Large aircraft APU

[balaton]

The APU engine on medium category aircraft is a simple, single stage (single spool) jet engine optimized to provide bleed air and rotational output to drive the APU generator.
On wide body, large aircraft the APU is a double stage (twin spool) jet engine. To my knowledge this type of engine was developed to provide forward thrust more efficiently and with less noise. Why is it used as APU on big airplanes? Does the N1 or N2 shaft drive the generator?
Cheers,

[NSEU]

We have P&W 901A's on our 747-400's.

The elec generators are driven by the N1 (load) gearbox. There are no Constant Speed Drives on this type, so N1 is kept close to 100% (8000rpm). N2 varies, depending on loads. The starter and accessory geabox is on the N2. The accessory gearbox drives the oil and fuel pumps.

Rgds.
NSEU

[120110]

On 330 the APU is quite similar to the 320 one.......a littel bigger and I means 3 turbine stages connected to 3 compressor stages with a shaft that moves the elec generator!

[rudderrudderrat]

Hi balaton,

Why is it used as APU on big airplanes

Besides electrics - the APU needs to deliver air for conditioning and engine start. A high bypass APU can deliver bigger quantities of LP air.

[balaton]

Oh yes.. thats the point! Thanks rudder.
b