[ QUOTE ]
Ok, I found it, Free Turbine Speed...but what is free turbine speed?
[/ QUOTE ] The first turbojets had all their rotating components on a common shaft. Once turbofans were adopted, and the fan section became larger, it became evident that for better efficiency, all the components should not rotate at the same speed. It was better if the larger fan section did not rotate as fast as the smaller compressor sections of the engine. So, the solution was to use multiple shafts. The Fan section and low pressure compressor was connected to the low pressure turbine by a common shaft, which is referred to as 'N1'. A hollow outer shaft was then used to support the high pressure compressor and high pressure turbine. This is the 'N2' section of the engine. On two shaft engines, N2 is also used to drive the accessory gearbox located on the bottom of the engine through a tower shaft. The gearbox usually contains the engine oil pump, multiple scavange pumps that recover oil from various engine bearings, the generator, and the starter. During engine starts the starter will spin up the N2 section to pressurize the combustion chambers prior to fuel being introduced.
On the largest engines, two shafts was not enough, and a third shaft 'N3' is also used. This is the innermost high pressure compressor and high pressure turbine and is called 'N3'.
Most business jets only have two shaft engines, and this is also true of the regional jets as well. Although, there are some smaller engines that have a reduction gear box between the N1 shaft and the fan.
On the larger jets, 3 shaft engines are more common due to the increased size and cost of the engines. Efficiency is also much more important on the larger the aircraft.
. Hope all is well man.