【63xf.com--百科】

　　airships were developed from principles

　　The large aircraft that are often seen floating over sporting events are commonly known as blimps. These are actually a type of lighter-than-air craft called an airship. Airships were developed from principles of ballooning, and like balloons they are filled with a gas that allows them to float in the air. Unlike balloons, however, airships are driven by engine-powered propellers and steered with rudders. For this reason they are also called dirigibles, from the French worddiriger, meaning ‘to steer'. Airships were used for air travel until they were replaced by aeroplanes in the early 20th century.

　　Types of Airships

　　Some airships have a rigid outer frame, while others are simply big balloons that rely on the gas to keep their shape. The nonrigid ships are also known as blimps. Most airships are long and cigar-shaped. The main section of an airship is filled with a gas that is lighter than air. Helium is the most commonly used gas, though hydrogen has also been used in airships. Hydrogen is the lightest known gas and therefore has great lifting capacity, but it also catches on fire easily and has caused many fatal airship disasters. Helium is not as light but is far safer than hydrogen because it does not burn. A gondola, also called a car, is often suspended below the gas container. This gondola holds the airship's crew and passengers. In some cases, the passenger compartment is actually within the frame of the airship.

　　How Airships Fly

　　Airships rely on several different features to enable them to fly. The gas inside the ship lifts the craft into the air and then engines propel the craft forward. The pilot uses the rudder to steer the craft. While most of the ship is filled with the lighter-than-air gas, there are also two compartments filled with normal air. The pilot can make the ship go up or down by controlling the amount of air in these compartments. To make the ship go up the pilot releases some of the normal air. Because that air is heavy, the ship becomes lighter and floats upward when it is released. To make the ship go down the pilot can fill the compartment with more air.

　　History

　　The first propeller-driven airship flew in 1852 in France. Its engine was powered by steam. After internal combustion engines were introduced later in the 19th century, progress came rapidly. The first rigid airship, with a hull of aluminium sheeting, was built in Germany in 1897. One of the most successful builders of rigid airships was Ferdinand von Zeppelin, a German nobleman. He completed his first rigid airship in 1900. His ships, called zeppelins, had a frame of girders and cross-braces, with gas kept in separate cells. They were used in bombing runs in World War I.

　　After the war, airships flew paying passengers across the Atlantic Ocean as well as on shorter trips. The largest zeppelin, the Hindenburg, was 245 metres (804 feet) long and carried as many as 110 people. However, several accidents, particularly one that destroyed the Hindenburg in 1937, and improvements in the aeroplane soon made the airship less useful for commercial use.

　　Although airships are no longer used as transportation, they are common sights above sporting events and other large gatherings. They are used for advertising as well as for providing television coverage of such events.

本文來源：http://63xf.com/content/105821.html