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Axial CompressorAn axial compressor is the name used in the aircraft industry to refer to a particular type of compressor used in jet engines. Engines using an axial compressor are known as axial-flow. Almost all modern engines are axial-flow, the notable exception being those used in helicopters. Axial compressors are essentially a steam turbine reversed; instead of high-pressure steam flowing into the turbine and forcing it to rotate to provide power, in the compressor role power is provided from an external source in order to spin the system and compress surrounding air. In a typical jet engine, the compressor is powered by a tubine placed in the hot exhaust, which contains considerably more energy than is needed to power the compressor. That said, axial compressors use between 60% and 65% of the engine's power in order to run. This explains why jet engines are not used in cars; even standing still at a red light the engine would be running almost full out just to idle. In aircraft this is not an issue, as the engine is running almost full out for almost the entire trip. Early axial compressors offered poor effeciency, so poor that in the early 1920s a number of papers claimed that a practical jet engine would be impossible to construct. Things changed dramatically after A. A. Griffith published a seminal paper in 1926, noting that the reason for the poor performance was that existing compressors used flat blades and were essentially "flying stalled". He showed that the use of airfoils instead of the flat blades would dramatically increase effeciency, to the point where a practical jet engine was a real possibility. He concluded the paper with a basic diagram of such an engine, which included a second turbine that was used to power a propeller. Although Griffith was well known due to his earlier work on metal fatigue and stress measurement, little work appears to have started as a direct result of his paper. The only obvious effort was a test-bed compressor built by Griffith's collegue at the RAE, Haine Constant. Other early jet efforts, notably those of Frank Whittle and Hans von Ohain, were based on the much better understood centrifugal compressor which was widely used in superchargers. Griffith had seen Whittle's work in 1929 and pooh-poohed it, noting an error in the math and going on to claim that the frontal size of the engine would make it useless on a high-speed aircraft. Real work on axial-flow engines started in the late 1930s, in several efforts that all started at about the same time. In England, Haine Constant reached an agreement with the steam turbine company Metropolitan Vickers (Metrovick) in 1937, starting their turboprop effort based on the Griffith design in 1938. In 1940, after the successful run of Whittle's centrifugal-flow design, their effort was re-designed as a pure jet, the Metrovick F.2. In Germany, von Ohain had produced several working centrifugal engines, some of which had flown, but all short-term development efforts had moved on to Junkers and BMW, who used axial-flow designs. In the United States, both Lockheed and General Electric were awarded contracts in 1941 to develop axial-flow engines, the former a pure jet, the later a turboprop. Northrop also started their own project to develop a turboprop, which the US Navy eventually contracted in 1943. Westinghouse also entered the race in 1942, their project proving to be the only successful one of the British or US efforts, later becoming the J30. By the 1950s every major engine development had moved on to the axial-flow type. As Griffith had originally noted in 1929, the large frontal size of the centrifugal compressor caused it to have higher drag than the "skinnier" axial-flow type. Additionally the axial-flow design could improve its compression ratio simply by adding additional stages and making the engine slightly longer. In the centrifugal-flow design the compressor itself had to be larger in diameter, which was much more difficult to "fit" properly on the aircraft. On the other hand, centrifugal-flow designs remained much less complex (the major reason they "won" in the race to flying examples) and therefore have a role in places where size and streamlining are not so important. For this reason they remain a major solution for helicopter engines, where the compressor lies flat and can be built to any needed size without upsetting the streamlining to any great degree. Axial compressors typically conisist of alternating rows of rotating fan blades and fixed ones, the later referred to as stators. The rotating disks essentially blow the air up against the stators to compress it, the stators being slighly smaller in order to keep the air compressed as it is passed through the compressor assembly. This process is repeated a number of times, with each set of fans and stators making up a single stage. Compressors typically have between 9 and 15 stages. Improvements can be made by replacing the stators with a second set of fans rotating in the opposite direction, but these designs have generally proven to be too complex to be worthwhile. All compressors have a sweet spot relating rotational speed and pressure, with higher compressions requiring higher speeds. Early engines were designed for simplicity, and used a single large compressor spinning at a single speed. Later designs added a second turbine and divided the compressor into "low pressure" and "high pressure" sections, the later spinning faster. This two-spool design resulted in increased effeciency. Even more can be squeezed out by adding a third spool, but in practice this has proven to be too complex to make it generally worthwhile. That said, there are several three-spool engines in use. As an aircraft changes speed or altitude, the pressure of the air at the inlet to the compressor will vary. In order to "tune" the compressor for these changing conditions, designs starting in the 1950s would "bleed" air out of the middle of the compressor in order to avoid trying to compress too much air in the final stages. This was also used to help start the engine, allowing it to be spun up without compressing anything by bleeding off as much air as possible. Bleed systems were already commonly used anyway, to provide airflow into the turbine stage where it was used to cool the turbine blades. A more advanced design, the variable stator, used blades that can be individually rotated around their axis, as opposed to the power axis of the engine. For startup they are rotated to "open", reducing compression, and then are rotated back into the airflow as the external conditions require. The General Electric J79 was the first major example of a variable stator design, and today it is a common feature of most military engines. The limiting factor in jet engine design is not the compressor, but the temperature at the turbine. It is fairly easy to build an engine that can provide enough compressed air that when burnt will melt the turbine; this was a major cause of failure in early German engines. Improvements in air cooling and materials have dramatically improved the temperature performance of turbines, allowing the compression ratio of jet engines to increase dramatically. Early test engines offered perhaps 3:1 and production engines like the [Jumo 004 were about 6:1, about the same as contemporary piston engines. Improvements started immediately and have not stopped; the latest Rolls-Royce Trent operates at about 40:1, far in excess of any piston engine. Since compression ratio is strongly related to fuel economy, this eightfold increase in compression ratio really does result in an eightfold increase in fuel economy for any given amount of power, which is the reason there is strong pressure in the airline industry to use only the latest designs. Mitigating this savings is the fact that drag increases exponentially at high speeds, so while the engine is able to operate far more effeciently, this typically translates into a smaller real-world effect. For instance, the latest Boeing 737's with high-bypass CFM56 engines operates at an overall effeciency about 30% better than the earlier models.
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