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I'm not an aviator, I guess I'd call myself an enthusiast. I'm also a physics nut. I've always wondered about this but I equally worried my question posed to a commercial airline crew would land me in the Homeland Security Suites.

On helicopters, torque along the vertical axis is countered by a smaller tail rotor (or a twin main rotor rotating in the opposite direction).

My question is whether the turbines/fans of multi-engine jet aircraft rotate in the same direction. I.e., do both the port engine and the starboard engine rotate uniformly clockwise or counterclockwise?

It seems like if that were the case then maintenance might be easier but it would create some instability along the longitudinal axis of the airframe due to torque and moment arms.

Follow up: I guess, then, that single engine propeller aircraft utilize trim presets to compensate for the longitudinal torque generated by their propeller–correct?

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Engines on airliners turn the same direction. The torque isn't as much an issue on jets as it is on props.

A lot of multiengine prop planes have propellers that turn in opposite directions. In turbine engines this can be done in a gearbox to allow the same engine to be used on both sides.

Some turbine engines, such as the PT6, have two turbines turning in opposite directions, but the propeller friction produces additional torque in one direction.

Single engine planes sometimes have the engine mounted at a slight angle to reduce the effect of the propwash (and maybe torque?). The propwash causes imbalance at low speed and high power when the descending blade tends to push air down over the wing on that side, and and vice versa for the ascending blade.

Immediately after takeoff, the rudder of a single engine plane is normally depressed, to some extent, to make up for the uneven torque and propwash. This may be done manually, with trim settings, or both.

At higher speeds, the effect of the propeller torque is not as noticeable because the wings are held "more tightly" by the airflow.

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  • $\begingroup$ Cheers, thanks for the extended explanation. "The torque isn't as much an issue on jets as it is on props"... is that due to the greater mass (in the case of an airliner) being acted on by the torque? $\endgroup$ – cfx Jan 8 '14 at 4:40
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    $\begingroup$ Nice answer! I like, not even to correct you but maybe make it some simpler at one point. At the end you wrote that the effect isn't noticeable, or less noticable at higher speeds. This is simply because the aircraft is in it's designed optimum speed range where it is build and trimmed for. See for example addionaly attached fixed trim tabs at the rudder - these changes are normaly made during flight testing when they notice that thei design isn't as well as they expected it to be ;) $\endgroup$ – Falk Jan 8 '14 at 4:49
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    $\begingroup$ I think it's a matter of rotational air friction rather than mass. Mass would only matter during acceleration. The prop blades have a larger radius (per plane size), so more torque from the air for a given horsepower. Also, behind the compressor fans of a jet there are vanes to straighten out the airflow longitudinally, causing the air to exit the jet with less rotation. This would reduce the torque from the air. $\endgroup$ – xpda Jan 8 '14 at 4:49
  • $\begingroup$ Good point, @falk. $\endgroup$ – xpda Jan 8 '14 at 4:50
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    $\begingroup$ Falk, there is another factor at work here. When speeds are low, the prop plane has a high deck angle (angle between the direction of flight and the floor of the plane. In other words, it is flying nose high. When this is true, the descending blade has a significantly higher angle of attack than does the ascending blade. Therefore the descending blade produces more thrust. This tends to turn the nose of the plane to the left (in most planes) requiring right rudder to maintain straight flight. As speed increases and the deck angle decreases, both sides of the prop pull evenly continued $\endgroup$ – Skip Miller Jan 29 '14 at 19:46
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Torque is less of a problem due to the effective roll damping of a wing, but gyro effects and prop wash are important. Gyro effects first became an issue with rotary engines in WW I. A rotary engine has its crankshaft fixed to the airplane, and both the cylinder block and the propeller rotate. This gives better cooling at low speed and produces a flywheel effect, so the engine runs more smoothly. But when you yaw, the gyro effect pitches the aircraft up or down, so any precise maneuvering becomes very hard.

With the increasing engine power in 1916 and 1917, this effect became so severe that geared engines were developed where the cylinders rotate in one direction and the propeller in the opposite direction. As a consequence, the propeller had only half the RPM in air as it had with the cylinder block. This gave great propeller efficiency, but also big propeller diameters, so airplanes with those engines needed a high landing gear. Below is a picture of a Roland D XVI with a Siemens & Halske III counter-rotating rotary engine from 1918 (source). This was an excellent fighter aircraft for its time with almost no gyro coupling.

Roland D XVI

Today, high power propeller aircraft tend to use identical engines but left- and right-handed gearboxes so the propellers run in both directions. This is less due to gyro effects and mostly to produce benign stall characteristics. The prop wash of a propeller increases the local angle of attack on the wing on one side and decreases it on the other side, so the wing will stall first on the side with bigger angle of attack. If this side is always to the right of the propellers, the aircraft will roll right in a stall. In the WW II period, a number of multi-engined aircraft used left-hand and right-hand turning engines in order to have the prop wash cancel out.

With jets, the rotating inertias are much smaller because the diameters are smaller. But there is one exception: The Bristol-Siddeley Pegasus engine of the Kestrel, Harrier and AV-8B jump jets needs to have the low pressure spool run in opposite direction of the high pressure spool to balance its gyro effects. If that would not be the case, a yawing motion would produce a pitching motion, and vice versa. When you sit on a jet, the thrust of which is equivalent to your weight, tilting this jet fore or aft only slightly will produce a quick shift of your location, so any maneuvering in hover will become extremely hard.

USMC AV-8B Harrier in hover

USMC AV-8B Harrier in hover (picture source)

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  • $\begingroup$ Nice bit of historical perspective and extra kudos for discussing the vtol craft. $\endgroup$ – CGCampbell Jun 21 '14 at 12:14
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Let's give a short and snappy answer regarding jets: it's all about efficiency.

If a part of an engine is damaged then the engine will most likely be replaced and repaired while the aircraft is flying with another engine. An engine turning clockwise and an engine turning counterclockwise are two different engines. Let's do some simple economics now: is it more efficient to have only one extra engine you can use regardless of which engine failed or if you need an extra engine for each side? Clearly the most economical maintenance solution is to ensure both jets, regardless of the side of the airframe they attach to, are identical and thus interchangeable.

Speaking as someone who's not a scientist, the torque effect at jet engines indeed is negligible.

How about props? The biggest factor affecting props is that an ascending blade delivers less lift (in forward direction) than the descending blade. If you now chose two engines both turning in the same direction and attach these to the wings of an aircraft, one side has the descending blade outboard and on the other side it will be inboard - now you have a critical engine. If the engine where the descending blade is inboard (critical engine) fails, it creates a greater yaw than if the other engine fails. Now you need to evaluate if safety and structural design allow you to have two engines turning in the same direction.

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    $\begingroup$ Ugh. Can you revisit your second paragraph? Do you really mean to imply a jet will be replace "while flying?" $\endgroup$ – CGCampbell Jun 21 '14 at 12:09
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    $\begingroup$ Well, this would be an incredibly good maintenance job, but probably not manageable. I think everybody can understand what I wanted to say, but if you have some better words to describe it, please feel free to edit my answe. $\endgroup$ – Falk Jun 23 '14 at 1:42
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Turbine engines of the same family, all rotate in the same direction. Imagine having to build a turbine engine that rotated opposite. The cost would be over the top. The airlines would be inventorying duplicate spares for the engines.

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