# How cfm 56 -3 turbofan engine start ?

## Main Question or Discussion Point

I know all procees of cfm 56 -3 . how its work and everything but i am not understanding starting system of cfm56 - 3.

cfm 56 -3 has two shaft n1 and n2 . N1 is coneected with low pressure compressor , lp turbine and connected with fan . N2 shaft is coneected with hp turbine and hp compressor.
In starting we have to give some external power to rotate engine until fuel fire.so first we have to rotate n2 HP turbine and high pressure compressor . but how n2 start to rotate? n1 and n2 are not mechanically connected. Gas stream passing across the turbine is only link between them.

if i want to design hydraulics starting system for cfm 56 -3 . how can we calculated how much horsepower required to rotate high spool ? how can we calculate moment of inertia of turbine ? can anybody explain me how starting system work for cfm 56 -3 ?

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Well one way to start a jet engine is to simply blow air through it. Get enough air mass moving through it and the engine will start.

However, on larger engines (well most modern ones anyway) I've always been under the impression they use a motor of some sort - usually electric - to turn the turbines to generate airflow. They work via a gearing system attached to the shaft.

In the latter case, you either need the APU running or you need a GPU in order to power the motor.

Haven't flown the heavy metal high bypass motors yet, but on the F404's in my jet, we fire up the APU, and then the air from that is diverted through the L and R bleed air circuits to spin the compressor section. Once spun up through about 15-20% N1, you have enough air flow to introduce fuel (ie put the throttle up into idle), and it will spool up and light off from there. Regardless, you start turbine engines by cranking the compressor section in some way, which pushes compressed air through the combustion chamber area, allowing for fuel and spark to get the cycle going. Not sure exactly what you are wondering here, but if you are thinking that the turbines are cranked in order to drive the compressor (as they are in steady state operation), then you have it backwards in terms of starting.

Previous poster is correct, that in older generation aircraft, normally a GPU or "huffer" is used to do the same thing. Normally some sort of a gas combustion engine on a start cart, that pushes airflow into a hose that is connected to the motor housing and feeds this air through the compressor to get it turning. Most modern aircraft have a self contained start capability for ease of ground handling and allowing for the ability to operate out of fields that don't have such equipment available.

but how n2 start to rotate? n1 and n2 are not mechanically connected. Gas stream passing across the turbine is only link between them.
That's it. Everything's in windmill until pressures build beyond certain levels. Start is done at that level, and things stabilize almost immediately.

n1 and n2 are coaxial, but not mechanically connected, as it's simply more efficient that way.

Very diminishing returns (as below optimum) to continue to an n3 stage using this approach or beyond.

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