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Old 16th June 2002 | 08:05
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Nick Lappos
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Yep!

The turbines that we use are free turbines, actually two turbines that are parked close together. The Gas Turbine is the one that makes the stream of hot gas that will eventualy make all the output power. The gas producer has a compressor that packs the air, a combustion section that fuels it and burns it, and a hot section that extracts some of the power so that the compressor can be run. Also, the accessories are run by the gas producer, so the oil pump, fuel pump and other housekeeping stuff draws its power. This gas producer spins independantly, and we call its speed the N1 or Ng. About 2/3 of all the energy the engine burns is consumed by the gas producer (in effect, this 2/3 is the overhead, none of it actually does anything else but keep the engine running.)

If the engine is a pure turbojet, it ends with the gas producer. the stream of hot gas that pours out of the gas producer section is guided through a nozzel and forms the "jet" in jet engine. This thrust pushes the airplane forward.

If the engine is a turbo shaft, there is a power section that backs a new wheel into the gas stream, and connects this wheel to the transmission of the helicopter, or the prop of the turboprop. This wheel has the job of extracting almost all the power from the gas stream, leaving it just enough energy to fall out of the engine by itself. The speed of this powersection is labeled N2, or Np. Often, for ease in packaging, the power turbine shaft is run up the engine inside the gas producer shaft and out the front end of the engine. This makes the engine more compact, and lighter, since there need not be a big stiff frame to hold everything in alignment. As a bit of trivia, the N2 runs in the opposite direction as the N1, so the torques of the two counteract and cancel. This releaves some of the stress on the engine mounts.

On most modern jet airliners, there is a big propellor at the front hidden inside the engine shroud, that's why the engine pods are so big these days. Nobody wants to call them propellors, because that would look like a step backwards, but they are big shrouded props. The ratio of amount of cold air the jet engine pushes via the prop, as compared to the air that goes all the way through the combustor and leaves as hot jet is called the bypass ratio. In a pure turbojet, like the old 707's engine, the bypass was 0%, everything went through the insides. In a modern engine, the bypass ratio can be 87%, so that 7/8 or the air that the engine move is never taken inside, it is just pushed by the prop. This is a big efficiency improver, done for better gas milage. The big fan is exactly analogous to a big helicopter rotor, where less power is needed to make the thrust if the disk loading is lower.