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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 Sep 4 '17 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 Sep 5 '17 at 4:42
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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.

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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.

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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.

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Location

The general design is to group elements that need to be driven by the engine around one "accessory gearbox" which is located away from the engine centerline. A driving axial shaft in therefore required. In some cases, there are multiple accessory gearboxes.

Usually the accessories are located at the bottom of the engine fan case, making them and the gearbox away from airflow and more accessible for maintenance.

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CFM56-3, source

Here a view of an accessory gearbox with its accessories in place:

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Source

When this is not possible to put the accessories here, because the clearance between the engine nacelle and the ground is too limited, they are moved sideway as in the B737 with a CFM56-7. This gives the nacelle of the B737 its characteristics squared profile:

enter image description here
The arrow points at the accessory gearbox (source: B737 AMM)

Indeed putting obstacles to the airflow is not good for the engine efficiency, so the radial driving shaft location is chosen to minimize aerodynamic effects, usually within, or behind, one of the struts of the fan frame.

A view of the driving shaft whole gearbox without the accessories:

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CFM56 Gearbox (without accessories) and radial drive shaft, source

Accessories

Accessories are driven by a radial shaft, or a set of shafts with angles and gearboxes as required. Energy is taken from the HP spool, or the IP (intermediate pressure in engines with 3 spools, like in many Rolls Royce engines).

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Source: The Jet Engine, Rolls Royce

The generator can be seen on the right of the accessory box.

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 engine being reversible. See How is a turbine engine rotated for starting and inspections?

By design, all accessories turn with the engine.

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  • $\begingroup$ You mean that itty-bitty thing...is the generator? Just how much power does it generate? $\endgroup$ – DrZ214 Sep 4 '17 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 Sep 4 '17 at 21:13
  • $\begingroup$ Yes, at the higher frequency, it has all the copper wire, but far less iron core. $\endgroup$ – Harper Sep 4 '17 at 23:30
  • $\begingroup$ @mins So you tell me that the handheld thingy in the photo generates over 100 kW?! That's incredible! $\endgroup$ – yo' Sep 5 '17 at 11:05
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    $\begingroup$ 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 Feb 24 '18 at 21:20

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