How do the blades in a turbofan engine spin?

Question

i know how a turbo fan works (sucks it in, compresses it, boom and out again) but how do the blades start spinning and what keeps them spinning?

Answer 1

Turbo impellers are like 2 fans stuck together.  The exhaust gas powers the turbo that turns the compressors.

http://en.wikipedia.org/wiki/Turbocharge...

Good Luck...

Answer 2

I'm not trying to be a smart @ss but I think your question in like asking why there can be a 'Cordless Phone'.  When you grow up in a generation that takes if for granted you may miss the meaning.

Jet efficiency sux at low altitude.  The good old 707 took miles of runway to become unstuck.  Fans give Propeller efficiency to Jet Engines.  They are only needed on take off.  After that they are basically a passenger.  They are connected to T1 shaft but their best contribution is in the suction they provide at low altitude.  

Fans used to be obvious because of the external ducting.  But after modifications to hang fans on the 737 they found more internal usage.

The 737 was a brilliant aircraft but not economical and if they would have put conventional fans on they would have become great FOD vacuums.  New design was required which improved the entire concept.

Answer 3

The engine is usually started with a motor that runs on compressed air.  The motor winds up the compressor fans inside the engine until enough air is flowing to start the engine, at which point fuel is introduced into the combustion can and igniters provide sparks to start combustion.  The engine then spools up to idle on its own.

The front fan is driven by a turbine separately from the compressors. It will start to turn as the engine is started, but it won't come up to speed until the engine actually starts running.

Answer 4

Most turbine engine are dual spool or twin spool, one for the LP (low pressure turbine which includes The fan blade) and another for the HPT (high pressure turbine) they are independent of each other but is both driven by the same hot compressed air. the LPT is your N1 and HPT is your N2.  the excess air is then rerouted back to exit at the casing for LPT cooling. Modern engines have HPT cooling as well which makes them more efficient.