How does a jet engine spin its rotors?

SuprSi

Ok, I understand the basics - air is sucked through the engine and compressed, mixed with fuel and ignited to create thrust. What I don't understand is what makes the rotors spin in the first place. I haven't been able to find any site that explains this particular part so any help would be appreciated!

alexonhisown

Air from a pneumatic starter which is a component mounted on the accessories gearbox. The source of air to the starter is ground equipment or an APU (extra little engine). Through mechanical linkage the accessories gearbox drives the transfer gearbox, the transfer gearbox drives the turbine shaft and the turbine shaft drives the compressor shaft. Hope this makes sense.

artanevilla

Engines on smaller jets can use a generator to start it, powered by the battery.

SuprSi

alexonhisown wrote: »

Air from a pneumatic starter which is a component mounted on the accessories gearbox. The source of air to the starter is ground equipment or an APU (extra little engine). Through mechanical linkage the accessories gearbox drives the transfer gearbox, the transfer gearbox drives the turbine shaft and the turbine shaft drives the compressor shaft. Hope this makes sense.

Thanks!! Ok so, there's a starter engine on the ground to get the rotors spinning, and once they start spinning they keep spinning as more air is sucked in and ignited?

Or is the mechanical linkage you mention above part of the aircraft engine?

alexonhisown

SuprSi wrote: »

Thanks!! Ok so, there's a starter engine on the ground to get the rotors spinning, and once they start spinning they keep spinning as more air is sucked in and ignited?

Or is the mechanical linkage you mention above part of the aircraft engine?

An air start unit (ASU) provides high pressure air at huge volumes. The auxiliary power unit (APU) is an extra little engine installed on the aircraft which provides air and electrics for ground operations of the aircraft. ASU or APU will be used for engine start (more often the APU). The volume of air is the kicker . The air from the ASU is ducted to the pneumatic starter (component installed on the engine). The starter is high rpm, low torque. At 22% N2, fuel is introduced. When the N2 (turbine shaft) reaches 56% speed, the pneumatic starter will automatically cutout and is then excluded from the operation of the engine until the next engine start. After 56% N2 the fuel and air sustain the engine. The mechanical linkage is gearboxes and shafts (not the main rotor shafts).

The ASU is not installed on the aircraft. The APU and pneumatic starter are installed on the aircraft

SuprSi

Ok, I'm getting there! The APU starts the engine, but disengages once the turbine shaft reaches 56% as at this point the compression and ignition of the fuel/air mixture is enough to keep spinning.

When an aircraft is taxiing, what sort of percentage is the shaft running at? Also, is the APU the cause of the exhaust at the rear of the plane while it's loading passengers/luggage?

paulmcgrath

The same thing that starts the spark in a ICE. A battery. Or in the case of an AC an APU.

alexonhisown

The APU provides air (40psi tops) and AC elec power on the ground and in the air up to a certain altitude (depends on the aircraft). The air from the APU (or ASU if the APU is not serviceable) is ducted to the engine pneumatic starter (installed on each engine). The starter winds up the gearboxes, the gearbox turns the rear shaft, the rear shaft turns the front shaft.
The N1 and N2 percentage depends on throttle setting (low during taxi, high at take-off).
The APU is the cause of the exhaust yep
Hope this has helped and not confused you

SuprSi

Definitely helped, thanks a million!!