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Why does the LP compressor rotate in the opposite direction of the HP compressor in some multi-spool turbine engines?

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    $\begingroup$ most likely to balance the gyro effects. Do you have an example of such an engine? $\endgroup$ – Federico Jun 26 '17 at 11:18
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    $\begingroup$ I'd like an example as well. All engines I know of, 2-spool or 3-spool, rotate in the same direction to minimize wear between stages. $\endgroup$ – MrBrushy Jun 26 '17 at 16:24
  • $\begingroup$ Related: aviation.stackexchange.com/questions/31613/… $\endgroup$ – Peter Kämpf Jun 26 '17 at 19:28
  • $\begingroup$ Honeywell TFE731 engines have spools that rotate in opposite directoins. $\endgroup$ – wbeard52 Jun 26 '17 at 20:41
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Contra-rotating spools are a very recent development, apart from the Bristol-Siddeley (later: Rolls-Royce) Pegasus engine for the Harrier. The Rolls-Royce Trent 900 started the trend when its high pressure spool was made to rotate in the opposite direction to the other two spools. From Wikipedia:

It is also the first member of the Trent family to feature a contra-rotating HP spool

Next was General Electric with the GEnx, a two-spool design with a high pressure compressor rotating clockwise while the low pressure compressor rotates counterclockwise.

Contra-rotation means that the relative speeds between the spools are increased, increasing the bearing losses between them. However, contra-rotation helps to increase efficiency - now the stator between the last low pressure stage and the first high pressure stage of the compressor and the stator between the last high pressure turbine stage and the first low pressure turbine stage can be left away which reduces parts count, engine length, engine mass and friction.

Again Wikipedia:

Counter-rotating spools for the reaction turbines to reduce load on guide vanes.

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  • $\begingroup$ I've heard somewhere that contra-rotation is done to counter the gyroscopic precision by the force ( thrust ) compressors have made , and as you told in your description the interacting stator vane rows have been omitted but what is the reason of this elimination? $\endgroup$ – Mehdi shelby Jun 27 '17 at 12:48
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    $\begingroup$ @Mehdishelby: It's about precession and concerns only the Harrier. Stators are necessary because of the flow direction after the preceding stage, but only when they have the same direction of movement. $\endgroup$ – Peter Kämpf Jun 27 '17 at 16:14

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