I assume that the thrust of engines on the two sides must be kept exactly equal otherwise the plane would start yawing. What mechanism is used to prevent this from happening?

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    $\begingroup$ Rudder trim? I'm speculating. I could be wrong. $\endgroup$ – curious_cat Jan 25 '16 at 5:24
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    $\begingroup$ Are you asking how to equalize the thrust of the engines, or how to counteract the resulting yaw? Both seem like interesting questions, but the question title asks the former while the body asks the latter. $\endgroup$ – Jeffrey Bosboom Jan 25 '16 at 6:26
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    $\begingroup$ Yaw will occur for other reasons than asymmetrical thrust, so equal thrust is not so critical. Such reasons include icing and cross-wind. $\endgroup$ – mins Jan 25 '16 at 7:17
  • $\begingroup$ @mins Cross wind during cruise is no factor on yaw, only during takeoff and landing $\endgroup$ – Chris V Jan 25 '16 at 15:41
  • $\begingroup$ Thought experiment: you're flying along at cruse in a twin-engine airliner when one of the engines suddenly fails. The thrust on each side is now very much asymmetric, yet the aircraft does not spiral out of control. Clearly, if this abnormal situation can be handled with relative ease, the thrust doesn't need to be kept exactly equal in normal operations. $\endgroup$ – Zach Lipton Jan 25 '16 at 16:20

It isn't. The rudder is used.

Yes, if there is asymmetry in thrust, the plane will yaw. And there is asymmetry in thrust. The engine thrust decreases slightly with wear and the engines are often worn out differently. There are effects like P-factor that shift the thrust axis depending on speed and attitude. And of course an engine may fail.

So thrust asymmetry happens and needs to be compensated. And it is compensated by generating some sideways force with the rudder. Either by pilot pushing the pedal, or by applying trim.

As @mins mentions, there are also effects that can cause asymmetry of drag, e.g. icing, and there are situations where the aircraft intentionally needs to fly somewhat sideways, e.g. cross-wind take-off and landing, so rudder is needed for many other things too. And when it's there, it can compensate thrust asymmetry (often, though, the engine failure dictates the minimum size of the vertical stabilizer and rudder).

Note, that P-factor occurs in single-engine propeller aircraft too, so they also have some thrust asymmetry.

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  • $\begingroup$ Don't know if this might be worth a different question, but wouldn't it be more efficient to be able to balance the thrust of the engines exactly, like with some kind of 'throttle trim'? Of course that would not work with an inoperative engine, but it could compensate for the small, wear related differences Jan Hudec mentioned. IMO deflecting rudder continuously means a lot of drag that could be avoided this way. $\endgroup$ – Rob Vermeulen Jan 25 '16 at 12:32
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    $\begingroup$ @RobVermeulen, there is no exactly in the physical world! You can only match the thrust as well as you can measure it and measuring thrust precisely is rather hard. Deflecting the rudder a little bit adds only a little bit of drag and the differences are small enough as they are that it's not worth a complicated device. $\endgroup$ – Jan Hudec Jan 25 '16 at 12:40
  • $\begingroup$ "the engines are often worn out differently" - a very good point. Most airliners will be flying with two(+) engines with very different numbers of flight hours. It's not unusual for an aircraft to have a 6,000 hour engine and a 25,000 hour engine, for example. $\endgroup$ – Jon Story Jan 25 '16 at 16:03
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    $\begingroup$ This is not true. It very much depends on the aircraft type. The B737 (NG) and B767 use both the rudder and thrust management to counter asymmetric thrust. The B787 does not use the rudder for this at all. $\endgroup$ – user3628960 Jan 25 '16 at 16:22
  • $\begingroup$ @Jan Hudec, I understand from your comment that measuring thrust exactly at the source isn't trivial. But is isn't necessary either. The input of such a feedback loop could be the resulting asymmetry as it results in yaw. Which is readily detectable: it is the same input the pilot now uses to apply the right amount of rudder trim. But well, this is merely theorizing, as I understand the amount of yaw/trim/drag is not worthwhile. And the 'complicated device' is already in place and paid for: the pilot. $\endgroup$ – Rob Vermeulen Jan 25 '16 at 18:51

This depends on the airframe and where the engines are mounted. The father inboard the engines the less of an issue a slight variation in thrust will cause. But as mentioned the rudder is the general correction for this yaw.

The closest thing I can think of (and its not really for thrust balance although that is a byproduct of its function) is a Synchronizer/Synchronizer which can be found on some multi engine propeller planes. These devices are mainly to prevent harmonic beats from becoming irritating to the occupants to do so they keep the propellors spinning at near identical RPM's. If you engines are in tune and generating about the same power you should get near identical thrust from them.

The MD80 did have an engine sync system that was able to match RPM's of the N1 or N2 stages of the jets. It seems that this may have been mainly for noise but symmetrical thrust would also have been a byproduct more or less.

Here is an interesting patent on engine sync systems

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  • $\begingroup$ Actually, for a propeller engine with constant-speed propeller the thrust and rpm are completely independent variables (or almost; rpm limits power, but high rpm with low power is perfectly normal and used during approach). So you still rely on the same power, and the same power is hard. $\endgroup$ – Jan Hudec Jan 25 '16 at 16:19

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