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Where are the alternators in jet planes?

  1. Mar 13, 2012 #1
    In jet aircrafts where are the alternators, how are they connected to the jet engines? Is it using belts? Are they on the wings?
    Thank you
  2. jcsd
  3. Mar 13, 2012 #2

    jim hardy

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    On Boeing 707 they were on the engines through a variable speed drive so they can be synchronized.

    Somebody may have newer infromation.
  4. Mar 13, 2012 #3
    Thank you for your answer. Can you develop it? What do you mean by "on the engines"?
  5. Mar 13, 2012 #4

    jim hardy

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  6. Mar 13, 2012 #5
    You mean inside the jet engines?
  7. Mar 13, 2012 #6

    jim hardy

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    Inside the engine pod but outside the engine.
    Probably one of those appendages surrounding engine just left of center in above image.

    A real turbine guy would know which one. Sorry, i don't.

    PrattWhitney site should have better details at their site..

    See anything here looks like an alternator?

    http://www.pw.utc.com/media_center/images_library/images/pw400094_1_high.jpg [Broken]

    http://www.pw.utc.com/media_center/images_library/images/pw400094_1_high.jpg [Broken]
    Last edited by a moderator: May 5, 2017
  8. Mar 13, 2012 #7
    I dont... Is it belt driven?
  9. Mar 13, 2012 #8

    jim hardy

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    no, aircraft design is for extreme reliability. They'll be direct driven through variable speed drive as described in second link above.

    As i said i'm not a turbine guy so can't point to the alternator.

    Search a while and you'll find service manuals for gas turbine engines.
  10. Mar 13, 2012 #9
    Thank you for your answers!
    I am reading some things about the constant speed drive unit for the alternator. I can't understand how the engine shaft will connect to a constant speed drive unit gear that then will link to the alternator, since these will have to be outside the engine. How can the engine remain airtight in the middle if it has some sort of gear to connect to the outside?
  11. Mar 13, 2012 #10

    jim hardy

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    I looked in those P&W cutaways for a gear driven shaft for auxilliaries, couldn't find one with cursory search.

    But it's easy to seal around a shaft, every automobile motor has at least two.

    here's a video for an old jet, F86 era acccessories on front end.

    and here

    and from a GE glossary
    http://www.geaviation.com/education/vocabulary.html [Broken]
    Spend some time at that site it's interesting.
    http://www.geaviation.com/engines/commercial/genx/2b_fett.html [Broken]
    Last edited by a moderator: May 5, 2017
  12. Mar 13, 2012 #11


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    Modern jets usually have a separate auxiliary power unit (APU) to generate electrical pwer. This is usually a small gas turbine whose only function is driving a generator. They are usually located in the tail of the aircraft.

    This has the advantage that the aircraft can operate under its own power on the ground, without the main engines running. It also removes the problem of running the generator at constant speed when the main engine speed changes.

    There are reliabilty consderations that have to be taken into account. The "ultimate backup generator" can be literally a wind turbine that is normally stowed in inside the wing or fuselage and deployed when all else fails. http://en.wikipedia.org/wiki/Ram_air_turbine

    The variable speed drive is not in the pressurized part of the engine. There is usually a radial drive shaft (with a gear drive) between one of the engine rotors and a unit mounted on the outside of the engine casing. The constant speed drive would be inside the external unit. This also drives the engine fuel and oil pumps, etc.
  13. Mar 13, 2012 #12
    Thank you both for your answers. jim hardy those are nice links, thank you.
    The "APUs are also used to run accessories while the engines are shut down." from http://en.wikipedia.org/wiki/Auxiliary_power_unit. Does this mean that during flight they have no use?
    Also I am trying to understand how the constant speed drive systems to link the engine to generator work. I am finding very little information, useful links just these:
    http://avstop.com/ac/apgeneral/alternator.html (sent by jim hardy)
    and there is this nice animation in http://17-11-1991.deviantart.com/art/constant-speed-drive-207261432
    But I don't seem to understand how the pump part works, or what rotates in sympathy with what in that part. If you can help...
  14. Mar 14, 2012 #13

    jim hardy

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    I dont know how quite they work either.
    But here's a thought experiment to help the mind accept that it's not too difficlut:
    A positive displacement pump can be run backwards as a motor.

    So by controlling flow through it you control its speed.

    That lets you make a very handy device - something that absorbs or delivers torque at controllable speed.

    Connect that to one axle of an automobile differential
    and to the other axle, your alternator.

    Now drive the input shaft with an engine at some nominal speed
    and you can control alternator speed by how much flow you allow through your pump.
    That's because a differential gear takes difference between two displacements.

    Differential gear was a calculating element before Henry Ford latched onto it. He was a mechanical genius. We forget what an elegant function it really performs.

    Now - if you run a positive displacement pump as a motor, its speed will be in proportion to flow through it.
    That means it can be used to measure flow.

    In 1930's a boiler fuel oil control system actually used a Model T differential to compute fuel flow. Two positive displacement pumps run as motors, one measuring supply to burners and one measuring return from burners, were geared into the axles but turned in opposite directions. Drive shaft spun at rate proportional to fuel consumed by burners and was used to set airflow to propoer air/fuel ratio..
    It was before my time but my mentor remembered it well.

    I think it was an old Smoot Controls system. Automatic Controls was not a so well developed field prior to WW2.
    Here's an old ad:

    Anyhow i was reminded of this by the two "displacement units" and differential near top of this link you gave:


    and this animation you linked - the swash plate probably controls flow through a positive displacement pump of some sort. Watch it move as you select speeds.
    It's the exact same basic idea as thought experiment above.

    Details improve but basic principles are long - lived.

    Sorry for long post, but wanted to get you thinking about workings of gears and hydraulic pumps.

    Hope this helps. I'll look at your links some more.
    THANKS !

    old jim
    Last edited: Mar 14, 2012
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