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I've been flying the Cirrus Vision SF-50 for quite some time on my X-plane 11.

Having a single turbofan engine mounted near the V-tail, I expected the requirement of at least a little right rudder. However, it seems to be perfectly aligned throughout the roll.

Why is a turbofan any different from a propeller aircraft in this regard?

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    $\begingroup$ Consider the reason that propeller-driven singles have that torque. Would you expect the same effect to apply to a jet? Also, "do all single engine..." has the answer no, for the case of an engine/propeller that rotates the other way. The torque in that case is opposite the normal case. But that's due not to the engine's rotation than to the propeller's. $\endgroup$ – Ralph J May 10 '20 at 11:47
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    $\begingroup$ This might be useful. aviation.stackexchange.com/questions/9153/… - In Europe, some engines spin the opposite way. $\endgroup$ – NadJ May 10 '20 at 15:54
  • $\begingroup$ You are overlooking what might be the most obvious explanation: the designers of the simulator may not have modelled the aircraft accurately. $\endgroup$ – jamesqf Jun 1 '20 at 17:23
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There are four reasons why a propeller aircraft will exhibit a left-turning tendency: torque, spiraling slipstream, P-factor, and gyroscopic precession.

Torque is due to Newton's third law of motion. As the engine spins the propeller, the propeller spins the airplane in the opposite reaction. Since most propellers spin clockwise as viewed from the cockpit, this results in a tendency to roll (and therefore turn) to the left. For jet engines, the rotation of the blades is cancelled by the stator blades redirecting the airflow the other way.

The spiraling slipstream is caused by the fact that the propeller is rotating, causing the air moving through it to also rotate. This spiral flows around the plane, eventually hitting the rudder. Because, again, the engine is rotating clockwise, it will tend to push the rudder to the right, forcing the nose left. Jets don't suffer from this, because, again, the internal stators make sure the exhaust gasses are (mostly) linear.

The P-factor is caused by the fact that the airplane isn't perfectly level. It will usually have a slight nose-up angle. This means that the descending blade has a higher angle of attack, and thus produces more "lift" (a.k.a. "thrust") than the rising blade. Since most engines rotate clockwise as seen from the cockpit, this means that the blade on the right will be the one producing more thrust. Jet engines don't have this because the airflow in the engine is redirected to be parallel to the engine centerline, regardless of the plane's actual angle of attack.

Gyroscopic precession is kind of complicated, so I can't really explain why this happens without a digression into a bunch of complicated physics. Just know that, if you push on the edge of a rotating disk, it will create a force 90 degrees ahead of the point you're pushing on. For a tail-dragger aircraft, lifting the tail is the same as pushing on the top of the propeller disk, thus, precession creates a left-turning force. But then, when either kind of airplane pulls up to take off, this actually causes a right-turning force. Jet engines are no different from propellers in this regard, and will produce these forces in exactly the same way. It's just that the gyroscopic precession in a typical airplane is much, much smaller than the other forces, so the creators of X-plane probably didn't think it was worth programming such a tiny force into the game.

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    $\begingroup$ All correct, but you haven't really answered the question. This is true IF the airplane has an engine that rotates clockwise (viewed from behind). If you fly an older (mostly likely European) airplane with an engine that rotates counterclockwise, the factors above will be reversed and the airplane will exhibit a right-turning tendency. $\endgroup$ – Gerry May 10 '20 at 16:52
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    $\begingroup$ The airplane in the question didn't have a right-turning tendency. It was staying straight without rudder input. So I interpreted it more along the lines of "why doesn't it have a turning tendency at all". $\endgroup$ – HiddenWindshield May 10 '20 at 16:57
  • $\begingroup$ You're right. To avoid nitpicks on "answering the question", I've edited the title to make it clear that I was looking for this answer. $\endgroup$ – William R. Ebenezer May 10 '20 at 18:51
  • $\begingroup$ @Gerry except if the engine is in the back (pusher) forces are reversed. The Cirrus Vision is a jet in propeller-land (sky?), guzzling 60-70 gallons per hour. Love the convenient placing of the jet, not much else. X-plane 11 is a simulator, helping hold fuel costs down. $\endgroup$ – Robert DiGiovanni May 10 '20 at 19:35
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    $\begingroup$ @Robert no, a pusher will have the same tendency for the same rotation direction. (Except that if you mount the same engine on the back, you will effectively reverse its direction). All these effects create torques around the prop attachment point, except for the slipstream. The latter is usually the strongest of all, but on most pushers the tail will be out of the slipstream anyway (and if it wasn't, the yaw torque would have the same direction again). $\endgroup$ – Zeus May 11 '20 at 0:46
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"Leftward Torque" or left-turning tendencies only affects piston engine aircraft. In order to understand why this doesn't happen to turbofan aircraft, you have to know what causes these left-turning tendencies.

There are actually four different reasons why this happens: Torque, P-Factor, Spiraling Slipstream, and gyroscopic precession. Torque is the reaction of the airplane to the rotation of the propeller as it turns clockwise (from the pilot seat), the airplane will want to turn the opposite way which is to roll to the left. P-Factor is because the propeller moves clockwise so the descending blade will have higher AOA than the ascending giving more thrust from the right resulting in left yawing. This is noticed mostly during a climb. Spiraling Slip Stream is because the wind generated by the prop rotates around the aircraft and hitting the left plane of the rudder resulting in left yaw. gyroscopic precession is very complicated but it affects conventional geared airplanes more than tricycle.

All these forces except for gyroscopic precession, which is very minimal, only affects aircraft with propellers. P-factor doesn't have an effect because turbofans have blades, not propellers. Also, turbofan engines are ducted so spiraling slipstream will not be a factor. And the torque is canceled out by the turbines at the back of the engine.

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    $\begingroup$ Although this answer is mostly right, there are a few major errors. Left-turning tendencies also affect turboprop airplanes, they're not restricted to piston engines. Also, the "blades not propellers" argument is flawed; "blades" would also be affected by P-factor in exactly the same way as the blades of a propeller if the airstream was coming in at an angle. $\endgroup$ – HiddenWindshield May 10 '20 at 16:45
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In ultralights and light-sport aircraft with pusher engines mounted in a central location (slightly aft of the CG in the fore-and-aft sense, but fairly high in the vertical sense), it seems that the spiral slipstream is the dominant factor, and it also seems that the vertical fin mainly experiences the LOWER half of the spiralling slipstream, especially if the vertical fin is not very tall (e.g. the Challenger ultralight aircraft.) That is the only way to explain how an prop that rotates clockwise (as viewed from behind) would generate a right turning tendency in such an aircraft, as is very often the case. Take a look at the fixed (bendable) trim tabs on the rudder in such aircraft-- if the prop rotates clockwise as viewed from behind, they are usually bent toward the right, to move the rudder toward the left and generate a left yaw torque. If the prop rotates counterclockwise as viewed from behind, they are usually bent the opposite way.

Clearly, in such a case, it could happen that at some airspeed and angle-of-attack, P-factor (which would generate a yaw torque to the left if the prop rotates clockwise as viewed from the rear, regardless of whether the prop is ahead of the CG or behind the CG or in line with the CG) could exactly counteract the rightwards yaw torque from the lower half of the spiralling slipstream striking the vertical fin.

So it is undoubtedly not true that all single-engined aircraft (with propellers) experience a turning tendency at all airspeeds and angles-of-attack encountered throughout a take-off roll and climb.

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  • $\begingroup$ Just noticed the question was about a turbofan-powered aircraft! Oh well... $\endgroup$ – quiet flyer May 12 '20 at 17:21

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