Jet Engine
IN the early part of the twentieth century, aricraft wre power by internal combustion engines. These engines became increasingly heavy and complex as efforts were made to create speedier planes, Aircraft engine designers realized the capabilities of the engines were limited. Attainable altitude and speed was capped because thin air reduced propeller and engine efficiency. As a heavier engine would only weigh down the craft, engineers turned to other ways to power aircraft.
Frank Whittle, a British engineer, worked on one possible solution: the jet engine. In 1930, Whittle patented a variation on a gas turbine engine already in commercial use. The engine used a turbocompressor to compress incoming air, which was mixed with fuel and ignited. The high-pressure exhaust blew out the back, producing forward thrust. The machine, however, received little attention in Britain because it was too heavy and inefficient to be mounted in a plane. After transferring its development to the United States early in World War II and making numerous modifications, Whittle saw the engine successfully power the Gloster Meteor in 1944. The Allies' first jet fighter, the Bell P-39, was not perfected in time to be used in the war.
At the time of Whittle's experiments, the Germans were independently working on a jet engine. A young engineer, Hans von Ohain, brought an idea for a jet engine to aircraft designer Ernst Heinkel (1888-1958), who was impressed enough to back Ohain. His modified engine, operating on kerosene, produced enough thrust to power the He 178, a small experimental aircraft that debuted in 1939. Later, the German military developed more powerful and reliable engines with greater fuel economy. The most successful German fighter with jet engines was the Messerschmitt Me 262, capable of flying over 550 miles per hour.
The British were the first to develop the turboprop engine, which added a propeller to the conventional jet engine. The original jet engine design, most efficient on long flights at high altitudes, wasted a great deal of power. The turboprop engine had better fuel efficiency, provided a smoother ride, and needed less maintenance than piston engines. The British design, with a Rolls Royce engine, was so successful on its emergence in 1940 that it was still in production 30 years later.
While the turboprops were superior to previous propeller-driven planes, however, they did little to relieve the inherent problems of the propeller. Rolls Royce developed a new, more fuel-efficient engine in the late 1950's with a large, propeller-like fan at the front. Much of the air drawn in by the fan actually never entered the combustion chamber, but bypassed the core engine and was expelled at the back, producing additional thrust at low speeds. An even more efficient turbofan system with two compressors was later developed. For the special needs of military aircraft, engineers devised a method to burn extra fuel in the rear of the jet pipe, called reheat, or after-burning. Though the innovation wasted fuel, it provided a big thrust increase for a small increase in engine weight. In 1950, planes like the United States' F-86 Saber and the Soviet Union's Mig 17 enabled pilots to travel at the speed of sound (Mach 1). Fighters developed in the 1960s and 70s were capable of speeds of Mach 2 and Mach 3.
The introduction of jet engines to commercial aviation cut travel time dramatically. After World War II, the British produced the world's first jet-powered airliner, the de Havilland Comet, making its maiden flight in 1949. The Boeing 707, the first widely-used jet airliner, revolutionized the industry. The cooperative British and French effort that resulted in the Concorde airliner used a Rolls Royce engine and speeded travel in Europe. The latest advances in commercial airliners deal with noise reduction, the biggest problem in modern jets. So severe is the noise problem on older, so-called "Stage II" jets (such as the Boeing 727), that the Federal Aviation Administration has required all such jets to be retired by the year 2000, or, if they remain in service, to be fitted with "hushkits" that reduce engine noise. Engineers have spent much effort on modern jets to make the noise level more comfortable for those at ground level, experimenting with several factors. Turbofans proved to be quieter than other jet engines. Some noise reduction was achieved by introducing sound-absorbing material in the intakes and nozzles of the engines; multi-lobe exhaust nozzles that mix exhaust with outside air also reduce noise levels. One has only to stand near an airport and listen to an old 727 and a newer 757 takeoff to hear the dramatic improvements in noise reduction technology since the 1970s.
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