generally given as the launch of the Luftschiff Zeppelin LZ1 in July of 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 some patents and the LZ1 (1900) and the LZ2 (1906). At the beginning of World War I the Zeppelin airships had a cylindrical aluminium alloy frame and a fabric-covered hull containing separate gas cells. Multi-plane 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.

It is worth noting that Count Zeppelin's designs were not entirely his own creation. In 1896, a rigid airship created by Croatian engineer David Schwarz made its first flight at Tempelhof field in Berlin. After Schwarz's death, Count von Zeppelin paid his widow a sum of 15,000 Marks for information about Schwarz's designs. Although some aspects of the early Zeppelin designs clearly reflect Schwarz's work, many aspects are novel.

Count Zeppelin and others in the German military believed they had found the ideal weapon with which to counteract British Naval superiority and strike at Britain itself. More realistic airship advocates believed the Zeppelin was a valuable long range scout/attack craft for naval operations. Raids began by the end of 1914, reached a first peak in 1915, and then were discontinued after 1917.

Zeppelins proved to be terrifying but inaccurate weapons. Navigation, target selection and bomb-aiming proved to be difficult under the best of conditions. The darkness, high altitudes and clouds that were frequently encountered by zeppelin missions reduced accuracy even further. The physical damage done by the zeppelins over the course of the war was trivial, and the deaths that they caused (though visible) amounted to a few hundred at most.

The wartime Zeppelins also proved to be vulnerable to attack by aircraft and antiaircraft guns, especially those armed with incendiary bullets. Several were shot down in flames by British defenders, and others crashed en route. In retrospect, advocates of the naval scouting role of the airship proved to be correct, and the land bombing campaign proved to be disastrous in terms of morale, men and material. Many pioneers of the German airship service died bravely, but needlessly in these propaganda missions. They also drew unwanted attention to the construction sheds which were bombed by the British Royal Naval Air Service.

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.

Several large rigid airships were built in the United States and United Kingdom in the 1920s and 1930s, mostly imitating original Zeppelin design derived from crashed or captured German World War I airships.

The first American-built rigid dirigible was the USS Shenandoah, which flew in 1923 and was the first to be inflated with helium rather than hydrogen.

The Zeppelin works were saved by the purchase of what became the USS Los Angeles by the United States Navy, paid for with "war reparations" money, owed according to the Versailles Treaty. The success of the Los Angeles encouraged the United States Navy to invest in larger airships of its own. Germany, meanwhile, was building the Graf Zeppelin, the first of what was intended to be a new class of passenger airships.

Hindenburg shortly after catching fire.

Initially airships met with great success and compiled an impressive safety record. The Graf Zeppelin, for example, flew over one million miles (including the first circumnavigation of the globe by air) without a single passenger injury. The expansion of airship fleets and the growing (sometimes excessive) self-confidence of airship designers gradually made the limits of the type clear, and initial successes gave way to a series of tragic rigid airship accidents.

The most spectacular and widely remembered airship accident, however, is the burning of the Hindenburg on 6 May 1937. Of the 97 people on board, there were 36 deaths: 13 passengers, 22 aircrew, and one American ground-crewman. (Much controversy persists as to the cause(s) of the accident. See Hindenburg for a discussion of this topic.)

The destruction of the Hindenburg shattered public faith in airships. The "Golden Age of Airships" ended with its passing.

Blimp operations

Although the days of large rigid airships were over, blimps remained. During the Second World War, the primary airship tasks were patrol and convoy escort near the US coastline. They were deemed quite successful in their duties with the highest combat readiness factor in the entire US air force (87%).

During the war some 532 ships were sunk near the coast by submarines. However, not a single ship of the 89,000 or so in convoys escorted by blimps was sunk by enemy fire.

In spite of their success in convoy duty, the US Navy shut down their airship operation in the 1950s.

Noteworthy historic prototypes and experiments

The Heli-Stat was an airship / helicopter hybrid built in New Jersey in 1986.

The Aereon was a hybrid aerostatic/aerodynamic craft built in the 1970s.

The Cyclocrane was a hybrid aerostatic/rotorcraft in which the entire airship 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 [1] sponsored by the US Department of Defense [2] 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 of 2006, DARPA announced the termination of work on WALRUS after completion of the current Phase I contracts.

Present day use

Although airships are no longer used for passenger transportation, they continued to be used for other purposes such as advertising and sightseeing.

In recent years, the Zeppelin company has reentered the airship business. Their new model, designated the Zeppelin NT made its maiden flight on September 18, 1997. There are currently three NT aircraft flying.

Blimps continue to be used for advertising and as TV camera platforms at major sporting events. The most iconic of these are the Goodyear blimps. Goodyear operates three blimps in the United States. In addition, the Lightship group operates up to 19 advertising blimps around the world.

Airship Management Services, Inc. operates three Skyship 600 blimps. Two operate as advertising and security ships in the North America and the Caribbean and one operates under the name "SkyCruizer", providing sightseeing tours in Switzerland.

Los Angeles based Worldwide Aeros Corp. produces a small blimp called the Aeros 40D Sky Dragon.

In May of 2006, press reports indicated that the US Navy has started to fly airships again after a hiatus of nearly 44 years. At present, the program utilizes a single American Blimp Company A-170 non-rigid airship. Operations will focus on crew training and research with Northrop Grumman as the platform integrator. The program is under the direction of the Naval Air Systems Command and is being carried out at NAES Lakehurst, the original center of US Navy lighter-than-air operations in previous decades.

In November of 2006, the US Army purchased an A380+ airship from American Blimp Corporation through a Systems level contract with Northrop Grumman and Booz Allen Hamilton. The airship will start flight tests in late 2007 with a primary goal of carrying 2,500lbs of payload to an altitude of 15,000kft under remote control and autonomous waypoint navigation. The program will also demonstrate carrying 1,000lbs of payload to 20,000kft. The platform could be used for Multi-Intelligence collections. Northrop Grumman (formerly Westinghouse) has responsibility for the overall program.

Several companies, such as Cameron Balloons in Bristol, United Kingdom, build hot-air airships. These combine the structures of both hot-air balloons and small airships. The envelope is the normal 'cigar' shape, complete with tail fins, but is not inflated by helium, but by hot air (as in a balloon), which provides the lifting force. A small gondola, carrying the pilot (and sometimes between 1 and 3 passengers), a small engine and the burners to provide the hot air is suspended below the envelope, below an opening through which the burners protrude.

Hot-air airships typically cost less to buy and maintain than modern Helium-based blimps, and they can be quickly deflated after flights. This makes them easy to carry in trailers or trucks and inexpensive to store. Such craft are usually very slow moving, with a typical top speed of 15-20 mph. They are mainly used for advertising, but at least one has been used in rainforests for wildlife observation, as they can be easily transported to remote areas.

Remote controlled (RC) airships, a type of Unmanned Aerial System (UAS), are sometimes used for commercial purposes such as advertising and aerial video/ photography as well as for recreational purposes. They are particularly common as an advertising mechanism at indoor stadiums.

Ongoing research projects

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. However no flight tests were made during 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.

Proposed designs and application

There are several proposed long-range/large-payload designs. All of these designs are still on the drawing board.

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.

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