enter image description here

So, where do you put the electrical generator?

If it's gonna be coaxial, the only place is the rear (it will be burnt) or the front (it will get in the way of intake).

If it's gonna be coupled to a gear, I can't see how to mesh a gear onto the spinning parts, because the bypass air surrounds the entire engine.

Note: I'm interested in large turbofans of large airliners like the A380 or Boeing 787, basically anything with turbofans carrying 200+ passengers.

(P.S., does the generator spin from the LP turbine or the HP one?)

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    $\begingroup$ "bypass air surrounds the entire engine" - I think this is your misconception, there is a set of support struts and bearings which are responsible for keeping the spinning parts centered in the turbine. $\endgroup$
    – pjc50
    Commented Sep 4, 2017 at 12:10
  • $\begingroup$ @pjc50 I figured there had to be support struts somewhere, but figured they would be streamligned (very thin) and not able to support a generator. Now I see there is a radial gearshaft to drive some aux stuff, which to me seems very exotic and prompts more questions. $\endgroup$
    – DrZ214
    Commented Sep 5, 2017 at 4:42

2 Answers 2


enter image description hereImage source

The generators are usually driven from the Accessory Gearbox. This CAD drawing is from the company that makes accessory gearboxes for the Rolls Royce Trent engines driving the A330 and A350, and it shows the connection of the accessory gearbox to the high pressure rotor.

enter image description here

This picture from the book The Jet Engine by Rolls Royce shows the same arrangement: a bevel gear drives off of the HP spool, then drives another bevel gear connected to the crescent shaped Accessory Gearbox underneath the engine.

enter image description here

The gearbox itself is shown above (from the same book), with the accessories connected to the LP spool:

  • The Engine Driven hydraulic Pump
  • Fuel pump
  • Oil pump
  • Pneumatic starter, requiring the highest loads and therefore dimensioning the gear strength.
  • The IDG: Integrated Drive Generator which provides the AC power.

The accessory gearbox contains the engine starter as well, pneumatic for older types and electrical for bleedless engines. The engines are started by turning the HP rotor, and the accessory gearbox must be connected to the HP rotor for this purpose. For this reason, the HP rotor drives the generator and other accessories, although it would be more beneficial for fuel economy to use the LP compressor. Source: The Jet Engine issue 5 by Rolls Royce.


IDG Location: Dedicated pad on the accessory gearbox

If it's gonna be coaxial, the only place is the rear (it will be burnt) or the front (it will get in the way of intake).

That's a good question and remark. Actually the generator (integrated drive generator or IDG) is off engine axis and goes on the corresponding IDG drive pad of the accessory gearbox (AGB).

The accessory gearbox is a gear train with multiple teethed wheels enclosed in a housing, located at the bottom of the fan case, or on its side when the engine nacelle would be too close to the ground. AGB for the CFM56-7B (Boeing 737 NG):

CFM56-7B AGB located on the side of the fan frame
CFM56-7B AGB located on the side of the fan frame. Source.

Same engine, with accessories in place:

Accessories in place on the CFM56-7B AGB
Accessories in place (NTSB inspection after fatal accident). Source.

The IDG is visible below the starter (the large cylinder with its long vertical bleed air pipe), identifiable by its 3 colored phase connections and wires also running vertically to the pylon connectors).

How is AGB driven by the engine?

Does the generator spin from the LP turbine or the HP one

HP (N2) shaft. I believe the first reason to do this is because the AGB is also used to start the engine, and the starter must drive the high pressure compressor rather than the booster.

If it's gonna be coupled to a gear...

Yes there are several gears in the way.

On the CFM56-7B engine, the N2 shaft speed is about 14,400 rpm, but the fuel pump works at 6,000 rpm, the IDG is more efficient at 8,000 and the hydraulic pump operates at 4,000. The purpose of the three gearboxes and the gear train in the AGB is to provide individual accessories with a support and a shaft at the desired rpm.

The wheels in the AGB are driven by the high pressure shaft (N2) using the inlet gearbox (IGB) located near the N2 shaft within the fan frame hub. Internal configuration, same engine:

enter image description here
How is the AGB driven by the engine high pressure shaft

  • The gearbox on N2 (inlet gearbox or IGB) rotates a radial shaft.

  • The radial shaft, which runs within one of the fan frame struts to cross both the core airflow and the fan airflow, drives a second gearbox located outside of the engine, attached to the fan frame case

  • The second right-angle gearbox (transfer gearbox or TGB) drives a horizontal shaft which itself drives the AGB gear train.

  • The AGB provides the drive pads with their shaft.

Same accessory gearbox with related components, ready to be driven by N2 shaft:

CFM56-7B AGB with its TGB and radial shaft
CFM56-7B AGB with its TGB and radial shaft. Source.

Allocation of dedicated pads on the AGB

Accessory pads on the CFM56-7B AGB
Accessory pads on the CFM56-7B AGB. Source: CFM56-7B Engine Manual

AGB variations

Each engine has a different, but similar accessory gearbox. Another example:

enter image description here
AGB on a Rolls-Royce engine. Source.

Two electric generators actually and possibly the starter used as a generator

Note that on both AGB, there are two electric generators:

  • One is the smaller engine electrical generator (control alternator on the first AGB) which is used by the engine itself to feed its own electric equipment (notably the FADEC/EEC which controls the engine). This generator takes over the aicraft electrical supply (IDG) as soon as the engine is started (after the speed has reached 15% N1). The engine will continue to run in the event of aircraft electrical failure.

  • The other is the larger IDG, the main source of electric power for the aircraft.

The starter is another accessory. When starting the engine, the radial driving shaft is used in the other direction, the accessory box drives the engine shaft. Sometimes the starter and the generator are a single component, an electric motor being reversible. See How is a turbine engine rotated for starting and inspections?

  • $\begingroup$ You mean that itty-bitty thing...is the generator? Just how much power does it generate? $\endgroup$
    – DrZ214
    Commented Sep 4, 2017 at 7:46
  • $\begingroup$ @DrZ214 compared with land based electricity supplies, aircraft supplies often run at much higher AC frequency (e.g. 400Hz not 50 or 60) which means the generator can spin faster. That reduces its size and weight, for a given power output. Nobody really cares how much the generators in a power station weigh, or how big they are, but weight is critical in plane design. $\endgroup$
    – alephzero
    Commented Sep 4, 2017 at 21:13
  • $\begingroup$ Yes, at the higher frequency, it has all the copper wire, but far less iron core. $\endgroup$ Commented Sep 4, 2017 at 23:30
  • 1
    $\begingroup$ It isn't so much frequency that allows the generator to be smaller, but its output voltage. The higher the voltage, the less current for the same power; the less current, the thinner the wires can be - less copper, so less mass and less volume. Higher frequency is beneficial because higher frequencies couple better in transformers, so they can be smaller and lighter too. $\endgroup$
    – Anthony X
    Commented Feb 24, 2018 at 21:20
  • 1
    $\begingroup$ What is itty bitty, and what is hand held? I don't see any scale shown in the photos or referenced, and some of those engines are huge! $\endgroup$ Commented Dec 23, 2019 at 18:28

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