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I have seen many jet engines' turbine blades rotate in different directions while they look on same shaft.

I wish to understand how they can rotate in different direction. The picture below explains my question more. Is there a gearbox that changes direction? Or do they use two shafts?

Still from a video on turbine engines, showing how the various stages rotate.

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  • $\begingroup$ No the two shafts are not connected mechanically. They rotate in different directions simply because the turbine blades are in different directions. $\endgroup$
    – user3528438
    Commented Jul 26 at 10:53
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    $\begingroup$ Hello eeesa, welcome to aviation.stackexchange. $\endgroup$
    – DeltaLima ♦
    Commented Jul 26 at 11:26

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The image shows a geared turbofan with two concentric shafts.

The outer shaft is connected to the final compressor stages and the first turbine stages. The inner shaft connects the first compressor stages and the final turbine stages.

Hot air from the combustor drives the first turbine stages (and thus the outer shaft) in a clockwise direction (when viewed from behind in the direction of flight). The vanes of the final turbine stages are the other way around; they rotate the inner shaft (an thus the first compressor stages) in a counter-clockwise direction.

The fan up front is connected through a gearbox to the inner shaft. The gearbox reduces the rotation speed and reverses the direction. Thus the fan rotates clockwise again.

The reason to have multiple shafts and a gear box is that each stage functions optimally at different rotation rates. You want to avoid supersonic speeds as much as possible, since the shockwaves of supersonic blade movement decrease the efficiency. Only the blade tips are allowed to go supersonic.

Therefore, the fan, which has the largest diameter should spin slowest.

The first compressor stages and the final turbine stages have lower diameter, and thus can go much faster.

The final compressor stages and the first turbine stages have even smaller diameter and see very hot air (and thus higher speed of sound) can rotate even faster.

By having two shafts and a gearbox, the design is optimised for every stage. (some designs use three shafts).

Reversing the direction of rotation helps keeping the general airflow aligned with the axis of the engine, which is more efficient than creating a swirling airflow.

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  • $\begingroup$ is that what happens in military engines too? is this also how the new engines designed? $\endgroup$
    – eeesa
    Commented Jul 26 at 11:43
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    $\begingroup$ @eeesa military engines are optimised for different requirements, depending on their intended use. Generally military engines will have different needs for efficiency, reliability, maintainability, maximum power, cruising speeds etc. than the engines used on civil aircraft. The engines on a military transport aircraft are comparable to those on civil airlines. The engines on a fighter jet are totally different, and so are those on cruise missiles. $\endgroup$
    – DeltaLima ♦
    Commented Jul 26 at 11:51
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    $\begingroup$ Flow isn't the primary reason for the high pressure and low pressure parts of the engine rotating in opposite directions Each row of rotating blades is followed by a row of stationary blades so direction of rotation of different sections doesn't matter much for flow. The main reason to reduce the gyroscope effect of the engine, which would otherwise interfere with control inputs (for example exerting a pitch torque when the pilot turns the aircraft in yaw.) To a lesser extent, power changes could exert a roll torque on the aircraft if the different sections didn;t rotate in opposite directions $\endgroup$
    – Level River St
    Commented Jul 27 at 1:12
  • $\begingroup$ @LevelRiverSt -- the RR Pegasus in the Harrier is an excellent example of how contrarotating spools helps controllability $\endgroup$
    – UnrecognizedFallingObject
    Commented Jul 27 at 3:40
  • $\begingroup$ @DeltaLima how can I read more on fighter engine exactly? any sources? books? recommendations? pls $\endgroup$
    – eeesa
    Commented Jul 27 at 10:04

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