Secret History of Airships
Secret History of Airships
An airship or dirigible is a type of aerostat or lighter-than-air aircraft that can be steered and propelled through the air using rudders and propellers or other thrust mechanisms. Unlike aerodynamic aircraft such as fixed-wing aircraft and helicopters, which produce lift by moving a wing through the air, aerostatic aircraft stay aloft by having a large “envelope” filled with a gas which is less dense than the surrounding atmosphere. The first lifting gas used was hydrogen, although this had well-known concerns over its flammability. Helium was rare in most parts of the world, but large amounts were discovered in the USA. This meant that this non-flammable gas was rarely used for airships outside of the USA. All modern airships, since the 1960s, use helium.[note 1]
The main types of airship are non-rigid (or blimps), semi-rigid and rigid. Blimps are pressure airships where internal pressure, maintained by forcing air into an internal ballonet, is used to maintain both the shape of the airship and its structural integrity. Semi-rigid airships maintain the envelope shape by internal pressure, but have some form of internal support such as a fixed keel to which control and engine gondolas, stabilizers, and steering surfaces are mounted. Rigid airships have a structural framework which maintains the shape and carries all loads such as gondolas and engines. The framework contains numerous balloons, known as “gas cells” or “gasbags” which supply static lift without having to bear any structural loading.[2] Rigid airships are often called Zeppelins, as the type was invented by Count Zeppelin and the vast majority of rigid airships built were manufactured by the firm he founded.
Airships were the first aircraft to enable controlled, powered flight, and were widely used before the 1940s, but their use decreased over time as their capabilities were surpassed by those of aeroplanes. Their decline continued with a series of high-profile accidents, including the 1937 burning of the hydrogen-filled Hindenburg, and the destruction of the USS Akron. In the 21st century airships have been used in advertising, tourism, camera platforms for sporting events, geological surveys, and aerial observation – applications where the ability to hover in one place for an extended period outweighs the need for speed and maneuverability.
In modern common usage, the terms Zeppelin, dirigible and airship are used interchangeably for any type of rigid airship, with the term blimp alone used to describe non-rigid airships. Although the blimp also qualifies as a “dirigible”, the term is seldom used with blimps. In modern technical usage, 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.
Types
In the background, ZR-3, in front of it, (l to r) J-3 or 4, K-1, ZMC-2, in front of them, “Caquot” observation balloon, and in foreground free balloons used for training. US Navy airships and balloons, 1931 Non-rigid airships (blimps) use a pressure level in excess of the surrounding air pressure to retain their shape during flight. Unlike the rigid design, the non-rigid airship’s gas envelope has no compartments. At sea level, the ballonets (internal flexible cells) are filled with air. As altitude is increased, the lifting gas expands and air from the ballonets is expelled through air valves to maintain the same hull shape. To return to sea level, the process is reversed. Air is forced back into the ballonets by both scooping air from the engine exhaust and using auxiliary blowers. 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. Rigid airships (Zeppelin is almost synonymous with this type) have rigid frames containing multiple, non-pressurized gas cells or balloons to provide lift. Rigid airships do not depend on internal pressure to maintain their shape and can be made to virtually any size. Metal-clad airships were of two kinds: rigid and non-rigid. Each kind used a thin gas-tight metal envelope, rather than the usual rubber-coated fabric envelope. Only four metal-clad ships are known to have been built, and only two actually flew: Schwarz’s first aluminum rigid airship of 1893 collapsed, while his second flew; the non-rigid ZMC-2 flew 1929 to 1941; while the 1929 non-rigid Slate City of Glendale collapsed on its first flight attempt.
Thermal airships use a heated lifting gas, usually air, in a fashion similar to hot air balloons.
