I have heard that they do rotate in opposite directions to maintain the equilibrium of the engine. That is the net force generated by the rotation is nearly equal to zero. But, I don't think the force generated by both compressor and turbine is exactly equal and gets canceled. Am I right?

$$ F_{net}= F_{c} - F_{t} \approx 0 $$

If I am correct up to this point, how do the compressor and turbine shafts connect so that they move in the opposite direction? What additional force is produced to maintain equilibrium? By combustor? A picture would be much appreciated.


1 Answer 1


Not usually, they are almost always fixed to the same shaft so that the turbine drives the compressor directly. That means, in the same direction.

The high-bypass geared turbofan does exist, but is unusual. And even there, the fan provides minimal compression; one or more further compressor stages are still mounted on the main shaft.

You may perhaps be thinking of the two-spool turbofan, in which a first-stage compressor, inner shaft and second-stage turbine are followed by a second-stage compressor, outer (cylindrical) shaft and first-stage turbine. The two spools spin at different speed.

But even here, they usually turn the same way. A few designs do buck the trend and spin them opposite ways, see Why are contra-rotating jet engines so rare?

The turbine blades tend to resist the air flow and deflect it tangentially, while the compressor blades tend to push it backwards and (if they spin the same way) deflect it in the opposite tangential direction. The fore-aft effects in part cancel out, leaving the rotor in a state of net tension (and net thrust on the fan/compressor blades). In steady operation at constant revs, the torque effects are (of necessity) equal and opposite. They do create a net torque on the shaft, which it must be strong enough to resist. Stator blades may protrude in from the casing to counter the tangential gas flows; again, if properly designed these will result in zero tangential flow and the casing will also experience some twisting but, overall, the air will not.

A counter-rotating fan pushes the air tangentially in the same direction as the turbine. If this is not carefully countered by compressor blades attached to the turbine spool, it may create wasteful spinning and turbulence in the engine wake. The fan transfers its torque through the gearbox to the driving turbine, and vice versa. Because the gearbox reverses the torque, perhaps paradoxically the torque acts in the same sense on both shafts. But overall, whatever the configuration, torque cannot be transmitted between the spool and the casing unless the engine revs are changing. The main difference is that the thrust of the fan is decoupled from the drag of the turbine. Depending on where the thrust bearings are located, this may affect whether the thrust loads are carried by the shaft, and what their path through the casing is to the wing mounting point.

  • 1
    $\begingroup$ @Bianfable Thanks for pointing that out, I have amended my answer. $\endgroup$ Commented Jun 1, 2021 at 12:42
  • $\begingroup$ If they don't then how are the force resolved in the gas-turbine engine? Won't there be high stress in only one direction on the pylon or the whole engine? $\endgroup$
    – Auberron
    Commented Jun 1, 2021 at 15:34
  • $\begingroup$ Can you explain what forces and stresses you are asking about? (longitudinal, rotational, etc.) $\endgroup$ Commented Jun 1, 2021 at 16:06
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    $\begingroup$ @Auberron OK, I have added some general remarks about that. OK yet? $\endgroup$ Commented Jun 1, 2021 at 17:15

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