Large aircraft APU
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,