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Does existing Avionics / Control Systems on large Commercial Jets make use of differential thrust of engines much?

e.g. I was contemplating applications of this sort: Normally the thrust provided by each of multiple jet engines will be different due to difference in age, wear, build etc. and this is usually compensated by rudder trim.

But instead, would it be feasible for a controller to detect the slight yaw moment and modulate the throttle setting to compensate thereby negating the use of rudder trim?

What about a more general strategy in which the use of rudder trim, or more extremely rudder itself is reduced via automated differential thrust modulation of jet engines? i.e. Using differential thrust not just yaw compensation but yaw creation as well?

Do such systems already exist? Or is there a fundamental flaw in my reasoning that make these ideas not viable?

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  • $\begingroup$ As far as I know, differential thrust is a last resort when hydraulics fail. See the A300 incident in Baghdad in 2003. If everything works, the control system does not use differential thrust. The pilot might do so manually when maneuvering on the ground, though. $\endgroup$ – Peter Kämpf Jan 28 '16 at 14:07
  • $\begingroup$ Some yaw is compensated for with the Yaw Damper as well which is capable of actuating the rudder its self. $\endgroup$ – Dave Jan 28 '16 at 14:29
  • $\begingroup$ @PeterKämpf But why not do things the other way around: i.e. You differential thrust in preference to rudder / rudder trim? Wouldn't there be an efficiency advantage? Every time you use the rudder you add drag, right? $\endgroup$ – curious_cat Jan 28 '16 at 17:45
  • $\begingroup$ @curious_cat: At small deflections the drag does not change. Only when the angle exceeds 10° would drag rise. Trimming small asymmetries with the rudder does not incur a drag penalty. $\endgroup$ – Peter Kämpf Jan 28 '16 at 21:05
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No, differential thrust is used in normal operations only when maneuvering on the ground, and then manually, not driven by a computerized control system.

It is preferable to run all engines at the same speed. Not only would you add additional frequencies, but also the beat frequency between pairs of two engines if speeds differ. This makes it harder to check with your ear that all engines are humming away nicely and generally creates more noise than when the engines run in sync. The answers to this question explain what the beat frequency is and how synchronization is achieved.

Next, the lag in engine response makes the control loop prone to oscillations. A rudder command is executed quickly and precisely, and when the deflections are below maybe 10°, no profile drag penalty is incurred. Induced drag due to lateral trim should be very small, especially at high speed. Trim deflections are normally much below that value, so an efficiency benefit from trimming with differential engine speeds is unlikely.

During take-off and initial climb both engines run at maximum thrust, and any trim requirement would reduce total thrust. This reduction is small, but sometimes every bit counts. During descent, the opposite happens and one engine might need to run at a higher thrust level than ideal. To decouple lateral trim and control from thrust makes sense in those cases.

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  • $\begingroup$ Thanks @Peter Kämpf, this made things much clearer for me too. What remains a little difficult to grasp is that rudder deflections <~10 degrees incur no drag penalty. How can lift (sideways in this case) be created without drag? How should I compare this for instance with the drag caused by aileron deflections that lead to adverse yaw? $\endgroup$ – Rob Vermeulen Jan 29 '16 at 0:17
  • $\begingroup$ @RobVermeulen: Oh, right, induced drag is indeed created. Silly me - I forgot. The statement is only true for viscous drag. $\endgroup$ – Peter Kämpf Jan 29 '16 at 10:15
  • $\begingroup$ @PeterKämpf Any quantification of how much the additional drag would be at various rudder deflections? $\endgroup$ – curious_cat Jan 29 '16 at 14:39

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